Variation between apple fruits and its relation to keeping quality. II. Between-tree variations due to cropping factors.

Abstract

Storage trials and chemical tests of fruit from individual trees of plots of apple varieties Cox and Cleopatra were made. Samples from each tree were picked on the same dates each year and treated in three ways: composite samples from 100 fruits of separate zones within the fruit were examined for acid and soluble solids content; samples of 25 fruits, at progressive stages of maturity, were examined for pressure, ground colour, starch conversion, total acid, and soluble solids of the mid-cortex region; and samples of 200 fruits were stored at 32-340°F for 10 weeks followed by 3 weeks at room temperature. The mean fruit size and total number of fruits per tree were found and from the latter over a 10-year period a crop index was developed. The data were given statistical treatment. Results from samples picked at successive maturities from light-crop and heavy-crop trees showed that up to a late stage of maturity light-crop fruit had a larger diameter, higher acidity, earlier colour change, and later starch conversion than heavy-crop fruit; the level of soluble solids in light-crop fruit was lower in the earlier stages of maturation, but rose faster and might eventually become higher than in heavy-crop fruit. This relationship was called the simple crop pattern, and though there were differences between the different zones within a fruit, these differences remained constant during maturation. Respiration per unit fresh weight was the same for different crop levels. Results from samples picked at a standard date in several years from trees having a range of cropping levels in each year gave measures of the variables, number of fruits per tree, crop index, mean diameter, acid, soluble solids, starch conversion, pressure, colour change, and incidence of the disorders pit and breakdown. The intercorrelation of these variables in each year and the partial correlation holding mean diameter constant between trees were examined. There was a correlation of all variables (except soluble-solids level) which had the same sign consistently in all years and there was evidence that in the changes of maturation at least three groups of processes followed independent courses: acid-starch-pressure, colour change, and soluble solids. The highest correlation of the disorder pit was with mean diameter and if there was another factor in addition, the most consistent seemed to be acid. The correlation of the disorder breakdown with diameter was extremely high and there was no advantage in choosing any of the other measures to increase the value of mean diameter alone as a measure of breakdown liability. The mean fruit diameter per tree is by far the best index of the physiological behaviour of the fruit from it, being more reliable than any measure of crop in terms of numbers or any of the common chemical or physical changes associated with ripening. An explanation for this relation is suggested in terms of the mean cell size per fruit and respiration per unit protein. There was a physiological interaction between pit and breakdown, each tending to suppress the other, which emphasized the importance of a respiratory phase in the development of pit.