Two mechanisms that reduce water and salt stress, respectively, are an increase in root hydraulic conductivity (LP) and reduction in Na and Cl absorption and transport to the leaf. NH4+-N decreased muskmelon LP 55-70% while under 100 mM NaCl stress and 40-50% in the absence of NaCl stress. A decrease in LP increases the rate of water stress development as the transpiration rate increases. Although dry weight decreased about 70%, with NO-3-N, muskmelon remained healthy green, while with NH+4-N they became chlorotic and necrotic with a 100% and 25% increase in leaf blade Na and Cl compared to NO-3-N, respectively. Further investigation indicated that NH+4-N increased muskmelon sensitivity to NaCl through both an increased rate of net Na influx and transport of Na to the leaf. Since Na influx partitioning is controlled by mechanisms K/Na selectivity and exchange across membranes, the NH+4-N inhibition of K absorption may impair K/Na exchange mechanisms. Reduced K/Na selectivity or Na efflux are implicated as the source of the increased net Na influx with NH+4-N. The importance of K in preventing Na partitioning to the leaf was confined through removal of K from the nutrient solution thereby simulating the NH+4-N-induced gradual K depletion in muskmelon. Our work indicates that at a given level of water or NaCl stress, NO-3-N reduces the level of stress experienced by muskmelon through increasing LP and reducing the net rate of Na influx and transport to the sensitive leaf blade. This avoidance mechanism should enable muskmelon plants fertilized with NO-3-N to tolerate greater levels of stress.
WHOLE PLANT MECHANISMS OF AMMONIUM INDUCED INCREASES IN WATER STRESS AND SENSITIVITY OF MUSKMELON TO NaCl
