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 matte r production was strongly depressed by ammonium nutrition in the hybri d 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 rat e (RGR) of roots and shoots of the sensitive hybrid Helga decreased si gnificantly under ammonium nutrition during the first 2 weeks of the e xperiment, while at the end of the experiment nitrogen form had no eff ect 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 accumula tion of twice the concentration of free ammonium in the shoot tissue r elative to the other hybrids. Helga was therefore unable to sufficient ly detoxify ammonia in the roots. Root concentrations of water soluble carbohydrates (WSC) in Helga and Melina in the early seedling stage d id not differ under ammonium and nitrate nutrition. In contrast, Ramse s and DK 261 both had elevated WSC concentrations in ammonium-fed root s. It is hypothesized that a sufficient supply of carbon skeletons for ammonium assimilation in the roots is required for maximum growth und er high ammonium concentrations, and that there is genotypic variabili ty in this physiological trait. (C) 1997 Annals of Botany Company