Drought stress is a serious threat to crop production in arid and semi-arid regions of the world. The research aimed to investigate the effects of putrescine on two commercial citrus rootstocks (Carrizo citrange and Volkameriana) grown under drought stress. Putrescine at three concentrations (0, 1, and 2 mM) was foliar-sprayed on pot-grown Carrizo citrange and Volkameriana seedlings under drought stress and normal conditions. Growth properties, physiological traits, and the expression of some stress-related genes were analyzed. Root growth and shoot growth were remarkably enhanced by putrescine treatment in both groups. Application of putrescine significantly increased root fresh weight, root length, and the percentage of fine roots in the seedlings. While growth, photosynthetic traits, leaf water status, and chlorophyll content decreased by drought, they were improved by putrescine application. The application of 2 mM putrescine yielded the optimal results. Furthermore, putrescine treatment significantly reduced root diameter, but increased chlorophyll fluorescence parameters, chlorophyll content, and proline and protein contents. The analysis of gene expression showed higher expression of CsNCED3 and RD22 genes in both groups under drought stress. Furthermore, the expression of these two genes was higher in putrescine-treated seedlings grown under drought stress. On the contrary, the expression of Chlase1 gene increased under drought stress but decreased as putrescine concentration increased. The adverse effects of drought stress were mitigated following the application of putrescine in both groups.
