Grain filling mechanisms in spring wheat [Triticum aestivum], 3: Effects of shadings on dry matter accumulation in grain and culm

To determine the effects of assimilate shortage to grain filling mechanisms in spring wheat canopy (cv. Haruyutaka), we examined the accumulation and translocation of dry matter in grain, along with water soluble sugar content (WSC) and structural material in culm under shading conditions during grain filling phases. Shading treatments were conducted using 95% shading cloth from -2 to 7 days after anthesis (DAA) (initial shading), 7 to 14 DAA (early shading) and 14 to 21 DAA (late shading). These shadings prevented the wheat canopy from producing any assimilate. However rates of increase in grain dry weights for all the shadings were 50-60% of the control (no shading), while the WSC in culm was used for the grain growth. During the initial grain filling phase, the WSC in culm for the initial shading decreased (-15mg pl(-1)day(-1)), whereas that for the control increased (20 mg pl(-1)day(-1)). The grain dry weight showed a high rate of increase (11 mg pl(-1)day(-1)) for the initial shading, being about 50% of the control (19 mg pl(-1)day(-1)). During the early phase, the WSC in culm for the early shading decreased (-15 mg pl(-1)day(-1)), although that for the control increased (12mg pl(-1)day(-1)). The rate of grain weight increase for the early shading (24 mg pl(-1)day(-1)). During the late phase, the WSC in culm for the late shading decreased (-33 mg pl(-1)day(-1)) five times as much as the control (-7 mg pl(-1)day(-1)). The rate of grain weight increase for the late shading (34 mg pl(-1)day(-1)) was 60% of that for the control (55 mg pl(-1)day(-1)). Although the culm lengths at harvest were no different between the initial shading and the control, the dry weight of culm structural material for the initial shading was lower than that for the control. The difference between the cases in which the partitioning rate to grain did or did not increase is discussed herein