GENOTYPIC DIFFERENCES IN NITRATE UPTAKE AND UTILIZATION EFFICIENCY INPUMPKIN HYBRIDS

The daily and cumulative nitrate (NO3) uptake was monitored over a 14- day period, including seedling establishment and early vine extension, in four recently released pumpkin (Cucurbita moschata Poir.) hybrids ('698', '401','424','190') grown in solution culture with nitrogen (N) supplied at either 1.0 or 10.0 mM NO3 (Ns). At the end of the time-co urse, plants were harvested and assayed for total N, reduced N, and dr y weight in roots, stems, and leaves. In each hybrid, NO3 removal from solution increased curvilinearly as days in the time-course increased . This pattern was consistent at both 1.0 and 10.0 mM Ns, although the magnitude of solution NO3 depletion differed depending on hybrid, and in each case, increased as Ns rate increased. Total dry matter produc tion was higher in '698','424', and '190' at 10.0 mM Ns, but was not a ffected by N supply in '401'. Nitrogen utilization efficiency (NLTE, d ry matter produced per unit of plant N) decreased in all hybrids at th e high level of Ns. Differences among hybrids for NUE were greater at 1.0 mM than 10.0 mM Ns. At both levels of Ns, the relative ranking of hybrids for root uptake efficiency (mg NO3-N absorbed/g root dry weigh t) differed markedly from that for NUE, indicating that differences in root absorption capacity among hybrids was not the primary factor inv olved in genotypic differences in NUE. In '424', high NLTE values at 1 0.0 mM Ns were directly related to enhanced N assimilation capacity. I n general, genotypic differences in some of the physiological paramete rs for N uptake and utilization were sufficiently broad to suggest the potential for genetic improvement for NUE in pumpkin hybrids.