Cell Division and Enlargement in Peach Mesocarp Cells Affected by Time of Thinning

Peach fruit size is affected by crop load, cultivar, and temperature during the Phase I growth stage.  Fruitlets from peach cultivars, ‘Carored’, ‘Summerprince’, ‘Redhaven’ and ‘Scarletprince’, ripening sequentially from mid-May to early July were sampled every 5 days from week 2 to week 5 post-bloom. Fruitlets were collected from the outside canopy of each tree on all four quadrants and were fixed with FAA (ethanol-formaldehyde-acetic acid) before mounting for sectioning.  Trees of each cultivar were either thinned one week or four weeks after petal fall with 4 replicates for each cultivar and treatment.  Mesocarp cell number and size were counted and calculated from the transverse sections using an Infinity Analyze software program.  Differences in cell number due to crop load were observed within days after the petal fall thinning.  Early thinned trees had significantly more cells than later thinned trees 3 weeks after the first thinning date.  At 4 weeks post-bloom, ‘Redhaven’ and ‘Scarletprince’ had more cells than earlier ripening ‘Carored’ and ‘Summereprince’.  Differences in cell size occurred within 5 days of thinning for ‘Carored’.  The other cultivars did not exhibit differences in cell size until 2 weeks after the earlier thinning treatment. Three weeks after the petal fall thinning, fruit from early thinned trees had mesocarp cells 5% to 43% larger than those in the later thinned trees.  At 5 weeks after bloom, ‘Carored’ and ‘Summerprince’ had larger but fewer cells than the later ripening ‘Redhaven’ and ‘Scarletprince’.  Fruit from the early thinned trees ripened earlier and were larger at harvest.