SENSITIVITY TO ABSCISIC-ACID AND OSMOTICUM CHANGES DURING EMBRYOGENESIS OF ALFALFA (MEDICAGO-SATIVA)

Developing embryos of alfalfa can be placed into nine stages of development according to their morphological characteristics. The intact seeds do not germinate when removed from the pod and placed on water until at least stage VI, and complete germinability is not achieved until the seeds are at the stage when they start to undergo maturation desiccation. Isolated embryos are germinable as early as stage III, and will germinate within 24 h on Murashige and Skoog medium containing 3% sucrose. Osmoticum can inhibit embryo germination, but only proper osmotic conditions can maintain development in vitro; development does not occur at germination-inhibiting concentrations of abscisic acid. The sensitivity to abscisic acid and osmoticum changes with stage of development. Early-stage embryos have the highest abscisic acid sensitivity and this declines to the extent that mature dry embryos require a high concentration (1.0 mol m-3) to prevent their germination. Sensitivity of the embryos to osmoticum is maximum at stage VII of development. The combined inhibitory effect of abscisic acid and osmoticum on germination during development is greater than their individual effects. This combined effect is stage-dependent. Thus studies on the effects of abscisic acid and osmoticum on embryogenesis and associated synthetic events should be expected to vary according to the sensitivity to these agents at different stages of development.