Mechanisms regulating germination in seeds of Stylosanthes

Abstract

Seed coat impermeability, embryo dormancy, decline in embryo vigour and the effect of adhering pods on germination were measured monthly on 15 lines of Stylosanthes which had weathered on the soil surface. The effect of pod and seed coat on imbibition was investigated in S. humilis over a range of soil water contents.

The level of impermeability of newly harvested seed was initially high for all lines, but the rates of loss varied both between and within species, which indicated the possibility of screening for residual hard seed. The lines also differed in their ability to maintain impermeability over a 14-day germination period.

The initial level of embryo dormancy varied between species but the protection afforded against germination was short lived. Decline in embryo vigour appeared to be partly caused by the testa restricting the radicle mechanically.

The presence of pods enclosing the seed reduced germination by a mean of 15.4%, but this ranged from 1% in a line of S. guyanensis with thin papery pods to 42% in a line of S. viscosa with thick strong pods. There was no evidence of germination inhibitors in either pods or seed coats, but both restricted the entry of water into the seed. Imbibed seed could be dehydrated without damage provided the radicle had not emerged from the seed coat. This occurred when the water content of the seed reached 90%.