- The basic relationships between cytokinin signaling, cell division and fruit growth are well established. Final fruit size depends on the combined effects of the number of cells present at fruit set, the number of subsequent cell divisions, and the extent of cell expansion. Most studies related to the cell cycle in developing fruits have been performed on cultured cells or model plants under optimal laboratory conditions. The question, however, of how cytokinin signaling and cell division regulate fruit growth under field conditions has not been addressed. The effect of the synthetic cytokinins N(2-chloro-4-pyridyl)-N'-phenylurea (CPPU) and N-phenyl-N'-1,2,3-thiadiazol-5- ylurea (TDZ), both derived from phenylurea, on fruit size of the small-fruited 'Spadona' pear was examined over a period of five consecutive years in Israel. CPPU (10-20 ppm) and TDZ (15-30 ppm), applied two weeks after full bloom, when typical fruitlet diameter was 10 mm, caused an appreciable increase in fruit size, without affecting fruit shape and seed number. Histological observations indicated that CPPU-and TDZ-treated fruit displayed a significant increase in the number of cells along the fruit radius compared to untreated fruit. This increase correlated with a prolonged phase of cell division activity, as demonstrated by histone H1 kinase assay, as well as by the extended presence of G2 nuclei, as was evident by fluorescence-activated cell sorter (FACS) analysis. In addition, CPPUand TDZ-treated fruit had smaller cells than untreated fruit did. Thus, a single spray of 'Spadona' pear fruitlets with CPPU or TDZ under field conditions induced an increase in pear fruit size by significantly prolonging the cell division phase.