Alleviatory activities of salicylic acid and chitosan in burdock plant (Arctium lappa L.) under drought stress

Salicylic acid and chitosan improve the plants’ resistance against drought stress through various mechanisms. To date, no information is available about the simultaneous effect of drought stress, chitosan, and salicylic acid onthe biochemical and physiological responses of burdock (Arctium lappa L.) plant. Therefore, a field experiment was conducted in a split plot form based on randomized complete block design with four replicates at the experimental farm of the Agriculture Faculty of Payame Noor University in Isfahan, during two successive years (2017-2018). Treatments included irrigation as the main factor at three levels (40%, 60%, and 80% hereafter called I1, I2 and I3, respectively), based on a predefined level of maximum allowable depletion of the threshold of available soil water, and four levels of foliar applications as the subplots (control, 5 g/liter of chitosan, 1 mg/liter of salicylic acid, and combined application of salicylic acid and chitosan hereafter called S1, S2, S3, and S4, respectively). Results indicated that the reduction of irrigation water, I2 and I3 treatments, compared to I1 treatment increased the activity of antioxidant enzymes, the content of reactive oxygen species, and content of mucilage in burdock plant. The mucilage content in root showed an increase by 61%, and 110% in I2 and I3, respectively, in comparison with I1 regardless of foliar applications. Also, application of chitosan and salicylic acid led to improved chlorophyll content, gas exchange parameters, plant water status, and the uptake of nutrients. Burdock photosynthesis rate was higher in S2-S4 than S1 regardless of irrigation levels (on the average 1.9, 2.9, and 2.4 μmol CO2 m-2 s-1, respectively). It is therefore suggested that application of chitosan and salicylic acid could be effective in growing burdock under drought stress.