Effects of drought on photosynthetic induction in leaves of different wheat genotypes under dark-to-light transition

Abstract

Adjustment of photosynthetic processes to an increase in irradiance constrains the CO2 assimilation and photosynthetic carbon gain compared to that which would be obtained if photosynthesis reached its terminal value instantaneously. Acceleration of photosynthesis induction under field conditions of fluctuating light opens up new perspectives for increasing yields. However, there is little information on response of photosynthetic processes to changes in the light under drought conditions. In a pot experiment, we have studied the peculiarities of response of CO2- and H2O-gas exchange parameters in flag leaves of 3 winter wheat genotypes to a transition from dark to bright light under drought conditions, to reveal the features of drought effect on photosynthetic induction processes. The plants were exposed to a 7-day drought (30% FC) during the flowering stage. After that, the watering of the treated plants was restored to the control level (70% FC), which was maintained until the end of the growing season. Induction curves of CO2 assimilation and transpiration of the flag leaves were recorded after keeping them in the darkness for 30 minutes, then the light was turned on. It was revealed that drought impacts the photosynthetic apparatus by reducing its maximum functional intensity as well as by diminishing its ability to respond to changing light conditions. Specifically, drought slows the rate of gas exchange increase during transitions from dark to light. High-yielding wheat genotypes, which had higher assimilation rates and stomatal conductance under optimal watering, showed increased sensitivity to drought. The reduction in the CO2 assimilation rate in wheat leaves under drought was primarily due to damage to the photosynthetic apparatus in mesophyll cells, rather than inhibition of stomatal conductance. This conclusion is supported by Ci value calculations, which were highest at the lowest CO2 assimilation rate at the beginning of light exposure and lowest at the highest CO2 assimilation rate when reaching a steady-state plateau. The stronger impact of the drought on the biochemical components of the photosynthetic apparatus than on stomata is also suggested by a decrease in instantaneous water use efficiency (WUEi) during photosynthesis. The genotypic differences in the effects of drought on the dynamics of photosynthetic induction parameters during dark-to-light transitions in wheat leaves suggest the potential of these traits for evaluating breeding material. This could enhance the ecological plasticity of new wheat varieties.