Involvement of endogenous plant hormones in the effect of mixed nitrogen source on growth and tillering of wheat

Abstract

A Chinese spring wheat (Triticum aestivum L. cv. Yang Mai 158) was grown hydroponically with nitrogen (N) supplied as all ammonium (NH₄ ⁺), all nitrate (NC₃ ^‐), and a mixture (NH₄ ⁺‐N:NO₃ ^‐‐N=30:70) of each form. Wheat plants produced more tiller and more dry matter, and showed a higher shoot/root dry matter ratio when grown with mixed N than with either N form alone. In addition, all‐NH₄ ⁺‐grown plants produced more tillers and dry matter as compared to all‐NO₃ ^‐‐grown plants. Subsequently, endogenous levels of isopentenyladenine and isopentenyladenosine, zeatin and zeatin riboside, dihydrozeatin and dihydrozeatin riboside, indole‐3‐acetic acid (IAA), gibberellin A, and gibberellin A₃ (GA₁₊₃) and abscisic acid (ABA) in the shoot and the root was determined by enzyme‐linked immunosorbent assays. It was found that the levels of cytokinin in both the shoot and the root of mixed‐N‐grown plants were higher than those of all‐NO₃ ^‐‐grown and all‐NH₄ ⁺‐grown plant, whereas ABA level was similar among plants grown under different N forms. On the other hand, plants grown with mixed N exhibited lower IAA level in the shoot, but higher IAA level in the root, compared to plants growth with all NH₄ ⁺ or all NO₃ ^‐. Mixed N use also elicited a higher level of GA₁₊₃ in the shoot. These data demonstrate that mixed N enhancement of tillering and dry matter in wheat may be due to the higher cytokinin/IAA ratio together with an increased GA₁₊₃ level in the shoot.