Annals of Botany 80 : 419–425, 1997 Cell Number and Cell Size in Parthenocarpic vs. Pollinated Blueberry (Vaccinium ashei) Fruits RAQUEL CANO-MEDRANO* and REBECCA L. DARNELL‡ Horticultural Sciences Department, Uniersity of Florida, Gainesille, FL 32611, USA Received : 26 February 1997 Accepted : 1 May 1997 Gibberellic acid (GA  ) promotes parthenocarpic fruit development and is used commercially to increase fruit set in many crops. However, fruit size is usually smaller than that of pollinated fruit. The purpose of this work was to determine the anatomical basis for differences in fruit size between pollinated and GA  -induced parthenocarpic blueberry (Vaccinium ashei Reade) fruits. Fresh weights at ripening averaged 16 and 25 g for GA  -treated s. pollinated fruits, respectively. In both pollinated and GA  -treated fruits, mesocarp cell number comprised about 75 % of the total pericarp cell number, and increased from  7000 cells per cross-sectional area at bloom to  9000 at harvest. The duration of the cell division period in pollinated and GA  -treated fruits was similar, with the majority of cell division ceasing by 24 d after bloom (DAB). Cell size in both middle and inner mesocarp of ripe pollinated fruits was significantly larger than in ripe GA  -treated fruits (31 000 s. 22000 μm). Differences in final fruit size between pollinated and GA  -induced parthenocarpic blueberry fruit are due to differences in cell enlargement rather than cell number.  1997 Annals of Botany Company Key words : Blueberry, cell number, cell size, gibberellic acid, parthenocarpy, Vaccinium ashei. INTRODUCTION In blueberry (Vaccinium spp.), parthenocarpic fruit set and development is induced by exogenous applications of gibberellic acid (GA  ) ; however, final fruit size of partheno- carpic fruits is smaller than that of pollinated fruits (Williamson et al., 1995). The smaller fruit size of GA  - induced parthenocarpic fruits has also been observed in other crops, including grape (Vitis inifera L.) (Iwahori, Weaver and Pool, 1968), cranberry (Vaccinium macrocarpon Ait.) (Devlin and Demoranville, 1967), peach (Prunus persica L.) (Stembridge and Gambrell, 1972), and citrus (Citrus reticulata Blanco) (Garcı  a-Martı  nez and Garcı  a-Papı  , 1979). Cell number at anthesis, the length of the cell division period after anthesis, and the extent of cell enlargement determine final fruit size in a number of fruits (Coombe, 1976). Plant growth regulators may induce parthenocarpic fruit set and development by directly affecting cell division andor cell enlargement. Cytokinin-induced parthenocarpic fruit development in cucumber (Cucumis satius L.) was accompanied by increased cell division in the pericarp compared to the non-pollinated fruits (Takeno et al., 1992). Exogenous auxin application to non-pollinated watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) flowers increased both cell division and cell enlargement in pericarp tissue over that of the non-pollinated control (Sedgley, Newbury and Possingham, 1977). In both instances, hormone-induced parthenocarpic fruits were similar in size to their pollinated counterparts. In tomato (Lycopersicon * Current address : Colegio de Postgraduados en Ciencias Agri- colas, Monticell-Texcoco, 56230 Edomex, Mexico. ‡ For correspondence. Fax 352 392 6479 esculentum Mill.) fruit, exogenous applications of auxins to emasculated flowers transiently increased the rate of cell division compared to the pollinated control, and resulted in fruit that was similar in size to the seeded fruit (Bunger- Kibler and Bangerth, 1983). On the other hand, application of GA  increased cell enlargement, but decreased cell division throughout tomato fruit development compared to the pollinated control, and resulted in fruit that was significantly smaller than the seeded fruit. In pea (Pisum satium L.) ovaries, GA  applied at anthesis induced mesocarp cell division and enlargement (Vercher et al., 1984), enhanced cell division in the endocarp (Vercher, Molowny and Carbonell, 1987), and promoted partheno- carpic fruit development compared to the non-pollinated ovaries. However, GA  -treated ovaries were significantly smaller than pollinated ovaries. It is unclear from this work if the smaller size of GA  -treated compared to pollinated ovaries was due to a decrease in cell size, cell number, or both. Although GA  -induced parthenocarpic fruit development occurs in a variety of crops, and development is significantly enhanced compared to non-pollinated fruits, growth is usually less than that observed in pollinated fruits. The smaller final fruit size of GA  -induced parthenocarpic blueberry fruit compared to pollinated fruit may be a result of decreased cell number andor decreased cell size. The objective of this study was to determine the anatomical basis for differences in fruit size in pollinated, GA  -induced parthenocarpic, and non-pollinated blueberry fruits. 0305-73649710041907 $25.000 bo970462  1997 Annals of Botany Company