| Summary: | Yield fluctuation in <i>Vicia faba</i> is due primarily to reproductive failure, which can occur as a result of bud abortion, flower shedding or pod and ovule abortion (Gates <i>et al</i>., 1983b). Flower drop, which accounts for the major proportion of total reproductive loss, contributes most to the reduction in potential yield. Application of artificial plant growth retardants (EL500, JF 10405 and Alar) were shown to increase the yield of broad beans (<i>Vicia faba</i> L. <i>major</i> cv. Threefold White), by up to 52%, mostly due to increased pod set. Experiments involving the application of plant growth substances directly to the flowers, suggested that increase in pod set was due to changes in intrinsic hormone levels. In particular, high levels of cytokinin are required at the pedicel:peduncle junction pre-pollination, to allow successful initiation of potential sinks, while increased levels of auxin are required after pollination in conjunction with cytokinin to allow cell division, pod expansion and vascular differentiation. Application of anti-gibberellin plant growth retardants appeared to alter internal hormone ratios, affecting the distribution of dry matter production during early flowering, i.e. treated plants had an increased root to stem dry matter growth rate. This suggested an increased cytokinin:gibberellin ratio had been achieved. Although it was shown that pod set could be enhanced by the application of either plant growth retardants and/or plant growth substances, yield was not always as high as anticipated due to increased levels of pod drop. Further applications of cytokinin and auxin to the pods reduced this drop. It followed therefore, that further applications could dramatically increase the yield potential of the plant due to better distribution of assimilates to pods. However, it would appear that the plant also suffers from source limitations and until these are successfully overcome, yield instability in the field environment is still likely.
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