NaCl pre-treatments mediate salt adaptation in melon plants through antioxidative system

Seeds of melon (Cucumis melo var. inodorus) cultivars Hasanbey and Kirkagac were primed with 18 dS m^-1 NaCl for 3 days at 20°C. During emergence and seedling growth, non-primed seeds were irrigated with local irrigation water (EC: 0.3 dS m^-1) whereas primed groups were treated with 9.0 dS m^-1 saline solution for 35 days (seedling conditioning). After transplanting, the seedlings derived from NaCl pre-treated (P) and non pre-treated (NP) groups were irrigated with or without 18 dS m^-1 NaCl solution for a period of 90 days. NaCl pre-treatments diminished inhibiting effect of salinity on plant dry matter production in Kirkagac and induced a significant recovery in survival in Hasanbey. NaCl pre-treatments mitigate the negative impacts of salt stress due to increased adaptation of melon plants to NaCl salinity by enhancing chlorophyll content, stomatal conductance and transpiration in both cultivars. Malondialdehyde (MDA) content decreased in salt-stressed P plants of both cultivars, but was practically unaffected in those of the NP ones. Comparing the response of P and NP plants in both cultivars, NaCl pre-treatments had no effect on SOD activity while they induced APX activity only in Hasanbey. However, NaCl pre-treatments enhanced the activities of CAT, POX and GR in salt-stressed plants of both cultivars. These results suggested that NaCl pre-treatments might be activating antioxidative enzymes, at least in part, thereby controlling overproduction of ROS, hence preventing lipid peroxidation resulting into improved salt adaptation of long term salinity stressed melon plants.