Exogenous Application of Humic Acid Mitigates Salinity Stress on Pittosporum (Pittosporum tobira) Plant by Adjusting the Osmolytes and Nutrient Homeostasis

Abstract Improving the physiological status expressed in adjusting the osmolytes and nutrients balance of plant cell is a crucial matter for ameliorating the hazards of salinity. In this context, humic substances have a significant role for stimulating the plant tolerance to various stresses. Therefore, the current study aimed to assess the importance of foliar spray of humic acid (0 and 150 mg L −1 ) for avoiding the effect of salt stress (0, 4000 and 8000 mg L −1 ) on vegetative growth of pittosporum plant, protein, proline, peroxidase activity and nutrient components related to salinity. The treatments were arranged in a randomized complete block design with 3 replicates. Results revealed that the highest vegetative growth was recorded with mg L −1 humic acid. While, salinity levels of 4000, and 8000 mg L −1 led to increases in protein, proline peroxidase activity, and chloride and sodium inions. Compared to humic acid-untreated plants, application of humic acid under salinity level of 4000 mg L −1 enhanced plant height, root fresh weight plant −1 , root dry weight plant −1 , shoot fresh weight plant −1 and shoot dry weight plant −1 by 12.6, 10.9, 17.7, 43.4, 19.4%, respectively, in the second season. Humic acid application under all salinity levels showed favorable effect for keeping leaves in both seasons, since fallen leaves number was reduced. The increases in potassium (K) content reached about 12.0 and 22.4% under 4000 mg L −1 and 8000 mg L −1 , respectively, owing to humic acid application. Protein, proline content and peroxidase activity showed the minimal values under humic acid × salinity level of 4000 mg L −1 . It could be concluded that application of humic mitigates the harmful effect of salinity and improves the vegetative growth parameters and physiological status of pittosporum plants while increases the uptake of beneficial nutrients.