Oxidative Stress and Changes in Antioxidant and Biochemical Constituents in Papaya ( Carica papaya L.) under Salt Stress

Six papaya cultivars viz., Pusa Dwarf, Surya, Solo, CO5, Tainan and Red Lady were subjected to saline water salt stress continuously for a period of six months with saline water irrigation having an EC value of 0.6, 2.0 and 4 dsm^-1. Among these, Red Lady was more sensitive while Tainan resisted salt stress. Under salt stress of 4 dsm^-1, yield reduced by 10% in Tainan and by 24% in Red Lady compared to unstressed controls. T.S.S. measurement showed that quality of fruits was not affected by saline irrigation in both cvs. Malondialdehyde levels estimated after six months period of stress, as thiobarbituric acid reacting substances, did not increase in Tainan in contrast to substantial increase in Red Lady under stress conditions. There was substantial increase in levels of antioxidant compounds namely, carotenoids, phenols and flavonoids in Tainan compared to Red Lady. In Tainan there were significant increases in reducing and total sugars and sucrose under conditions of stress in contrast to sharp decreases in Red Lady. Under conditions of stress, there was considerable accumulation of total and reducing sugars and sucrose, across the varieties, possibly contributing to osmotic adjustment. Association of salt stress tolerance in Tainan with soluble sugar accumulation could be used as a breeding tool for selecting salt tolerant papaya genotypes.