Respiratory Loss of Recently Assimilated Carbon in Wheat

Abstract

A series of experiments was undertaken to assess the amount of respiration associated with the growth of wheat at different stages. Plants (or in some cases just the flag leaf) were labelled with 14CO2 and the amount of 14CO2 respired during the subsequent 48 or 72 h was measured. The evolution of 14C, expressed as a percentage of the amount initially assimilated (referred to as the R/A value) was used as a measure of the overall efficiency of dry matter production. Respiratory 14CO2 evolution from labelled plants was most rapid in the first 12 h after labelling, thereafter declining rapidly. Evolution was also more rapid following labellings at the end of the light period (dusk) than at the beginning of it (dawn). The R/A values were greatest (42 and 50 per cent respectively for dawn- and dusk-labelled plants) for young plants and least (13 and 28 per cent respectively) for plants during mid grain filling. When flag leaves, as distinct from whole plants, were labelled, R/A values were lower still (9 and 21 per cent respectively), indicating that flag leaf assimilate was used efficiently in grain production. The calculated minimum R/A for the formation of grain material (10 per cent protein, 90 per cent starch) was 6.2 per cent. That the experimentally determined values were greater than this is attributed to the turnover of carbon in enzymes, to other maintenance processes, and possibly to the operation of the pentose phosphate pathway of glucose oxidation. R/A values were lower in those plants labelled at the beginning than those at the end of the photoperiod. This was considered to be a consequence of refixation of respiratory 14CO2 during the light. The higher R/A values found for young plants were considered to be a consequence of the greater percentage of 14C translocated to the roots (roots being unable to refix respired CO2) and of greater turnover of enzymes associated with more active metabolism.