THE INFLUENCE OF CYTOKININS IN NITRATE REGULATION OF NITRATE REDUCTASE-ACTIVITY AND EXPRESSION IN BARLEY

The responses of nitrate reductase (NR) activity and levels of NR-mRNA to environmental nitrate and exogenous cytokinins are characterised i n roots and shoots of barley (Hordeum vulgare L., cv. Golf), using a c hemostate-like culture system for controlling nitrate nutrition. Exper iments were mainly performed with split root cultures where nitrate-N was supplied at a constant relative addition rate of 0.09 day(-1), and distributed between the subroots in a ratio of 20%:80%. The subroot N R-mRNA level and NR activity, as well as the endogenous level of zeati n riboside (ZR), increased when the local nitrate supply to one of the subroots was increased 4-fold by reversing the nitrate addition ratio (i.e. from 20%:80% to 80%:20%). Also shoot levels of ZR, NR-mRNA and NR activity increased in response to this treatment: even though the t otal nitrate supply remained unaltered. External supply of ZR at 0.1 m u M caused an approximately 3-fold increase in root ZR levels within 6 h, which is comparable to the nitrate-induced increase in root ZR. Ex ternal application of ZR, zeatin, isopentenyl adenine or isopentenyl a denosine at 0.1 mu M caused from insignificant to 25% increases in NR- mRNA and activity in roots and up to 100% stimulation in shoots, where as adenine or adenosine had no effect. No synergistic effects of pertu rbed nitrate supply and cytokinin application were detected in either roots or shoots. The translocation of nitrate from the root to the sho ot was unaffected by application of ZR or switching the nitrate distri bution ratio between subroots. The data give arguments for a physiolog ical role of cytokinins in the response of root and shoot NR to enviro nmental nitrate availability. The nature and limitations of the physio logical role of cytokinins are discussed.