Photosynthetic Gas Exchange Characteristics of Wheat Flag Leaf Blades and Sheaths during Grain Filling

The rate of net CO(2) assimilation (A), the stomatal (g(s)) and residual (g(r)) conductances to CO(2), the intercellular CO(2) concentration, the CO(2) compensation points at 21% O(2) (Gamma(21)) and at 2% O(2) (Gamma(2)), and the amounts of dry matter, nitrogen, and carbohydrates were determined, from anthesis through grain filling, in the flag leaf blade and sheath of spring wheat (Triticum aestivum L. cv Kolibri). The nitrogen content and the rate of net CO(2) assimilation declined slowly until the onset of senescence in both organs, about 3 weeks after anthesis. During senescence the reduction of A in both organs was not primarily caused by a decrease in g(s); the main factor is the decrease in g(r). From values of Gamma(21) and Gamma(2) it is suggested that the rate of respiration in the light contributing to the CO(2) compensation point is higher in sheaths than in blades irrespective of the O(2) level considered. The role of sheaths storing and later transporting assimilates to the developing grains seems to be more important for shoot yield than that of sheaths functioning as photosynthetic organs after the onset of senescence occurs. It is suggested that accumulation of carbohydrates in leaves might somehow trigger senescence in the flag leaf blade and sheath simultaneously.