, high nitrogen supply increases cytokinin level,
but decreases auxin and NO levels in roots of maize. Besides, it
was well documented ethylene has a negative effect on root elongation
of various plants.25-27 Exogenous supply of cytokinin increase
ethylene production (Stenlid 1982; Bertell et al., 1990). Recently,it was demonstrated in Arabidopsis that auxin transport from the
root apex via the lateral root cap is required for ethylene-mediated
inhibition of root growth.28 Therefore, a complex multiple siganlling
pathways may be involved in N-mediated root elongation
(Fig.2). Further study is required to understand how these pathways
interact with each other to reduce root elongation in response to high
nitrate supply.