Hormonal Regulation of Morphogenesis and Cold-resistance

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Abstract
The endogenous levels of gibberellin and abscisic acid were determined in extracts from seedlings of alfalfa (Medicago sativa L.) ov. Hairy Peruvian and cv. Ranger, growing under long days and high temperatures (not inducing cold-hardiness), or short days and low temperatures (inducing cold-hardiness). Under inductive conditions only Ranger was coldacclimated, exhibiting a rosette growth; non-acclimated seedlings of Ranger and Hairy Peruvian developed an elongated shoot. Under non-inductive conditions gibberellin A3 (GA3)-like activity was found in both cultivars. Under inductive conditions GA3-like activity increased in Hairy Peruvian and was almost non-existent in Ranger. In spite of morphological modification, ABA-like activity was hardly affected by thermophotoperiod conditions. Addition of ABA to the nutrient solution of seedlings growing under non-inductive conditions simulated the effects of short days and of low temperatures. It diminished GA3 content, and affected morphological modification of the seedlings. It is concluded that the modification of the ABA/GA balance, through the decrease of the GA level, monitors the capacity of the twoalfalfa cultivars to become cold-acclimated when exposed to low temperatures.

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