Improving photosynthesis and drought tolerance in Nicotiana tabacum L. by foliar application of salicylic acid

Drought causes a decrease in tobacco yield and quality as well as a reduction in net photosynthetic rate. Salicylic acid (SA) is involved in the regulation of many physiological processes and has been widely used to improve plant drought tolerance. The hydroponic experiment was carried out on flue-cured tobacco variety K326. SA solutions (0.3 mM) were sprayed on the leaves every day for three days, after which 15% PEG-6000 was used to simulate moderate drought. The results showed that drought stress reduced the photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr), while it enhanced the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), malondialdehyde (MDA), proline and protein content. Compared with PEG drought stress, SA improved the Pn, Gs and Tr by 45.74%, 26.82%, and 52.22% respectively. Besides, the activities of SOD, POD, CAT, protein and proline content increased by 44.27%, 50.18%, 25%, 34.69% and 24.88%, respectively, while the MDA content decreased by 23.89%. Furthermore, SA spraying increased the drought tolerance of tobacco by modulating the expression level of the genes associated with photosynthesis, carbon metabolism, and photosynthesis-antenna proteins. Hence, SA spraying reduced the inhibition of drought on photosynthesis and stimulated the antioxidant system, thus improving the drought resistance in tobacco.