Asynchronous sexual development determines the breeding system in field beans

Abstract

THE breeding system of varieties of field bean (Vicia faba minor L.) has long been known to be intermediate between allogamy and autogamy, with approximately 1/3 of the seeds set as a result of cross-fertilisation and 2/3 by self-fertilisation¹. Cross-fertilisation is due to bees visiting open flowers. Most inbred lines show poor seed set unless their flowers are visited by bees or artificially manipulated (tripped) and are therefore called autosterile; a few lines which do not require tripping are called autofertile^2,3. Irrespective of variety, so far as is known, the field bean does not possess a self-incompatibility system for regulating fertilisation^4–6; also anther dehiscence always precedes flower opening by one or two days. Why then does the temporal advantage of self pollen over cross pollen not lead to obligate autogamy under field conditions? In an attempt to answer this question, it has been suggested that a spatial separation between discharged pollen and the stigma is normally present, but is abolished by bee visits which simultaneously bring about pollen–stigma contact and chance cross-pollination. According to this hypothesis, autosterile lines have a serious impediment to pollen–stigma contact, whereas, due to a different floral architecture, autofertile lines do not⁷, although intermediate floral structures also occur. Here we report results of a scanning electron microscopic (SEM) study of the stigmas of an autosterile line (T2) and an autofertile line (T51), which lead to an alternative explanation for autosterility and autofertility.