The influence of leaf age, leaf position and sinks on the rate of export and partition of14C at different stages of development following assimilation of14CO2by a single leaf ofVicia fabaL.

Abstract

The third leaf of a broad bean plant at four different stages of development was allowed to assimilate ¹⁴ CO₂ for 1 h. The plants were selected so that leaf 3 was at four different positions throughout its life. The distribution of ^l4C products was determined sequentially at intervals of 1, 3, 6 and 24 h and 7 days after the ¹⁴ CO₂ was supplied. The fed leaf was found to retain substantial amounts of the radiocarbon it initially fixed. The presence of strong sinks, e.g. fruits, significantly decreased the amount of ¹⁴C retained. No readily separable phases for the export of assimilates were observed. Although the rate of export continuously declined with time, but not with age, export from the fed leaf continued for several days. Upward or downward movement of the exported C was influenced by the position of the source in relation to the sink. The role of the stem in the C economy of the plant included a function as a temporary storage organ. The significance of these results is discussed and comparisons made with other crops.