The development of the fruit of the Washington Navel orange

A study is presented on the development of the Washington Navel orange fruit in a factorial field experiment. The development of the fruit was similar in the two cultural systems studied, but the level of nitrogen supply had pronounced effects in the later stages of development. Peel thickness reached a peak early in January, but became thinner till the end of March. The rate of thinning was higher, the lower the level of nitrogen supply. From March onwards the peel thickness increased again at the highest nitrogen level, but it continued to decrease at the lowest level. Dry weight of the peel increased much faster at the higher than at the lower nitrogen levels. The level of nitrogen supply had a relatively small effect on nitrogen moving to the pulp. Considerable differences, however, were found in the nitrogen going to the peel, particularly during the period in which major differences in peel thickness and weight developed. Leaf analyses suggested that at least part of the nitrogen going to the peel at the highest nitrogen levels came from the leaves. Total phosphorus in the pulp was much higher than in the peel during the later stages of development. The quantities of phosphorus moving to the pulp were considerably higher at the lower than at the higher nitrogen levels. The results are discussed in relation to some cultural and fertilizer practices. These should aim at a nitrogen supply just sufficient to maintain yields, and so avoid the excessive accumulation of nitrogen in the tree, with its consequent detrimental effect on fruit quality.