-(Effect of abscisic acid on the mobilisation of galactomannan and embryo development of Sesbania virgata (Cav.) Pers. (Leguminosae -Faboideae)). Galactomannans (GM) are storage cell wall polysaccharides present in endospermic seeds of legumes. They are thought to be storage polymers, since it has been observed for a few species (among them Sesbania virgata) that they are completely broken down after germination and their products are transferred to the growing embryo. We examined the effect of 10 -4 M abscisic acid (ABA) on the degradation of galactomannan in isolated endosperms and intact seeds of S. virgata. We found that after seed germination the initial embryo growth was retarded. Ultrastructural analysis showed that the embryo is completely surrounded by an endosperm which displays very thick galactomannan-containing cell walls. Although an inhibitory effect has been observed on the increase of fresh mass of the embryo, the effect of ABA on the dry mass was weaker and transitory (from 48 to 96 h). Endosperm dry mass and galactomannan degradation were significantly inhibited and the activity of α-galactosidase was strongly affected. The addition of ABA before and/or after the start of mobilisation in intact seeds or isolated endosperms, showed that whereas addition before mobilisation did not affect dry mass decrease in intact seeds, it was strongly affected in isolated endosperms. On the other hand, whereas it affected embryo fresh mass increase in intact seeds, but not in isolated embryos, no significant effect was observed on dry mass. These results suggest that ABA affects galactomannan degradation and by doing so, prevents water absorption by the embryo, rather than affect its dry mass. As ABA has been detected in the endosperm of seeds of S. virgata, it is proposed that it probably acts as a modulator of galactomannan mobilisation and consequently synchronises it with early growth of the embryo.RESUMO -(Efeito do ácido abscísico sobre a mobilização de galactomanano e desenvolvimento do embrião de Sesbania virgata (Cav.) Pers. (Leguminosae -Faboideae)). As sementes de Sesbania virgata (Leguminosae -Faboideae) acumulam galactomanano no endosperma, o qual é degradado após a germinação e seus produtos de degradação são transferidos para o embrião em crescimento. Neste trabalho verificou-se que o ácido abscísico (ABA 10 -4 M) retarda a germinação de sementes de S. virgata e inibe o crescimento do embrião. Sobre as massas fresca e seca deste, o ABA teve efeito transitório sobre o segundo e efeito drástico sobre o primeiro, sugerindo que sua ação seja na absorção de água pelo embrião em crescimento. Um efeito parcial do ABA foi verificado sobre a degradação do galactomanano e sobre a atividade de uma das enzimas de degradação, a α-galactosidase. O ABA atuou sobre a mobilização quando aplicado antes ou durante o período experimental e menos quando aplicado depois. O efeitos sobre a massa fresca do embrião foram similares aos observados para o endosperma, porém, não houve efeito sobre a massa seca do...

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