Effects of Nano-Silicon and Irrigation Regimes on Drought-Responsive Gene Expression in Capsicum annuum at Two Sampling Times

Deficit irrigation is increasingly used to conserve water, although it may adversely affect crop productivity. Nano-silicon has been investigated primarily for its morphological and physiological effects under drought stress, while its influence on drought-responsive gene expression in pepper remains poorly understood. This study evaluated the effects of three irrigation regimes (100%, 75%, and 50%) and three nano-silicon doses (0, 50, and 100 ppm) on the expression of CaDREB32, CaLOX1, CaNHX1, and CaP5CS in Capsicum annuum L. Gene expression was determined by RT-qPCR using three biological replicates per treatment, and relative expression was calculated using the 2^−ΔΔCt method. The two sampling times were analyzed and interpreted separately. On day 15, 100 ppm NanoSi resulted in significantly higher CaDREB32 expression than the 0 and 50 ppm treatments. CaLOX1 expression was significantly affected by irrigation regime, NanoSi dose, and their interaction, whereas CaP5CS expression differed significantly among irrigation × NanoSi treatment combinations. No significant treatment effects were detected for CaNHX1. On day 30, no significant treatment effects were detected for CaDREB32. Irrigation regime, NanoSi dose, and their interaction significantly affected CaLOX1 expression, with untreated plants showing higher expression than NanoSi-treated plants. CaNHX1 expression was significantly higher under 100 ppm NanoSi than under the 0 and 50 ppm treatments. CaP5CS expression was significantly affected by irrigation regime and the irrigation × NanoSi interaction, while the main effect of NanoSi dose was not significant. Overall, the statistically supported responses varied among genes and sampling times, and NanoSi did not exert a consistent stimulatory effect across all genes or irrigation regimes. These findings demonstrate treatment-associated changes in specific drought-responsive genes but do not alone establish improved drought tolerance.