Growth of the grain of wheat (Triticum aestivum L.). The significance and timing of greening

Abstract

The sink capacity of grains is determined early in development when endosperm cell production stops and net water accumulation in the grain ends. Cessation of net water accumulation in grains is preceded immediately by the onset of degeneration of the outer pericarp parenchyma. It is not known if factors emanating from outer pericarp degeneration affect endosperm cell production and/or water accumulation in the grain. Investigations of possible effects would be facilitated by a simple method for the determination of outer pericarp degeneration. Experiments were performed (i) to test the hypothesis that the greening of the wheat grain is an indicator of outer pericarp degeneration (expt. I) and (ii) to investigate the relationship between greening and grain development in a range of spring wheat genotypes differing in grain size (expt. II). Plants were grown in field plots. Greenness of the dorsal surface of grains was determined by use of an interactive true color image analysis system. Grains retained a whitish appearance until 16 days after anthesis, although chlorophyll density (defined as gg chlorophyll mm-2 projected grain area) increased greatly immediately after anthesis (expt. 1). Hence, the greening of grains was not due to the synthesis of chlorophyll (which is restricted to tissue of the inner pericarp) but to the outer pericarp becoming translucent during its degeneration. In a cultivar grown in two contrasting seasons and in a range of large and small grained cultivars grown in one season, greening consistently started shortly before the termination of rapid net water uptake into grains (expts. I and 11). The time interval between anthesis and the onset of greening was shorter in 1989 (expt.11) than in 1988 (expt.1). This was accompanied by a shorter period of water accumulation in the grain, a lower maximum water content and a lower final dry weight of grains in 1989. Large-grained genotypes had a higher grain water content (and hence larger volume) at the onset of greening than small grained genotypes).