Determination of the effects of exogenous melatonin applications on drought stress to raspberry (Rubus idaeus L.) plants reproduced in in vitro conditions

This study was conducted between 2023 and 2025 to investigate the responses of two raspberry cultivars (Ruby and Heritage), propagated under in vitro conditions, to subject different drought levels and to elucidate the mechanisms of melatonin applications aimed at enhancing drought tolerance. Under controlled conditions, twelve treatments were applied to the seedlings using three different drought levels (full irrigation, moderate drought, and severe drought) and four melatonin doses (0, 5, 10, and 15 µM). Morphological, physiological, and biochemical parameters were evaluated, including drought severity, relative growth rate, change in leaf number, change in shoot thickness, leaf area, relative water content of the leaves, membrane permeability, chlorophyll and carotenoid contents, MDA and proline content, total phenolic compounds, total soluble protein content, and antioxidant enzyme activities (SOD, CAT, APX). The study found that drought stress significantly increased membrane damage, and the growth and pigment parameters decreased, especially under severe drought conditions without melatonin application. However, particularly the 10 µM melatonin treatment effectively reduced drought-induced oxidative stress, improved the antioxidant enzyme activity, and helped maintain the chlorophyll content and growth performance. Among the two cultivars, 'Ruby' exhibited higher drought tolerance than 'Heritage'; this was indicated by the lower accumulation of stress markers, stronger antioxidant responses and better preservation of physiological traits under drought conditions. Overall, the 10 µM melatonin dose was found to be the most effective in enhancing drought tolerance in both cultivars, with 'Ruby' responding more positively in almost all parameters. These findings demonstrate the protective role of melatonin against drought stress in raspberry and emphasize that cultivar-specific differences should be considered in future stress management strategies.