A study has been made of growth in the fruit of apricot, Prunus armeniaca L. cv. Moorpark, in Adelaide, S.A. Morphological changes in the seed and mesocarp from anthesis to maturity are described. Significant positive correlations have been shown between the diameter of fruit at early pit-hardening, and that at the end of pit-hardening and at maturity. Large fruit at pit-hardening were also shown to ripen earlier. Other factors such as leaf/fruit ratio, fruit or leaf number per branch, branch diameter, and amount of light received by branches could not be correlated with fruit growth or final size. Differences were found in the rate of fruit growth during the first 3 or 4 weeks after anthesis between seasons, and also between fruit from early and late flowers. These differences were positively correlated with air temperature during this period; a constant number of "degree-days" above 5°C had accumulated when the fruit had grown to 2.3 cm in diameter. Fruit were shown to vary in volume and in the number and volume of mesocarp cells, both within and between trees. Volume differences in fruit within a tree were mainly due to differences in cell number, but between trees the contribution of cell volume was relatively more important.
The effect of temperature and gibberellic acid (GA3) applications on apricot fruit have been determined by measurements of fruit size and shape, mesocarp cell number, size, and shape, and endogenous gibberellin. Application of heat during the first 10 nights after anthesis increased the initial growth rate of fruit and of cells in the mesocarp and produced more rapid cell division in this tissue. It did not affect final fruit size or the number and diameter of cells in the mesocarp. Higher temperatures did, however, hasten maturity of fruit. GA3 perfused into branches before anthesis produced an increased drop of flower buds and fruit, raised the ratio of flower buds to leaf buds initiated that season, and resulted in elongated pedicels. Initially, fruit growth rate was increased by GA3, but subsequently it was depressed and final size was below normal. These effects on fruit size were mainly due to effects on the rate of cell division. Some differences were noted in the dimensions of cells but final radial cell diameter did not differ from untreated fruit. GA3-treated fruit ripened sooner than controls. Neither heating nor GA3 treatments affected the level of endogenous gibberellin-like substances in the fruit or their RF on paper chromatograms. There were no significant interactions between temperature and gibberellin in any parameter of apricot fruit growth.
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