Differences in fruit development among large- and small-fruited cultivars of saskatoon (Amelanchier alnifolia)

Fruit growth in saskatoons (Amelanchier alnifolia Nutt,), an emerging horti cultural crop across the Canadian prairies, results from development of the mesocarp and the endocarp-locular-ovular structure which includes the deve loping seeds. Contribution of these tissues to fruit size was assessed usin g transverse sections of ovaries sampled at six developmental stages among large- and small-fruited cultivars, Mesocarp development was similar among the larger-fruited cultivars (Thiessen, Northline, and Smoky); the number o f cells increased rapidly through Stage I [162 to 293 growing degree days ( GDDs)] of fruit growth, and cell number increase was minimal during Stages II (293 to 577 GDDs) and III (577 to 747 GDDs), In 'Regent' fruit (a small- fruited cultivar), the maximal rate of cell division was delayed until Stag e II and the mesocarp contained fewer cells than the larger-fruited cultiva rs at harvest maturity, Mesocarp cell enlargement was similar among all of the cultivars studied where cell expansion was maximal during Stage I and c ontinued at a slower rate during Stages II and m. The area of the endocarp- locular-ovular structure was greatest for 'Thiessen' and 'Northline', midra nge for 'Smoky', and smallest for 'Regent'. Data suggest that a minimum num ber of mesocarp cells early in fruit development is required to attain maxi mal mesocarp size, and that differences in cultivar fruit size are a functi on of both the mesocarp and the endocarp-locular-ovular structure.