Sources, Fluxes, and Sinks of Nitrogen during Early Reproductive Growth of Maize (Zea mays L.)

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Abstract
A study was designed to identify sources and sinks of N in the maize (Z. mays L.) shoot, by estimating net N fluxes for each of 7 parts of the shoot, determine effects of N entering the plant upon fluxes of N absorbed before reproductive growth and determine the effects of the opaque-2 gene on N fluxes in the maize shoot during early reproductive growth. Plants of a maize hybrid (Pioneer 3369A) and its opaque-2 counterpart (Pioneer L3369) were grown in a greenhouse using nutrient solution/sand culture, with NO3- as the N source during the vegetative growth phase. Beginning at the time of pollination, the same nutrient regime was continued, except that some plants received no N and others received 3.75 mM 15N as NO3--N. Stalk and leaves were primary N sources for the grain, while shank, husk and cob acted first as N sinks, then as N sources during reproductive growth. Net fluxes of N for each plant part were estimated by calculating the 1st derivatives of regression equations used to fit data for N contents of each plant part as functions of time. All parts of the shoot were sinks for exogenous N (absorbed after pollination). A total of 36 days after pollination, the grain contained 60% endogenous N (absorbed before pollination) when 3.75 mM NO3--N was supplied after pollination. Rates of total N influx to the grain were identical whether or not N was supplied in the nutrient solution during reproductive growth. At 36 days after pollination, less N had accumulated in the grain of the opaque-2 gentoype, but otherwise there were no differences in N content or dry weights of the shoots due to the opaque-2 gene. Absence of N from the rooting medium significantly affected N fluxes throughout the shoot during reproductive growth, but there were no detectable effects of the opaque-2 gene on N fluxes in parts of the plant other than the grain.