Morphological studies on the development of citrus fruits I

Morphological studies on the fruit development of the Early Satsuma orange (Citrus unshiu MARC. var. praecox TANAKA), the Hassaku orange (C. Hassaku Y. TANAKA) and certain other species were made respectively. By comparing these results together with those obtained previously with the Satsuma orange, the general characteristics and specific or varietal differences in the citrus fruit growth were investigated. Noteworthy features of this investigation were as follows:1. In the growing citrus fruit, the peel and the pulp seem to show different growth cycles. At first, the peel increased in thickness rapidly, next came a vigorous growth of the pulp, which was again followed by a decrease in the peel thickness. Thereafter the peel and the pulp grew in harmony with one another.The peel continued to grow for a longer period of time than did the pulp. As the fruit became mature, the decline and the cessation of growth occurred earlier in the pulp than in the peel. It seems that the peel maintains its growth activity even after the fruit maturation, although the extent of this post-maturation activity varies among different forms. This characteristic of citrus was most clearly indicated by the conspicuous growth of the peel in the mature fruit of Shuto, a variety of C. aurantium LINN. during spring and summer.2. In young citrus fruit the peel increased its thickness rapidly when the cell multiplication in the albedo tissue became very vigorous. In the Hassaku orange, which has a thick peel, the albedo tissue cells multiplied more vigorously, continued to do so for a longer period, and as a result became greater in number than in the Satsuma orange.3. The development of citrus fruit was divided into three stages as follows: Stage I, the cell division period; Stage II, the cell enlargement period; and Stage III, the maturation period. The duration of each stage could be determined by external changes associated with the pulp growth. The period from the emergence of juice sacs to the beginning of their remarkable elongation was referred to as Stage I. In the Satsuma orange, it was observed previously that when the juice sacs began to enlarge abruptly most of their epidermal cells also initiated to elongate. This period was therefore referred to as the cell enlargement period, Stage II. Stage III was defined by a conspicuous decline of the growth in width of the pulp segments.The species and varieties of citrus fruits used in the present study differed in the durations of these three stages, whereby they could be arranged in the following order (from the longest to the shortest of each stage): Stage I-Hassaku orange, Early Satsuma orange, Satsuma orange; Stage II-Satsuma orange, Hassaku orange, Early Satsuma orange; Stage III- Hassaku orange, Satsuma orange, Early Satsuma orange. The fruit with longer duration of Stage I grew rapidly in Stage II and attained larger size. The duration of Stage II was not directly related either to the final fruit size or to the time of fruit maturity, while the latter was proportional to the duration of Stage III.4. Despite their larger size, the fruits of Hassaku and Iyokan (C. Iyo TANAKA) contained much fewer juice sacs than did those of the Early Satsuma orange and Satsuma oranges. Consequently, their juice sacs were relatively large, especially so in Hassaku. On the other hand, in every kind of citrus under observation there was a tendency within the same form that the larger the fruit the more juice sacs were found in it. However, the degree of this association was not uniform among different kinds. Thus there were greater regressions of the number of juice sacs on the pulp weight in the Early Satsuma orange or the Satsuma orange than in Hassaku or Iyokan.