A comparison of nitrate transport in four different rice (Oryza sativa L.) cultivars.

As rice can use both nitrate (NO(3)(-)) and ammonium (NH(4)(+)), we have tested the hypothesis that the shift in the pattern of cultivars grown in Jiangsu Province reflects the ability of the plants to exploit NO(3)(-) as a nitrogen (N) source. Four rice cultivars were grown in solution culture for comparison of their growth on NO(3)(-) and NH(4)(+) nitrogen sources. All four types of rice, Xian You 63 (XY63), Yang Dao 6 (YD), Nong Keng 57 (NK) and Si You 917 (SY917), grew well and produced similar amounts of shoot biomass with 1 mmol/L NH(4)(+) as the only N source. However, the roots of NK were significantly smaller in comparison with the other cultivars. When supplied with 1 mmol/L NO(3)(-), YD produced the greatest biomass; while NK achieved the lowest growth among the four cultivars. Electrophysiological measurements on root rhizodermal cells showed that the NO(3)(-)-elicited changes in membrane potential (Delta E(m)) of these four rice cultivars were significantly different when exposed to low external NO(3)(-) (<1 mmol/L); while they were very similar at high external NO(3)(-) (10 mmol/L). The root cell membrane potentials of YD and XY63 were more responsive to low external NO(3)(-) than those of NK and SY917. The Delta E(m) values for YD and XY63 rhizodermal cells were almost the same at both 0.1 mmol/L and 1 mmol/L NO(3)(-); while for the NK and SY917 the values became larger as the external NO(3)(-) increased. For YD cultivar, Delta E(m) was measured over a range of NO(3)(-) concentrations and a Michaelis-Menten fit to the data gave aK(m) value of 0.17 mmol/L. Net N(3)(-) uptake depletion kinetics were also compared and for some cultivars (YD and XY63) a single-phase uptake system with first order kinetics best fitted the data; while other cultivars (ND and SY917) showed a better fit to two uptake systems. These uptake systems had two affinity ranges: one had a similar K(m) in all the cultivars (0.2 mmol/L); the other much higher affinity system (0.03 mmol/L) was only present in NK and SY917. The expression pattern of twelve different N(3)(-) transporter genes was tested using specific primers, but only OsNRT1. 1 and OsNRT2.1 expression could be detected showing significant differences between the four rice cultivars. The results from both the physiological and molecular experiments do provide some support for the hypothesis that the more popular rice cultivars grown in Jiangsu Province may be better at using NO(3)(-) as an N source.