Abstract
Pre-harvest fruit softening, so-called “On-tree early softened fruit”, in which maturation is abruptly accelerated and over-ripened prior to harvest for certain physiological reasons, was compared among 6 strains of the “Early B” types of Japanese persimmon ‘Saijo’. We also examined the cause and methods of preventing this phenomenon. Pre-harvest fruit softening was less frequently observed in the “Endo”, “Yamasaka”, and “Abe” strains than in “Koto” strain, even in years with heavy rain during July to September. In these strains, relatively higher activities were found in the fine roots, and except in “Koto”, there were lower levels of ethylene evolution in the fruit during early September to late October. A remarkable ethylene evolution was observed in fruit on about day 10 of flooding treatment, then heavy falls of leaf and fruit were observed. On the contrary, however, shaded trees did not show increased ethylene production in fruit, and caused few or no falling leaves and fruit. This was also observed in untreated trees. Accordingly, the disorder might be promoted not by shortage of sunshine, but by high moisture in the soil. Soil moisture in late July to October was little changed by a “White porous water proof sheet” covering the soil under the tree crown, the pF value of soil at a depth of 15 cm was 2.3 through out the period. Ethylene release from fruit, and the occurrence of pre-harvest fruit softening were considerably reduced by mulching with a water proof sheet. From the above results, it was considered that the pre-harvest fruit softening was induced mostly through the following 2 process; ① At the late stage of fruit maturity, flooding condition by continuous rainfall blocks aeration in root, and the ethylene precursors (ACC) are accumulated in fruit. ② Thereafter, waters are actively vaporized from fruits by exposure to high temperature during the following fine and windy days, and ethylene release in fruit under water stress, resulting in rapid fruit softening.
