The size of the sink on two indeterminate (cv. Hampton and cv. TGM 119) and two determinate (cv. Bossier and cv. TGM 294) cultivars of soya beans were reduced by continuously limiting the number of pods developing at each fruiting node to either one or two, starting from 2 weeks after anthesis, in three field experiments (early 1980, late 1980 and early 1981). Thiswas in order to evaluate the effects of sink size on the development of pod and seed characteristics as well as to quantify the inter-relationships between these characters under humid tropical conditions.
Persistently reducing the number of pods developing at each fruiting node consistently significantly reduced the number of pods per plant, the pod and seed dry weights per plant, the total number of seeds per plant and the harvest index (seed weight as a proportion of total dry weight) in all cultivars when compared with the control. On the other hand, the treated plants of all cultivars generally had longer pods, heavier individual pods and seeds, more seeds per podand greater leaf area and leaf dry weight as well as greater specific leaf weight than the control plants. Compensation indices (percentage increases in size over control) were 25–30% for weight per pod and seed weight per pod; 11–16% for weight per seed and number of seeds per pod and 2–5% for pod length in the treated plants.
Most of the characters measured (except individual weight of pods and seeds, seed weight per pod and number of seeds per pod) had significant interaction effects of sink size, cultivar and season; season had the highest correlation ratio, followed by cultivar, and sink size had the least. Also, nearly all components of seed yield were positively highly significantly correlated with one another; the number of seeds per pod and the number of pods per plant accounted for 76% of the variation in seed dry weight per plant.
It would seem therefore that under the prevailing humid tropical conditions, the most important component of seed yield in soya bean is the number of seeds per pod, as affected by the sink size.