Differential impact of decreasing relative humidity on photosynthesis under fluctuating light between maize and tomato

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.