Exogenous proline modulates heat shock and dehydrin protein expression in drought-stressed Capsicum annuum L. cv. Yalova Çorbacı 12

Drought stress is a major abiotic constraint limiting crop productivity worldwide, necessitating effective strategies to enhance plant tolerance. This study investigated the effects of exogenous proline application (0, 5, 10, or 20 mM) on physiological responses and stress-associated protein accumulation in pepper (Capsicum annuum L.) under drought stress conditions simulated by 10% polyethylene glycol (PEG-6000). Non-stressed or drought-stressed plants were treated with 0, 5, 10, or 20 mM proline. Drought stress significantly increased ion leakage and turgor loss while reducing relative water content (RWC) and total soluble protein (TSP) levels, indicating membrane destabilization and impaired protein homeostasis. Exogenous proline mitigated these effects in a concentration-dependent manner, with 5 and 10 mM treatments most effectively improving membrane stability and plant water status under drought conditions. Endogenous proline accumulation markedly increased under drought stress; however, exogenous application resulted in comparatively lower endogenous levels than drought-only plants. Immunoblot analyses revealed that drought strongly induced HSP60, two HSP23 isoforms (19 and 23 kDa), and 33 and 36 kDa dehydrin proteins. Proline treatments differentially modulated these stress-associated proteins, with moderate concentrations optimizing their accumulation, whereas higher doses amplified or suppressed specific responses depending on the protein examined. Notably, proline also altered protein expression patterns under non-stress conditions, indicating a regulatory or priming-like effect. These findings demonstrate that proline-mediated drought tolerance in pepper involves integrated osmotic regulation and stress-protein modulation, highlighting its potential as a concentration-dependent agronomic strategy to enhance crop resilience under water-limited conditions.