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The photosynthetic mechanism of crop yields in fluctuating light environments in the field remains controversial. To further elucidate this mechanism, we conducted field and simulation experiments using maize (Zea mays) plants. Increased planting density enhanced the light fluctuation frequency and reduced the duration of daily high light, as well as the light-saturated photosynthetic rate, biomass, and yield per plant. Further analysis confirmed a highly significant positive correlation between biomass and yield per plant and the duration of photosynthesis related to daily high light. The simulation experiment indicated that the light-saturated photosynthetic rate of maize leaves decreased gradually and considerably when shortening the daily duration of high light. Under an identical duration of high light exposure, increasing the fluctuation frequency decreased the light-saturated photosynthetic rate slightly. Proteomic data also demonstrated that photosynthesis was mainly affected by the duration of high light and not by the light fluctuation frequency. Consequently, the current study proposes that an appropriate duration of daily high light under fluctuating light environments is the key factor for greatly improving photosynthesis. This is a promising mechanism by which the photosynthetic productivity and yield of maize can be enhanced under complex light environments in the field., Competing Interests: Conflict of interest statement. The authors declare no conflicts ofinterest., (© American Society of Plant Biologists 2022. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Chlorophyll a fluorescence induction kinetics (CFI) is an important tool that reflects the photosynthetic function of leaves, but it remains unclear whether it is affected by leaf structure. Therefore, in this study, the leaf structure and CFI curves of sunflower and sorghum seedlings were analyzed. Results revealed that there was a significant difference between the structures of palisade and spongy tissues in sunflower leaves. Their CFI curves, measured on both the adaxial and abaxial sides, also differed significantly. However, the differences in the leaf structures and CFI curves between both sides of sorghum leaves were not significant. Further analysis revealed that the differences in the CFI curves between the adaxial and abaxial sides of sunflower leaves almost disappeared due to reduced incident light scattering and refraction in the leaf tissues; more importantly, changes in the CFI curves of the abaxial side were greater than the adaxial side. Compared to leaves grown under full sunlight, weak light led to decreased differences in the CFI curves between the adaxial and abaxial sides of sunflower leaves; of these, changes in the CFI curves and palisade tissue structure on the adaxial side were more obvious than on the abaxial side. Therefore, it appears that large differences in sunflower leaf structures may affect the shape of CFI curves. These findings lay a foundation for enhancing our understanding of CFI from a new perspective.

A leaf structure with high porosity is beneficial for lateral CO 2 diffusion inside the leaves. However, the leaf structure of maize is compact, and it has long been considered that lateral CO 2 diffusion is restricted. Moreover, lateral CO 2 diffusion is closely related to CO 2 pressure differences (ΔCO 2 ). Therefore, we speculated that enlarging the ΔCO 2 between the adjacent regions inside maize leaves may result in lateral diffusion when the diffusion resistance is kept constant. Thus, the leaf structure and gas exchange of maize (C 4 ), cotton (C 3 ), and other species were explored. The results showed that maize and sorghum leaves had a lower mesophyll porosity than cotton and cucumber leaves. Similar to cotton, the local photosynthetic induction resulted in an increase in the ΔCO 2 between the local illuminated and the adjacent unilluminated regions, which significantly reduced the respiration rate of the adjacent unilluminated region. Further analysis showed that when the adjacent region in the maize leaves was maintained under a steady high light, the photosynthesis induction in the local regions not only gradually reduced the ΔCO 2 between them but also progressively increased the steady photosynthetic rate in the adjacent region. Under field conditions, the ΔCO 2 , respiration, and photosynthetic rate of the adjacent region were also markedly changed by fluctuating light in local regions in the maize leaves. Consequently, we proposed that enlarging the ΔCO 2 between the adjacent regions inside the maize leaves results in the lateral CO 2 diffusion and supports photosynthesis in adjacent regions to a certain extent under fluctuating light.

Low light conditions not only induce leaf senescence, but also photosynthetic acclimation. This study aimed to determine whether plants exhibit photosynthetic acclimation during low-light-induced leaf senescence. The influences of shading on leaf senescence and photosynthetic acclimation were explored in post-anthesis maize plants. The results showed that whole shading (WS) of maize plants accelerated leaf senescence, whereas partial shading (PS) slowed leaf senescence. WS led to larger decreases in the photosynthetic rate (P n ) and stomatal conductance (G s ) compared to those of the PS treatment. Interestingly, chlorophyll a fluorescence (ChlF) demonstrated that the absorption flux (ABS/CS o ) and trapped energy flux (TR o /CS o ) per cross section in leaves remained relatively stable under WS, whereas significant decreases in the active PSII reaction centers (RC/CS o ) resulted in considerable increases in absorption (ABS/RC) and trapped energy flux (TR o /RC) per reaction center. ABS/CS o , TR o /CS o , ABS/RC, and TR o /RC increased markedly under PS, whereas there were slight decreases in RC/CS o and electron transport activity. These results suggest that the PS treatment resulted in obvious improvements in the absorption and capture of light energy in shaded leaves. Further analysis demonstrated that both the WS and PS treatments resulted in a greater decrease in the activity of Rubisco compared to that of phosphoenolpyruvate carboxylase (PEPC). Moreover, PEPC activity in PS was maintained at a high level. Consequently, the current study proposed that the improvement of the absorption and capture of light energy and the maintenance of PEPC activity of mesophyll cells were due to photosynthetic acclimation of low-light-induced leaf senescence in maize plants. In addition, the rate of senescence of vascular bundle cells in maize leaves exceeded that of mesophyll cells under low light, showing obvious tissue specificity., (© 2021. The Author(s), under exclusive licence to Springer Nature B.V.)