The effects of the duration and timing of drought plus heat plus gibberellin A4/7 on apical meristem development and coning in Sitka spruce [Picea sitchensis (Bong.) Carr

Abstract

Coning was induced on container-grown grafts of Picea sitchensis (Bong.) Carr. by stem injection of 2 mg per month or part of a month of gibberellin A4/7 in combination with drought and high temperatures. The treatments were applied in a polyethylene-covered greenhouse at five different times or durations during spring and summer. Coning response was determined the following spring. Shoots were sampled every fortnight from early May until late August and buds were dissected and fixed for anatomical study of apical development. The best coning resulted from the May to June, and June treatments with lesser coning occurring in the May, July, and May (2 wk only) treatments (all received heat + drought + gibberellin A4/7 ). Both pollen cones and seed cones were induced. Apices from control trees began to initiate bud scales in early May and leaf primordia about mid-July. Apices from May-treated trees began to differentiate into vegetative, pollen-cone or seed-cone buds in mid-June. Apices from the June-treated trees showed the earliest stages of vegetative, seed-cone and pollen-cone bud differentiation in mid-July, about 2 wk after treatment stopped. Apices from the May to June, and the July-treated trees began to differentiate into vegetative, pollen-cone and seed-cone buds in mid-June and mid-July respectively. The 2 wk May treatment induced too few cones to warrant anatomical study. The long period (May-July), during which apices can be induced to differentiate into cone buds, may be explained partly by variation in the stage of bud development and partly because treatments alter the time of differentiation and synchronize apical development. Thus the earliest treatment (May) produced earlier differentiation and promoted more buds to develop in a uniform and vigorous manner to the stage where differentiation occurs. Other treatments (May-June, and June) which preceded the natural time of bud differentiation also promoted early and uniform bud development and the longer treatment stimulated many apices, which would otherwise have slowed or stopped their development, to continue developing. Later treatment, in July, delayed apical differentiation and allowed lagging and retarded apices to catch up and thus be susceptible to the inductive treatment.