Daily changes in nitrate influx, efflux and metabolism in maize and pearl millet

Maize (Zea mays L.) and pearl millet (Pennisetum americanum (L.) Leeke) seedlings were exposed to [(15)N]nitrate for 1-h periods at eight times during a 24-h period (16-8 h light-dark for maize; 14-10 h for millet). Influx of [(15)N]nitrate as well as its reduction and translocation were determined during each period. The efflux of previously absorbed [(14)N]nitrate to the uptake solution was also estimated. No marked diurnal changes in [(14)N]nitrate efflux or [(15)N]nitrate influx were evident in maize. In contrast, [(14)N]nitrate efflux from millet increased and eventually exceeded [(15)N]nitrate influx during the late dark and early light periods, resulting in net nitrate efflux from the roots. The dissimilarity of their diurnal patterns indicates that influx and efflux are independently regulated. In both species, [(15)N]nitrate reduction and (15)N translocation to shoots were curtailed more by darkness than was [(15)N]nitrate influx. In the light, maize reduced 15% and millet 24% of the incoming [(15)N]nitrate. In darkness, reduction dropped to 11 and 17%, respectively. Since the accumulation of reduced-(15)N in shoots declined abruptly in darkness, whereas that in roots was little affected, it is suggested that in darkness [(15)N]nitrate reduction occurred primarily in roots. The decrease in nitrate uptake and reduction in darkness was not related to efflux, which remained constant in maize and did not respond immediately to darkness in pearl millet.