Identification of active substances in Chinese chive and rakkyo plants responsible for breaking bud dormancy in grape cuttings

Abstract

The active substances in volatiles from two Allium species, Chinese chive (A. tuberosum Rottler) and rakkyo (A. chinense G. Don), responsible for breaking bud dormancy in grapevines, were investigated. Among several gas-chromatographic peaks, two major ones in Chinese chive and one in rakkyo were detected. Based on a comparison of the retention time with authentic chemicals and GC-MS analysis, the compounds were identified as methyl mercaptan (CH3SH) and allyl mercaptan (CH2=CHCH2SH) in Chinese chive, and dimethyl disulfide (CH3SSCH3) in rakkyo. Dormant 'Kyoho' grape cuttings were exposed for different durations to these and related compounds at varying concentrations in late November, late December and mid January. Methyl mercaptan promoted budbreak in late December and mid January, although a lower concentration resulted in a uniform rate of budbreak. Allyl mercaptan was effective in late December but inhibitory in mid January. The effect of dimethyl disulfide seemed to vary with concentration, time and duration of exposure. In late November, 24-hr exposure to 99% dimethyl disulfide greatly promoted budbreak, whereas 12-hr exposure had a slight effect. Moreover, no budbreak was observed in cuttings exposed to 30% dimethyl disulfide for 12 or 24 hr. In late December, a 24-hr exposure to the 10% suspension of the same compound promoted budbreak but the same exposure to a 75% suspension inhibited it. Therefore, it is highly probable that these substances are effective for breaking dormancy of grapevine buds but their time course effects and optimum concentrations remain to be investigated in relation to the dormancy stages.