NITROGEN-INDUCED CHANGES IN THE GROWTH AND METABOLISM OF DEVELOPING MAIZE KERNELS GROWN-INVITRO

Cereal kernel growth and grain yield are functions of endosperm starch accumulation. The objective of this study was to examine how various metabolic factors in developing maize (Zea mays L.) endosperm influence starch deposition. Kernels were grown in vitro on medium with: (a) zero N (-N), (b) optimum N (+N), or (c) -N from 3 to 20 days after pollination followed by +N until maturity (±N) to produce different degrees of endosperm growth and to promote an enhancement of starch synthesis midway through development. At intervals, kernels were harvested and levels of enzyme activities and carbohydrate and N constituents examined. Endosperm starch and protein accumulation were decreased in -N compared to +N kernels, but relief of N starvation increased both constituents. With greater movement of N into ±N kernels, endosperm sugar concentrations declined suggesting an inverse relationship between C and N transport. Unusually high concentrations of sugar in N stressed kernels did not appear to limit or enhance starch production. Rather, increased accumulation of starch in ±N endosperm was correlated with significant increases in the enzymatic activities of sucrose synthase and PPi-linked phosphofructokinase, and to a lessor extent hexokinase. In addition, the occurrence of specific proteins of the albumin/globulin fraction either increased, decreased, or remained unchanged in relation to starch synthesis. These data suggest that lack of N limits starch deposition in maize endosperm primarily through an influence on synthesis of key proteins.