The utilization of recently assimilated carbon in graminaceous plants

Abstract

SUMMARYIsotopic carbon and infra‐red gas analysis techniques were used to measure the following growth attributes in maize, sorghum, winter wheat and perennial ryegrass: the rate of entry of carbon into each main shoot leaf; the rate of translocation of leaf assimilate to meristems; the fraction of leaf and total shoot assimilate respired in one diurnal period; and the distribution of residual assimilate to new leaf, stem, axillary shoots and root.The two tropical plants possessed higher leaf assimilation rates and larger leaves than the temperate species, but their efficiency of translocation was only marginally superior in the experimental conditions. In all species, c. 25% of the assimilate generated in the 8·4h photoperiod was respired in in the same diurnal period. Maize and sorghum partitioned a greater proportion of their total shoot assimilate to new leaf tissue at the main shoot apex and to root than wheat and ryegrass. On the other hand, wheat and ryegrass exported up to 30% of their assimilate to axillary shoots; in sorghum, little assimilate was translocated to axillary shoots, while in maize this activity was completely absent.Plant habit, as exemplified by the contrast between the annual, single‐axis maize plant and the perennial, multi‐tillering ryegrass plant, appears to be a reflexion of the pattern of assimilate distribution to areas of potential growth. With the exception of superior leaf assimilation rates in maize and sorghum, the four species showed no marked differences in respect of the production, transport and respiratory utilization of assimilates.