Ethylene production in xylem of Pinus radiata in relation to heartwood formation

The production of ethylene by xylem tissues of Pinus radiata was monitored throughout the year. Statistically, significantly greater quantities of ethylene were produced during the winter by transition zones separating sapwood from heartwood than by transition zones during the rest of the year and by middle and inner sapwood during the entire year. Ethylene (5 ppm in air), but not carbon dioxide (27.5% in air), stimulated the production of pinosylvin in sapwood blocks. Phenol synthesis appeared to be preceded by desiccation during both heartwood formation and wound response. Tests with cycloheximide indicated that transition zones are capable of protein synthesis. It was suggested that the auxin-ethylene balance could contribute to quantitative and seasonal differences in ethylene production between the wound and senescent responses (heartwood formation). These and other results suggest that (i) ethylene is involved in stimulation of respiration and phenol synthesis during the wound and senescent responses of xylem tissues of this species, and (ii) heartwood formation takes place mainly in the dormant season during increased metabolic activity in the transition zone.