Carbohydrate regulation of photosynthesis and respiration from branch girdling in four species of wet tropical rain forest trees

Ordered rise and disordered fall: dynamic changes of membrane lipids during girdling-induced tree mortality in Populus yunnanensis

Understanding the mechanisms behind drought-induced tree mortality is crucial for predicting the impact of global climate change on forests. We studied the mechanism at the cellular level in Populus yunnanensis by profiling membrane lipid molecules in leaves, branch phloem, top and bottom trunk phloem under trunk-girdling-induced drought conditions. We found that both lipid composition and content changed, depending on the tree's tissue positions and the progression of the girdling effect. The compositional changes were similar between the leaves and branches and between the top and bottom trunk phloem. The lipid content initially increased and then decreased until complete degradation, with similar fold increases between leaves and branch phloem, and between top and bottom trunk phloem. However, the fold increase in the former two was significantly lower than that in the latter two. The lipid composition remained stable during the increase but changed during the decrease. The decrease in phloem lipids occurred later than in leaves and simultaneously across positions. Our findings provide novel insights into the mechanisms of water deficit and carbohydrate allocation in drought-induced tree mortality, and suggest that the onset of phloem lipid degradation could serve as a threshold for predicting tree mortality.

Divergent responses of woody plant leaf and root non-structural carbohydrates to nitrogen addition in China: Seasonal variations and ecological implications

Plant non-structural carbohydrates (NSCs), which largely comprise starch and soluble sugars, are essential energy reserves to support plant growth and physiological functions. While it is known that increasing global deposition of nitrogen (N) affects plant concentration of NSCs, quantification of seasonal responses and drivers of woody species leaf and root NSCs to N addition at larger spatial scales remains lacking. Here, we systematically analyzed data from 53 field experiments distributed across China, comprising 1202 observations, to test for effects of N addition on woody plant leaf and root NSCs across and within growing and non-growing seasons. We found (1) no overall effects of N addition on the concentrations of leaf and root NSCs, soluble sugars or starch during the growing season or the non-growing season for leaves. However, N addition decreased root NSC and starch concentrations by 13.8 % and 39.0 %, respectively, and increased soluble sugars concentration by 15.0 % during the non-growing season. (2) Shifts in leaf NSC concentration under N addition were driven by responses by soluble sugars in both seasons, while shifts in root NSC were driven by soluble sugars in the non-growing season and starch and soluble sugars in the growing season. (3) Relationships between N, carbon, and phosphorus stoichiometry with leaf and root NSCs indicated effects of N addition on woody plant NSCs allocation through impacts on plant photosynthesis, respiration, and growth. (4) Effects of N addition on leaf and root NSCs varied with plant functional types, where effects were more pronounced in roots than in leaves during the non-growing season. Overall, our results reveal divergent responses of woody plant leaf and root NSCs to N addition within non-growing season and highlight the role of ecological stoichiometry and plant functional types in woody plant allocation patterns of NSCs in response to ongoing N deposition under global change.

Selenium Nanoparticles Regulate Antioxidant Enzymes and Flavonoid Compounds in Fagopyrum dibotrys

Fagopyrum dibotrys is a herbal plant. Selenium (Se) is a beneficial element for plants; selenium nanoparticles (SeNPs) are gaining importance in food and agriculture due to their low toxicity and high activity. This study revealed that foliar application of SeNPs enhanced superoxide dismutase, glutathione peroxidase, and peroxisome activities and significantly enhanced the flavonoid compound content in F. dibotrys. SeNPs with a concentration of 5.0 mg L-1 also promoted the growth of F. dibotrys. The foliar application of SeNPs could be absorbed by pores in leaves of F. dibotrys and mainly transformed to selenomethionine (32.5-43.2%) and selenocysteine (23.4-38.4%) in leaves and tubers of F. dibotrys. Consequently, this study offers a profound understanding of plants' uptake and biotransformation of SeNPs. Furthermore, the findings of this study have suggested that SeNPs can be applied to improve the quantity and quality of the herbal plant of F. dibotrys.

Seasonal dynamics and punctuated carbon sink reduction suggest photosynthetic capacity of boreal silver birch is reduced by the accumulation of hexose

The 'assimilates inhibition hypothesis' posits that accumulation of nonstructural carbohydrates (NSCs) in leaves reduces leaf net photosynthetic rate, thus internally regulating photosynthesis. Experimental work provides equivocal support mostly under controlled conditions without identifying a particular NSC as involved in the regulation. We combined 3-yr in situ leaf gas exchange observations (natural dynamics) in the upper crown of mature Betula pendula simultaneously with measurements of concentrations of sucrose, hexoses (glucose and fructose), and starch, and similar measurements during several one-day shoot girdling (perturbation dynamics). Leaf water potential and water and nitrogen content were measured to account for their possible contribution to photosynthesis regulation. Leaf photosynthetic capacity (A/Ci) was temporally negatively correlated with NSC accumulation under both natural and perturbation states. For developed leaves, leaf hexose concentration explained A/Ci variation better than environmental variables (temperature history and daylength); the opposite was observed for developing leaves. The weaker correlations between NSCs and A/Ci in developing leaves may reflect their strong internal sink strength for carbohydrates. By contrast, the strong decline in photosynthetic capacity with NSCs accumulation in mature leaves, observed most clearly with hexose, and even more tightly with its constituents, provides support for the role of assimilates in regulating photosynthesis under natural conditions.

Elevated atmospheric CO2 concentration mitigates salt damages to safflower: Evidence from physiological and biochemical examinations

The physiological and biochemical responses of salt-stressed safflower to elevated CO2 remain inadequately known. This study investigated the interactive effects of high CO2 concentration (700 ± 50 vs. 400 ± 50 μmol mol-1) and salinity stress levels (0.4, 6, and 12 dS m-1, NaCl) on growth and physiological properties of four safflower (Carthamus tinctorius L.) genotypes, under open chamber conditions. Results showed that the effects of CO2 on biomass of shoot and grains depend on salt stress and plant genotype. Elevated CO2 conditions increased shoot dry weight under moderate salinity stress and decreased it under severe stress. The increased CO2 concentration also increased the safflower genotypes' relative water content and their K+/Na + concentrations. Also enriched CO2 increased total carotenoid levels in safflower genotypes and improved membrane stability index by reducing H2O2 levels. In addition, increased CO2 level led to an increase in seed oil content, under both saline and non-saline conditions. This effect was particularly pronounced under severe saline conditions. Under conditions of high CO2 and salinity, the Koseh genotype exhibited higher grain weight and seed oil content than other genotypes. This advantage is due to the higher relative water content, maximum quantum efficiency of photosystem II (Fv/Fm), and K+/Na+, as well as the lower Na+ and H2O2 concentrations. Results indicate that the high CO2 level mitigated the destructive effect of salinity on safflower growth by reducing Na + uptake and increasing the Fv/Fm, total soluble carbohydrates, and membrane stability index. This finding can be used in safflower breeding programs to develop cultivars that can thrive in arid regions with changing climatic conditions.

Precipitation pattern changed the content of non-structural carbohydrates components in different organs of Artemisia ordosica

BACKGROUND

Non-structural carbohydrates (NSC) play a significant role in plant growth and defense and are an important component of carbon cycling in desert ecosystems. However, regarding global change scenarios, it remains unclear how NSCs in desert plants respond to changing precipitation patterns. [Methods] Three precipitation levels (natural precipitation, a 30% reduction in precipitation, and a 30% increase in precipitation) and two precipitation intervals levels (5 and 15 d) were simulated to study NSC (soluble sugar and starch) responses in the dominant shrub Artemisia ordosica.

RESULTS

Precipitation level and interval interact to affect the NSC (both soluble sugar and starch components) content of A. ordosica. The effect of precipitation on NSC content and its components depended on extended precipitation interval. With lower precipitation and extended interval, soluble sugar content in roots increased and starch content decreased, indicating that A. ordosica adapts to external environmental changes by hydrolyzing root starch into soluble sugars. At 5 d interval, lower precipitation increased the NSC content of stems and especially roots.

CONCLUSIONS

A. ordosica follows the "preferential allocation principle" to preferentially transport NSC to growing organs, which is an adaptive strategy to maintain a healthy physiological metabolism under drought conditions. The findings help understand the adaptation and survival mechanisms of desert vegetation under the changing precipitation patterns and are important in exploring the impact of carbon cycling in desert systems under global environmental change.

Photosynthetic carbon and nitrogen metabolism of Camellia oleifera Abel during acclimation to low light conditions

Tea-oil tree (Camellia oleifera Abel) is an important woody oil crop with high economic value. However, it has low photosynthetic production considering the low light intensity of its growth environment. To understand the acclimation mechanism of tea-oil trees to low light conditions, three light intensity treatments were conducted: high light (450-500 μmol. m^-2. s^-1), medium light (180-200 μmol. m^-2. s^-1), and low light (45-50 μmol. m^-2. s^-1). The carbon (C) and nitrogen (N) metabolism network were constructed by investigating the leaf anatomy, photosynthetic characteristics, N partitioning, transcriptome and metabolome. Results demonstrated that a larger proportion light energy was used for photochemical reactions in an environment with lower light intensity, which resulted in an increase in photosystem II photochemical efficiency and instantaneous light use efficiency (LUE) at the leaf level. As the light intensity increased, decreased electron transfer and carboxylation efficiencies, photorespiration and dark respiration rates, LUE at plant level, and N use efficiency (P_NUE) were observed. Leaves trended to harvest more light using higher expression levels of light-harvesting protein genes, higher chlorophyll content, more granum and more tightly stacked granum lamella under lower light intensity. At transcriptional and metabolic levels, the TCA cycle, and the synthesis of starch and saccharides were weakened as light intensity decreased, while the Calvin cycle did not show the regularity between different treatments. Less N was distributed in Rubisco, respiration, and cell wall proteins as light decreased. Storage N was prominently accumulated in forms of amino acids (especially L-arginine) and amino acid derivatives as under medium and low light environments, to make up for C deficiency. Therefore, tea-oil trees actively improve light-harvesting capacity and enlarges the storage N pool to adapt to a low light environment, at the cost of a decrease of photosynthetic C assimilation and P_NUE.

Exogenous aspartic acid alleviates salt stress-induced decline in growth by enhancing antioxidants and compatible solutes while reducing reactive oxygen species in wheat

Salinity is the primary environmental stress that adversely affects plants' growth and productivity in many areas of the world. Published research validated the role of aspartic acid in improving plant tolerance against salinity stress. Therefore, in the present work, factorial pot trials in a completely randomized design were conducted to examine the potential role of exogenous application of aspartic acid (Asp) in increasing the tolerance of wheat (Triticum aestivum L.) plants against salt stress. Wheat plants were sown with different levels of salinity (0, 30, or 60 mM NaCl) and treated with three levels of exogenous application of foliar spray of aspartic acid (Asp) (0, 0.4, 0.6, or 0.8 mM). Results of the study indicated that salinity stress decreased growth attributes like shoot length, leaf area, and shoot biomass along with photosynthesis pigments and endogenous indole acetic acid. NaCl stress reduced the total content of carbohydrates, flavonoid, beta carotene, lycopene, and free radical scavenging activity (DPPH%). However, Asp application enhanced photosynthetic pigments and endogenous indole acetic acid, consequently improving plant leaf area, leading to higher biomass dry weight either under salt-stressed or non-stressed plants. Exogenous application of Asp, up-regulate the antioxidant system viz. antioxidant enzymes (superoxide dismutase, peroxidase, catalase, and nitrate reductase), and non-enzymatic antioxidants (ascorbate, glutathione, total phenolic content, total flavonoid content, beta carotene, lycopene) contents resulted in declined in reactive oxygen species (ROS). The decreased ROS in Asp-treated plants resulted in reduced hydrogen peroxide, lipid peroxidation (MDA), and aldehyde under salt or non-salt stress conditions. Furthermore, Asp foliar application increased compatible solute accumulation (amino acids, proline, total soluble sugar, and total carbohydrates) and increased radical scavenging activity of DPPH and enzymatic ABTS. Results revealed that the quadratic regression model explained 100% of the shoot dry weight (SDW) yield variation. With an increase in Asp application level by 1.0 mM, the SDW was projected to upsurge through 956 mg/plant. In the quadratic curve model, if Asp is applied at a level of 0.95 mM, the SDW is probably 2.13 g plant^-1. This study concluded that the exogenous application of aspartic acid mitigated the adverse effect of salt stress damage on wheat plants and provided economic benefits.

The effect of inter-varietal variation in sugar hydrolysis and transport on sugar content and photosynthesis in Vitis vinifera L. leaves

Sugar synthesis from photosynthesis and its utilization through sugar metabolism jointly determine leaf sugar content, and in contrast, excess sugar represses leaf photosynthesis. Although plant photosynthesis is affected by leaf sugar metabolism, the relationship between sugar metabolism and photosynthetic capacity of different grape genotypes remains unclear. In this study, two grape (Vitis vinifera L.) genotypes 'Riesling' (RI, high sugar content in leaf) and 'Petit Manseng' (PM, low sugar content in leaf) were used to evaluate the relationship between sugar metabolism and photosynthesis. Sugar content, chlorophyll content, photosynthetic parameters, enzyme activity, and gene expression related to sucrose metabolism in leaves were measured, and the correlations between photosynthesis and sugar metabolism were assessed. The contents of sucrose and glucose were significantly higher in RI leaves than in PM leaves, while the fructose content pattern was reversed. Cell wall invertase activity for sucrose hydrolysis and the transcript levels of VvCWINV, VvHTs, VvTMT1, VvFKs, and VvHXK2 were also higher in RI leaves than in PM leaves, whereas that of VvHXK1 mediating glucose phosphorylation, was lower in RI leaves than in PM leaves. Net photosynthetic rate, stomatal conductance, transpiration rate, and chlorophyll content were lower in RI leaves than in PM leaves and negatively correlated with glucose content, and the transcript levels of VvCWINV, VvHTs, VvTMT1, and VvHXK2. In conclusion, this study indicates that leaf sugar metabolism and transport are related to photosynthesis in Vitis vinifera L., which provides a theoretical basis for improving grape photosynthesis.

Genome-wide association studies reveal the coordinated regulatory networks underlying photosynthesis and wood formation in Populus

Photosynthesis and wood formation underlie the ability of trees to provide renewable resources and perform ecological functions; however, the genetic basis and regulatory pathways coordinating these two linked processes remain unclear. Here, we used a systems genetics strategy, integrating genome-wide association studies, transcriptomic analyses, and transgenic experiments, to investigate the genetic architecture of photosynthesis and wood properties among 435 unrelated individuals of Populus tomentosa, and unravel the coordinated regulatory networks resulting in two trait categories. We detected 222 significant single-nucleotide polymorphisms, annotated to 177 candidate genes, for 10 traits of photosynthesis and wood properties. Epistasis uncovered 74 epistatic interactions for phenotypes. Strikingly, we deciphered the coordinated regulation patterns of pleiotropic genes underlying phenotypic variations for two trait categories. Furthermore, expression quantitative trait nucleotide mapping and coexpression analysis were integrated to unravel the potential transcriptional regulatory networks of candidate genes coordinating photosynthesis and wood properties. Finally, heterologous expression of two pleiotropic genes, PtoMYB62 and PtoMYB80, in Arabidopsis thaliana demonstrated that they control regulatory networks balancing photosynthesis and stem secondary cell wall components, respectively. Our study provides insights into the regulatory mechanisms coordinating photosynthesis and wood formation in poplar, and should facilitate genetic breeding in trees via molecular design.

Acclimation of leaf photosynthesis and respiration to warming in field-grown wheat

Climate change and future warming will significantly affect crop yield. The capacity of crops to dynamically adjust physiological processes (i.e., acclimate) to warming might improve overall performance. Understanding and quantifying the degree of acclimation in field crops could ensure better parameterization of crop and Earth System models and predictions of crop performance. We hypothesized that for field-grown wheat, when measured at a common temperature (25°C), crops grown under warmer conditions would exhibit acclimation, leading to enhanced crop performance and yield. Acclimation was defined as (a) decreased rates of net photosynthesis at 25°C (A²⁵ ) coupled with lower maximum carboxylation capacity (V_cmax²⁵ ), (b) reduced leaf dark respiration at 25°C (both in terms of O₂ consumption R_dark _O₂²⁵ and CO₂ efflux R_dark _CO₂²⁵ ) and (c) lower R_dark _CO₂²⁵ to V_cmax²⁵ ratio. Field experiments were conducted over two seasons with 20 wheat genotypes, sown at three different planting dates, to test these hypotheses. Leaf-level CO₂ -based traits (A²⁵ , R_dark _CO₂²⁵ and V_cmax²⁵ ) did not show the classic acclimation responses that we hypothesized; by contrast, the hypothesized changes in R_{dark_} O₂ were observed. These findings have implications for predictive crop models that assume similar temperature response among these physiological processes and for predictions of crop performance in a future warmer world.

Aboveground Wood Production Is Sustained in the First Growing Season after Phloem-Disrupting Disturbance

Carbon (C) cycling processes are particularly dynamic following disturbance, with initial responses often indicative of longer-term change. In northern Michigan, USA, we initiated the Forest Resilience Threshold Experiment (FoRTE) to identify the processes that sustain or lead to the decline of C cycling rates across multiple levels (0, 45, 65 and 85% targeted gross leaf area index loss) of disturbance severity and, in response, to separate disturbance types preferentially targeting large or small diameter trees. Simulating the effects of boring insects, we stem girdled > 3600 trees below diameter at breast height (DBH), immediately and permanently disrupting the phloem. Weekly DBH measurements of girdled and otherwise healthy trees (n > 700) revealed small but significant increases in daily aboveground wood net primary production (ANPPw) in the 65 and 85% disturbance severity treatments that emerged six weeks after girdling. However, we observed minimal change in end-of-season leaf area index and no significant differences in annual ANPPw among disturbance severities or between disturbance types, suggesting continued C fixation by girdled trees sustained stand-scale wood production in the first growing season after disturbance. We hypothesized higher disturbance severities would favor the growth of early successional species but observed no significant difference between early and middle to late successional species’ contributions to ANPPw across the disturbance severity gradient. We conclude that ANPPw stability immediately following phloem disruption is dependent on the continued, but inevitably temporary, growth of phloem-disrupted trees. Our findings provide insight into the tree-to-ecosystem mechanisms supporting stand-scale wood production stability in the first growing season following a phloem-disrupting disturbance.

Girled-induced anthocyanin accumulation in red-leafed Prunus cerasifera: Effect on photosynthesis, photoprotection and sugar metabolism

The feedback regulation of photosynthesis depends on the cooperation of multiple signals, including sugars. Herein, the effect of shoot girdling was monitored on a daily basis for three days in green- and red-leafed Prunus cerasifera plants (GLP and RLP, respectively). The effect of anthocyanin presence was investigated in terms of photosynthesis, sugar metabolism and photoprotection. Net photosynthesis (A₃₉₀) and stomatal conductance were reduced on the first day at 12:00 only in the girdled GLP (29 and 33 %, respectively). Moreover, the girdled GLP displayed at 12:00 higher sucrose, glucose and fructose concentrations than control leaves. Conversely, girdled RLP showed the first reduction of A₃₉₀ at 18:00, with no significant differences at 12:00 in sucrose and glucose concentrations. The increased biosynthesis of anthocyanins that was only detected in girdled RLP contributed to lowering the accumulation of hexoses. Overall, these results revealed a sugar-buffering role exerted by anthocyanins that positively influence the feedback regulation of photosynthesis. Moreover, non-photochemical quenching, namely pNPQ, revealed the ability of anthocyanins to photoprotect photosystem II from supernumerary photons reaching the chloroplast, whose function was compromised by girdling. The present study provides a starting point to understand the possible link between photosynthesis regulation through sugar signalling and anthocyanin upregulation.

Modeling the Effects of Global Change on Ecosystem Processes in a Tropical Rainforest

Research Highlights: Ongoing land-use change and climate change in wet tropical forests can potentially drive shifts in tree species composition, representing a change in individual species within a functional group, tropical evergreen trees. The impacts on the global carbon cycle are potentially large, but unclear. We explored the differential effects of species within this functional group, in comparison with the effects of climate change, using the Century model as a research tool. Simulating effects of individual tree species on biome-level biogeochemical cycles constituted a novel application for Century. Background and Objectives: A unique, long-term, replicated field experiment containing five evergreen tree species in monodominant stands under similar environmental conditions in a Costa Rican wet forest provided data for model evaluation. Our objectives were to gain insights about this forest’s biogeochemical cycles and effects of tree species within this functional group, in comparison with climate change. Materials and Methods: We calibrated Century, using long-term meteorological, soil, and plant data from the field-based experiment. In modeling experiments, we evaluated effects on forest biogeochemistry of eight plant traits that were both observed and modeled. Climate-change simulation experiments represented two climate-change aspects observed in this region. Results: Model calibration revealed that unmodeled soil processes would be required to sustain observed P budgets. In species-traits experiments, three separate plant traits (leaf death rate, leaf C:N, and allocation to fine roots) resulted in modeled biomass C stock changes of >50%, compared with a maximum 21% change in the climate-change experiments. Conclusions: Modeled ecosystem properties and processes in Century were sensitive to changes in plant traits and nutrient limitations to productivity. Realistic model output was attainable for some species, but unusual plant traits thwarted predictions for one species. Including more plant traits and soil processes could increase realism, but less-complex models provide an accessible means for exploring plant-soil-atmosphere interactions.

Asymmetric pruning reveals how organ connectivity alters the functional balance between leaves and roots of Chinese fir

The functional balance between leaves and roots is believed to be mediated by the specific location of shoots and roots, i.e. differences in transport distances and degrees of organ connectivity. However, it remains unknown whether the adaptive responses of trees to biomass removal depend on the relative orientation of leaf and root pruning. Here, we applied five pruning treatments to saplings of Cunninghamia lanceolata (Chinese fir) under field and glasshouse conditions, namely no pruning (control), half of lateral branches pruned, half of lateral roots pruned, half of the branches and roots pruned on the same side of the plant, and half of the branches and roots pruned on opposite sides of the plant. The effects of pruning on the growth, carbon storage and allocation, and physiology of leaves and fine roots on the same and opposite sides of the plant were investigated. Compared with the effect of root-pruning on leaves, fine roots were more limited by carbon availability and their physiological activity was more strongly reduced by shoot pruning, especially when branches on the same side of the plant were removed. Pruning of branches and roots on the opposite side of the plant resulted in the lowest carbon assimilation rates and growth among all treatments. The results of a stable-isotope labeling indicated that less C was distributed to fine roots from the leaves on the opposite side of the plant compared to those on the same side, but N allocation from roots to leaves depended less on the relative root and leaf orientation. The results collectively indicate that the functional responses of C. lanceolata to pruning are not only determined by the source-sink balance model but are also related to interactions between leaves and fine roots. We argue that the connectivity among lateral branches and roots depends on their relative orientation, which is therefore critical for the functional balance between leaves and fine roots.

Modelling temporal variation of parameters used in two photosynthesis models: influence of fruit load and girdling on leaf photosynthesis in fruit-bearing branches of apple

BACKGROUND AND AIMS

Several studies have found seasonal and temporal variability in leaf photosynthesis parameters in different crops. This variability depends upon the environment, the developmental stage of the plant and the presence or absence of sinks. Girdling involves the removal of the bark and phloem down to the youngest xylem all around the stem and prevents export of photoassimilates out of the stem. The load of developing fruits has often been reported to influence the individual net leaf photosynthesis rate (Pn) in tree crops. In this study, we chose (1) to model the key parameters of photosynthesis models of leaves (Pgmax, Rd, α and θ) as a function of time and using these two means (girdling and low fruit load) to alter the source-sink balance and (2) to compare three models: the rectangular and non-rectangular hyperbola model by Thornley, as well as the non-rectangular hyperbola model by Marshall and Biscoe.

METHODS

Six-year-old fruit-bearing branches of 10-year-old apple trees were used to study and model the seasonal variation of photosynthetic parameters in leaves of vegetative shoots, as a function of global fruit load (at the branch level), with or without girdling, during the growing season of 2015. Three treatments were applied: control, low load (LL) or low load + girdling (LLG). For each fruit-bearing branch, light-response curves of Pn for two leaves of vegetative shoots were measured at two different positions, proximal and distal.

KEY RESULTS

The model of Marshall and Biscoe was the most accurate for the simulation of Pn in fruit-bearing branches of apple trees with time (season) and the three treatments applied.

CONCLUSION

The present study proposed a way to model the photosynthesis rate by temporal and environmental variables only. A proper validation of this model will be necessary to extend its utilization and appreciate its predictive capacity fully.

Modelling temporal variation of parameters used in two photosynthesis models: influence of fruit load and girdling on leaf photosynthesis in fruit-bearing branches of apple

Background and Aims

Several studies have found seasonal and temporal variability in leaf photosynthesis parameters in different crops. This variability depends upon the environment, the developmental stage of the plant and the presence or absence of sinks. Girdling involves the removal of the bark and phloem down to the youngest xylem all around the stem and prevents export of photoassimilates out of the stem. The load of developing fruits has often been reported to influence the individual net leaf photosynthesis rate (Pn) in tree crops. In this study, we chose (1) to model the key parameters of photosynthesis models of leaves (Pgmax, Rd, α and θ) as a function of time and using these two means (girdling and low fruit load) to alter the source-sink balance and (2) to compare three models: the rectangular and non-rectangular hyperbola model by Thornley, as well as the non-rectangular hyperbola model by Marshall and Biscoe.

Methods

Six-year-old fruit-bearing branches of 10-year-old apple trees were used to study and model the seasonal variation of photosynthetic parameters in leaves of vegetative shoots, as a function of global fruit load (at the branch level), with or without girdling, during the growing season of 2015. Three treatments were applied: control, low load (LL) or low load + girdling (LLG). For each fruit-bearing branch, light-response curves of Pn for two leaves of vegetative shoots were measured at two different positions, proximal and distal.

Key Results

The model of Marshall and Biscoe was the most accurate for the simulation of Pn in fruit-bearing branches of apple trees with time (season) and the three treatments applied.

Conclusion

The present study proposed a way to model the photosynthesis rate by temporal and environmental variables only. A proper validation of this model will be necessary to extend its utilization and appreciate its predictive capacity fully.

Cambial response of Norway spruce to modified carbon availability by phloem girdling

We tested the hypothesis that increase in carbon (C) availability in Norway spruce saplings (Picea abies (L.) Karst.) intensifies cambial cell division and increases cell lumen diameter (CLD) and cell wall thickness (CWT) when water availability is adequate. To accomplish this, we experimentally subjected 6-year-old P. abies saplings (n = 80 trees) to two levels of soil humidity (watered versus drought conditions) and manipulated tree C status by physically blocking phloem transport at three girdling dates (GDs). Stem girdling occurred in mid-March (day of the year (doy) 77) and in mid-May (GD doy 138), i.e., ~4 weeks before the onset of bud break and during vigorous stem growth, respectively, and in early July (GD doy 190), i.e., 6 and 4 weeks after cessation of radial growth in drought-stressed trees and shoot growth in both soil humidity (SH) treatments, respectively. In response to phloem blockage a striking increase in the number of xylem cells at all GDs and reactivation of cambial activity in drought-stressed trees was detected above the girdling zone, while below girdling xylem formation stopped in both SH-treatments. Although girdling differently affected wood anatomical parameters (CLD, CWT and CLD:CWT ratio) during earlywood and latewood formation, GD had a minor effect on cambial cell division and xylem cell differentiation. Results also revealed that phloem girdling outweighed drought effects imposed on cambial activity. We explain our findings by accumulation of carbohydrates, osmotically active sugars and/or C based signaling compound(s) in response to girdling. Altogether, we conclude that wood formation in P. abies saplings is limited by C availability, which is most likely caused by high C demand belowground especially under drought.

Source-sink interaction: a century old concept under the light of modern molecular systems biology

Many approaches to engineer source strength have been proposed to enhance crop yield potential. However, a well-co-ordinated source-sink relationship is required finally to realize the promised increase in crop yield potential in the farmer's field. Source-sink interaction has been intensively studied for decades, and a vast amount of knowledge about the interaction in different crops and under different environments has been accumulated. In this review, we first introduce the basic concepts of source, sink and their interactions, then summarize current understanding of how source and sink can be manipulated through both environmental control and genetic manipulations. We show that the source-sink interaction underlies the diverse responses of crops to the same perturbations and argue that development of a molecular systems model of source-sink interaction is required towards a rational manipulation of the source-sink relationship for increased yield. We finally discuss both bottom-up and top-down routes to develop such a model and emphasize that a community effort is needed for development of this model.

Sucrose transporters and plasmodesmal regulation in passive phloem loading

An essential step for the distribution of carbon throughout the whole plant is the loading of sugars into the phloem in source organs. In many plants, accumulation of sugars in the sieve element-companion cell (SE-CC) complex is mediated and regulated by active processes. However, for poplar and many other tree species, a passive symplasmic mechanism of phloem loading has been proposed, characterized by symplasmic continuity along the pre-phloem pathway and the absence of active sugar accumulation in the SE-CC complex. A high overall leaf sugar concentration is thought to enable diffusion of sucrose into the phloem. In this review, we critically evaluate current evidence regarding the mechanism of passive symplasmic phloem loading, with a focus on the potential influence of active sugar transport and plasmodesmal regulation. The limited experimental data, combined with theoretical considerations, suggest that a concomitant operation of passive symplasmic and active phloem loading in the same minor vein is unlikely. However, active sugar transport could well play an important role in how passively loading plants might modulate the rate of sugar export from leaves. Insights into the operation of this mechanism has direct implications for our understanding of how these plants utilize assimilated carbon.

Photosynthesis and aboveground carbon allocation of two co-occurring poplar species in an urban brownfield

Phytoremediation, a technique used to reclaim heavy metal-contaminated soils, requires an understanding of plant physiological responses to heavy metals. However, the majority of studies documenting heavy metal impact on plant functioning have been performed in laboratory or greenhouse settings. We predicted that increased soil heavy metal concentrations reduce photosynthesis and biomass production in trees growing in metal contaminated soil in a naturally re-vegetated urban brownfield. Leaf gas exchange, leaf carbon and nitrogen concentration, and tree biomass were recorded and compared for Populus deltoides and Populus tremuloides growing in an urban brownfield. The CO₂ compensation point (CCP) differed significantly between soil metal concentrations and species, with P. deltoides displaying a greater CCP and P. tremuloides displaying a lower CCP as soil metal concentration increased, despite no changes in dark respiration for either species. In terms of biomass, only total branch weight (TBW) and leaf area (LA) differed significantly between soil metal concentrations, though the difference was largely attributable to variation in diameter at breast height (DBH). Furthermore, TBW and LA values for P. deltoides did not decrease with increasing soil metal concentration. Soil metal concentration, thus, had minimal effect on the relationship between tree age and DBH, and no effect on relationships of tree age and height or LA, respectively. Significant differences between soil metal concentrations and species were found for δ¹⁵N (isotopic nitrogen ratio) while leaf nitrogen content (% N) also differed significantly between species. Long-term water use efficiency derived from carbon isotope analysis (iWUE_isotope) differed significantly between trees grown on different soil metal concentrations and a significant species-metal concentration interaction was detected indicating that the two study species responded differentially to the soil metal concentrations. Specifically, P. tremuloides enhanced while P. deltoides reduced long-term iWUE_isotope as soil metal concentration increased, further emphasizing the importance of species and possible genotype selection for phytoremediation.

Proteomic and Metabolomic Analyses of Leaf from Clematis terniflora DC. Exposed to High-Level Ultraviolet-B Irradiation with Dark Treatment

Clematis terniflora DC. has potential pharmaceutical value; on the contrary, high-level UV-B irradiation with dark treatment led to the accumulation of secondary metabolites. Metabolomic and proteomic analyses of leaf of C. terniflora were performed to investigate the systematic response mechanisms to high-level UV-B irradiation with dark treatment. Metabolites related to carbohydrates, fatty acids, and amino acids and/or proteins related to stress, cell wall, and amino acid metabolism were gradually increased in response to high-level UV-B irradiation with dark treatment. On the basis of cluster analysis and mapping of proteins related to amino acid metabolism, the abundances of S-adenosylmethionine synthetase and cysteine synthase as well as 1,1-diphenyl-2-picrylhydrazyl scavenging activity were gradually increased in response to high-level UV-B irradiation with dark treatment. Furthermore, the abundance of dihydrolipoyl dehydrogenase/glutamate dehydrogenase and the content of γ-aminobutyric acid were also increased following high-level UV-B irradiation with dark treatment. Taken together, these results suggest that high-level UV-B irradiation with dark treatment induces the activation of reactive oxygen species scavenging system and γ-aminobutyric acid shunt pathway in leaf of C. terniflora.