Ethylene-enhanced chlorophyll catabolism in Citrus unshiu fruit: effects of inhibitors on the activity of enzymes involved in type I reactions

摘要

The mechanism of ethylene (C2H4) action on type I reactions (the loss of phytol, the loss of magnesium, and the modification of the side chains of an isocyclic ring of the chlorophyll (Chl) structure) during Chl catabolism in satsuma (cv. Nichinan No.1) fruits was investigated using protein synthesis inhibitors. C2H4 treatment promoted degreening through increased chlorophyllase (Chlase) activity of satsuma fruit peel held in the dark. Cycloheximide (CH), an inhibitor of cytoplasmic protein synthesis, inhibited the C2H4-enhanced degreening, but chloramphenicol (CP), an inhibitor of plastid protein synthesis, did not inhibit the effects of C2H4. However, C2H4 treatment did not significantly increase the chlorophyllide (Chlide)-degrading enzyme (CDE) nor C132-carboxyl-pyropheophorbide (Pyrophed)-forming enzyme (CPFE) activities of satsuma fruit peels. Likewise, CDE and CPFE activities in C2H4-treated fruits were unaffected by CH- and CP-treatments. A high positive correlation between the degree ofpeel greenness and Chlase activity was obtained: Y = 1.368 X - 16.038, (r = 0.992), where X and Y are intensity of the green colour of the peel and Chlase activity, respectively. However, correlations between the degree of greenness of the peel and (1),CDE activity (Y = 0.024 X + 98.030, r = 0.460) and (2) CPFE activity (Y =0.015 X + 103.581, r = 0.424) are low. These results indicate that C2H4 appears to enhance the degreening of the peel through de novo synthesis of Chlase, and that Chlase is the "key" enzyme in the degreening process.