PHOSPHATE INTERACTION WITH UPTAKE AND LEAF CONCENTRATION OF MAGNESIUM, CALCIUM, AND POTASSIUM IN WINTER-WHEAT SEEDLINGS

Low concentrations of Mg and Ca in cool-season grasses in late fall and early spring are a primary cause of grass tetany and wheat pasture poisoning in grazing cattle. Our objective was to investigate phosphate interaction with the uptake and leaf concentration of Mg, Ca, and K in winter wheat (Triticum aestivum L., cv. Larned) seedlings. Seedlings were grown hydroponically or in pots containing perlite with a solution containing nutrient element concentrations that mimicked the soil solution of a typical midwestern alfisol. Phosphate concentration of this solution was low so that wheat seedlings depleted solution P within a few hours. At low solution P concentrations, Mg and Ca were effluxed from roots. As solution P was increased from 25 to over 100-mu-M, net uptake of both Mg and Ca was observed over a 48 h period. In 41-d greenhouse experiments, Mg and Ca concentrations increased and K concentration decreased in the leaf as P treatments were increased. Tetany is likely to occur in ruminants consuming grass with equivalent ratios of K/(Ca + Mg) > 2.2. Increasing P from 50 to 400-mu-M lowered this ratio from 1.8 to 1.0 and from 1.7 to 1.2 in separate greenhouse experiments. Concentrations of P above 100-mu-M stimulated net uptake, increased leaf Mg and Ca concentration, and produced leaf tissues with a very low K/(Ca + Mg) ratio.