Estimation of assimilation activity by daily increase in water soluble sugar content in spring wheat [Triticum aestivum] canopy

The ability or activity of assimilated organs in wheat canopy was estimated by measuring nitrogen content and water soluble sugar content (WSC) of leaves and ear in spring wheat canopy (cv. Haruyutaka) under field conditions. The relation of these organs to assimilation activity was also determined. Nitrogen content of leaf was closely related with carbon exchange rate (GER) in leaf, when measured under intensive radiation of more than 1500 mu-mol m(-2)s(-1) of PAR (r=0.963** ). This indicated that nitrogen content showed potential photosynthetic ability. Daily increase of WSC of leaf was closely related with CER in leaf under natural canopy (r=0.760** ), showing the real-time activity of assimilation in canopy. The activity of assimilation in second leaf was lower than in flag leaf owing to shading by the latter, although photosynthetic ability in both was the same. Although CER of ear was not measured in this study, we assumed that photosynthesis in ear contributed considerably to grain growth as found in other recent work, because the amount of nitrogen and the daily increase of WSC in ear were higher than those in leaf