COMPENSATORY EFFECTS OF HUMIC ACID ON PHYSIOLOGICAL CHARACTERISTICS OF PISTACHIO SEEDLINGS UNDER SALINITY STRESS

Low soil quality caused by low organic compounds and high salinity are the major natural limitations of pistachio orchard productivity in Iran. Humic acid (HA), the major component of soil organic matter, can be of major benefit for soil chemistry and fertility, plant physiology and plant growth under salinity stress. HA was soil applied at four levels: 0, 500, 1000 and 2000 ppm to one-year-old ‘Badami’ pistachio seedlings. The NaCl salinity including 0, 25, 50 and 100 mM applied into irrigation water for 45 days. The results showed significant interaction effects of HA and salinity on plant physiology and growth. Significant increase in proline accumulation was revealed under high salinity conditions (50 and 100 mM NaCl). HA treatments at low levels tend to moderate salinity stress and decrease leaf proline accumulation. The HA treatments also decreased levels of ABA (a stress-induced hormone) by their stimulatory effects on plant growth and development. As a conclusion, HA had a compensatory effect on plant growth under salinity stress.