An exploratory steady-state redox model of photosynthetic linear electron transport for use in complete modelling of photosynthesis for broad applications

A photochemical model of photosynthetic electron transport (PET) is needed to integrate photophysics, photochemistry, and biochemistry to determine redox conditions of electron carriers and enzymes for plant stress assessment and mechanistically link sun-induced chlorophyll fluorescence to carbon assimilation for remotely sensing photosynthesis. Towards this goal, we derived photochemical equations governing the states and redox reactions of complexes and electron carriers along the PET chain. These equations allow the redox conditions of the mobile plastoquinone pool and the cytochrome b₆ f complex (Cyt) to be inferred with typical fluorometry. The equations agreed well with fluorometry measurements from diverse C₃ /C₄ species across environments in the relationship between the PET rate and fraction of open photosystem II reaction centres. We found the oxidation of plastoquinol by Cyt is the bottleneck of PET, and genetically improving the oxidation of plastoquinol by Cyt may enhance the efficiency of PET and photosynthesis across species. Redox reactions and photochemical and biochemical interactions are highly redundant in their complex controls of PET. Although individual reaction rate constants cannot be resolved, they appear in parameter groups which can be collectively inferred with fluorometry measurements for broad applications. The new photochemical model developed enables advances in different fronts of photosynthesis research.

Tissue culture coupled with a gas exchange system offers new perspectives on phenotyping the developmental biology of Solanum lycopersicum L. cv. 'MicroTom'

Solanum lycopersicum L. cv. 'Microtom' (MicroTom) is a model organism with a relatively rapid life cycle, and wide library of genetic mutants available to study different aspects of plant development. Despite its small stature, conventional MicroTom research often requires expensive growth cabinets and/or expansive greenhouse space, limiting the number of experimental and control replications needed for experiments, and can render plants susceptible to pests and disease. Thus, alternative experimental approaches must be devised to reduce the footprint of experimental units and limit the occurrence problematic confounding variables. Here, tissue culture is presented as a powerful option for MicroTom research that can quell the complications associated with conventional MicroTom research methods. A previously established, non-invasive, analytical tissue culture system is used to compare in vitro and conventionally produced MicroTom by assessing photosynthesis, respiration, diurnal carbon gain, and fruit pigments. To our knowledge, this is the first publication that measures in vitro MicroTom fruit pigments and compares diurnal photosynthetic/respiration responses to abiotic factors between in vitro and ex vitro MicroTom. Comparable trends would validate tissue culture as a new benchmark method in MicroTom research, as it is like Arabidopsis, allowing replicable, statistically valid, high throughput genotyping and selective phenotyping experiments. Combining the model plant MicroTom with advanced tissue culture methods makes it possible to study bonsai-style MicroTom responses to light, temperature, and atmospheric stimuli in the absence of confounding abiotic stress factors that would otherwise be unachievable using conventional methods.

Granal thylakoid structure and function: explaining an enduring mystery of higher plants

In higher plants, photosystems II and I are found in grana stacks and unstacked stroma lamellae, respectively. To connect them, electron carriers negotiate tortuous multi-media paths and are subject to macromolecular blocking. Why does evolution select an apparently unnecessary, inefficient bipartition? Here we systematically explain this perplexing phenomenon. We propose that grana stacks, acting like bellows in accordions, increase the degree of ultrastructural control on photosynthesis through thylakoid swelling/shrinking induced by osmotic water fluxes. This control coordinates with variations in stomatal conductance and the turgor of guard cells, which act like an accordion's air button. Thylakoid ultrastructural dynamics regulate macromolecular blocking/collision probability, direct diffusional pathlengths, division of function of Cytochrome  b₆ f complex between linear and cyclic electron transport, luminal pH via osmotic water fluxes, and the separation of pH dynamics between granal and lamellar lumens in response to environmental variations. With the two functionally asymmetrical photosystems located distantly from each other, the ultrastructural control, nonphotochemical quenching, and carbon-reaction feedbacks maximally cooperate to balance electron transport with gas exchange, provide homeostasis in fluctuating light environments, and protect photosystems in drought. Grana stacks represent a dry/high irradiance adaptation of photosynthetic machinery to improve fitness in challenging land environments. Our theory unifies many well-known but seemingly unconnected phenomena of thylakoid structure and function in higher plants.

A Noninvasive Gas Exchange Method to Test and Model Photosynthetic Proficiency and Growth Rates of In Vitro Plant Cultures: Preliminary Implication for Cannabis sativa L

Simple Summary

The gas exchange system presented herein integrates open-flow/force ventilation, LED technology, and micropropagation to determine the impact of environmental factors (e.g., [CO₂], sucrose, light intensity) on the photosynthetic capacity of cultured plantlets. This system was developed and tested on Cannabis sativa L., an emerging crop of high economic value, for which micropropagation has become an important aspect of production. Since conventional micropropagation avenues can minimize photosynthetic performance, this system offers fresh opportunities to examine the role of light signaling and photosynthesis in micropropagation to investigate and overcome in-vitro-associated morphophysiological disorders. By maintaining [CO₂] at controlled levels (400 and 1200 ppm) with calibrated light intensities, photosynthetic light response curves were prepared based on net carbon exchange rates (NCERs) to paint a picture of the dynamic, combinational influences of irradiance, [CO₂], and additional factors on photosynthetic performance. Additionally, NCERs were continuously monitored during a 24 h light/dark period under standard conditions to provide estimates of relative growth rates (daily C-gain). Thus, a system is presented with the ability to answer questions about the nature of in vitro plant physiology related to carbon dynamics, that would otherwise be difficult to assess.

Abstract

Supplemental sugar additives for plant tissue culture cause mixotrophic growth, complicating carbohydrate metabolism and photosynthetic relationships. A unique platform to test and model the photosynthetic proficiency and biomass accumulation of micropropagated plantlets was introduced and applied to Cannabis sativa L. (cannabis), an emerging crop with high economic interest. Conventional in vitro systems can hinder the photoautotrophic ability of plantlets due to low light intensity, low vapor pressure deficit, and limited CO₂ availability. Though exogenous sucrose is routinely added to improve in vitro growth despite reduced photosynthetic capacity, reliance on sugar as a carbon source can also trigger negative responses that are species-dependent. By increasing photosynthetic activity in vitro, these negative consequences can likely be mitigated, facilitating the production of superior specimens with enhanced survivability. The presented methods use an open-flow/force-ventilated gas exchange system and infrared gas analysis to measure the impact of [CO₂], light, and additional factors on in vitro photosynthesis. This system can be used to answer previously overlooked questions regarding the nature of in vitro plant physiology to enhance plant tissue culture and the overall understanding of in vitro processes, facilitating new research methods and idealized protocols for commercial tissue culture.

A Perspective Emphasizing Circadian Rhythm Entrainment to Ensure Sustainable Crop Production in Controlled Environment Agriculture: Dynamic Use of LED Cues

World-wide, sustainable crop production is increasingly dependent on the protection of crops from adverse local climate conditions by using controlled environment agriculture (CEA) facilities. Today's greenhouses and plant factories are becoming very technologically advanced. Important breakthroughs in our understanding of the deployment of affordable artificial lighting systems that can supplement and even replace solar radiation is the subject of this perspective article. The key to improving sustainable CEA is to synchronize those environmental cues that best entrain the natural circadian rhythm of the crop. Patterns of circadian rhythms reflect the balance of daily metabolic cycles and phenological stages of development that integrate and anticipate environmental changes for all complex organisms. Within the last decade, our understanding of the use of light-emitting diodes (LEDs) as spectrally tunable tools for stimulating plant responses has expanded rapidly. This perspective proposes that extending the photoperiod in CEA is an economically sustainable goal to for year-round productivity of tomato, using dynamic LED shifts that entrain the circadian rhythm. When the photoperiod is extended too far, tomato experiences injury. To avoid yield reduction, we look to nature for clues, and how circadian rhythms evolved in general to long-photoperiods during the summer in high-latitudes. It follows that circadian rhythm traits are good targets for breeders to select new tomato cultivars suitable for CEA. Circadian rhythm entrainment, using dynamic LED cues, can be tailored to any latitude-of-origin crop, and thus expands the strategies ensuring sustainable food security including healthy diets locally in any region of the world.

Light Spectra and Root Stocks Affect Response of Greenhouse Tomatoes to Long Photoperiod of Supplemental Lighting

Plant biomass and yield are largely dictated by the total amount of light intercepted by the plant (daily light integral (DLI)—intensity × photoperiod). It is more economical to supply the desired DLI with a long photoperiod of low-intensity light because it uses fewer light fixtures, reducing capital costs. Furthermore, heat released by the light fixtures under a long photoperiod extended well into the night helps to meet the heating requirement during the night. However, extending the photoperiod beyond a critical length (>17 h) may be detrimental to production and lead to leaf chlorosis and a reduction in leaf growth and plant vigor in greenhouse tomato production. It is known that red light can increase leaf growth and plant vigor, as can certain rootstocks, which could compensate for the loss in plant vigor and leaf growth from long photoperiods. Therefore, this study investigated the response of tomatoes grafted onto different rootstocks to a long photoperiod of lighting under red and other light spectra. Tomato plants ‘Trovanzo’ grafted onto ‘Emperator’ or ‘Kaiser’ were subjected to two spectral compositions—100% red or a mix of red (75%), blue (20%), and green (5%) light for 17 h or 23 h. The four treatments supplied similar DLI. Leaf chlorosis appeared in all plants under 23 h lighting regardless of spectral compositions between 20 and 54 days into the treatment. The yield for 23 h mixed lighting treatment was lower than both 17 h lighting treatments. However, the 23 h red lighting treatment resulted in less leaf chlorosis and the plants grafted onto ‘Emperator’ produced a similar yield as both 17 h lighting treatments. Therefore, both spectral compositions and rootstocks affected the response of greenhouse tomatoes to long photoperiods of lighting. With red light and proper rootstock, the negative yield impact from long photoperiod lighting can be eliminated.

Continuous Light Does Not Compromise Growth and Yield in Mini-Cucumber Greenhouse Production with Supplemental LED Light

Continuous lighting (CL, 24 h) can reduce the light intensity/light capital costs used to achieve the desired amount of light for year-round greenhouse vegetable production in comparison to short photoperiods of lighting. However, growth under CL has led to leaf injury characterized by chlorosis unless a thermoperiod or alternating light spectrum during CL is used. To date, there is no literature relating to how cucumbers (Cucumissativus) respond to CL with LEDs in a full production cycle. Here, we evaluated a mini-cucumber cv. "Bonwell" grown under 4 supplemental lighting strategies: Treatment 1 (T1, the control) was 16 h of combined red light and blue light followed by 8 h of darkness. Treatment 2 (T2) had continuous (24 h) red light and blue light. Treatment 3 (T3) was 16 h of red light followed by 8 h of blue light. Treatment 4 (T4) was 12 h of red light followed by 12 h of blue light. All treatments had a supplemental daily light integral (DLI) of ~10 mol m-2 d-1. Plants from all treatments showed similar growth characteristics throughout the production cycle. However, plants grown under all three CL treatments had higher chlorophyll concentrations from leaves at the top of the canopy when compared to T1. The overall photosynthetic capacity, light use efficiency, and photosynthetic parameters related to light response curves (i.e., dark respiration, light compensation point, quantum yield, and photosynthetic maximum), as well as the quantum yield of photosystem II (PSII; Fv/Fm) were similar among the treatments. Plants grown under all CL treatments produced a similar yield compared to the control treatment (T1). These results indicate that mini-cucumber cv. "Bonwell" is tolerant to CL, and CL is a viable and economical lighting strategy for mini-cucumber production.

An Overview of the Sociological and Environmental Factors Influencing Eating Food Behavior in Canada

This review extensively discusses various socio environmental factors affecting eating behavior of the general public within Canada including the development and implementation of national policies. A framework representing the determinants of healthy eating can be grouped into four categories i.e., the individual determinants, the economic environment, the social environment and the physical environment. This framework allowed for addressing food insecurity and social economic ecosystem of Canadians. Lastly, we investigate the role in which biotechnology plays in improving food security and addresses the significant impact biotechnology has contributed toward on agriculture and the food market. Overall, this review using such sources as Web of Science, Pub Med and Scopus provides significant contribution toward understanding the social economic environment and eating behavior of people living in Canada. In conclusion, this has led to identify a research gap as there is a significant need to address the development and implementation of policies in the food and nutrition environment.

Machine Vision System for 3D Plant Phenotyping

Machine vision for plant phenotyping is an emerging research area for producing high throughput in agriculture and crop science applications. Since 2D based approaches have their inherent limitations, 3D plant analysis is becoming state of the art for current phenotyping technologies. We present an automated system for analyzing plant growth in indoor conditions. A gantry robot system is used to perform scanning tasks in an automated manner throughout the lifetime of the plant. A 3D laser scanner mounted as the robot's payload captures the surface point cloud data of the plant from multiple views. The plant is monitored from the vegetative to reproductive stages in light/dark cycles inside a controllable growth chamber. An efficient 3D reconstruction algorithm is used, by which multiple scans are aligned together to obtain a 3D mesh of the plant, followed by surface area and volume computations. The whole system, including the programmable growth chamber, robot, scanner, data transfer, and analysis is fully automated in such a way that a naive user can, in theory, start the system with a mouse click and get back the growth analysis results at the end of the lifetime of the plant with no intermediate intervention. As evidence of its functionality, we show and analyze quantitative results of the rhythmic growth patterns of the dicot Arabidopsis thaliana (L.), and the monocot barley (Hordeum vulgare L.) plants under their diurnal light/dark cycles.

Alternating Red and Blue Light-Emitting Diodes Allows for Injury-Free Tomato Production With Continuous Lighting

Plant biomass is largely dictated by the total amount of light intercepted by the plant [daily light integral (DLI) - intensity × photoperiod]. Continuous light (CL, 24 h lighting) has been hypothesized to increase plant biomass and yield if CL does not cause any injury. However, lighting longer than 18 h causes leaf injury in tomato characterized by interveinal chlorosis and yield is no longer increased with further photoperiod extension in tomatoes. Our previous research indicated the response of cucumbers to long photoperiod of lighting varies with light spectrum. Therefore, we set out to examine greenhouse tomato production under supplemental CL using an alternating red (200 µmol m-2 s-1, 06:00-18:00) and blue (50 µmol m-2 s-1, 18:00-06:00) spectrum in comparison to a 12 h supplemental lighting treatment with a red/blue mixture (200 µmol m-2 s-1 red + 50 µmol m-2 s-1 blue, 06:00-18:00) at the same DLI. Our results indicate that tomato plants grown under supplemental CL using the red and blue alternating spectrum were injury-free. Furthermore, parameters related to photosynthetic performance (i.e., Pnmax, quantum yield, and Fv/Fm) were similar between CL and 12 h lighting treatments indicating no detrimental effect of growth under CL. Leaves under CL produced higher net carbon exchange rates (NCER) during the subjective night period (18:00-06:00) compared to plants grown under 12 h lighting. Notably, 53 days into the treatment, leaves grown under CL produced positive NCER values (photosynthesis) during the subjective night period, a period typically associated with respiration. At 53 days into the growth cycle, it is estimated that leaves under CL will accumulate approximately 800 mg C m-2 more than leaves under 12 h lighting over a 24 h period. Leaves grown under CL also displayed similar diurnal patterns in carbohydrates (glucose, fructose, sucrose, and starch) as leaves under 12 h lighting indicating no adverse effects on carbohydrate metabolism under CL. Taken together, this study provides evidence that red and blue spectral alternations during CL allow for injury-free tomato production. We suggest that an alternating spectrum during CL may alleviate the injury typically associated with CL production in tomato.

Effect of elevated CO2 and spectral quality on whole plant gas exchange patterns in tomatoes

In controlled environment plant production facilities, elevating either light or CO2 levels generally has led to increased biomass and yield due to enhanced canopy photosynthesis. Today, advancements in light-emitting diodes (LEDs) have made this technology a viable option for both supplementary lighting in greenhouses and a sole lighting source in controlled environment chambers. Our study used tomato plants grown under both ambient CO2 (AC) and elevated CO2 (EC) conditions then exposed them to various CO2 and lighting treatments during both whole plant and leaf level measurements. Plants grown under EC reached the first flower developmental stage 8 days sooner and were approximately 15cm taller than those grown under AC. However, under AC plants had more leaf area while their dry weights were similar. Of note, under EC chlorophyll a and b were lower, as were carotenoids per unit leaf area. Whole plant analyses, under all CO2 challenges, showed that plants exposed to high-pressure sodium (HPS), red-blue LED, and red-white LED had similar photosynthesis, respiration, and daily carbon gain. Under different light qualities, day-time transpiration rates were similar among CO2 conditions. Day-time water-use efficiency (WUE) was higher in plants grown and exposed to EC. Similarly, WUE of plants grown under AC but exposed to short-term elevated CO2 conditions was higher than those grown and tested under AC during all light treatments. Under all CO2 conditions, plants exposed to red-white and red-blue LEDs had lower WUE than those exposed to HPS lighting. Assessing alterations due to CO2 and light quality on a whole plant basis, not merely on an individual leaf basis, furthers our understanding of the interactions between these two parameters during controlled environment production. Principle component analyses of both whole plant and leaf data indicates that increasing CO2 supply has a more dramatic effect on photosynthesis and WUE than on transpiration.

Effects of Light Quality and Intensity on Diurnal Patterns and Rates of Photo-Assimilate Translocation and Transpiration in Tomato Leaves

Translocation of assimilates is a fundamental process involving carbon and water balance affecting source/sink relationships. Diurnal patterns of CO2 exchange, translocation (carbon export), and transpiration of an intact tomato source leaf were determined during 14CO2 steady-state labeling under different wavelengths at three pre-set photosynthetic rates. Daily patterns showed that photosynthesis and export were supported by all wavelengths of light tested including orange and green. Export in the light, under all wavelengths was always higher than that at night. Export in the light varied from 65-83% of the total daily carbon fixed, depending on light intensity. Photosynthesis and export were highly correlated under all wavelengths (r = 0.90-0.96). Export as a percentage of photosynthesis (relative export) decreased as photosynthesis increased by increasing light intensity under all wavelengths. These data indicate an upper limit for export under all spectral conditions. Interestingly, only at the medium photosynthetic rate, relative export under the blue and the orange light-emitting diodes (LEDs) were higher than under white and red-white LEDs. Stomatal conductance, transpiration rates, and water-use-efficiency showed similar daily patterns under all wavelengths. Illuminating tomato leaves with different spectral quality resulted in similar carbon export rates, but stomatal conductance and transpiration rates varied due to wavelength specific control of stomatal function. Thus, we caution that the link between transpiration and C-export may be more complex than previously thought. In summary, these data indicate that orange and green LEDs, not simply the traditionally used red and blue LEDs, should be considered and tested when designing lighting systems for optimizing source leaf strength during plant production in controlled environment systems. In addition, knowledge related to the interplay between water and C-movement within a plant and how they are affected by environmental stimuli, is needed to develop a better understanding of source/sink relationships.

The Effect of Spectral Quality on Daily Patterns of Gas Exchange, Biomass Gain, and Water-Use-Efficiency in Tomatoes and Lisianthus: An Assessment of Whole Plant Measurements

Advancements in light-emitting diode (LED) technology have made them a viable alternative to current lighting systems for both sole and supplemental lighting requirements. Understanding how wavelength specific LED lighting can affect plants is thus an area of great interest. Much research is available on the wavelength specific responses of leaves from multiple crops when exposed to long-term wavelength specific lighting. However, leaf measurements do not always extrapolate linearly to the complexities which are found within a whole plant canopy, namely mutual shading and leaves of different ages. Taken together, both tomato (Solanum lycopersicum) leaves under short-term illumination and lisianthus (Eustoma grandiflorum) and tomato whole plant diurnal patterns of plants acclimated to specific lighting indicate wavelength specific responses of both H2O and CO2 gas exchanges involved in the major growth parameters of a plant. Tomato leaves grown under a white light source indicated an increase in transpiration rate and internal CO2 concentration and a subsequent decrease in water-use-efficiency (WUE) when exposed to a blue LED light source compared to a green LED light source. Interestingly, the maximum photosynthetic rate was observed to be similar. Using plants grown under wavelength specific supplemental lighting in a greenhouse, a decrease in whole plant WUE was seen in both crops under both red-blue (RB) and red-white (RW) LEDs when compared to a high pressure sodium (HPS) light. Whole plant WUE was decreased by 31% under the RB LED treatment for both crops compared to the HPS treatment. Tomato whole plant WUE was decreased by 25% and lisianthus whole plant WUE was decreased by 15% when compared to the HPS treatment when grown under RW LED. The understanding of the effects of wavelength specific lighting on both leaf and whole plant gas exchange has significant implications on basic academic research as well as commercial greenhouse production.

Increased mtPDH Activity Through Antisense Inhibition of Mitochondrial Pyruvate Dehydrogenase Kinase Enhances Inflorescence Initiation, and Inflorescence Growth and Harvest Index at Elevated CO2 in Arabidopsis thaliana

Mitochondrial pyruvate dehydrogenase (mtPDH) is a key respiratory enzyme that links glycolysis and the tricarboxylic acid cycle, and it is negatively regulated by mtPDH kinase (mtPDHK). Arabidopsis lines carrying either a constitutive or seed-specific antisense construct for mtPDHK were used to test the hypothesis that alteration of mtPDH activity in a tissue- and dosage-dependent manner will enhance reproductive growth particularly at elevated CO2 (EC) through a combined enhancement of source and sink activities. Constitutive transgenic lines showed increased mtPDH activity in rosette leaves at ambient CO2 (AC) and EC, and in immature seeds at EC. Seed-specific transgenic lines showed enhanced mtPDH activity in immature seeds. A strong relationship existed between seed mtPDH activity and inflorescence initiation at AC, and at EC inflorescence stem growth, silique number and seed harvest index were strongly related to seed mtPDH activity. Leaf photosynthetic rates showed an increase in rosette leaves of transgenic lines at AC and EC that correlated with enhanced inflorescence initiation. Collectively, the data show that mtPDHK plays a key role in regulating sink and source activities in Arabidopsis particularly during the reproductive phase.

The effects of phenotypic plasticity on photosynthetic performance in winter rye, winter wheat and Brassica napus

The contributions of phenotypic plasticity to photosynthetic performance in winter (cv Musketeer, cv Norstar) and spring (cv SR4A, cv Katepwa) rye (Secale cereale) and wheat (Triticum aestivum) cultivars grown at either 20°C [non-acclimated (NA)] or 5°C [cold acclimated (CA)] were assessed. The 22-40% increase in light-saturated rates of CO₂ assimilation in CA vs NA winter cereals were accounted for by phenotypic plasticity as indicated by the dwarf phenotype and increased specific leaf weight. However, phenotypic plasticity could not account for (1) the differential temperature sensitivity of CO₂ assimilation and photosynthetic electron transport, (2) the increased efficiency and light-saturated rates of photosynthetic electron transport or (3) the decreased light sensitivity of excitation pressure and non-photochemical quenching between NA and NA winter cultivars. Cold acclimation decreased photosynthetic performance of spring relative to winter cultivars. However, the differences in photosynthetic performances between CA winter and spring cultivars were dependent upon the basis on which photosynthetic performance was expressed. Overexpression of BNCBF17 in Brassica napus generally decreased the low temperature sensitivity (Q₁₀) of CO₂ assimilation and photosynthetic electron transport even though the latter had not been exposed to low temperature. Photosynthetic performance in wild type compared to the BNCBF17-overexpressing transgenic B. napus indicated that CBFs/DREBs regulate not only freezing tolerance but also govern plant architecture, leaf anatomy and photosynthetic performance. The apparent positive and negative effects of cold acclimation on photosynthetic performance are discussed in terms of the apparent costs and benefits of phenotypic plasticity, winter survival and reproductive fitness.

S-alk(en)yl-L-cysteine sulfoxides and relative pungency measurements of photosynthetic and nonphotosynthetic tissues of Allium porrum

Three standard assays for pyruvate gave equivalent measurements of relative pungency for two leek cultivars ( 'Tadorna' and 'Ramona'). Background pyruvate levels varied depending on the assay used, ranging from 0.4 (lactate dehydrogenase) to 1.5 (high-performance liquid chromatography, HPLC) micromol g(-1) fresh weight (FW) on average. The relative pungencies of the two leek cultivars were also compared to total concentrations of the S-alk(en)yl-L-cysteine sulfoxides (RCSOs). The average ratio of EPy to total RCSOs was 10.9, indicating that standard pungency assays underestimate the levels of RCSOs in the tissue. A detailed analysis of 'Tadorna' leaves showed that total RCSO concentrations decreased acropetally. Profiles were composed of (-/+)-methyl-, (-/+)-ethyl-, (+)-propyl-, and (+)-1-propenyl-L-cysteine sulfoxide (MCSO, ECSO, PCSO, and 1-PeCSO, respectively). (+)-PCSO was the most prominent in green (2.4 mg g (-1) FW), yellow (5.5 mg g (-1) FW), and white (3.8 mg g (-1) FW) tissues. The prop(en)yl-L-cysteine sulfoxide derivatives were dominant in tissues that had photosynthetic capacity. The (+)-MCSO levels were high in the bulb (3.6 mg g (-1) FW). Interestingly, detectable levels of (-/+)-ECSO were measured in the leaves ( approximately 0.5 mg g (-1) FW). RCSO profiles of the different tissue regions were similar, but more (+)-PCSO and (+)-1-PeCSO were detected in the bulb. In general, mature upper leaf tissues had lower levels of total RCSOs. Overall, mild extraction methods and a low-temperature HPLC protocol (preferably with long retention times) achieved adequate compound separation and resolution of the diastereomers.

The distribution of two major Iridoids in different organs of Antirrhinum majus L. at selected stages of development

Two iridoid glucosides isolated from leaves of Antirrhinum majus L. were identified as the known compounds antirrhinoside and antirrhide. Plants grown hydroponically demonstrated that antirrhinoside is present in all plant organs including the roots. In contrast, antirrhide is found only in leaves. Furthermore, both iridoids were identified in the main stem axillary leaves and leaves on the lateral branches. The highest concentrations of antirrhinoside were found in the main and lateral stems as well as the buds and flowers. As leaves age, for both cultivars, the levels of antirrhinoside drop significantly, and there is a corresponding increase in antirrhide. In spite of the different genetic backgrounds of the two cultivars, the overall distribution of the iridoids was similar during vegetative and flowering development. Radiolabeling of recently expanded axillary leaves with (14)CO(2) showed that both antirrhinoside and antirrhide were prominently labeled in the laminar tissue. However, only (14)C-antirrhinoside was recovered in the subtending petiole tissue, consistent with the suggestion that it is a phloem mobile compound.

Energy balance, organellar redox status, and acclimation to environmental stress

In plants and algal cells, changes in light intensity can induce intrachloroplastic and retrograde regulation of gene expression in response to changes in the plastoquinone redox status. We review the evidence in support of the thesis that the chloroplast acts as a general sensor of cellular energy imbalance sensed through the plastoquinone pool. Alteration in cellular energy balance caused by chloroplast or mitochondrial metabolism can induce intracellular signalling to affect chloroplastic and nuclear gene expression in response, not only to light intensity, but to a myriad of abiotic stresses. In addition, this chloroplastic redox sensing also appears to have a broader impact, affecting long-distance systemic signalling related to plant growth and development. The organization of the respiratory electron transport chains of mitochondria and heterotrophic prokaryotes is comparable to that of chloroplast thylakoid membranes, and the redox state of the respiratory ubiquinone pool is a well-documented cellular energy sensor. Thus, modulation of electron transport component redox status by abiotic stress regulates organellar as well as nuclear gene expression. From the evidence presented, we suggest that the photosynthetic and respiratory machinery in prokaryotic and eukaryotic organisms have a dual function: primary cellular energy transformation, and global environmental sensing.

Diel patterns of leaf C export and of main shoot growth for Flaveria linearis with altered leaf sucrose-starch partitioning

Diel C export from source leaves of two Flaveria linearis lines [85-1: high cytosolic fructose-1,6-bisphosphatase (cytFBPase) and 84-9: low cytFBPase] were estimated using three methods, including leaf steady-state (14)CO(2) labelling, leaf metabolite analysis, and leaf dry mass analysis in conjunction with leaf CO(2) exchange measurements. Synthesis and accumulation of starch during the daytime were much higher in 84-9. Relative (14)C-export (export as a % of photosynthesis) in the light was 36% higher in 85-1. The diel export patterns from (14)C-analyses correlated with those based on metabolite or dry weight/gas exchange analyses during the daytime, but not during the night. Night-time export estimated from (14)C-disappearance was 3.6 times lower than those estimated using the other methods. Even though the starch degradation at night was greater for 84-9, night-time export in 84-9 was similar to 85-1, since 84-9 showed both higher respiration and accumulation of soluble sugars (i.e. glucose) at night. Patterns of (14)C allocation to sink organs were also different in the two lines. Main stem growth was less in 84-9, being reduced most in the light when leaf export was lower relative to 85-1. Supplementation with sucrose for 1 h daily via the roots at a time when leaf export in 84-9 was low relative to 85-1 increased the stem growth rate of 84-9 to a level similar with that of 85-1. This study provides evidence that diel C availability predicted by source strength (e.g. C-export rate) influences main stem extension growth and the pattern of sink development in F. linearis.

An improved method for constructing and selectively silanizing double-barreled, neutral liquid-carrier, ion-selective microelectrodes

We describe an improved, efficient and reliable method for the vapour-phase silanization of multi-barreled, ion-selective microelectrodes of which the silanized barrel(s) are to be filled with neutral liquid ion-exchanger (LIX). The technique employs a metal manifold to exclusively and simultaneously deliver dimethyldichlorosilane to only the ion-selective barrels of several multi-barreled microelectrodes. Compared to previously published methods the technique requires fewer procedural steps, less handling of individual microelectrodes, improved reproducibility of silanization of the selected microelectrode barrels and employs standard borosilicate tubing rather than the less-conventional theta-type glass. The electrodes remain stable for up to 3 weeks after the silanization procedure. The efficacy of a double-barreled electrode containing a proton ionophore in the ion-selective barrel is demonstrated in situ in the leaf apoplasm of pea (Pisum) and sunflower (Helianthus). Individual leaves were penetrated to depth of approximately 150 microm through the abaxial surface. Microelectrode readings remained stable after multiple impalements without the need for a stabilizing PVC matrix.

Net Carbon Gain and Growth of Bell Peppers, Capsicum annuum 'Cubico', Following Root Infection by Pythium aphanidermatum

ABSTRACT The first characterization of alterations in whole-plant photosynthetic rate and carbon assimilation of bell peppers associated with infection by Pythium aphanidermatum is described. Relationships of root disease caused by P. aphanidermatum to whole-plant net carbon exchange rate (NCER), total carbon accumulation, dark respiration rates, water loss, and destructive growth parameters were quantified in vegetative, hydroponically grown pepper plants (Capsicum annuum 'Cubico'). Inoculated plants displayed lower whole-plant NCER. This translated into a loss of 28% in cumulative C gain during 7 days after inoculation and occurred before visible shoot symptoms developed. Leaf area and dry weight of shoots and roots were significantly decreased and the shoot/root ratio was higher in inoculated plants than in noninoculated plants. We propose that reduced NCER in inoculated plants was mainly due to restricted development of leaf area, because no differences in NCER and evapotranspiration were observed between control and inoculated plants when expressed based on leaf area and root dry mass, respectively. These findings indicate that Pythium infection did not affect the photosynthetic apparatus directly and that the reductions in photosynthesis and growth were not caused by inefficient water transport by diseased roots. These results enlarge on the understanding of physiological responses of host plants to early stages of root disease.

A Flavanone and Two Phenolic Acids from Chrysanthemum morifolium with Phytotoxic and Insect Growth Regulating Activity

Leaves of Chrysanthemum morifolium cv. Ramat were extracted sequentially with hexane, ethyl acetate, and methanol. The methanol fraction, when incorporated into artificial diet was found to reduce the growth of cabbage looper (Trichoplusia ni Hubner) larvae at concentrations between 500 and 5000 ppm of diet. Fractionation of the methanol extract on a Sephadex column yielded five fractions, three of which reduced the weight of larvae relative to the control. One fraction was analyzed using high performance liquid chromatography (HPLC) and found to contain three main constituents. These compounds were purified using a combination of gel permeation chromatography on Sephadex LH20 and HPLC, and analyzed by 1H and 13CNMR as well as undergoing chemical and physical analyses. The compounds were identified as: 1, chlorogenic acid (5-O-caffeoylquinic acid); 2, 3,5-O-dicaffeoylquinic acid; and 3, 3', 4',5-trihydroxyflavanone7-O-glucuronide (eriodictyol7-O-glucuronide). At concentrations between 100 to 1000 ppm these compounds reduced both growth and photosynthesis of Lemna gibba L. with the order of efficacy being: flavanone > chlorogenic acid > 3,5-O-dicaffeoylquinic acid. Furthermore, when incorporated separately into artificial diet these compounds, at 10 to 1000 ppm, enhanced or reduced growth of the cabbage looper (Trichoplusia ni) and gypsy moth (Lymantria dispar L.).

Daily photosynthetic and C-export patterns in winter wheat leaves during cold stress and acclimation

Diurnal patterns of whole-plant and leaf gas exchange and 14C-export of winter wheat acclimated at 20 and 5 degrees C were determined. The 5 degrees C-acclimated plants had lower relative growth rates, smaller biomass and leaf area, but larger specific leaf weight than 20 degrees C plants. Photosynthetic rates in 20 degrees C and 5 degrees C-acclimated leaves were similar; however, daytime export from 5 degrees C-acclimated leaves was 45% lower. Photosynthesis and export remained steady in 20 degrees C and 5 degrees C-acclimated leaves during the daytime. By comparison, photosynthesis in 5 degrees C-stressed leaves (20 degrees C-acclimated plants exposed to 5 degrees C 12 h before and during measurements) declined from 70 to 50% of the 20 degrees C-acclimated leaves during the daytime, while export remained constant at 35% of the 20 degrees C-acclimated and 60% of the 5 degrees C-acclimated leaves. At high light and CO2, photosynthesis and export increased in both 20 degrees C and 5 degrees C-acclimated leaves, but rates in 5 degrees C-stressed leaves remained unchanged. At all conditions daytime export was greater than nighttime export. Taken together, during cold acclimation photosynthesis was upregulated, whereas export was only partially increased. We suggest that this reflects a requirement of cold-acclimated plants to both sustain an increased leaf metabolic demand while concomitantly supporting translocation of photoassimilates to overwintering sinks.

Photosynthesis and carbon partitioning in transgenic tobacco plants deficient in leaf cytosolic pyruvate kinase

Whole-plant diurnal C exchange analysis provided a noninvasive estimation of daily net C gain in transgenic tobacco (Nicotiana tabacum L.) plants deficient in leaf cytosolic pyruvate kinase (PKc-). PKc- plants cultivated under a low light intensity (100 &mgr;mol m-2 s-1) were previously shown to exhibit markedly reduced root growth, as well as delayed shoot and flower development when compared with plants having wild-type levels of PKc (PKc+). PKc- and PKc+ source leaves showed a similar net C gain, photosynthesis over a range of light intensities, and a capacity to export newly fixed 14CO2 during photosynthesis. However, during growth under low light the nighttime, export of previously fixed 14CO2 by fully expanded PKc- leaves was 40% lower, whereas concurrent respiratory 14CO2 evolution was 40% higher than that of PKc+ leaves. This provides a rationale for the reduced root growth of the PKc- plants grown at low irradiance. Leaf photosynthetic and export characteristics in PKc- and PKc+ plants raised in a greenhouse during winter months resembled those of plants grown in chambers at low irradiance. The data suggest that PKc in source leaves has a critical role in regulating nighttime respiration particularly when the available pool of photoassimilates for export and leaf respiratory processes are low.

Sealed environment chamber for canopy light interception and trace hydrocarbon analyses

Two sealed chambers were constructed, each measuring approximately 4.5 m x 3 m x 2.5 m (LxWxH). Heat exchangers and air handling components were integrated within the sealed environment. Construction materials were chosen to minimize off-gassing and oxidation. Acceptable materials included stainless steel, Teflon (TM), glass and Heresite (TM) or baked enamel coated metal parts. The glass-topped chambers have externally mounted microwave powered light sources providing minimum PAR at canopy level of 1000 micrometers m-2 s-1. Major gases (CO2, O2) were monitored. Other environmental variables relevant to plant production (humidity, temperature, nutrient solution) were monitored and controlled continuously. Typical environment control capability and system specifications are presented. The facility is described as a venue ideally suited to address specific research objectives in plant canopy light interception, such as the roles of novel microwave powered overhead and inner-canopy light sources for dense plant canopies. In addition, control of recycled hydroponic nutrient solutions and analysis of trace atmospheric hydrocarbons in the context of sealed environment life support can be concurrently monitored.

Regulating plant/insect interactions using CO2 enrichment in model ecosystems

The greenhouse environment is a challenging artificial ecosystem in which it is possible to study selected plant/insect interaction in a controlled environment. Due to a combination of "direct" and "indirect" effects of CO2 enrichment on plant photosynthesis and plant development, canopy productivity is generally increased. In this paper, we discuss the effects of daytime and nighttime CO2 enrichment protocols on gas exchange of pepper plants (Capsicum annuum L, cv Cubico) grown in controlled environments. In addition, we present the effects of thrips, a common Insect pest, on the photosynthetic and respiratory activity of these plant canopies. Carbon dioxide has diverse effects on the physiology and mortality of insects. However, our data indicate that thrips and whiteflies, at least, are not killed "directly" by CO2 levels used to enhance photosynthesis and plant growth. Together the data suggest that the insect population is affected "indirectly" by CO2 and that the primary effect of CO2 is via its effects on plant metabolism.

Increasing plant productivity in closed environments with inner canopy illumination

Due to the high cost of habitable real estate associated with space travel and colonization, and the ultimate use of plants as the primary method of life support, it is necessary to develop cultivation methods whereby the highest sustainable level of productivity is achieved within the least amount of space. It is well known that in a dense plant canopy, lower leaves become shaded from above and eventually no longer contribute to carbon gain. In fact, they contribute to net respiratory carbon losses. One method of improving biomass production is to introduce light of suitable quantity and quality to the inner canopy, thereby utilizing unused photosynthetic capacity. By coupling microwave-powered lights to 100-mm-diameter glass tubes lined with 3M Optical Lighting Film, light with a spectral quality similar to that of sunlight was delivered to the inner canopy of a developing soybean crop. Results indicated that increases in productivity of 23-87%, as measured by CO2 assimilation, can be achieved in dense plant canopies (LAI approximately 6) when overhead lighting (40O-1200 micromoles m-2 s-1) is supplemented with inner canopy illumination.

Growth and maintenance respiration of leaflet, stipule, tendril, rachis, and petiole tissues that make up the compound leaf of pea (Pisum sativum)

Respiratory changes during development, as well as growth and maintenance coefficients, were measured in organs of a typical compound leaf at the seventh node position of a pea (Pisum sativum) plant. The leaf consists of both laminar (leaflets and stipules) and cylindrical organs (tendrils, rachis, and petiole). Young tissue of each organ had relatively high respiration rates that declined as the tissue expanded. The respiration rates of leaflet, stipule, and tendril tissue throughout maturation were significantly greater than those of the other organs. The growth respiration coefficients were not significantly different among laminar and cylindrical organs. Maintenance respiration, expressed on a total dry mass basis and on a carbohydrate-corrected dry mass basis, as well as in vitro photosynthetic rates, were significantly lower in petioles and rachises than in tendrils or the leaflets and stipules. No difference in maintenance respiration of organs was observed when rates were expressed on a protein basis. A linear relationship between mass-based respiration and organ protein concentration was observed, suggesting that the energy costs involved in protein turnover may account, in part, for the differences in maintenance respiration among the organs. Taken together, our data show that although the tendril is structurally similar to the rachis, petioles, and stem, which have a role in supporting the canopy of this climbing plant, the respiratory properties of tendrils are more like those of leaflets and stipules, thus parallelling the photosynthetic characteristics of these organs in the compound leaf. Keywords: development, leaflets, Pisum sativum, respiration, stipules, tendrils.

The Effect of Leaf Temperature and Photorespiratory Conditions on Export of Sugars during Steady-State Photosynthesis in Salvia splendens

Export and photosynthesis in leaves of Salvia splendens were measured concurrently during steady-state 14CO2 labeling conditions. Under ambient CO2 and O2 conditions, photosynthesis and export rates were similar at 15 and 25[deg]C, but both declined as leaf temperature was raised from 25 to 40[deg]C. Suppressing photorespiration between 15 and 40[deg]C by manipulating CO2 and O2 levels resulted in higher rates of leaf photosynthesis, total sugar synthesis, and export. There was a linear relationship between the rate of photosynthesis and the rate of export between 15 and 35[deg]C. At these temperatures, 60 to 80% of the carbon fixed was readily exported with sucrose, raffinose, and stachyose, which together constituted over 90% of phloem mobile assimilates. Above 35[deg]C, however, export during photosynthesis was inhibited both in photorespiratory conditions, which inhibited photosynthesis, and in nonphotorespiratory conditions, which did not inhibit photosynthesis. Sucrose and raffinose but not stachyose accumulated in the leaf at 40[deg]C. When leaves were preincubated at 40[deg]C and then cooled to 35[deg]C, export recovered more slowly than photosynthesis. These data are consistent with the view that impairment of export processes, rather than photosynthetic processes associated with light trapping, carbon reduction, and sucrose synthesis, accounted for the marked reduction in export between 35 and 40[deg]C. Taken together, the data indicated that temperature changes between 15 and 40[deg]C had two effects on photosynthesis and concurrent export. At all temperatures, suppressing photorespiration increased both photosynthesis and export, but above 35[deg]C, export processes were more directly inhibited independent of changes in photorespiration and photosynthesis.

Plant responses to short- and long-term exposures to high carbon dioxide levels in closed environments

When higher plants are exposed to elevated levels of CO2 for both short- and long-term periods photosynthetic C-gain and photoassimilate export from leaves are generally increased. Water use efficiency is increased on a leaf area basis. During long-term exposures, photosynthesis rates on leaf and whole plants bases are altered in a species specific manner. The most common pattern in C3 plants is an enhanced rate of whole plant photosynthesis in a well irradiated canopy. Nevertheless, in some herbaceous species prolonged exposure to high CO2 results in remobilization of nitrogenous reserves (i.e., leaf protein degradation) and reduced rates of mature leaf photosynthesis when assayed at ambient CO2 and O2 levels. Both short- and long-term exposures to those CO2 levels (i.e., 100 to 2,000 microliter l-1) which modify photosynthesis and export, also modify both endogenous ethylene gas (C2H4) release, and substrate, 1-aminocyclopropane-1-carboxylic acid (ACC), saturated C2H4 release rates from irradiated leaves. Photosynthetically active canopy leaves contribute most of the C2H4 released from the canopy. Prolonged growth at high CO2 results in a persistent increase in the rate of endogenous C2H4 release from leaves which can, only in part, be attributed to the increase of the endogenous pools of C2H4 pathway intermediates (e.g., methionine, M-ACC, and ACC). The capacity for increasing the rate of C2H4 release in response to short-term exposures to varying CO2 levels does not decline after prolonged growth at high CO2. When leaves, whole plants, and model canopies of tomato plants are exposed to exogenous C2H4 a reduction in the rate of photosynthesis can, in each case, be attributed to the classical effects of C2H4 on plant development and morphology. The effect of C2H4 on CO2 gas exchange of plant canopies is shown to be dependent on the canopy leaf area index.

Sink to Source Transition in Tendrils of a Semileafless Mutant, Pisum sativum cv Curly

Sink to source transition parallels loss of thigmotropic capacity in tendrils of a semileafless mutant, Pisum sativum cv Curly. Macroscopic tendril development is subdivided based on thigmotropic capacity. Stage I is the elongation stage and, although the rate of photosynthesis is similar to that of stage II and III tendrils, dark respiration rates are higher in stage I. During stage II, tendrils are thigmotropic and act as a sink. Even though stage II tendrils have CO(2) exchange characteristics similar to those of stage III tendrils, which are coiled, our fluorescein, (14)C-partitioning, and (11)C-translocation experiments suggest that stage I and II tendrils do not export carbon. Only stage III tendrils act as sources of newly fixed carbon. Export from them is blocked by cold, heat girdling of the petiole, or anoxia treatment of the tendrils. A late stage II tendril complex, in which coiling is occurring, may be exporting photoassimilates; however, this phenomenon can be attributed to the fact that the pea leaf is a compound structure and there may be one or more stage III tendrils, no longer thigmotropic, within the tendril complex. Photosynthetic maturity in pea tendrils occurs at stage III and is characterized by the ability of these tendrils to export photoassimilates.

Comparative leaf development of conventional and semileafless peas (Pisum sativum)

Comparative leaf development of conventional (cv. Improved Laxton's Progress) and semileafless (cv. Curly) peas was studied three-dimensionally, from initiation to maturity. The pattern of initiation of leaf primordia, stipules, and the pairs of lateral leaflet and tendril primordia is the same for both cultivars. However, their respective developmental pathways diverge by the time four pairs of lateral primordia have formed. In the conventional cultivar, the basal lateral primordia become increasingly dorsiventral as they develop into leaflets. Distal lateral primordia retain a cylindrical form and develop into tendrils. In contrast, basal first-order lateral primordia of the semileafless cultivar retain a cylindrical form and initiate second-order primordia, first in pairs, then in an alternate pattern. These second-order primordia develop into tendrils. Distal lateral primordium initiation and development are the same in both cultivars. Macroscopic development was subdivided into three stages based on tendril function. Stage I is an elongation phase during which the coiling response is not yet exhibited. During stage II, the tendrils are thigmotropic and retain their capacity to elongate. By stage III the tendrils have completed coiling and they no longer respond to thigmotropic stimuli. Stage I lasts an average of 1.4 ± 0.1 days in 'Improved Laxton's Progress' and 2.1 ± 0.1 days in 'Curly' from emergence from the stipule. Stage II may last up to 8 days, with an average of 6.4 ± 0.2 and 6.9 ± 0.3 days for 'Improved Laxton's Progress' and 'Curly', respectively, under greenhouse conditions for both cultivars. Key words: peas, Pisum sativum, leaf development, tendrils, afila.

Whole Plant and Leaf Steady State Gas Exchange during Ethylene Exposure in Xanthium strumarium L

The effects of ethylene evolved from ethephon on leaf and whole plant photosynthesis in Xanthium strumarium L. were examined. Ethylene-induced epinasty reduced light interception by the leaves of ethephon treated plants by up to 60%. Gas exchange values of individual, attached leaves under identical assay conditions were not inhibited even after 36 hours of ethylene exposure, although treated leaves required a longer induction period to achieve steady state photosynthesis. The speed of translocation of recently fixed (11)C-assimilate movement was not seriously impaired following ethephon treatment; however, a greater proportion of the assimilate was partitioned downward toward the roots. Within 24 hours of ethephon treatment, the whole plant net carbon exchange rate expressed on a per plant basis or a leaf area basis had dropped by 35%. The apparent inhibition of net carbon exchange rate was reversed by physically repositioning the leaves with respect to the light source. Ethylene exposure also inhibited expansion of young leaves which was partially reversed when the leaves were repositioned. The data indicated that ethylene indirectly affected net C gain and plant growth through modification of light interception and altered sink demand without directly inhibiting leaf photosynthesis.

Whole Plant CO(2) Exchange Measurements for Nondestructive Estimation of Growth

A computer controlled semiclosed net CO(2) exchange measurement system, employing an infrared gas analyzer and mass flow controllers to inject pure CO(2) at preset rates, has been developed for measuring whole plant net CO(2) exchange and net C gain in a controlled environment (i.e. CO(2), light, and temperature). Data for tomato (Lycoperscicon esculentum cv Campbell 19 VF) and rose (Rosa hybrida cv Samantha) plants grown for 4 and 17 day periods, respectively, clearly show that net C gain measured and computed using nondestructive CO(2) analysis equaled the increase in C content determined by chemical analysis following destruction of the test plants. The analysis of C gain based on CO(2) exchange allows estimation of biomass production and growth of a single population of plants under varying light and CO(2) conditions without physically handling the test plants.

Effect of Oxygen Concentration on C-Photoassimilate Transport from Leaves of Salvia splendens L

Partitioning and transport of recently fixed photosynthate was examined following (14)CO(2) pulse-labeling of intact, attached leaves of Salvia splendens L. maintained in an atmosphere of 300 microliters per liter CO(2) and 20, 210, or 500 milliliters per liter O(2). Under conditions of increasing O(2) (210, 500 milliliters per liter), a smaller percentage of the recently fixed (14)C in the leaf was allocated to starch, whereas a greater percentage of the fixed (14)C appeared in amino acids, particularly serine. The increase in (14)C in amino acids was reflected in material exported from source leaves. A higher percentage of (14)C in serine, glycine, and glutamate was recovered in petiole extracts when source leaves were maintained under elevated O(2) levels. Although pool sizes of these amino acids were increased in both the leaves and petioles with increasing photorespiratory activity, no significant changes in either (14)C distribution or concentration of transport sugars (i.e. stachyose, sucrose, verbascose) were observed. The data indicate that, in addition to being recycled intracellularly into Calvin cycle intermediates, amino acids produced during photorespiration may also serve as transport metabolites, allowing the mobilization of both carbon and nitrogen from the leaf under conditions of limited photosynthesis.

Enhancement of Ethylene Release from Leaf Tissue during Glycolate Decarboxylation : A Possible Role for Photorespiration

When leaf discs of Xanthium strumarium L. and Salvia splendens L. are incubated in sealed flasks in the light, more C(2)H(4) gas is released in the presence of added CO(2) (30-200 millimolar NaHCO(3)) than without CO(2). In Salvia, the maximum rate of C(2)H(4) release occurs when sufficient CO(2) (above 125 millimolar NaHCO(3)) is added to saturate photosynthesis confirming previous studies. The maximum rate of C(2)H(4) release from illuminated discs is similar to the rate in the dark with or without CO(2) in both species. Glycolate enhances a CO(2)-dependent C(2)H(4) evolution from illuminated leaf discs. However, the maximum rate of C(2)H(4) release with glycolate is the same as that observed with saturating CO(2). When photosynthesis is inhibited by darkness or by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, glycolate has no effect.Studies with [2,3-(14)C]-1-aminocyclopropane-1-carboxylic acid (ACC) show that the pattern of C(2)H(4) release and the specific activity of the (14)C(2)H(4) in the presence and absence of glycolate is similar to that described above, indicating that glycolate does not alter uptake of the exogenously supplied precursor (ACC) or stimulate C(2)H(4) release from an endogenous source at appreciable rates. Glycolate oxidase in vitro generates H(2)O(2) which stimulates a slow breakdown of ACC to C(2)H(4), but since exogenous glycolate is oxidized to CO(2) in both the light and the dark it is argued that the glycolate-dependent increase in C(2)H(4) release from illuminated leaf discs is not mediated directly by the action of enzymes of glycolate catabolism. The effects of glycolate and CO(2) are not easily explained by changes in stomatal resistance. The data support the view that glycolate decarboxylation at subsaturating levels of CO(2) in the light stimulates C(2)H(4) release by raising the CO(2) level in the tissue.

Effects of Glycolate Pathway Intermediates on Glycine Decarboxylation and Serine Synthesis in Pea (Pisum sativum L.)

Glycine decarboxylation and serine synthesis were studied in pea (Pisum sativum L.) leaf discs, in metabolically active intact chloroplasts, and in mitochondria isolated both partially by differential centrifugation (i.e. ;crude') and by further purification on a Percoll gradient. Glycolate, glyoxylate, and formate reduced glycine decarboxylase activity ((14)CO(2) and NH(3) release) in the crude green-colored mitochondrial fractions, and in the leaf discs without markedly altering serine synthesis from [1-(14)C]glycine. Glycolate acted because it was converted to glyoxylate which behaves as a noncompetitive inhibitor (K(i) = 5.1 +/- 0.5 millimolar) on the mitochondrial glycine decarboxylation reaction in both crude and Percoll-purified mitochondria. In contrast, formate facilitates glycine to serine conversion by a route which does not involve glycine breakdown in the crude mitochondrial fraction and leaf discs. Formate does not alter the conversion of two molecules of glycine to one CO(2), one NH(3), and one serine molecule in the Percoll-purified mitochondria. In chloroplasts which were unable to break glycine down to CO(2) and NH(3), serine was labeled equally from [(14)C]formate and [1-(14)C]glycine. The maximum rate of serine synthesis observed in chloroplasts is similar to that in isolated metabolically active mitochondria. Formate does not appear to be able to substitute for the one-carbon unit produced during mitochondrial glycine breakdown but can facilitate serine synthesis from glycine in a chloroplast reaction which is probably a secondary one in vivo.

Acclimation to High CO(2) in Bean : Carbonic Anhydrase and Ribulose Bisphosphate Carboxylase

Young bean plants (Phaseolus vulgaris L. cv Seafarer) grew faster in air enriched with CO(2) (1200 microliters per liter) than in ambient CO(2) (330 microliters per liter). However, by 7 days when increases in overall growth (dry weight, leaf area) were visible, there was a significant decline (about 25%) in the leaf mineral content (N, P, K, Ca, Mg) and a drop in the activity of two enzymes of carbon fixation, carbonic anhydrase and ribulose 1,5-bisphosphate (RuBP) carboxylase under high CO(2). Although the activity of neither enzyme was altered in young, expanding leaves during the acclimation period, in mature leaves the activity of carbonic anhydrase was reduced 95% compared with a decline of 50% in ambient CO(2). The drop in RuBP carboxylase was less extreme with 40% of the initial activity retained in the high CO(2) compared with 50% in the ambient atmosphere. While CO(2) enrichment might alter the flow of carbon into the glycolate pathway by modifying the activities of carbonic anhydrase or RuBP carboxylase, there is no early change in the ability of photosynthetic tissue to oxidize glycolate to CO(2).

Regulation of chloroplastic carbonic anhydrase : effect of magnesium

It was previously reported that magnesium ion inhibited carbonic anhydrase (Bamberger and Avron 1975 Plant Physiol 56: 481-485). Studies with partially purified carbonic anhydrase from spinach (Spinacia oleracea L.) chloroplasts show that the effect was the result of the chloride counterion and not the magnesium ion. Enzyme activity was reduced 50% upon addition of 3 to 10 millimolar MgCl(2) or KCl while all additions of MgSO(4) between 0.3 and 10 millimolar were mildly stimulatory.

Light Stimulation of Ethylene Release from Leaves of Gomphrena globosa L

The effect of light and CO(2) on both the endogenous and 1-aminocyclopropane-1-carboxylic acid (ACC)-dependent ethylene evolution from metabolically active detached leaves and leaf discs of Gomphrena globosa L. is reported. Treatment with varying concentrations of ACC did not appear to inhibit photosynthesis, respiration, or stomatal behavior. In all treatments, more ethylene was released into a closed flask from ACC-treated tissue, but the pattern of ethylene release with respect to light/dark/CO(2) treatments was the same.Leaf tissue in the light with a source of CO(2) sufficient to maintain photosynthesis always generates 3 to 4 times more ethylene than tissue in the dark. Conversely, the lowest rate of ethylene release occurs when leaf tissue is illuminated and photosynthetic activity depletes the CO(2) to the compensation point. Ethylene release in the dark is also stimulated by CO(2) either added to the flask as bicarbonate or generated by dark respiration. Ethylene release increases dramatically and in parallel with photosynthesis at increasing light intensities in this C(4) plant. Ethylene release appears dependent on CO(2) both in the light and in the dark. Therefore, it is suggested that the important factor regulating the evolution of ethylene gas from leaves of Gomphrena may be CO(2) metabolism rather than light per se.

A Study of Formate Production and Oxidation in Leaf Peroxisomes during Photorespiration

When glycolate was metabolized in peroxisomes isolated from leaves of spinach beet (Beta vulgaris L., var. vulgaris) formate was produced. Although the reaction mixture contained glutamate to facilitate conversion of glycolate to glycine, the rate at which H(2)O(2) became "available" during the oxidation of [1-(14)C]glycolate was sufficient to account for the breakdown of the intermediate [1-(14)C]glyoxylate to formate (C(1) unit) and (14)CO(2). Under aerobic conditions formate production closely paralleled (14)CO(2) release from [1-(14)C]glycolate which was optimal between pH 8.0 and pH 9.0 and was increased 3-fold when the temperature was raised from 25 to 35 C, or when the rate of H(2)O(2) production was increased artificially by addition of an active preparation of fungal glucose oxidase.When [(14)C]formate was added to these preparations it was oxidized directly to (14)CO(2) by the peroxidatic action of peroxisomal catalase; however, the breakdown of formate was slow relative to the rate of formate production. For example, when [(14)C]formate was generated from [2-(14)C]glycolate it was not readily oxidized to (14)CO(2) in these organelles. Because the activity of formate-NAD(+) dehydrogenase in cell-free leaf extracts was low compared with that of formyl tetrahydrofolate synthetase it is suggested that most of the formate produced during glycolate oxidation could be metabolized via the one carbon pool and not oxidized directly to CO(2).At 25 C the rate of release of (14)CO(2) from [2-(14)C]glycolate in leaf discs was 40 to 50% of the rate from [1-(14)C]glycolate. Isonicotinyl hydrazide inhibited (14)CO(2) release from both [1-(14)C]- and [2-(14)C]glycolate; but this inhibitor was more effective in blocking (14)CO(2) release from [2-(14)C]glycolate. It is argued that the oxidation of the methylene carbon group of glycolate does not occur as a direct consequence of formate (C(1) unit) breakdown, but is a product of the further metabolism of formate and glycine, possibly, via serine.

Glyoxylate decarboxylation during photorespiration

At 25° C under aerobic conditions with or without gluamate 10% of the [1-(14)C]glycollate oxidised in spinach leaf peroxisomes was released as (14)CO2. Without glutamate only 5% of the glycollate was converted to glycine, but with it over 80% of the glycollate was metabolised to glycine. CO2 release was probably not due to glycine breakdown in these preparations since glycine decarboxylase activity was not detected. Addition of either unlabelled glycine or isonicotinyl hydrazide (INH) did not reduce (14)CO2 release from either [1-(14)C]glycollate or [1-(14)C]glyoxylate. Furthermore, the amount of "available H2O2" (Grodzinski and Butt, 1976) was sufficient to account for all of the CO2 release by breakdown of glyoxylate. Peroxisomal glycollate metabolism was unaffected by light and isolated leaf chloroplasts alone did not metabolise glycollate. However, in a mixture of peroxisomes and illuminated chloroplasts the rate of glycollate decarboxylation increased three fold while glycine synthesis was reduced by 40%. Although it was not possible to measure "available H2O2" directly, the data are best explained by glyoxylate decarboxylation. Catalase reduced CO2 release and enhanced glycine synthesis. In addition, when a model system in which an active preparation of purified glucose oxidase generating H2O2 at a known rate was used to replace the chloroplasts, similar rates of (14)CO2 release and [(14)C]glycine synthesis from [1-(14)C]glycollate were measured. It is argued that in vivo glyoxylate metabolism in leaf peroxisomes is a key branch point of the glycollate pathway and that a portion of the photorespired CO2 arises during glyoxylate decarboxylation under the action of H2O2. The possibility that peroxisomal catalase exerts a peroxidative function during this process is discussed.

The effect of temperature on glycollate decarboxylation in leaf peroxisomes

[1-(14)C]glycollate was oxidised to(14)CO2 by peroxisomes isolated from leaves of spinach beet about 3 times as rapidly at 35°C as at 25°C; the rate was further increased with rise in temperature to a maximum at 55°C. These increases are shown to be mainly due to the increased H2O2 available to oxidise glyoxylate non-enzymically as a result of the higher temperature coefficient of glycollate oxidase activity relative to that of catalase. These results are compared with similar increases in the rate of(14)CO2 release between 25°C and 35°C when [1-(14)C]glycollate was supplied to leaf discs in light or darkness. The role of these reactions in accounting for the temperature effect on the release of photorespiratory CO2 is discussed.

Intracellular localization of glycolate dehydrogenase in a blue-green alga

Glycolate dehydrogenase activity was detected in cell-free extracts of Oscillatoria sp. prepared by osmotic lysis of spheroplasts in 0.05 m potassium phosphate buffer, pH 7.5, containing 0.3 m mannitol. Most of the enzyme activity was found in a particulate fraction and localized in the photosynthetic lamellae after centrifugation in a discontinuous sucrose density gradient. Enzyme activity was detected in this fraction both in the presence and absence of the artificial electron acceptor 2,6-dichlorophenolindophenol (DPIP) and a low rate of O(2) uptake was detected in this lamellar fraction. Activity was lost from the lamellar fraction by repeated washing or by treatment with 0.005% Triton X-100 and the solubilized enzyme activity was DPIP-dependent. The data indicate that both glycolate dehydrogenase and its natural electron acceptor are bound to the photosynthetic lamellae in vivo. In contrast, catalase activity was found in the soluble cytoplasmic fraction.

Hydrogen peroxide production and the release of carbon dioxide during glycollate oxidation in leaf peroxisomes

The rate at which H2O2 becomes available during glycollate oxidation for further oxidation reactions, especially that of glyoxylate to formate and CO2, in peroxisomes from spinach-beet (Beta vulgaris L., var. vulgaris) leaves has been determined by measuring O2 uptake in the presence and absence of added catalase. The rates observed under air and pure O2 were sufficient to account for the (14)CO2 released from [l-(14)C]glycollate under these conditions; the two reactions showed similar characteristics. In the course of the reaction, a fall in catalase activity was observed concomitant with an increase in (14)CO2 release. There is no evidence that catalase was disproportionately lost from the peroxisomes during isolation, and it is argued that the CO2 release observed contributes to the photorespiratory CO2 loss in intact leaves.