The Light Dependence of Mesophyll Conductance and Relative Limitations on Photosynthesis in Evergreen Sclerophyllous Rhododendron Species

Light Intensity Dependence of CO2 Assimilation Is More Related to Biochemical Capacity Rather than Diffusional Conductance

The response of CO2 assimilation rate (AN) to incident light intensity reflects the efficiency of light utilization. The light intensity dependence of AN varies widely among different plant species, yet the underlying mechanisms remain poorly understood. To elucidate this issue, we measured the light intensity dependence of gas exchange and chlorophyll fluorescence in twelve tree species. The results indicated that (1) with increasing light intensity, the variation in AN was closely related to stomatal conductance (gs), mesophyll conductance (gm), the maximum velocity of Rubisco carboxylation (Vcmax), and electron transport rate (ETR); (2) compared with AN at sub-saturating light, the increase in AN at saturating light was more strongly associated with Vcmax and ETR than with gs and gm; and (3) the increase in Vcmax and AN from 600 to 2000 μmol photons m-2 s-1 were positively correlated with the maximum capacity of Vcmax. These findings suggest that Vcmax is an energy-dependent process that significantly regulates the light intensity dependence of AN in plants. This provides valuable insights for crop improvement through the manipulation of Vcmax.

CO2 mesophyll conductance regulated by light: a review

MAIN CONCLUSION

Light quality and intensity regulate plant mesophyll conductance, which has played an essential role in photosynthesis by controlling leaf structural and biochemical properties. Mesophyll conductance (g_m), a crucial physiological factor influencing the photosynthetic rate of leaves, is used to describe the resistance of CO₂ from the sub-stomatal cavity into the chloroplast up to the carboxylation site. Leaf structural and biochemical components, as well as external environmental factors such as light, temperature, and water, all impact g_m. As an essential factor of plant photosynthesis, light affects plant growth and development and plays a vital role in regulating g_m as well as determining photosynthesis and yield. This review aimed to summarize the mechanisms of g_m response to light. Both structural and biochemical perspectives were combined to reveal the effects of light quality and intensity on the g_m, providing a guide for selecting the optimal conditions for intensifying photosynthesis in plants.

Photosynthetic Induction Under Fluctuating Light Is Affected by Leaf Nitrogen Content in Tomato

The response of photosynthetic CO₂ assimilation to changes of illumination affects plant growth and crop productivity under natural fluctuating light conditions. However, the effects of nitrogen (N) supply on photosynthetic physiology after transition from low to high light are seldom studied. To elucidate this, we measured gas exchange and chlorophyll fluorescence under fluctuating light in tomato (Solanum lycopersicum) seedlings grown with different N conditions. After transition from low to high light, the induction speeds of net CO₂ assimilation (A _N ), stomatal conductance (g _s ), and mesophyll conductance (g _m ) delayed with the decline in leaf N content. The time to reach 90% of maximum A _N , g _s and g _m was negatively correlated with leaf N content. This delayed photosynthetic induction in plants grown under low N concentration was mainly caused by the slow induction response of g _m rather than that of g _s . Furthermore, the photosynthetic induction upon transfer from low to high light was hardly limited by photosynthetic electron flow. These results indicate that decreased leaf N content declines carbon gain under fluctuating light in tomato. Increasing the induction kinetics of g _m has the potential to enhance the carbon gain of field crops grown in infertile soil.

The Effect of Low Irradiance on Leaf Nitrogen Allocation and Mesophyll Conductance to CO2 in Seedlings of Four Tree Species in Subtropical China

Low light intensity can lead to a decrease in photosynthetic capacity. However, could N-fixing species with higher leaf N contents mitigate the effects of low light? Here, we exposed seedlings of Dalbergia odorifera and Erythrophleum fordii (N-fixing trees), and Castanopsis hystrix and Betula alnoides (non-N-fixing trees) to three irradiance treatments (100%, 40%, and 10% sunlight) to investigate the effects of low irradiance on leaf structure, leaf N allocation strategy, and photosynthetic physiological parameters in the seedlings. Low irradiance decreased the leaf mass per unit area, leaf N content per unit area (N_area), maximum carboxylation rate (V_cmax), maximum electron transport rate (J_max), light compensation point, and light saturation point, and increased the N allocation proportion of light-harvesting components in all species. The studied tree seedlings changed their leaf structures, leaf N allocation strategy, and photosynthetic physiological parameters to adapt to low-light environments. N-fixing plants had a higher photosynthesis rate, N_area, V_cmax, and J_max than non-N-fixing species under low irradiance and had a greater advantage in maintaining their photosynthetic rate under low-radiation conditions, such as under an understory canopy, in a forest gap, or when mixed with other species.