Alleviation of Drought Stress Effects by Exogenous Application of Spermidine and Salicylic Acid on Hollyhock (Alcea rosea)

Drought is one of the most important environmental factors limiting the cultivation of ornamental plants in green spaces. The effects of spermidine and salicylic acid were investigated on hollyhock (Alcea rosea L.) under drought stress. At first, the plants were sprayed with three doses of spermidine and three doses of salicylic acid (control, 100, 200 and 400 μM) for three consecutive days and they were then subjected to drought stress (40, 60, 80 % FC) for two weeks. The results showed that the increase in drought stress up to 40% FC increased electrolyte leakage, proline, and superoxide dismutase enzyme activity compared to the control plants. Also, the application of 100 μM spermidine and salicylic acid at different concentrations of spraying solutions significantly reduced electrolyte leakage and catalase enzyme activity and increased relative water content (RWC), proline, protein, number of flowers, leaf area, and superoxide dismutase enzyme activity, but the higher concentration (400 μM) was ineffective or had inhibitory effects. The plants treated with 100 μM spermidine and salicylic acid showed higher tolerance to drought stress (up to 40% FC) with regard to lower electrolyte leakage (by 5%) and higher relative water content (by 11 and 9%), proline content (by 31 and 21%), SPAD (by 18 and 5%), and dry weight (by 3%) compared with the non-treated plants under 40% FC. Hollyhock growth was severely damaged by water deficit, but the application of spermidine and salicylic acid promoted RWC, proline, and protein content under water deficit conditions. Foliar application of spermidine and salicylic acid can be considered an economical practice to increase hollyhock performance under water deficit conditions.