Cropload Affects the Size but Not the Number of Cells in Sweet Cherry Fruit

High crop loads in sweet cherry typically result in a large proportion of under-sized fruit. However, the anatomical basis for fruit size has not been established raising the question “Is the potential for fruit size determined by cell number or cell size?” Because horticultural practices can affect both cell division and cell enlargement, we investigated the timing of cell division and enlargement to identify critical timing when horticultural practices could be employed. Early season ‘Chelan’, mid-season ‘Bing’ and late-season ‘Sweetheart’ were evaluated for fruit growth and anatomy beginning at full-bloom and ending one week after commercial harvest. At about 40% full-bloom, balloon-stage flowers were identified and all other opened or un-opened flowers were removed leaving only two to four flowers per spur. This ‘bloom-setting’ procedure reduced the variability in the timing of fruit-set to about 4 days compared to about 9 days for the whole orchard. This was shown with detailed growth analysis from samples taken daily of the ‘set’ and ‘un-set’ fruits.  Additionally in ‘Sweetheart’, crop load was reduced by bud-thinning in the dormant stage. For the high crop load treatment in ‘Sweetheart’, flowers of un-thinned spurs were marked with tape at a similar balloon-stage and no flowers were removed. Light-microscopy of whole and dissected fruit was used for growth analysis and scanning electron microscopy (SEM) was used for cell number and size measurements. High crop loads reduced the growth of fruits and pits beginning as early as 18 days after full bloom. We documented spatial and temporal changes in cell number, shape and size of cells of the mesocarp tissue (flesh). Roughly half the number of cells comprising an individual fruit at harvest was already present at full bloom.  The cell division period was completed during mid-stage I growth, at about the same time as the crop load effects on fruit size were first detected. Cells of the outer region of the mesocarp were elongated during most of the growing season, but during the final fruit swelling became rounded as they expanded tangentially. Cells of the inner region of the mesocarp remained elongated at harvest. Final fruit size was more strongly correlated with cell size than cell number and cell size differences were most apparent in the outer mesocarp during final fruit swelling.