Changes in gibberellin levels in the flowering shoot of Phalaenopsis hybrida under high temperature conditions when flower development is blocked

The ability of exogenous gibberellin (GA) A(3) to substitute for low temper atures in inducing flowering in Phalaenopsis hybrida under high temperature s suggests a similar role for endogenous GAs in normal (low temperature) de velopment of the inflorescence and floral buds. Changes in endogenous GAs i n shoot-tips (about 2.0 cm long) were examined when the potentially floweri ng shoot was 2-3 cm and 7-10 cm long, respectively. Treatments included war m control (30/25 degreesC day/night, non-flowering), GA(3)-treated (30/25 d egreesC day/night, flowering) and cool-induced (25/20 degreesC day/night, f lowering) plants. GA(3) at 0.5 mug.shoot(-1) was injected into the cavity o f the second visible bud scale below the flowering shoot apex. The contents in the tips of potentially flowering shoots of GA(1), GA(20), GA(19) and G A(53) tended to be highest in cool-induced plants, intermediate in GA(3)-tr eated plants, and lowest in the warm controls (non-flowering). Tips of pote ntially flowering shoots in warm controls also contained higher GA(8) relat ive to GA(3)-treated and cool-induced plants. Tips of potentially flowering shoots (2-3 cm in length with no flower primordium), contained low levels of GAs than older 7-10 cm long flowering shoots (with flower primordia) und er cool-inductive conditions. These results suggest that continued inflores cence development and flower bud initiation are closely associated with inc reases in endogenous GAs, even in GA(3)-treated plants. They also suggest t hat a more rapid biosynthetic flow-through from GA(19)-->GA(20)-->GA(1)-->G A(8) leaving reduced levels of GA(1) may be involved in the inhibition of f lowering and shoot elongation seen in warm (30/25 degreesC day/night) contr ol plants. A reduced conversion of 'active' GA(1) to inactive GA(8) may thu s be required for successful floral induction and development whether induc ed by cool temperatures or by applied GA(3). These results imply that the h igh temperature inhibitory effect on flowering is mediated through its effe ct on lowering the level of endogenous GA(1) and its precursors. (C) 2001 E ditions scientifiques et medicales Elsevier SAS.