Morphological, anatomical, and physiological changes in the developing fruit of the Valencia orange, Citrus sinensis (L) Osbeck

Measurements of fruit radius and peel and pulp width, as well as determinations of fresh weight, dry weight, moisture content total and protein nitrogen content, and respiration rate were made throughout two growing seasons on Valencia oranges from the Gosford district of New South Wales. Soluble solids, sugar, and acid were also determined in the juice. Anatomical changes during development were investigated throughout one season. Development could be divided into three stages, corresponding with changes in growth rate and coinciding on a calendar basis in both seasons. Stage I varied in length according to the date of the blossom, but was completed by mid December. This was the cell division stage; by mid December cell division was completed in all tissues except the outermost cell layers. Increase in fruit size at this stage was mainly due to increased peel thickness. Stage 11, a period of very rapid growth from mid December to mid July, was the critical period for growth and was distinguished as the cell enlargement period, rapid morphological and physiological changes occurring in the absence of cell division. The growth of the pulp was responsible for most of the increase in fruit size during Stage 11; the peel reached a maximum width early in this stage and then became thinner with very little subsequent change in thickness as the pulp continued to increase in size. Stage 111, the maturation period, lasted from mid July until the fruit was ripe, or approximately 7 months. Fruit continued to grow for as long as it was left on the tree but at a very reduced rate compared with Stage 11. Ripening occurred during Stage 111.