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Cellulase activity in detached avocado (Persea americana Mill.) fruits was found to be directly correlated with ripening processes such as climacteric rise of respiration, ethylene evolution, and softening. This activity in the pericarp could be induced by ethylene treatment, and the more mature the fruit-the faster and the greater was the response. Only a very low cellulase activity could be detected in hard avocado fruit right after harvest. Cellulase activity was highest at the distal end of the fruit, lower in the midsection, and lowest at the proximal end. The enzyme is heat-labile and appeared to have activity of an endocellulase nature mainly. Electron micrographs of cell walls from hard and soft fruits are presented.The most obvious feature of avocado fruit ripening is softening, and the most common physiological parameters for determining avocado ripening are ethylene evolution and respiration rate (4). It is generally believed that softening of fruits during ripening is related to alteration in the pectic substances through action of pectic enzymes (4). A clear correlation between the activities of polygalacturonase and pectin methylesterase and fruit softening in avocado has been shown (3, 23). Also, differences in polygalacturonase activity (17) and in ethylene production and respiration (21) between the different parts of avocado fruit were reported. It has been suggested that cellulase, in addition to pectic enzymes, might contribute to softening of tomato fruits (7,13,15) and of dates (12). Cellulose was reported to be the main constituent of young avocado fruit cell walls (19). Cellulase activity in freezeinjured avocado fruits has been reported (10). The ultrastructure of plant cell walls has been described in detail (2, 14), and some ultrastructural aspects of avocado fruit have been reported (19).It was of interest to study what might be the involvement of cellulase during normal and ethylene-induced ripening processes in avocado. Therefore, changes in cellulase activity were investigated in fruits which were harvested at various stages of maturity and treated with ethylene. Some characteristics of the partially purified enzyme and some ultrastructural changes in the cell walls were studied.MATERIALS AND METHODS Plant Material. Avocado (Persea americana Mill.) fruits of the 'Fuerte' cultivar were harvested periodically, starting in June with very young fruit (62 g and 1.6% oil content), and ending in December with completely mature fruit (313 g and 14.9% oil content). These fruits were stored at 20 C. Starting on the day of harvest, ethylene was applied for 48 hr in a flow system, at a concentration of 50 ,ul/l; pure air was supplied to the controls. and 8 parts (w/v) of acetone at -20 C were homogenized in a blender. After vacuum filtration the powder was blended a few more times with cold acetone until a bright colorless powder was obtained. The powder was dried at room temperature and then milled in a coffee mill and stored at -20 C until extraction.Tissue for electron microscopy wa...

Subjecting some chilling-sensitive commodities to various moderate stresses can cause them to acquire resistance to chilling injury. Jasrnonates have been implicated in playing an integral role in the signal transduction cascade that operates in plants to induce responses to stress. Therefore, it was hypothesized that exogenous application of jasrnonic acid or methyl jasmonate to chilling-sensitive commodities might replace the moderate stress treatment and reduce their chilling symptoms. To test this hypothesis, three chilling-susceptible fruits, avocado (Persea americana Mill., cvs. Hass, Etinger, and Fuerte), grapefruit (Citrus paradisi cv. Marsh seedless), and red bell pepper (Capsicum annum cv. Maor), were treated with various concentrations of methyl jasmonate prior to 4-10 weeks of storage at 2°C. Results show that methyl jasmonate dipping (for 30 s) of avocado 'Fuerte' and 'Hass' (2.5 pM), avocado 'Etinger' or grapefruit (10 pM), and red bell pepper fruits (25 pM) significantly reduced both the severity of their chilling injury symptoms and the percentage of injured fruits. Application of methyl jasrnonate by gassing for 24 h was similarly effective. In avocado 'Etinger', methyl jasmonate dipping was effective in alleviating chilling injury either after immediate storage at 2OC or after the fruits had been held for 1 day at 20°C prior to their storage at 2OC. The results suggest that methyl jasmonate, which is receiving renewed biological interest as a potentially important signaling molecule in plants, might mediate the plant's natural response to chilling stress, and by its application might provide a simple means to reduce chilling injuries in chilling-susceptible commodities. Risumi: Le fait de soumettre certaines denrtes sensibles au froid B divers stress modtrCs peut les arnener B dCvelopper une rtsistance aux dommages par le froid. Les auteurs ont formulC l'hypothkse qu'une application exogene d'acide jasmonique, ou de jasmonate de rnCthyl, B des denrCes sensibles aux dommages par le froid, pourrait remplacer le traitement par des stress modtrts et rCduire les syrnptBrnes de refroidissement. Afin de vtrifier cette hypothese, trois fruits sensibles au froid, l'avocado (Persea americana Mill., cvs. Hass, Etinger et Fuerte), le pamplemousse (Citrus paradisi cv. Marsh seedless), et le pirnent doux rouge (Capsicum annum cv. Maor), ont Ctt traitCs avec diverses concentrations de jasmonate de mtthyl, avant de les conserver pendant 4-10 sernaines B 2°C. Les rtsultats montrent que l'imrnersion dans le jasmonate de mtthyl (pendant 30 s) des fruits d'avocados 'Fuerte' et 'Haas' (2,5 pM), d'avocado 'Etinger' ou de pamplemousse (10 pM), et de pirnent doux rouge (25 pM) rtduit significativement la gravitC des symptBmes de refroidissement et le pourcentage des fruits endommagks. L'application du jasmonate de rntthyl sous forme gazeuse pendant 24 h est Cgalement efficace. Chez l'avocado 'Etinger' l'imrnersion dans le jasmonate de methyl est efficace B prCvenir les dommages par le froid, qu'elle soit suivie irnrntd...

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