Potential for Phosphorus Toxicity in Zinc-Stressed Corn and Potato

Corn (Zea mays L., ‘Illinois WF 9 × 38-11’) and potato (Solanum tuberosum L., ‘Russet Burbank’) were grown in dithizone purified nutrient solutions to contrast relationships between dry matter yield, tissue phosphorus (P) concentration, and tissue zinc (Zn) concentration as they influence the development of Zn deficiency and P toxicity. Treatments consisted of Zn at 0, 0.14, and 0.41 µM in factorial combination with P at 0.02, 0.10, 1.0, and 3.0 mM. Yield and tissue concentration of P and Zn were affected by statistically significant P-Zn treatment interactions in both species. Corn plants developed Zn deficiency symptoms and responded to nutrient solution Zn when plant growth was not limited by inadequate nutrient solution P. Corn plants grown in nutrient solutions containing 0 µM Zn and 3.0 mM P contained 3.39% P in leaf tissue and had significantly lower yields than did plants grown in nutrient solutions containing 1.0 mM P. Even though no visual symptoms were evident, P toxicity probably was responsible for the reduction in yield. Potato plants responded to nutrient solution Zn when growth was not limited by inadequate P but did not exhibit visual symptoms of Zn deficiency. Zinc-deficient potato plants containing in excess of 2.22% P in leaf tissue exhibited probable P toxicity symptoms characterized by puckering of leaf edges, thickening and upward curling of leaves, and leaf necrosis. Excessive concentration of P in leaf tissue of Zn-deficient potato was the result of increased mobilization and translocation of P from roots to above-ground parts, increased total P uptake, and concentration of P resulting from restricted growth. Excessive levels of P in corn leaf tissue were primarily the result of concentration of P due to restricted growth. Zinc concentration in tissue of both species was reduced by growth response dilution, but Zn uptake by plants receiving low levels of Zn was not reduced by increasing nutrient solution P concentrations. Apparent accentuation of Zn deficiency by P application is explained on the basis of accumulation of toxic levels of P in Zn-deficient plants. Species such as potato which show increased P uptake and increased mobilization and translocation of P to tops when under Zn stress are probably more susceptible to P toxicity.