CORN SEED MATURITY INDICATORS AND THEIR RELATIONSHIP TO UPTAKE OF C-14 ASSIMILATE

Various maturity rating systems have been proposed to evaluate seed development in corn (Zea mays L.). The objective of this research was to determine the relationship of seed assimilate uptake and dry weight to three indicators of physiological maturity: moisture concentration, black layer, and milk line development. Assimilate movement into the seed was determined by exposing the leaf subtending the ear to (CO2)-C-14 at regular intervals during seed development and measuring the C-14 24 h later in F2 seed of B73 x Mo17 (1984, 1985) and the stay-green hybrids FS854 and Pioneer Brand 3358 (1985). In 1986 and 1987, plants of the same F2 hybrids and the inbreds B73 and Mo17 and their F1 cross (B73 x Mo17) were defoliated at almost-equal-to 20 d after pollination and seed development was compared to that of undefoliated plants. Seed dry weight, moisture concentration, and milk line and black layer development (using a five-stage rating system) were determined at 3- to 5-d intervals. Uptake of C-14-assimilate by seed and cob tissue decreased sharply near the end of the linear phase of seed dry weight accumulation when the milk line and black layer ratings were at Stage 3 (mid-milk line) or greater. Near Stage 4 of both black layer and milk line development, C-14 in the seed had fallen to the lowest levels, maximum seed weight had been reached, and the seed were physiologically mature. Both black layer and milk line followed consistent trends throughout seed development for all hybrids (including the stay-green genotypes), defoliation treatments, and years. Seed moisture during seed development was highly correlated with both black layer and milk line, but the concentration at black layer Stage 4 was more variable (293-415 g kg-1) across genotypes and years. The data suggest that Stage 4 of both black layer and milk line development represents a useful and reliable indicator of physiological maturity.