The Role of Brassinosteroids and Nano-Encapsulated Brassinosteroids in Capsicum Pepper Growth and Physiological Adaptations to High-Temperature Stress

Much is unknown about the positive effect of plant growth regulators, such as brassinosteroids, on high-temperature stress tolerance in pepper plants. This study aimed to reveal the effect of exogenous brassinosteroids (BRs) on high-temperature-stressed bell peppers by foliar application. BR treatments (1 µM), in both bulk (Br) and nano-encapsulated (N-Br) forms, were applied to pepper plants subjected to high-temperature stress (35 °C). The results indicated an increase in plant biomass, number of fruits, and relative water content and a significantly lower flower abscission in response to Br and N-Br under high temperatures. Br and N-Br applications at high temperatures resulted in the lowest leaf water potential. Br and N-Br, especially N-Br, were more effective than Br in the upregulation of the antioxidant enzyme activity, such as catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), and the overall antioxidant capacity of heat-stressed plants. MDA and electrolyte leakage significantly declined as a result of BR application and the proline content of the leaves was significantly higher in Br and N-Br treatments at high temperatures. Further analysis of the data pointed to SOD, DPPH, proline, RWC, and leaf water potential in pepper leaves as the most affected traits in response to brassinosteroid application under high temperature. Glucose and fructose levels also increased under high temperature, and only N-Br administration showed a significant effect on reducing the sugar levels. At high temperatures, the ratio of saturated to unsaturated fatty acids was greatest when neither Br nor N-Br was present. N-Br could reduce this ratio effectively. Conclusively, the overall performance of bell pepper improved in response to both types of BR application with no significant discrimination being found to prioritize the encapsulated form of BR application.