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Under the background of global climate change, crop leaves usually experience fluctuating light (FL) under compound stresses of drought and heat in the field. However, photoinhibition and photosynthetic regulation in FL under these compound stresses are little known. To elucidate this, we measured gas exchange, chlorophyll fluorescence, P700 redox state and electrochromic shift signals for tomato (Solanum lycopersicum) leaves exposed to FL under individual and compound stresses of moderate drought and heat stresses. After treatment with short‐term FL, photosystem I (PSI) activity decreased slightly in all treatments. By comparison, photosystem II (PSII) activity was reduced slightly under individual stress but largely decreased by 46% under compound stresses. Therefore, PSI is more tolerant than PSII when exposed to FL under compound stresses of drought and heat. Under compound stresses, PSII photoinhibition largely downregulated linear electron flow and photorespiration became a major electron sink downstream. Concomitantly, cyclic electron flow (CEF) was highly enhanced to help the building up of trans‐thylakoid proton gradient. This study highlights the importance of PSII activity and CEF in crop tolerance under future compound climate extremes such as drought and heat. [ABSTRACT FROM AUTHOR]

Photosynthetic CO2 assimilation (AN) under fluctuating light (FL) plays a critical role in plant growth and crop yield. C4 crops usually reach greater yields than C3 crops when grown in regions with low relative humidity, which might be partly due to the CO2 concentration mechanism and higher AN under FL of C4 crops. To test this hypothesis, we measured gas exchange and chlorophyll fluorescence under different changing frequencies of FL at contrasting relative air humidity (60% and 15%) for leaves of maize (Zea mays, C4) and tomato (Solanum lycopersicum, C3). In general, a more rapid changing frequency of FL would cause a larger loss in AN during shade‐sun transitions, with 20–38% in maize and 16–52% in tomato. Low relative humidity induced a larger decrease of stomatal conductance (gs) under FL in tomato than in maize, leading to a much stronger loss in AN in tomato than in maize. After transition from low to high light, AN of maize was mainly limited by gs with negligible non‐stomatal limitation, and the stomatal limitation was slightly affected by relative humidity. By comparison, low relative humidity significantly increased the stomatal limitation of AN in tomato but hardly affected non‐stomatal limitation. These results highlight the differential impact of decreasing relative humidity on dynamic photosynthesis and productivity between C3 and C4 crops. [ABSTRACT FROM AUTHOR]

Photosynthetic efficiency under both steady-state and fluctuating light can significantly affect plant growth under naturally fluctuating light conditions. However, the difference in photosynthetic performance between different rose genotypes is little known. This study compared the photosynthetic performance under steady-state and fluctuating light in two modern rose cultivars (Rose hybrida), "Orange Reeva" and "Gelato", and an old Chinese rose plant Rosa chinensis cultivar, "Slater's crimson China". The light and CO2 response curves indicated that they showed similar photosynthetic capacity under steady state. The light-saturated steady-state photosynthesis in these three rose genotypes was mainly limited by biochemistry (60%) rather than diffusional conductance. Under fluctuating light conditions (alternated between 100 and 1500 μmol photons m−2 m−1 every 5 min), stomatal conductance gradually decreased in these three rose genotypes, while mesophyll conductance (gm) was maintained stable in Orange Reeva and Gelato but decreased by 23% in R. chinensis, resulting in a stronger loss of CO2 assimilation under high-light phases in R. chinensis (25%) than in Orange Reeva and Gelato (13%). As a result, the variation in photosynthetic efficiency under fluctuating light among rose cultivars was tightly related to gm. These results highlight the importance of gm in dynamic photosynthesis and provide new traits for improving photosynthetic efficiency in rose cultivars. [ABSTRACT FROM AUTHOR]

Fluctuating light (FL) can cause the selective photoinhibition of photosystem I (PSI) in angiosperms. In nature, leaves usually experience FL conditions with the same low light and different high light intensities, but the effects of different FL conditions on PSI redox state and PSI photoinhibition are not well known. In this study, we found that PSI was highly reduced within the first 10 s after transition from 59 to 1809 μmol photons m−2 s−1 in tomato (Solanum lycopersicum). However, such transient PSI over-reduction was not observed by transitioning from 59 to 501 or 923 μmol photons m−2 s−1. Consequently, FL (59-1809) induced a significantly stronger PSI photoinhibition than FL (59-501) and FL (59-923). Compared with the proton gradient (∆pH) level after transition to high light for 60 s, tomato leaves almost formed a sufficient ∆pH after light transition for 10 s in FL (59-501) but did not in FL (59-923) or FL (59-1809). The difference in ∆pH between 10 s and 60 s was tightly correlated to the extent of PSI over-reduction and PSI photoinhibition induced by FL. Furthermore, the difference in PSI photoinhibition between (59-923) and FL (59-1809) was accompanied by the same level of linear electron flow. Therefore, PSI photoinhibition induced by different intensities of FL is more related to the kinetics of ∆pH formation rather than linear electron flow. [ABSTRACT FROM AUTHOR]

Photosynthesis in fluctuating light requires coordinated adjustments of diffusion conductance and biochemical capacity, but the role of chloroplast ATP synthase activity (g H +) in dynamic photosynthesis is not well understood. In this study, we measured gas exchange, chlorophyll fluorescence and electrochromic shift signals in fluctuating light for leaves of tomato (Solanum lycopersicum) and maize (Zea mays). During the transition from sun to shade, simultaneous increases in g H +, effective quantum yield of PSII, and net CO 2 assimilation rate (A N) occurred in tomato but uncoupled in maize, indicating that g H + limited A N during the sun-to-shade transition in tomato but not in maize. During the shade-to-sun transition, g H + increased simultaneously with stomatal conductance, mesophyll conductance and Rubisco carboxylation capacity in tomato, suggesting that g H + is an overlooked factor affecting light induction of A N in tomato. By comparison, g H + maintained at high levels in maize and its A N was mainly restricted by stomatal conductance. Our results reveal that the kinetics of g H + in fluctuating light differs between species, and chloroplast ATP synthase may be a potential target for improving dynamic photosynthesis in crops such as tomato. • The kinetics of g H + under fluctuating light differ between tomato and maize. • g H + limits A N during the sun-to-shade transition in tomato but not in maize. • g H + limits A N during photosynthetic induction in tomato but not in maize. • g H + coordinates with g s , g m and V camx to regulate A N under fluctuating light. • Chloroplast ATP synthase is a potential target for improving dynamic photosynthesis. [ABSTRACT FROM AUTHOR]