Whole plant mechanisms of ammonium-induced increases in water stress and sensitivity of muskmelon to salt

Paul Robert Adler, Purdue University

Abstract

Many researchers have focused on trying to understand the physiological basis for and breeding to increase drought and salt tolerance. Two mechanisms that would reduce water and salt stress, respectively, would be to increase root hydraulic conductivity and reduce Na and Cl absorption and transport to the leaf. In arid climates, frequent application of N through trickle irrigation increases efficiency of N and water application and use. Nitrification of NH$\sb4\sp+$-N occurs rapidly under most soil conditions. However, with frequent N applications, the conversion of NH$\sb4\sp+$-N to NO$\sb3\sp-$-N can be considered negligible. NH$\sb4\sp+$-N decreased muskmelon root hydraulic conductivity $(L\sb{p}$) 55% while under NaCl stress and 48% in the absence of NaCl stress. A decrease in $L\sb{p}$ increases the rate of water stress development as the transpiration rate increases. Although muskmelon dry weight decreased about 70% with 200 mM NaCl, with NO$\sb3\sp-$-N, muskmelon remained healthy green, while with NH$\sb4\sp+$-N they became chlorotic and necrotic with a 100% and 25% increase in leaf blade Na and Cl compared to NO$\sb3\sp-$-N, respectively. Although NR is more sensitive to stress than GS or GOGAT, N was not deficient and, therefore, the rate of NO$\sb3\sp-$-N assimilation was apparently not limiting plant growth. Further investigation indicated that NH$\sb4\sp+$-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 and partitioning is controlled by mechanisms of K/Na selectivity and exchange across membranes, the NH$\sb4\sp+$-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$\sb4\sp+$-N. The importance of K in preventing Na partitioning to the leaf was confirmed through removal of K from the nutrient solution thereby simulating the NH$\sb4\sp+$-N-induced gradual K depletion in muskmelon. Our work indicates that at a given level of water or NaCl stress, NO$\sb3\sp-$-N reduces the level of stress experienced by muskmelon through increasing root hydraulic conductivity 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$\sb3\sp-$-N to tolerate greater levels of stress.

Degree

Ph.D.

Advisors

Wilcox, Purdue University.

Subject Area

Botany|Plant propagation

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