Effect of GA3 treatment timing in relation to natural day length on flowering and sex expression in Chamaecyparisnootkatensis

Abstract

Experiments on field-grown and potted trees investigated the effect of timing of gibberellin A3 treatment in relation to natural day length on flowering and sex expression in Chamaecyparisnootkatensis. When gibberellin A3 treatment began was more important than the number of weekly sprays (two to eight), and the optimal time of treatment was similar for promotion of seed and pollen cone buds. The optimal treatment time was independent of photoperiod or its direction of change, but instead appeared to be related to a specific stage of ontogeny during early development of the bud apex. Trees growing at progressively lower elevations attained this stage earlier in spring, suggesting that the mechanism triggering initiation of bud development involved heat sums. A higher concentration of gibberellin A3 was required to initiate seed cone buds (200 mg•L−1) than pollen cone buds (100 mg•L−1), and the average weekly dosage was more important than whether gibberellin A3 was applied weekly or biweekly. Young, potted trees within a greenhouse flowered more profusely than larger but similarly treated field-grown trees, indicating that the species is a good candidate for containerized seed orchards.