Leaf aging reduces photosynthetic and stomatal kinetics under fluctuating irradiance, but not steady-state traits

In both natural and greenhouse environments, irradiance fluctuates frequently, differentially impacting leaves at various layers inside crop canopies. It is unclear whether leaf age affects the response of photosynthesis and photoprotection to fluctuations in irradiance, as in previous studies leaf age and other environmental factors were often entangled. To elucidate the effect of leaf age on photosynthesis, cucumber plants were grown horizontally under constant irradiance (250 μmol m^-2 s^-1) for ~50 days. Then, steady-state and dynamic photosynthesis were characterized in leaves of different ages within the same plant to isolate the effect of leaf age. We found that leaf expansion progressed as leaf age increased (range: 3-32 days), while age had little impact on steady-state photosynthesis. However, leaf aging slowed down both photosynthetic induction and stomatal kinetics, thereby reducing the ability of older leaves to utilize fluctuating irradiance. However, the relaxation rate of non-photochemical quenching after a stepwise reduction in irradiance increased as leaves became older. Moreover, biochemical limitations in older leaves also increased within the first 20 min of photosynthetic induction. Our findings suggest that steady-state and dynamic traits underlying photosynthetic regulation are differentially regulated by leaf aging.