The effect of Ca on senescence was followed in detached cucumber (Cucumis sativus L.) cotyledons floating on various solutions in the dark.Compared with those in water, cotyedons in 10-molar CaCI2 exhibited reduced chlorophyll loss and H202 production, reduced and delayed ethylene production, and did not undergo a burst in CO2 production. In contrast, Mg had Uttle effect on cotyledon senescence, whereas K stimulated chlorophyll loss but did not increase H202 accumulation of ethylene and CO2 production. This reduction in the rate of senescence by Ca could also be achieved by increasing the endogenous levels ofCa in the cotyledons before excision, although the reduction was less than that with Ca in the external solution. The addition of H202 to the soluftons on which cotyledons were floated stimulated chlorophyll breakdown, but effects on ethylene and CO2 were not consistent.Ca is known to delay senescence of plant tissue. This is especially evident in fruit, where respiration rates (and thus rates of ripening) are reduced in tissue having high Ca concentrations (4). Other parameters of fruit ripening such as solute leakage can be reduced by Ca as well (9). Ca also lowers the rate of Chl and protein degradation in leaf discs of corn and Rumex (29), indicating an effect on vegetative senescence.In senescing leaf discs, respiration and ethylene production both undergo a lag period followed by stimulation similar to that of the climacteric rise in ripening fruit (1,3,16,31). This burst of activity is concomitant with the period of most rapid Chl breakdown. Recently, ethylene has been strongly implicated as a dominant hormone in the regulation of leaf senescence (1)(2)(3)14), although the exact nature of control of the process is still unknown. However, if Ca is to have an effect on leaf senescence, it should be manifest in a change in the pattern of ethylene production.There is strong evidence that oxidative metabolism also plays a role in senescence of both fruit and leaf tissue. Frenkel and coworkers (5, 6, 13) have demonstrated an association between the ripening of pears and tomatoes, and production of H202 and H202-producing enzyme systems. Increased levels of endogenous H202 or inhibition ofcatalase activity also enhanced fruit ripening. Other workers have demonstrated that exogenous H202 enhances leaf senescence (26,30), and the activity of glycolate oxidase, a major peroxide-generating enzyme, has been found to increase during senescence of sunflower leaves (30), and may have allowed H202 to accumulate. It is therefore possible that there is an involvement of H202 in leaf senescence similar to that proposed for ripening fruit (5, 6).We have used detached cucumber cotyledons to examine the effects of Ca on senescence. Such cotyledons undergo senescence in a manner similar to that of excised leaves, both in terms of Chl and protein breakdown (21) and in a decline in catalase activity (28). We show here that Ca has a substantial inhibitory effect on ethylene production, respiration, Chl breakdown, and H...

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The growth and chemical composition of feijoa fruit (Feijoa se/lowiana Berg or Acca sellowiana) harvested at different stages of maturity were examined. Fruit growth showed a double sigmoid pattern characterised by a rapid final growth phase which lasted until abscission and fruit fall from the tree. The respiration rate of young fruit was high (120 mg C02 kg -1 h -1) but declined to a preclimacteric minimum (20 -30 mg C02 kg -1 h -1) 80-100 days after anthesis, and then increas~d as the fruit matured and ripened. Sugar and orgamc acid content of fruit harvested less than 80 days after anthesis was low but increased after 90 days, until fruit fall. Calcium, magnesium, and potassium content of feijoa fruit increased throughout maturation.

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The effect of calcium (Ca) concentration on the fmnness of kiwifruit during cool-storage was investigated by (a) postharvest dipping of fruit in CaCI 2 , and (b) comparing endogenous Ca and frimness of fruit which differed in one of the following orchard or fruit variables: fruit size; crop load; position of the fruit on the vine; exposure to sunlight during growth; and irrigation treatments. Postharvest dipping of kiwifruit in 2, 3, 4, or 5% CaCl 2 reduced the rate of fruit softening early in storage, resulting in fruit which were fmner after storage than control fruit. However, severe pitting of the fruit skin developed during storage as a result of dipping in 5 % CaCI 2 . Total endogenous Ca levels in kiwifruit from several orchard sources ranged from 1.7 to 2.7 mg/g dry weight, but were not correlated with fruit firmness during storage. Furthermore, lines of kiwifruit which became particularly soft during storage had Ca concentrations similar to or even higher than lines of fruit which had remained fmner. These results are discussed in relation to the prediction of fruit firmness during storage, and the potential of developing commercial postharvest treatments for maintaining fmnness of kiwifruit. H90030