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 μ 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 μ 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 μ M stimulated net uptake, increased leaf Mg and Ca concentration, and produced leaf tissues with a very low K/(Ca + Mg) ratio.