The signal effect of nitrate supply enhances active forms of cytokinins and indole acetic content and reduces abscisic acid in wheat plants grown with ammonium

Ammonium can result in toxicity symptoms in many plants when supplied as a sole nitrogen source. Nitrate reduces the negative effects caused by ammonium and promotes plant growth. In order to explore the mechanism responsible of this beneficial effect, we investigated whether nitrate application causes significant changes in the indoleacetic acid (IAA)-and cytokinin-plant distribution and abscisic acid (ABA) accumulation in wheat (Triticum aestivum L.) plants grown with ammonium. Two different doses of nitrate were supplied to ammonium-fed plants (100 μM and 5 mM), to determine whether the effects of nitrate require significant doses (nutritional character), or can be promoted by very low doses (signal effect). The results showed that the presence of NO3- was associated with clear increases in the active forms of cytokinins (zeatine (Z), trans-zeatine riboside (tZR), isopentenyl adenosine (IPR)) and reduction of the levels of the lower active forms (cis-zeatine riboside (cZR)), independently of the dose applied. Likewise, the presence of nitrate also enhanced IAA shoot content, which correlated with higher cytokinin levels and a tendency toward lower ABA concentration. This study presents further evidence that the possible signal effect of NO3- involved in its beneficial effect on the growth of wheat plants fed with NH4+ could be mediated by a coordinated action of the levels of cytokinins, IAA and ABA in the shoot.