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 grow n hydroponically with nitrogen (N) supplied as all ammonium (NH4+), al l nitrate (NO3-), and a mixture (NH4+-N:NO3- -N=30:70) of each form. W heat plants produced more tiller and more dry matter, and showed a hig her shoot/root dry matter ratio when grown with mixed N than with eith er N form alone. In addition, all-NH4+-grown plants produced more till ers and dry matter as compared to all-NO3--grown plants. Subsequently, endogenous levels of isopentenyladenine and isopentenyladenosine, zea tin and zeatin riboside, dihydrozeatin and dihydrozeatin riboside, ind ole-3-acetic acid (IAA), gibberellin A(1) and gibberellin A(3) (GA(1+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 c ytokinin in both the shoot and the root of mixed-N-grown plants were h igher than those of all-NO3--grown and all NH4+-grown plant, whereas A BA level was similar among plants grown under different N forms. On th e other hand, plants grown with mixed N exhibited lower IAA level in t he 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 GA(1+3) in the shoot. These data demonstrate that mixed N enhancement of tillering and dry matter in wheat may be due to the higher cytokin in/IAA ratio together with an increased GA(1+3) level in the shoot.