Estimating mesophyll and biochemical limitations to carbon assimilation of potato crop during drought

Understanding how plant crops respond to drought is essential for both improving photosynthesis modelling and predicting the impacts of climate change on agricultural production. Over the past years, researches have focused on identifying the stomatal processes that restrict the net photosynthetic rate and quantifying the importance and how different factors limit it. However, the constraints to photosynthesis coming from perturbation in the mechanisms taking place from sub-stomatal cavities to carboxylation sites in chloroplasts are not yet fully understood especially in plant crops. The aim of our study was to investigate the impact of drought on the light-limited photosynthesis rate for potato (Solanum Tubersosum) by measuring the photosynthesis limitations and partitioning them between stomatal, mesophyll and biochemical constrains during a field-experiment that took place in Wallonia during the summer 2020. Gas-exchange and fluorescence techniques were used to quantify mesophyll conductance (g_m), stomatal conductance (g_s), Rubisco carboxylation rate (V_cmax) and electron transport rate (J_max) in response to low soil water content during the tuber development stage. We obtained a clear reduction of the leaf assimilation and performed a limitation analysis identifying which factor contributed the most to the light-saturated photosynthetic rate (A_n) decrease. During the one-month drought treatment, A_n, g_m, J_max and V_cmax significantly decreased when the relative extractable water (REW) passed below a threshold ranging from 0.5 to 0.7 . On the opposite, g₁, the slope of the g_s dependence on environmental factors, remained constant. When soil water was not limiting, most of the light-saturated photosynthetic rate variation was explained by VPD while mesophyll and biochemical influence progressively increased when soil water content declined. At the maximum drought intensity, g_m and V_cmax reduction explained respectively 40 % and 30% of the light-saturated photosynthetic rate decrease. The coexistence of aerial drought (high VPD) accounted only for 3% of the total limitation. This highlights the importance of mesophyll and biochemical limitations on potato photosynthesis and development.