Development of ovules in relation to premature fruit drop in Prunus-species

In the genus Prunus fertilisation of the ovule and normal seed development are a prerequisite for fruit set and growth. But despite fertilisation and beginning of seed development premature fruit drop may occur. The aim of th e present work was to investigate in some plum and cherry cultivars differe nces in the seed development of fruits which are shed prematurely and norma lly developing ones until maturity. Seeds of fruits which are shed, show a delayed development compared to norm ally growing fruits. Besides that falling fruits are back in their differen tiation: The transition from nuclear to cellular endosperm and the growth o f the haustorium to the chalaza occured later than in fruits reaching norma l maturity. In the plum variety 'Hauszwetsche, Rheinland' and the ornamental froms of P runus (e.g. I! cerasifera, P sargentii), all with very low fertility, degen eration of the ovules, as expressed by the deposition of callose at the cha laza, occured already in the bud stage of the flowers. Pollen tubes did gro w into the ovules, and in Il cerasifera a also cell divisions in the endosp erm in degenerating ovules were observed but despite that no fruit set was evident. In some seeds an abnormal haustoria or endosperm development was observed. Haustoria may show ramifications or they may be wider or narrower than norm al ones. According to year and variety they may reach up to 75 % of the inv estigated seeds. Abnormal endosperm may have a granular structure, form pre mature cell walls or deposit callose. The abnormalities in endosperm develo pment are below 1 % of the ovules investigated. Since natural fruit drop ma y be over 50 % these abnormalities can not be the main reason for early fru it shedding. In seeds of prematurely dropping fruits an increased deposition of callose at the nucellus basis and chalaza is evident. The depositions mostly increa se at the end of the fruit fall period. It is assumed that at a locus such as the basis of nucellus and chalaza, where symplastic transport from cell to cell into the seeds and to the growing embryo occurs, callose deposits i n the cell walls inhibit or decrease transport of nutrients. Seeds of early shed fruits show, in contrast to seeds of fruits which remai n on the tree untill maturity, symptoms of degeneration, more frequently ab normal development of haustoria and endosperm, and they stay back in their stage of growth and differentiation. All these factors may lead to a decrea sed sink activity of the seeds for nutrients. It is not clear if these fact ors cause fruit drop or if they are the concequence of still unknown factor s. Further investigations on transport phenomena into and out of the seed c ould clarify this problem.