Effect of Plant Growth Regulators on Nitrate Utilization by Roots of Nitrogen-Depleted Dwarf Bean

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Abstract
We examined the effect of pretreatments (18 h at 5 μmol dm−3) with abscisic acid, the ethylene-releasing substance ‘Ethephon’, gibberellic acid, indoleacetic acid, kinetin and zeatin on nitrate uptake and in vivo nitrate reductase activity (NRA) in roots of nitrogen-depleted Phaseolus vulgaris L. Nitrate uptake showed an apparent induction pattern with a steady state after about 6 h, in all treatments. The nitrate uptake rate after 6 h was unaffected or at most 30% lower after treatments with the plant growth regulators. Gibberellic acid, kinetin and zeatin induced substantial NRA in roots in the absence of nitrate, whereas Ethephon enhanced NRA only during nitrate nutrition. Kinetin-induced NRA (Ki-NRA) was maximal after a pretreatment at 1 μmol dm−3, and showed a lag phase of 6–8 h. Ki-NRA was additive to nitrate-induced NRA (NO−3-NRA) for at least 24 h, independent of the induction sequence. After full induction, Ki-NRA approximated 20% of NO-3-NRA. Abscisic acid counteracted the development of Ki-NRA, but not of NO−3-NRA. Cycloheximide and tungstate were equally effective to suppress the development of nitrate reductase activity after supply of kinetin or NO−3. Our data are consistent with the operation of two independent enzyme fractions (Ki-NRA and NO−3-NRA) with apparently identical properties but with separate control mechanisms. The absence of major effects of plant growth regulators on the time-course and rate of nitrate uptake suggests that exogenous regulators, and possibly endogenous phytohormones are of minor importance for initial nitrate uptake. The differential effect of some regulators on nitrate uptake and root NRA furthermore indicates that the processes of uptake and reduction of NO−3 are not obligatory or exclusively coupled to each other.

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