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Hisashi Yamada and George C. Martin

The addition of Al2O3 to 8-hydroxyquinoline citrate (8-HQC) solution did not alter the sensitivity of the leaf abscission zone to external ethylene treatment. Exogenous ethylene at 791 nl·l-1 for 72 to 120h and at 193 nl·l-1 for 120h induced leaf abscission whereas at 47 nl·l-1 for 72 to 120h no leaf abscission occurred. Ethylene treatment at 791 nl·l-1 for 72 to 120h increased ethylene evolution, but the amount of ethylene evolved from the explants does not seem to be enough for leaf abscission induction. Three different ethylene inhibitors, aminooxyacetic acid (AOA), CoCl2 and am inoethoxyvinylglycine (AVG), were used to determine whether phosphorus-induced leaf abscission was mediated through elevated ethylene evolution. Although AOA and CoCl2 failed to inhibit ethylene evolution from the explants stem-fed with NaH2P O4, AVG inhibited ethylene evolution. Each of the inhibitors except for 5 mM CoCl2 promoted leaf abscission when administered alone or with phosphorus. Our results reveal that phosphorus induced olive leaf abscission occurs without elevated ethylene evolution, but that oxygen is required.

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Hisashi Yamada and George C. Martin

Adding Al2O3 to 8-hydroxyquinoline citrate (8-HQC) solution did not alter the sensitivity of the leaf abscission zone to external ethylene. Exogenous ethylene at 791 nl·liter-1 for 72 to 120 hours and at 193 nl·liter-1 for 120 hours induced leaf abscission, whereas no leaf abscission occurred at 47 nl·liter-1 for 72 to 120 hours. Ethylene at 791 nl·liter-1 for 72 to 120 hours increased ethylene evolution, but the amount of ethylene evolved from the explants does not seem to be enough to induce leaf abscission. Three different ethylene inhibitors—aminooxyacetic acid (AOA), CoCl2, and aminoethoxyvinylglycine (AVG)—were used to determine whether P-induced leaf abscission was mediated through elevated ethylene evolution. Although AOA and CoCl2 failed to inhibit ethylene evolution from the explants stem-fed with NaH2PO4, AVG inhibited ethylene evolution. Each inhibitor, except 5 mm CoCl2, promoted leaf abscission when administered alone or with P. Our results reveal that P-induced olive leaf abscission may occur without elevated ethylene evolution. At 40 or 75 mm NaH2PO4, abscission did not occur until explants were removed from N2 and placed in ambient air.

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Hisashi Yamada, Hirokazu Ohmura, Chizuru Arai, and Makoto Terui

The influence of controlling temperature during apple (Malus domestica Borkh.) maturation on sugars, fruit maturity, and watercore occurrence was investigated in watercore-susceptible `Himekami' and `Fuji' apples. The incidence of watercore at 13/5 and 23/15C was greater than at 33/25C or ambient temperatures in `Himekami' apples in 1991 and was greater at 18/10C than at other temperatures in `Fuji' apples in 1990. In 1992, the extent of watercore increased as fruit temperature decreased from 28 to 14C and 23 to 9C in `Himekami' and `Fuji' apples, respectively. Watercore occurrence affected by fruit temperature was not related to fruit maturity, as judged by ethylene evolution. The effect of fruit temperature on sorbitol was relatively small compared with that on other sugars, and no relationship was found between watercore development and sugars. These results suggest that fruit temperature affects watercore expression independently of fruit maturity or sorbitol metabolism in the fruit in watercore-susceptible apple cultivars.

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Tjasa Burnik-Tiefengraber, Kitren G. Weis, Barbara D. Webster, George C. Martin, and Hisashi Yamada

When continuously stem-fed with 75 mm NaH2PO4, `Manzanillo' olive explants showed significant leaf abscission after 48 hours; by that time 1.042 mg·g-1 fresh weight P had accumulated in the abscission zone (AZ). The potential contribution of ethylene to phosphate-enhanced abscission was investigated using aminooxyacetic acid (AOA), an ethylene-synthesis inhibitor, and by measuring ethylene evolution in phosphate-treated explants. In combination with NaH2PO4, AOA did not affect leaf abscission. Though ethylene evolution from explants increased as leaf abscission was initiated, it was about two orders of magnitude less than the concentration necessary to induce leaf abscission as judged by exogenous treatments. Based on leaf-abscission kinetics, we have concluded that the mechanism of P-induced abscission is independent of gross measurement of evolved ethylene, but we cannot rule out ethylene confined to the AZ itself. When evaluated for P-induced leaf abscission, leaves of `Manzanillo' and `Sevillano' abscised earlier than `Ascolano' and `Mission'.