Effect of plant density on the uptake and distribution of 14C in the field bean, Vicia faba

Whole bean plants, ev. Cockfield, grown in pots crowded or well-spaced (50 or 10 plants m², respectively) were treated with ¹⁴CO₂ at the pod-fill stage (25 modes) and the radioactivity in each leaf was determined after 30 min. With spaced plants the uptake was greatest in the mid-stem leaves and was proportional to leaf area. In contrast, 70% of the total assimilation took place in the upper six leaves of crowded plants and there was a steady decrease down the stem.
When ¹⁴CO₂ was fed to single leaves of similar crowded plants the resultant distribution of labelled assimilates varied with the position of the treated leaf. After 6 h, 67% of the ¹⁴C fixed by a mid-stem leaf (node 13) was recovered from the beans, whereas 76% of that from an upper leaf (node 23) had accumulated along the stem. Due to the shading of mid-stem leaves at the higher planting densities, seed yield becomes increasingly dependent upon re-distribution of assimilates from stem to beans.