Flowering response of western hemlock seedlings to gibberellin and water-stress treatments

Abstract

Western hemlock (Tsugaheterophylla (Raf.) Sarg.) seedlings, 1–5 years old, were treated with various numbers and durations of soil-drying cycles alone or in combination with a weekly foliar spray of 200 mg/L of a gibberellin A_4/7 mixture. Control plants did not flower and water stress alone did not promote flowering. Only sparse flowering was produced with GA_4/7 alone but when this was applied with two soil-drying cycles during the 7-week treatment period from mid-June through July an average of seven strobili per plant were induced on one-half of the 1-year-old seedlings (during their second growing season). Two drying cycles were more effective than one, though similar maximum plant water stress at predawn, −25 bars (1 bar = 100 kPa), was attained in both treatments. For the 4-year-old seedlings, 60% flowered, with an average of 64 strobili per plant. As many male as female strobili were produced on the average for all seedling groups. An operational application of these treatments to substantially shorten the natural breeding cycle in tree improvement programs is feasible.