S06 - Session P4 - CO2 concentration and air humidity interactively affect the rate of photosynthetic induction, but not loss of induction

Authors: Ningyi Zhang *, Yunke Chen, Sarah Berman, Leo Marcelis, Elias Kaiser

Irradiance frequently fluctuates in greenhouses due to sun and cloud movement, shading by greenhouse structure, and supplemental lighting. Photosynthesis gradually increases upon an increase in irradiance ('photosynthetic induction') instead of immediately reaching steady-state, and this rate is strongly affected by environmental conditions. Various climate factors such as [CO 2 ] and relative humidity (RH) fluctuate simultaneously in greenhouses, potentially causing interactive effects on dynamic photosynthesis. We investigated the interactive effects of [CO 2 ] and RH on photosynthetic induction and loss of induction in tomato ( Solanum lycopersicum L. ). Photosynthetic induction was measured by exposing shade-adapted leaves (50 μmol m -2 s -1 ) to a high irradiance (1000 μmol m -2 s -1 ) until photosynthesis reached steady-state at three [CO 2 ] (400 ppm, 800 ppm and 1200 ppm) and three RH levels (30%, 50% and 75%; resulting in nine [CO 2 ] and RH combinations). Loss of induction was measured by exposing high irradiance-adapted leaves to different shade durations (30 s, 1 min, 5 min and 10 min) and re-exposing the leaf to the high irradiance until photosynthesis reached steady-state. Time to reach 20%, 50% and 90% photosynthetic induction (T 20 , T 50 and T 90 respectively) and induction states reached at 60 s in high irradiance for leaves under different shade durations (IS 60 ) were quantified. Under low RH (30-50%), T 20 , T 50 and T 90 decreased with increases in [CO 2 ]. Under 75% RH, T 20 was the highest at the intermediate [CO 2 ] (800 ppm) whereas T 50 and T 90 were the lowest. The rate of loss of induction was not affected by [CO 2 ] nor by RH, excepted for IS 60 under 10 min shade, which increased with increases in [CO 2 ] at 30% and 50% RH, but declined with [CO 2 ] at 75% RH. Our results reveal the importance of multiple climate factors interactively affecting dynamic photosynthesis, which deserves more attentions in optimizing light use efficiency in greenhouses.