和ナシ果実の日肥大周期に関する研究(第4報)

Abstract

In the first paper of this series, highly significant correlations were reported between hours of sunshine and diurnal increment of growth throughout the later stages of fruit development. Such a fact was confirmed with the results obtained on days of different hours of sunshine by comparing the diurnal increments of growth at each stage of fruit development. The present paper, however, deals with the results obtained by comparing the diurnal increments of growth on a given day at each stage of development. Cumulative effects of sunshine on the size and organic constituents of fruit were simultaneously examined. The fruit-bearing branches of mature Nijusseiki pear trees remained covered with cheesecloth, then diurnal fluctuations of fruit diameter under the screens and in normal sunlight were compared. The results obtained are as follows:1) Prior to July, diurnal fluctuations (both growth and contraction) were considerably depressed by artificial shading. In and after August, however, seasonal pattern of diurnal fluctuation was altered, that is, the artificially shaded fruits exhibited the fluctuation pattern of Stage II though the fruits in full sunlight already showed the fluctuation pattern of Stage III. Such a tendency was more conspicuous of the foliage-shaded fruits. There was observed a daytime decrease of fruit diameter without moisture reduction.2) After the end of June, fruit growth seemed to depend significantly upon the intensity of sunshine and more prominent decrease in fruit size was caused by shading after the middle of August. Relative transverse diameters of harvested fruits were 100, 85. 1, and 76.0 in normally irradiated, artificially shaded, and foliage-shaded branches, respectively. Likewise, relative fresh weight of harvested fruit was 100, 61.7, and 42.9 in normally irradiated, artificially shaded, and foliage-shaded fruits, respectively.3) Chemical characters such as total soluble solids (refractometer readings), titratable acidity (malic acid equivalent), and ascorbic acid contents were highest in the fruit exposed to full sunlight, followed by those of artificially shaded and then foliage-shaded fruits in decreasing order. These facts indicate that restricted illumination exerted some retarding effects on the maturation of Japanese pear fruits.4) On the basis of results obtained in the present investigation, it may reasonably be concluded that during later stages of fruit development the diurnal fluctuation of fruit diameter and consequently the progress of fruit growth are closely associated with the accumulation of elaborated materials, mainly sugars, within the fruit.