Response of two leafy vegetables grown at high salinity to supplementary potassium and phosphorus during different growth stages
An outdoor pot experiment was carried out in sand culture to investigate the response of spinach (Spinacia oleracea) cv. "Matador" and lettuce (Lactuca sativa L.) cv. "Paris Island" grown at high salinity to supplementary phosphorus (P) and potassium (K). Plants were tested during a period from germination to vegetative growth stage. Treatments in germination stage were (1) tap water alone (TW); (2) TW plus 60mM NaCl (TW + S); and (3) TW + S plus supplementary 1 mM KHPO and 2mM KSO in tap water (TW + S + PK). Treatments initiated for seedling and vegetative growth stages were (1) complete nutrient solution (C); (2) C plus 60 mM NaCl (C + S); and (3) C + S plus supplementary 1 mM KHPO and 2 mM KSO in nutrient solution (C + S + PK). High salinity in tap water delayed germination and reduced the final germination percentages for both species compared to control values. High salinity also decreased root elongation compared to the control while the TW + S + PK treatment was similar to the control for both species. Seedling growth, vegetative growth, total chlorophyll and water usage were significantly reduced in both species by high salinity and membrane permeability increased relative to control values. Supplementary K and P (C + S + PK treatment) produced fresh weight, chlorophyll concentrations, water usage and membrane permeability values similar to or slightly lower than the controls. Spinach growth parameters appeared to be less affected than those of lettuce by salinity. Sodium (Na) concentration in plant tissues increased for both species, especially in lettuce, in the elevated NaCl treatment and leaf P and K was lowered compared to control values. Supplementary P and K produced leaf and root Na levels that were much higher than control values but significantly lower than in the saline treatment in most cases. These results suggest that supplementary P and K can reduce the adverse effects of high salinity on plant growth and physiological development in these leafy vegetables.