Ammonium tolerance and carbohydrate status in maize cultivars

Four maize (Zea mays L.) hybrids were grown hydroponically for 4 weeks with 20 mM ammonium or nitrate as the sole nitrogen source. Dry matter production was strongly depressed by ammonium nutrition in the hybrid Helga relative to plants grown on nitrate, and moderately decreased in the hybrid Melina. Ammonium had no inhibitory effect on total yield in the other two hybrids (Ramses and DK 261). The relative growth rate (RGR) of roots and shoots of the sensitive hybrid Helga decreased significantly under ammonium nutrition during the first 2 weeks of the experiment, while at the end of the experiment nitrogen form had no effect on the RGR in any of the four hybrids. The strong reduction in RGR of Helga in the early seedling stage was correlated with the accumulation of twice the concentration of free ammonium in the shoot tissue relative to the other hybrids. Helga was therefore unable to sufficiently detoxify ammonia in the roots. Root concentrations of water soluble carbohydrates (WSC) in Helga and Melina in the early seedling stage did not differ under ammonium and nitrate nutrition. In contrast, Ramses and DK 261 both had elevated WSC concentrations in ammonium-fed roots. It is hypothesized that a sufficient supply of carbon skeletons for ammonium assimilation in the roots is required for maximum growth under high ammonium concentrations, and that there is genotypic variability in this physiological trait. (C) 1997 Annals of Botany Company