Seedling Nitrate Reductase Activity and Nitrate Partitioning in Maize (Zea maysL.) Strains Divergently Selected For Post-anthesis Leaf-Lamina Nitrate Reductase Activity

Abstract

Two experiments were conducted to evaluate the effects of phenotypic recurrent selection for high and low post-anthesis leaf lamina in vivo NRA on nitrate uptake, nitrate partitioning and in vitro NRA of seedling roots and leaves. In Experiment 1, intact plants of cycle 0, 4, and 6 of the high and low NRA strains were grown on NH4-N for 11 d, then exposed to 10 mol m 3 KN03, and cultures sampled at 6 h and 28 h (induction and post-induction periods). Nitrate uptake, tissue nitrate concentration and in vitro NRA were determined. The pattern of response to selection in seedling leaf NRA was similar to that observed for in vivo NRA of field grown plants. Leaf NRA increased between 6 h and 28 h. Root NRA was not affected by selection or sampling time. Treatments differed in total fresh weight but not in reduction or uptake of nitrate per unit weight, indicating a lack of correspondence between NRA and reduction and supporting the idea that concomitant reduction by NR is not obligatorily linked to nitrate influx in the intact plant. In Experiment 2, dark-grown plants of cycle 0, and 6 of the high and low NRA strains were cultured without N, detopped on day 6, transferred the following day to 0-75 mol m~3 KN03 and sampled at 6 h and 28 h. In contrast to Experiment 1, selection populations differed in nitrate reduction and root NRA, which by 28 h reached higher average levels than root NRA of intact plants. Translocation and reduction were inversely related among strains within each sampling time. The high level of translocation in detopped plants of the low NRA strain was difficult to reconcile with its low leaf NRA level of Experiment 1. It is suggested that nitrate transport in detopped roots is altered relative to the intact system in a way which permits greater NRA induction and nitrate reduction. The results indicate that nitrate partitioning by detopped root systems should be interpreted with caution.