AMMONIUM INCREASES THE NET RATE OF SODIUM INFLUX AND PARTITIONING TO THE LEAF OF MUSKMELON

The influx and partitioning of sodium (Na) is controlled by potassium (K)/Na selectivity/exchange mechanisms. Since ammonium-nitrogen (NH4-N) has been shown to inhibit K absorption and K/Na selectivity/exchange mechanisms control Na influx and partitioning, our objective was to observe if NH4-N affects Na influx and partitioning in muskmelon. Muskmelon (Cucumis melo L.) were grown in a pH controlled nutrient solution with 100 mM NaCl in a complete nutrient solution containing either 10 mM nitrate-nitrogen (NO3-N) or NH4-N. With NH4-N, Na accumulation and partitioning to the leaf blade increased while K absorption was almost completely inhibited. A second study omitted K to simulate the inhibition of K absorption by NH4-N and monitored Na accumulation and partitioning as K was depleted in the plant. Sodium accumulation and partitioning to the leaf increased as K decreased in the plant, mirroring the effect of NH4-N. Roots appeared healthy in both studies. Our work indicates that at a given level of NaCl stress, NO3-N reduces the level of stress experienced by muskmelon plants through reducing the net rate of Na influx and transport to the sensitive leaf blade, not by reducing chloride (Cl) absorption, thereby permitting these plants to ''avoid'' this stress.