Studies on the Daily Change in Fruit Size of the Japanese Pear

A series of investigations was undertaken to obtain some useful indices for the rationalization of orchard practices which are closely related to environmental conditions. From this point of view, the present investigation was conducted from 1967 to 1969 to clarify the relation between seasonal patterns of diurnal fluctuation in fruit diameter, mainly transverse one, and climatic factors with respect to mature Nijusseiki pear trees. The results obtained are summarized as follows:1) The seasonal trend of diurnal fluctuation in fruit diameter of Nijusseiki pear fruit was divided into four patterns according to its developing stages: namely Stage I (early growth), Stage II (retarded growth), Stage III (maximum growth) and Stage IV (nearly mature). The conspicuous characteristics at each stage are described as follows:Stage I: The diurnal growth was greater than the contraction, and the change in longitudinal diameter of fruit was greater than that of the transverse one.Stage II: This stage corresponds to the so-called June rainy season. Fruit showed. the least diurnal growth in contrast with the exceeding contraction.Stage III: The rate of growth was remarkable, and the diurnal growth attained to a maximum, though the degree of contraction was nearly the same as that of Stage II.Stage IV: The fruit continuously grew night and day, that is, the contraction in the daytime disappeared and the process turned into enlargement.2) The typical diurnal fluctuation of fruit diameter was closely related to the change of atmospheric humidity but showed such an opposite tendency to that of solar radiation and temperature that a considerable increase in fruit diameter occurred at night and contraction usually appeared in the daytime. On fine days the contraction was usually conspicuous except when the fruit became nearly mature. By sprinkling with water, however, the enlargement, instead of contraction, was observed even on a fine day as well as on a rainy day.3) Before Stage III, the phase of diurnal fluctuation in fruit size was greatly influenced by climatic conditions ; especially diurnal growth being affected quantitatively by the environmental conditions. Accordingly, in Stage I a highly significant positive correlation coefficient was obtained between temperature and diurnal amplitude of growth fluctuation. A significant correlation was also observed between diurnal hours of sunshine and diurnal growth increment of Stage III.4) Similarly a high correlation coefficient was found between the contraction and the decreasing rate of atmospheric humidity. The magnitude of contraction became greater as the temperature rose. Under these experimental conditions, however, it was deduced that the fruit growth was not influenced by atmospheric humidity which is usually considered to be a dominant factor for the fruit growth, since the deficiency of soil moisture was not observed throughout the period of this investigation.5) The diurnal fluctuations in fruit diameter were influenced not only by the concurrent fluctuation of climatic factors, but to some extent by the environmental conditions encountered several days before the measurement.6) Types of fruit enlargement were classified in two categories according to the stagnation or rather slight decrease of fruit diameter which was usually observed at midnight. A highly significant positive correlation coefficient was obtained between the increase in fruit diameter attained before the stagnation and the daily growth increment. This enlargement before stagnation was scarcely influenced by the atmospheric humidity, while the enlargement after the stagnation was closely related to high atmospheric humidity. This fact implies that the former enlargement may correspond to the fruit growth which is due mainly to the accumulation of water-soluble photosynthates within a fruit, and the latter may be caused by the swelling of fresh pulp.