CH $ Cl emissions by freshly harvested tubers of 61 potato cultivars ranged from less than 4 to 650 ng g −" f. wt d −" . In experiments with Anna, the cultivar displaying the highest release rate, maximum emission of CH $ Cl occurred at 20mC but the rate diminished after 48 h, a decrease due in part to inhibition of CH $ Cl release by the accumulation of CH $ Cl in the headspace. In 1995 and 1996, CH $ Cl emission by tubers was first detectable 6 wk before the normal harvest date. If tubers were stored at 20mC immediately following harvest, release attained a maximum of 350 ng g −" f. wt d −" about 4 d after harvest, falling to less than 4 ng g −" f. wt d −" within 3 wk of harvest. On storage at 6mC post-harvest, maximum release of CH $ Cl (c. 600 ng g −" ) was delayed until 5 wk after harvest and release fell below 4 ng g −" f. wt d −" after 200 d. Release of CH $ Cl was not significantly affected by cutting or bruising tuber tissue, and CH $ Cl biosynthesis occurred in both core and superficial tissues of the tuber. The Smethyl group of either L-or D-methionine acted as a precursor of CH $ Cl in sliced tubers, but the methylating system was specific for Cl − . An isotope dilution technique using C#H $ Cl demonstrated that the amount of CH $ Cl metabolized by the intact tuber was approximately 36% of the net release. However, investigations with "%CH $ Cl showed that only a small proportion (7%) of the CH $ Cl metabolized was fixed in involatile form in the tuber. In both skin and core tissues, fixed "%C was located primarily (80%) in the fraction soluble in phosphate buffer, but the specific activity of skin tissue was about twice that of core tissue. Autoradiography demonstrated that "%C fixation in the tuber was greatest within the lenticels, possibly indicating a microflora adapted to use the locally high concentrations of CH $ Cl. Significant "%C fixation was also associated with the periderm but was not attributable to labelling of the O-methyl groups of the phenolic components of suberin. Within the core of the tuber, "%C fixation was located primarily in the phloem, pith and medullary rays.

A methyltransferase enzyme catalysing the 3-0-methylation of isovanillic acid (3-h ydroxy-4-met hoxybenzo ic acid) by S-adenosyl met h ion ine (SAM) was identified in Phanerochaete chrysosporium and purified. Gel filtration indicated an M, of 71 000 and SDS-PAGE showed that the enzyme was composed of two subunits of M, approximately 36000. Substrate utilization studies demonstrated that the enzyme was highly specific, displaying an exclusive preference for the methylation of the 3-hydroxyl group of several substituted benzoic acids. 3-Hydroxybenzoic acids with a methoxyl or hydroxyl substituent in the 2 or 4 position were the best substrates with isovanillic and 3,4-dihydroxybenzoic acids showing the highest rates of methylation. The 3-0-methyltransferase enzyme was induced later in the growth cycle than the 4-0-methyltransferase previously isolated from this fungus, which is believed to have a role in the 4-O-methylation of lignin degradation products. However the function of this mefa-specif ic enzyme, the first phenolic 3-0-methyltransferase isolated from a fungus, remains unclear. The combined activities of the 3-and 4-0-methyltransferase enzymes satisfactorily account for the pattern of SAM-dependent methylating activity shown by whole mycelia to phenolic substrates.