The modulating effect of the perisperm-endosperm envelope on ABA-inhibition of seed germination in cucumber

Abscisic acid (ABA) markedly reduced the germination of developing seeds at much lower concentrations (ABA(50)=0.1 mM) compared with that of mature seeds (ABA(50)=1.6 mM) in cucumber (Cucumis sativus L. cv. Green long). The perisperm-endosperm (PE) envelope in developing seeds showed partly differentiated lipid and callose layers, considerable ABA biosynthetic activity in endosperm cells, and appreciable permeability to applied ABA. The decrease in the sensitivity of seeds to applied ABA was coincident with the complete development of lipid and callose layers, diminished ABA biosynthetic activity in endosperm cells in imbibed mature seeds, and moderate permeability of the PE envelope to applied ABA. Decoated seeds pretreated with chloroform showed decreased germination (ABA(50)=0.4 mM) in response to applied ABA and increased ABA permeation through the PE envelope. ABA thus allowed to permeate into embryonic tissues substantially reduced the pregerminative activity of beta-glucanase in the radicles. The structure and biophysical/biochemical properties of the PE envelope seem to modulate the effect of ABA on the germination of developing and mature cucumber seeds.