NITRATE REGULATION OF ZEATIN RIBOSIDE LEVELS IN BARLEY ROOTS - EFFECTS OF INHIBITORS OF N-ASSIMILATION AND COMPARISON WITH AMMONIUM

The zeatin riboside (ZR) response to environmental nitrate is characte rized in barley roots, using a chemostate-like culture system for cont rolling nitrate nutrition. Experiments were mainly performed with two- compartment split-root cultures, where nitrate-N was added at a consta nt relative addition rate of 0.09/day, and distributed between the sub roots in a ratio of 80:20. ZR responses were recorded following revers al of the addition ratio, i.e., the local nitrate dose was increased b y a factor of 4 in one subroot, and decreased to one quarter of the or iginal value in the other subroot. The endogenous levels of ZR in the subroot subjected to an elevated nitrate dose increased three-fold in less than 6 h. A decreased nitrate dose was without effect. Pretreatme nt with tungstate, methionine sulfoximine or cycloheximide inhibited t he nitrate-induced ZR response, but there was no decline in the level of ZR compared to the control. The data indicate that the root ZR leve l of N-limited barley is largely non-responsive to nitrate dose in the long term, but transiently responding (positively) to increased nitra te supply. An accelerated rate of N flux via glutamine synthetase appe ars to be required for this response. A further, nitrate-specific, eff ect (as compared to ammonium) is also observed.