Effect of shading before and after heading time on growth and yield of spring wheat

Three levels of shading (70, 50 and 25% of natural light condition) were used in the field-grown spring wheat (cv. Haruyutaka) during three weeks before and after heading time in order to determine the difference in effects for time of shading on dry matter production, partitioning, and yield. Effect of shading during three weeks before heading. Dry weights of each part (ears, leaves, culms including leaf sheath and roots) decreased with decreases in light intensity (Fig. 1). As shading increasing, however, distribution ratio of dry matter increased in leaves and ears, but decreased in roots (Table 3), resulted in greater reduction in root dry weight. Total dry weights were 75, 52 and 40% of that of unshaded (light intensity=100%) in 70, 50 and 25% light plots, respectively. Leaf area and surface area of ears and culms were also decreased by shading (Fig. 1). The decreasing percentages of leaf and culm area, however, were lower compared with those of dry weights, because of increasing in area per unit dry weight (cm2/g) as shading increasing (Table 3). After shading treatments were stopped, crop growth rates (CGR) of shade imposed increased more than that of unshaded, mainly due to increasing in net assimilation rates (NAR) (Table 4). This was also accompanied with drastic increasing in dry weights of culms and roots, and NAR was closely correlated with their dry weights (r=0.998). Grain yields were reduced 15% significantly only by 75% shade (25% light plot) (Table 6). Effect of shading during 22 days after heading. Green area indices were not different except that of 25% light plot, but dry weights of ears, culms and roots decreased with decreases in light intensity (Fig. 2). The restricting light during the post-heading stage resulted in greater yield reduction than for the pre-heading stage. Yields were 88, 78 and 51% of that of unshaded in 70, 50 and 25% light plots, respectively (Table 6). CGRs and NARs during both shading periods decreased in parallel (r≥998) with decreases in light intensity in accordance with sigmoid curves. The saturation point of NAR was lower for the post-heading stage (at about 15 MJ/m2/day) (Fig. 3). It is interesting that root growth was very flexible in response to light intensity regardless of stages of plant development.