Emissions, biogenesis and metabolic utilization of chloromethane by tubers of the potato (Solanum tuberosum)

Abstract

CH₃Cl emissions by freshly harvested tubers of 61 potato cultivars ranged from less than 4 to 650 ng g⁻¹ f. wt d⁻¹. In experiments with Anna, the cultivar displaying the highest release rate, maximum emission of CH₃Cl occurred at 20°C but the rate diminished after 48 h, a decrease due in part to inhibition of CH₃Cl release by the accumulation of CH₃Cl in the headspace. In 1995 and 1996, CH₃Cl emission by tubers was first detectable 6 wk before the normal harvest date. If tubers were stored at 20°C immediately following harvest, release attained a maximum of 350 ng g⁻¹ f. wt d⁻¹ about 4 d after harvest, falling to less than 4 ng g⁻¹ f. wt d⁻¹ within 3 wk of harvest. On storage at 6°C post-harvest, maximum release of CH₃Cl (c. 600 ng g⁻¹) was delayed until 5 wk after harvest and release fell below 4 ng g⁻¹ f. wt d⁻¹ after 200 d. Release of CH₃Cl was not significantly affected by cutting or bruising tuber tissue, and CH₃Cl biosynthesis occurred in both core and superficial tissues of the tuber. The S-methyl group of either L- or D-methionine acted as a precursor of CH₃Cl in sliced tubers, but the methylating system was specific for Cl⁻. An isotope dilution technique using C²H₃Cl demonstrated that the amount of CH₃Cl metabolized by the intact tuber was approximately 36% of the net release. However, investigations with ¹⁴CH₃Cl showed that only a small proportion (7%) of the CH₃Cl metabolized was fixed in involatile form in the tuber. In both skin and core tissues, fixed ¹⁴C 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 ¹⁴C fixation in the tuber was greatest within the lenticels, possibly indicating a microflora adapted to use the locally high concentrations of CH₃Cl. Significant ¹⁴C 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, ¹⁴C fixation was located primarily in the phloem, pith and medullary rays.