XYLOGLUCAN ENDOTRANSGLYCOSYLASE ACTIVITY DURING FRUIT-DEVELOPMENT ANDRIPENING OF APPLE AND KIWIFRUIT

Xyloglucan endotransglycosylase (XET) activity was measured in apple ( Malus domestica Borkh. cv. Braeburn) pericarp and kiwifruit (Actinidia deliciosa [A. Chev.] C. F. Liang et A. R. Ferguson var. deliciosa cv. Hayward) outer pericarp and core tissues in order to establish whethe r a correlation exists between the activity of the enzyme anti differe nt stages of fruit development. Whereas the growth rate of kiwifruit p aralleled changes in XET activity throughout fruit growth, that of app le did not. Both fruits showed the highest XET activity, on a fresh we ight basis, in the first two weeks after anthesis when cell division w as at its highest. XET activity then decreased sharply, but as the fru it increased in size (4-8 weeks after anthesis) there was a concomitan t increase in XET activity in both fruits. In the latter stage of frui t development (16-26 weeks after anthesis) XET activity increased to p eak at harvest in apple fruit. During this time there was relatively l ittle increase in fruit size and presumably therefore minimal cell exp ansion. XET activity then declined as fruit softened after harvest. In core tissue from kiwifruit, XET activity increased throughout the lat er stages of fruit growth to harvest maturity in a similar manner to a pple, but continued to increase after harvest until fruit were ripe. I n contrast, XET activity in the outer pericarp of kiwifruit did not in crease until ripening after harvest. In apple tissue up to 30% of the XET activity was cell wall bound and could not be solubilised, even in buffer containing 2 M NaCl. The results implicate XET in cell wall as sembly during cell division and expansion early in apple and kiwifruit growth. However, the disparity between apple and kiwifruit with respe ct to XET activity late in fruit development and ripening and the diff erent affinities of the enzyme for the cell wall in each fruit, sugges t that XET has several roles in plant development, not all of which ar e related to cell wall loosening during periods of accelerated growth.