Cytokinins in Enhanced Growth and Tillering of Wheat Induced by Mixed Nitrogen Source

Abstract

Supplying wheat plants with mixtures of NO3‐ and NH4+ increases growth and tiller production, but the physiological basis is unclear. Our objectives were to evaluate the roles for cytokinins and N use (e.g., accumulation, distribution, and utilization of absorbed N to produce tillers) in eliciting this response. Two species of spring wheat (Triticum aestivum L. cv. Len and Triticum durum Desf. cv. Inbar) were grown hydroponically in the greenhouse with N supplied as all NO3‐, all NH4+, or an equal mixture of each form. In one experiment, the synthetic cytokinin N6‐benzylaminopurine (100 µM) was applied repeatedly (a total of five times) to the shoots during tiller initiation and development, while in another, xylem exudate was collected from cut stumps during the early stages of tiller development. Plants not sprayed with cytokinins produced more tillers and accumulated more N when grown with mixed N than with either N form alone. Cytokinin sprays did not affect tillering of mixed N plants but increased tillers in all‐NO3‐grown or all‐NH4+‐grown plants to levels close to that obtained with mixed N. For unsprayed plants, the presence of NH4+ increased the shoot/root ratio, the proportion of N in the shoot, and the efficiency with which absorbed N was used to produce tillers, compared with plants grown with all NO3‐. Exogenous cytokinin appeared to mimic the effects of mixed N (and in some cases NH4+) in altering the distribution and use of N by the plant. In addition, plants grown with mixed N or all NH4+ exhibited higher concentrations and mass transfer of cytokinins in the xylem than those supplied with all NO3‐. These data are consistent with the interpretation that mixed N enhancement of tillering in wheat is due to increased synthesis of cytokinins, induced in part by use of NH4+‐N.