Abstract
Effects of nitrate, chloride and chlorate ions upon nitrate and chlorate uptake by roots of maize (Zea mays L., cv. B73) seedlings were examined. Net nitrate uptake, 36ClO3− influx and 36Cl− influx (the latter two in a background of 0.5 mM KNO3) displayed similar pH profiles with optima at pH 5.5 and below. External, non‐labeled chloride had little effect on the accumulation of 36ClO3− (both in 5 h and 20 min uptake assays), while nitrate and chlorate had almost identical, marked inhibitory effects. Nitrate pretreatment caused an apparent induction of both 36ClO3− and 15NO3− uptake activities. After 5 h of treatment in nitrate, the uptake activities of chloride‐ and chlorate‐pretreated plants increased to that of nitrate‐pretreated plants. During 6 h exposure to chlorate, 36ClO3− uptake activity of nitrate‐pretreated plants decreased to that of chlorate‐ and chloride‐pretreated plants. The results support the existence of a shared nitrate/chlorate transport system in maize roots which is not inhibited by external chloride, and which is induced by nitrate, but not by chlorate or chloride. The suggestion is made that selection of chlorate‐resistant mutants of maize can identify nitrate uptake as well as nitrate reductase mutants.
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