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 characterized in barley roots, using a chemostate-like culture system for controlling nitrate nutrition. Experiments were mainly performed with two-compartment split-root cultures, where nitrate-N was added at a constant relative addition rate of 0.09/day, and distributed between the subroots in a ratio of 80:20. ZR responses were recorded following reversal of the addition ratio, i.e., the local nitrate dose was increased by a factor of 4 in one subroot, and decreased to one quarter of the original 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. Pretreatment with tungstate, methionine sulfoximine or cycloheximide inhibited the 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 level of N-limited barley is largely non-responsive to nitrate dose in the long term, but transiently responding (positively) to increased nitrate supply. An accelerated rate of N flux via glutamine synthetase appears to be required for this response. A further, nitrate-specific, effect (as compared to ammonium) is also observed.