RESUMO
Mobilization of free sugars from vegetative tissues to grain and their transformation to starch in relation to activities of some relevant enzymes during growth and development were investigated in wheat (Triticum aestivum L.). Vegetative tissues, viz. flag-leaf, flag-leaf sheath, nodes and internodes contained high concentration of free sugars from 70 DAS to 18 DPA and that was in the order of accumulation--flag-leaf sheath> flag-leaf and internodes > nodes. In these tissues, major portion of 14C appeared in endogenous sucrose, irrespective of the nature of (U-14C]-sugars supplied. In photosynthetic structures above flag-leaf node, namely peduncle, rachis and bracts, the free sugar make-up was maximum at anthesis (90 DAS). Activity of soluble acid invertase (EC 3.2.1.26) was high in these tissues during early stages of grain growth but reverse was true for soluble neutral invertase (EC 3.2.1.27) activity. In apical and basal portions of grain, free sugars were more or less similarly distributed in concentration. Linear and rapid accumulation of starch in endosperm paralleled with a decline in accumulation of this polymer in pericarp-aleurone. In the latter tissue, the activities of starch hydrolyzing enzymes, i.e alpha- and beta-amylases (3.2.1.1 and 3.2.1.2) were high during initial stages of grain growth. During active grain-filling, alkaline inorganic pyrophosphatase (EC 3.6.1.1) seemed to play a vital role during starch accumulation in endosperm, whereas the involvement of 3-PGA phosphatase (EC 3.1.3.38) was almost confined to pericarp-aleurone. Impairement of ear head photosynthesis by shading depressed starch synthesis (approximately 50%) indicating, thereby, the significant role of current photosynthates during grain-filling. The results suggested that grain growth in wheat was influenced by an efficient operation of source as well as regulatory factors, including enzymes, constituting intrinsic potential of grain sink.