Source: sink ratio and leaf senescence in maize: II. Nitrogen metabolism during grain filling

Leaf senescence in a recent maize (Zea mays L.) hybrid is delayed relative to that in an older maize hybrid and the trait is associated with an improv ement of the ratio of assimilate supply (i.e., source) and demand (i.e., si nk) during grain filling. This study examined whether effects of source : s ink ratio of leaf longevity in an old and more recent hybrid are associated with changes in leaf nitrogen (N) concentration and N uptake during grain filling. A 3-year field study was conducted with maize hybrids Pride 5 (old ) and Pioneer 3902 (recent) grown at two soil-N levels: 150 kg(-1) N ha(-1) was broadcast in the high N treatment while none was added to the low N tr eatment. Four imposed source : sink treatments ranged from partial defoliat ion to no grain. Leaf N of the control treatments did not differ between th e two hybrids, but the decline in leaf N from the control to the no-sink tr eatment was larger for Pioneer 3902 than for Pride 5. Total N uptake in abo ve-ground portions was 10 and 18% greater in the new than in the old hybrid under low and high soil-N conditions, respectively. The difference in the total N uptake between the two hybrids could be attributed to post-silking N uptake. The proportion of N in the grain derived from post-silking N upta ke was 60% for Pioneer 3902 and 40% for Pride 5 and this proportion was pos itively associated with the source : sink ratio. Higher rates of N uptake i n Pioneer 3902 vs. Pride 5 appear to be, in part, the result of higher rate s of dry matter accumulation of the newer hybrid during grain filling. (C) 1999 Elsevier Science B.V. All rights reserved.