The interrelation of susceptibility to breakdown, cell size, and nitrogen and phosphorus levels in Jonathan apple fruits

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Abstract
A block of Jonathan apple trees were treated for 4 years as individual tree plots with nil, nitrogen, nitrogen + phosphorus, and nitrogen + potassium applied as sprays during two periods, either in spring before bud differentiation or after bud differentiation in mid January. The treatments had no effect on the levels of phosphorus or potassium in the fruit. Spring applications had no significant effect on the nitrogen content, but summer applications a marked one, though there was no evidence of a cumulative build-up or any effect on cell number per fruit. These increases in nitrogen content were accompanied by small increases in breakdown level, but only when an inherent seasonal susceptibility existed. Only a diminishing proportion of the additional nitrogen was incorporated into the protein fraction, and the preclimacteric respiration rate was not affected significantly by the increase in protein. Except for percentage dry matter, there was a wide variation in the values between trees of the variables, which permitted a multiple regression analysis. The regression of breakdown and cell volume was positive and highly significant in all years. Except for the year of very low incidence, there was a significant positive relationship between breakdown and nitrogen level, and a consistent negative relationship between breakdown and phosphorus level which was significant in the years of high breakdown incidence. No relationship could be shown between breakdown and potassium. Similar results were obtained by using a percentage fresh weight or percentage dry weight basis for the nitrogen, phosphorus, and potassium contents, or by using actual values or a log (x + 1) transformation for the variables, though there were some advantages in using the latter. The results suggest an important physiological interaction between breakdown and cell volume, nitrogen, and phosphorus.