Physiological aspects associated with mixed-nitrogen-induced increases in growth and tillering of wheat

Supplying N as mixtures of NO$\\sb{3\\sp-},$ shown as NO$\\sb3,$ and NH$\\sb{4\\sp+},$ shown as NH$\\sb4,$ (mixed N nutrition) enhances growth and tillering of wheat. However, despite the potential benefits to productivity, limited information is available on the physiological basis for this response. Thus, the overall goal of this work was to determine the physiological aspects associated with mixed-N-induced increases in growth and tillering of wheat. Specific experiments focused on the roles of: (1) cytokinins; (2) N use; (3) photoassimilate production and partitioning; and, (4) the accumulation and partitioning of other mineral ions in the enhanced growth and tillering of spring wheat induced by mixed N nutrition. Another experiment sought to verify that the tillering and yield response to N form observed with spring wheat also occurs for winter wheat. Plants of different wheat cultivars were grown with N as all NO$\\sb3,$ all NH$\\sb4,$ or an equal mixture of the two forms; and in some cases were treated with exogenous applications of a synthetic cytokinin--N$\\sp6$-benzylaminopurine (BAP). The level of endogenous cytokinins in the xylem exudate was also investigated. Similar to spring wheat, winter wheat plants produced more tillers, accumulated more whole plant N, and had higher grain yield when grown with the mixture of NO$\\sb3$ and NH$\\sb4$ than with either N form alone. Mixed-N-grown plants not sprayed with cytokinins produced more tillers, accumulated more N, and had greater total assimilation of $\\sp{14}$CO$\\sb2$ than NO$\\sb3$- or NH$\\sb4$-grown plants. The presence of NH$\\sb4$ (either with mixed N or all NH$\\sb4$) increased the shoot/root ratio, the partitioning of N to the shoot and of $\\sp{14}$C to the root, and the tillering efficiency for N use. N as NH$\\sb4$ also enhanced amino acid and protein synthesis by diverting carbon from carbohydrate metabolism. The cultivar which exhibited the most tillering also accumulated and partitioned more N to the shoots, and assimilated more $\\sp{14}$CO$\\sb2.$ Cytokinin sprays had no effect on the tillering of mixed-N-grown plants, but increased it in all NO$\\sb{3\\sp-}$- or NH$\\sb4$-grown plants to levels close to that obtained with mixed N. Cytokinin sprays also mimicked the effects of mixed N (and in some cases NH$\\sb4$) in altering the partitioning and use of N, and the assimilation and partitioning of $\\sp{14}$C. Plants grown with mixed N or all NH$\\sb4$ exhibited higher concentrations and mass transfer of cytokinins in the xylem exudate than those grown with all NO$\\sb3.$ Similar to N, the accumulation and partitioning of mineral nutrients was affected by N form. Along with increased tillering, mixed N increased the accumulation and partitioning of P, K, Cu, and B to the shoots compared to plants grown with either all NO$\\sb3$ or all NH$\\sb4.$ Based on the data from these studies, mixed-N induced increases in the growth and tillering of wheat appear to be associated with: (1) enhanced cytokinin production; (2) improvements in the accumulation and use of N and certain other mineral nutrients; and, (3) an alteration in the production and partitioning of photoassimilates.