The causes for barley root growth retardation in the presence of ammonium and glutamate

Abstract

In barley (Hordeum vulgare L.) seedlings, the rate of root growth, osmotic pressure (Π), hydraulic conductance (L p), and longitudinal (δl) and transverse (δD/D) extensibility of root cells were measured. The seedlings were grown on Knop solution with nitrate or without nitrate with addition of 5–10 mM NH 4 + or 0.5–1.0 mM glutamate. Root growth retardation on the 1st–4th days of exposure to NH 4 + was determined by a decrease in δl in the zone of elongation, whereas root thickening was evidently related to an increase in Π. Biphasic dynamics of δl in the presence of NH 4 + was imitated by medium acidification near the root surface to pH 3.7, which confirms a conclusion, we have done earlier, about a non-monotonous pH-dependence of longitudinal extensibility. Root growth retardation during the first day of exposure to Glu was also determined by a decrease in the δl, which was, however, accompanied by an increase in the δD/D and L p. Fast Glu-induced changes of measured root parameters were imitated by root exposure to oryzalin, ionomycin, and inhibitors of the H+-pump. It was supposed that a decrease in δl in the presence of NH 4 + and Glu was related to cortical microtubule disorganization with the involvement of cytosolic calcium Ca cyt 2+ . A decrease in the δD/D and L p in the presence of NH 4 + was related to apoplast acidification and a high activity of the plasmalemmal H+-pump. An increase in the δD/D and L p in the presence of Glu indicates the inhibition of the plasmalemmal H+-pump. On the 2nd–4th days of exposure to Glu, root growth ceased, as distinct from treatment with NH 4 + . This complete root growth inhibition by Glu was possibly related to a rapid uptake of Ca2+ through Glu-sensitive Ca2+-channels, Ca2+-dependent inhibition of the plasmalemmal H+-pump, and a decrease in mitotic activity.