Abstract
Wall bound acid invertase is thought to be of importance for apoplastic phloem unloading in assimilatory sink tissues. However, the occurrence of an insoluble form of this enzyme in grape homogenates has been attributed to polyphenol induced protein precipitation rather than to an in vivo binding to the cell wall structures. But even after exhaustive extraction of soluble invertase with polyethylene glycol (PEG) 4000, a polymer which proved most effective in counteracting tannic aggregation of proteins, considerable sucrose-hydrolysing activity remained in the particulate fraction of extracts from young grapevine leaves. Incubation of the thoroughly washed, insoluble sediment with walldegrading enzymes (cellulase, pectinase) solubilized most of the wall associated invertase, thus making it amenable to direct assessment and characterization. The activities of both enzymic forms as well as the sucrose contents of grapevine leaves were assayed at four different stages of development and foundto change in a manner consistent with the ‘phloem unloading hypothesis’.
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