Assessing stomatal and non-stomatal limitations to carbon assimilation under progressive drought in peanut (Arachis...

Drought is known to limit carbon assimilation in plants. However, it has been debated whether photosynthesis is primarily inhibited by stomatal or non-stomatal factors. This research assessed the underlying limitations to photosynthesis in peanuts (Arachis hypogaea L.) grown under progressive drought. Specifically, field-grown peanut plants were exposed to either well-watered or drought-stressed conditions during flowering. Measurements included survey measurements of gas exchange, chlorophyll fluorescence, PSII thermotolerance, pigment content, and rapid A-C_i response (RACiR) assessments. Drought significantly decreased stomatal conductance with consequent declines in photosynthesis (A_N), actual quantum yield of PSII, and electron transport rate (ETR). Pigment contents were variable and depended on stress severity. Stomatal closure on stressed plants resulted in higher leaf temperatures, but F_v/F_m and PSII thermotolerance were only slightly affected by drought. A strong, hyperbolic relationship was observed between stomatal conductance, A_N, and ETR. However, when RACiR analysis was conducted, drought significantly decreased A_N at C_i values comparable to drought-stressed plants, indicating non-stomatal limitations to A_N. The maximum rate of carboxylation and maximum electron transport rate were severely limited by drought, and chloroplast CO₂ concentration (C_C) declined substantially under drought along with a comparable increase in partitioning of electron flow to photorespiration. Thus, while stomatal conductance may be a viable reference indicator of water deficit stress in peanut, we conclude that declines in A_N were largely due to non-stomatal (diffusional and metabolic) limitations. Additionally, this is the first study to apply the rapid A-C_i response method to peanut, with comparable results to traditional A-C_i methods.