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 ¹⁵N]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 ¹⁵N]nitrate as well as its reduction and translocation were determined during each period. The efflux of previously absorbed ¹⁴N]nitrate to the uptake solution was also estimated. No marked diurnal changes in ¹⁴N]nitrate efflux or ¹⁵N]nitrate influx were evident in maize. In contrast, ¹⁴N]nitrate efflux from millet increased and eventually exceeded ¹⁵N]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, ¹⁵N]nitrate reduction and ¹⁵N translocation to shoots were curtailed more by darkness than was ¹⁵N]nitrate influx. In the light, maize reduced 15% and millet 24% of the incoming ¹⁵N]nitrate. In darkness, reduction dropped to 11 and 17%, respectively. Since the accumulation of reduced-¹⁵N in shoots declined abruptly in darkness, whereas that in roots was little affected, it is suggested that in darkness ¹⁵N]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.Paper No. 6722 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh