Steady-state stomatal responses of C3 and C4 species to blue light fraction: Interactions with CO2 concentration

A comparison of stomatal conductance responses to blue and red light between C3 and C4 photosynthetic species in three phylogenetically-controlled experiments

Introduction

C4 photosynthesis is an adaptation that has independently evolved at least 66 times in angiosperms. C4 plants, unlike their C3 ancestral, have a carbon concentrating mechanism which suppresses photorespiration, often resulting in faster photosynthetic rates, higher yields, and enhanced water use efficiency. Moreover, the presence of C4 photosynthesis greatly alters the relation between CO2 assimilation and stomatal conductance. Previous papers have suggested that the adjustment involves a decrease in stomatal density. Here, we tested if C4 species also have differing stomatal responses to environmental cues, to accommodate the modified CO2 assimilation patterns compared to C3 species.

Methods

To test this hypothesis, stomatal responses to blue and red-light were analysed in three phylogenetically linked pairs of C3 and C4 species from the Cleomaceae (Gynandropsis and Tarenaya), Flaveria, and Alloteropsis, that use either C3 or C4 photosynthesis.

Results

The results showed strongly decreased stomatal sensitivity to blue light in C4 dicots, compared to their C3 counterparts, which exhibited significant blue light responses. In contrast, in C3 and C4 subspecies of the monocot A. semialata, the blue light response was observed regardless of photosynthetic type. Further, the quantitative red-light response varied across species, but the presence or absence of a significant stomatal red-light response was not directly associated with differences in photosynthetic pathway. Interestingly, stomatal density and morphology patterns observed across the three comparisons were also not consistent with patterns commonly asserted for C3 and C4 species.

Discussion

The strongly diminished blue-light sensitivity of stomatal responses in C4 species across two of the comparisons suggests a common C4 feature that may have functional implications. Altogether, the strong prevalence of species-specific effects clearly emphasizes the importance of phylogenetic controls in comparisons between C3 and C4 photosynthetic pathways.

Photosynthesis in sun and shade: the surprising importance of far-red photons

The current definition of photosynthetically active radiation includes only photons from 400 up to 700 nm, despite evidence of the synergistic interaction between far-red photons and shorter-wavelength photons. The synergy between far-red and shorter-wavelength photons has not been studied in sunlight under natural conditions. We used a filter to remove photons above 700 nm to quantify the effects on photosynthesis in diverse species under full sun, medium light intensity and vegetation shade. Far-red photons (701 to 750 nm) in sunlight are used efficiently for photosynthesis. This is especially important for leaves in vegetation shade, where far-red photons can be > 50% of the total incident photons between 400 and 750 nm. Far-red photons accounted for 24-25% of leaf gross photosynthesis (P_gross ) in a C₃ and a C₄ species when sunlight was filtered through a leaf, and 10-14% of leaf P_gross in a tree and an understory species in deep shade. Accounting for the photosynthetic activity of far-red photons is critical for accurate measurement and modeling of photosynthesis at single leaf, canopy and ecosystem scales. This, in turn, is crucial in understanding crop productivity, the global carbon cycle and climate change impacts on agriculture and ecosystems.

Testing unified theories for ozone response in C4 species

There is tremendous interspecific variability in O₃  sensitivity among C₃  species, but variation among C₄  species has been less clearly documented. It is also unclear whether stomatal conductance and leaf structure such as leaf mass per area (LMA) determine the variation in sensitivity to O₃ across species. In this study, we investigated leaf morphological, chemical, and photosynthetic responses of 22 genotypes of four C₄ bioenergy species (switchgrass, sorghum, maize, and miscanthus) to elevated O₃ in side-by-side field experiments using free-air O₃ concentration enrichment (FACE). The C₄  species varied largely in leaf morphology, physiology, and nutrient composition. Elevated O₃ did not alter leaf morphology, nutrient content, stomatal conductance, chlorophyll fluorescence, and respiration in most genotypes but reduced net CO₂ assimilation in maize and photosynthetic capacity in sorghum and maize. Species with lower LMA and higher stomatal conductance tended to show greater losses in photosynthetic rate and capacity in elevated O₃ compared with species with higher LMA and lower stomatal conductance. Stomatal conductance was the strongest determinant of leaf photosynthetic rate and capacity. The response of both area- and mass-based leaf photosynthetic rate and capacity to elevated O₃ were not affected by LMA directly but negatively influenced by LMA indirectly through stomatal conductance. These results demonstrate that there is significant variation in O₃  sensitivity among C₄  species with maize and sorghum showing greater sensitivity of photosynthesis to O₃ than switchgrass and miscanthus. Interspecific variation in O₃  sensitivity was determined by direct effects of stomatal conductance and indirect effects of LMA. This is the first study to provide a test of unifying theories explaining variation in O₃  sensitivity in C₄ bioenergy grasses. These findings advance understanding of O₃ tolerance in C₄  grasses and could aid in optimal placement of diverse C₄ bioenergy feedstock across a polluted landscape.

Mild-Intensity UV-A Radiation Applied Over a Long Duration Can Improve the Growth and Phenolic Contents of Sweet Basil

UV-A radiation (320-400 nm) is an abiotic stressor that may be used to enhance the production of beneficial secondary metabolites in crops such as leafy vegetables. However, tradeoffs between enhanced phytochemical contents and overall growth/yield reductions have been reported. The responses varied depending on the UV-A intensity, spectral peak, exposure time, species, and varieties. We quantified the changes in growth, morphology, photosynthesis, and phenolic contents of sweet basil grown under a base red/blue/green LED light with four supplemental UV-A intensity treatments (0, 10, 20, and 30 W·m^-2) in an indoor environment over 14 days. The objective was to determine whether UV-A radiation could be utilized to improve both yield and quality of high-value sweet basil in a controlled production environment. Biomass harvested at 14 days after treatment (DAT) was highest under mild-intensity UV-A treatment of 10 W·m^-2 and lowest under high-intensity UV-A treatment of 30 W·m^-2. The total leaf area and the number of leaves were significantly lower under the 30 W·m^-2 treatment than under the 10 and 20 W·m^-2 treatments at 14 DAT. The maximum quantum efficiency of photosystem II (PSII) for photochemistry (F_v/F_m ) showed a gradual decrease under the 20 and 30 W·m^-2 treatments from 3 to 14 DAT, whereas F_v/F_m remained relatively constant under the 0 and 10 W·m^-2 treatments over the entire 14 days. The leaf net photosynthesis rate showed a significant decrease of 17.4% in the 30 W·m^-2 treatment compared to that in the 10 W·m^-2 treatment at 14 DAT. Phenolic contents (PAL enzyme activity, total phenolic concentration, and antioxidant capacity) were the highest under the 20 W·m^-2 treatment, followed by the 10, 30, and 0 W·m^-2 treatments. Overall, our results indicate that the biomass production and accumulation of beneficial phenolic compounds in sweet basil varied depending on the intensity and duration of UV-A application. Mild UV-A radiation (10-20 W·m^-2) can be a beneficial stressor to improve sweet basil yield and quality over relatively long-term cultivation.

Light Intensity Influence on Growth and Photosynthetic Characteristics of Horsfieldia hainanensis

Due to both anthropogenic and natural causes, the number of Horsfieldia hainanensis has been decreasing each year in the Tongza Branch nursery (109.534 525°E, 18.763 516°N) of the Hainan Academy of Forestry, China. Consequently, the protection of H. hainanensis is urgent, as is that of most rare tree species. To develop a more comprehensive understanding of the H. hainanensis growth environment, we took 3-year-old H. hainanensis saplings as the research object. We controlled the light intensity by setting different shade amounts to explore the growth and photosynthetic characteristics of H. hainanensis under different light intensities. We found that shade can promote growth and increase the contents of certain substances. Light transmittance of 44.41% can increase plant height (by 29.545%) and biomass (by 66.676%). Light transmittance of 16.19% can increase the pigment content; Chl increased by 40.864%, Chl a increased by 38.031%, and Chl b increased by 48.412%. Light transmittance of 7.30% can increase the soil plant analysis development (SPAD) value of each part of the leaf; the leaf base increased by 41.000%, the leaf margin increased by 32.574%, the blade tip increased by 49.003%, and the leaf average increased by 40.466%. The specific leaf area can reduce the specific leaf weight. We also found that compared to full light, reducing the light transmittance can increase the total chlorophyll (Chl), chlorophyll A (Chl a), and chlorophyll B (Chl b) contents, and the Chl-SPAD-leaf base, leaf edge, leaf tip, average content, and light-saturated net photosynthetic rate. This can in turn reduce the apparent quantum efficiency (AQY), light compensation point (LCP), and dark respiration rate (Rd). In addition, we found a strong correlation between seven of the photosynthetic pigment indicators (Chl, Chla, Chl b, Chl-SPAD-leaf base, leaf margin, leaf tip, and mean) and the three photosynthesis physiological parameters (AQY, LCP, and Rd). The light transmittance of 44.41% (one layer of shading net) treatment group was conducive to the growth of H. hainanensis and photosynthetic characteristic improvement. Therefore, our light transmittance selection of approximately 44.4% is significant for the natural return of H. hainanensis.

Bioenergy sorghum maintains photosynthetic capacity in elevated ozone concentrations

Elevated tropospheric ozone concentration (O3 ) significantly reduces photosynthesis and productivity in several C4 crops including maize, switchgrass and sugarcane. However, it is unknown how O3 affects plant growth, development and productivity in sorghum (Sorghum bicolor L.), an emerging C4 bioenergy crop. Here, we investigated the effects of elevated O3 on photosynthesis, biomass and nutrient composition of a number of sorghum genotypes over two seasons in the field using free-air concentration enrichment (FACE), and in growth chambers. We also tested if elevated O3 altered the relationship between stomatal conductance and environmental conditions using two common stomatal conductance models. Sorghum genotypes showed significant variability in plant functional traits, including photosynthetic capacity, leaf N content and specific leaf area, but responded similarly to O3 . At the FACE experiment, elevated O3 did not alter net CO2 assimilation (A), stomatal conductance (gs ), stomatal sensitivity to the environment, chlorophyll fluorescence and plant biomass, but led to reductions in the maximum carboxylation capacity of phosphoenolpyruvate and increased stomatal limitation to A in both years. These findings suggest that bioenergy sorghum is tolerant to O3 and could be used to enhance biomass productivity in O3 polluted regions.