Abstract
Effects of exogenous gibberellins GA53, GA44, GA19, GA20 and GA1 on photoperiodically controlled shoot elongation in seedlings of Salix pentandra L. were studied. Gibberellins GA20 and GA1 induced shoot elongation under short days (SD) and could substitute for a transfer to long day (LD), while gibberellins A53, A44 and A19 were inactive. In seedlings exposed to a prolonged SD‐treatment (30 days) there was a significant positive interaction between a transfer to LD and a treatment with GA20 and GA1 on shoot elongation. In addition, GA19 enhanced the growth promotive effect of LD in these seedlings. The results are compatible with the suggestion that conversion of GA19 to GA20 is blocked under SD. This effect is supposed to be an early process leading to the cessation of shoot elongation under SD. Responsiveness of the seedlings to LD and to a GA‐treatment gradually decreased with an increasing length of exposure to SD.
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