Physiological Response of the Pea Crop (Pisum sativum L.) Variety ‘San Isidro’ to Water Deficit and the Application of Brassinosteroids

The pea is a crop of great economic and nutritional importance due to its protein contribution. However, its productivity is affected by water deficit, which is an increasingly frequent phenomenon due to rising climate variability. Brassinosteroids (BRs) are plant hormones that regulate multiple and diverse physiological processes and can mitigate the effects of water stress. The objective was to evaluate the physiological response of the ‘San Isidro’ pea crop variety to water deficit and the application of BRs during flowering. A completely randomized design was used with five treatments: well-watered control (CK) and four treatments with water stress (imposed by withholding irrigation until volumetric water content reached 5–9%, maintained for 21 days) and BR doses of 0, 0.5, 1.0, and 1.5 mg per plant. On average, water deficit reduced shoot dry biomass and pod biomass by 82% and 77%, respectively, and decreased the number of pods per plant (49%) and per branch (41%) compared to the well-watered control. It also significantly decreased water potential, net photosynthetic rate (An), stomatal conductance (gs), transpiration (E), and intercellular CO2 concentration, while foliar temperature increased. The application of BRs (especially the 0.5 mg per plant dose) increased 100-seed mass by 91.4% compared with the water-stressed 0-BR treatment. Although no statistically significant differences were detected, this dose also increased pod biomass by 130% and the number of pods by 73% compared with the water-stressed 0-BR treatment. BR application alleviated the impact of water stress by increasing qL in leaves (at 85 DAP) and ΦPSII in stipules (at 95 DAP) and by enhancing ATP synthase activity (gH+) in both organs and promoting recovery of gas exchange (An, gs, E) and water potential. The stipules showed significant differences in more physiological variables than the leaves, suggesting potentially higher sensitivity; however, formal organ × treatment interaction analyses are needed to confirm this. It should be noted that the experimental design included only one well-watered control and did not include a factorial combination of water status × BR dose; consequently, the scope of our conclusions is restricted to the evaluation of BR effects under water-stress conditions. Further studies with a complete factorial design are required to fully disentangle the interactions between water status and BR dose.