Photosynthesis Response of Tomato Plants Subjected to Drought Treatments That Differed in the Rate but Not Severity or Duration of Exposure to Drought Stress

Photosynthesis is highly sensitive to the rate, duration and severity of drought stress. The objective of this study is to understand acclimation of photosynthesis to two drought stress treatments that differed only in the rate of exposure to drought stress. Tomato (Solanum lycopersicum L.) plants were subjected to different rates of exposure to drought stress either by maintaining substrate moisture content (θ) at a low level (WL1) or allowing θ to dry down from a higher to lower θ level (WL2). The average severity (θ = 15%) and duration (2 weeks) of the drought stress was similar between two treatments. An automated irrigation controller interfaced to ECH2O moisture sensors was used to accurately maintain the two drought treatments. Measurements included leaf photosynthesis rate (A), stomatal conductance (g_s), quantum yield of photosystem II (Φ_PSII), relative electron transport rate (ETR) and response of leaf photosynthesis to internal CO₂ concentration (A-C_i response curves). Leaf A and g_s did not change in the WL1 treatment, whereas they decreased linearly in the WL2 treatment over time. Both Φ_PSII and ETR were not statistically different between WL1 and WL2, indicating that light reactions of photosynthesis were largely unaffected by the rate of exposure to drought stress. However, mesophyll conductance to CO₂ and Rubisco efficiency determined from the A-C_i response curves were lower in the WL2 than WL1 treatment indicating that rate of drought stress mostly affected Calvin cycle of photosynthesis. These results support that the acclimation of photosynthesis to drought stress was more pronounced in WL1 than WL2 treatment, likely due to prolonged exposure to a similar rate of drought stress in WL1.