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

A Chinese spring wheat (Triticum aestivum L. cv. Yang Mai 158) was grown hydroponically with nitrogen (N) supplied as all ammonium (NH₄⁺), all nitrate (NO₃^-), 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 A₁ 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 allNH₄⁺-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.