Growth and physiology of ?Sunrise? papaya seedlings in response to salinity and humic acid

Salinity is one of the major obstacles to agriculture in semi-arid regions, as it affects the physiological processes, growth, and yield of numerous crops. Hence, the application of salt stress attenuators is of paramount importance, as it enables the use of saline water for agricultural purposes. Among these, humic acid promotes the growth as well as water and nutrient uptake of plants. To this end, the present study evaluated the effects of humic acid on the growth and physiology of Carica papaya seedlings irrigated with saline water. The experiment followed the randomized block design with five levels of electrical conductivity (0.5, 1.15, 2.75, 4.35, and 5.0 dS m-1) and five concentrations of humic acid (0.5, 0.94, 2.0, 3.06, and 3.5 mL L-1). Growth, gas exchange, chlorophyll content, and chlorophyll a fluorescence were evaluated. Irrigation with 4.9 dS m-1 water and application of 3.05 mL L-1 humic acid promoted the growth of papaya seedlings. Irrigation with high-salinity water (4.96 and 3.09 dS m-1) coupled with the application of 3.05 and 2.21 mL L-1humic acid increased internal CO2 concentration, transpiration, instantaneous water use efficiency, carboxylation efficiency, and chlorophyll b content. Overall, humic acid (3.5 mL L-1) attenuated the deleterious effects of salt stress, promoting the growth and improving the performance of papaya seedlings under moderate salinity (4 dS m-1).