THE EFFECT OF APPLIED HEAT ON GRAFT UNION FORMATION IN DORMANT PICEA-SITCHENSIS (BONG) CARR

The effect of application of heat on the development of graft unions in dormant Picea sitchensis (Bong.) Carr (Sitka spruce) was observed at monthly intervals from October to March. No advantage over controls was gained in terms of final graft success rates. It is suggested that this may have been a result of the mild winter conditions during the course of the experiment, which enabled callus formation to occur in unheated controls. On the other hand, scions of grafts which had been heated for 3 weeks, and then returned to ambient conditions showed considerably greater growth than unheated controls during the following spring and summer. The amount of extension depended on how late in the winter the graft had been prepared and heated, with feast growth by grafts made in October, and most by grafts made in March. Microscopical examination showed that callus formation was more rapid in heated grafts than in controls, although callus formation occurred in all control grafts examined. Heat also induced cambial reactivation and tracheid formation, particularly in the vicinity of needle or bud traces in the scion. In keeping with this, it was found that the presence of needles and buds was essential for cambial reactivation in normal dormant stems and that reactivation was restricted to the heated region.