Germination involves the imbibition of water, a rapid increase in respiratory activity, the mobilization of nutrient reserves and the initiation of growth in the embryo. It is an irreversible process; once germination has started the embryo is committed irrevocably to growth or death. Externally, germination is marked by the bursting of the testa and the extrusion of the plumule or radicle. In this chapter, we examine the influence that various environmental factors have on the process.

Temperature and germination

Constant temperatures

Quite apart from its well-documented effects on the induction and breaking of dormancy, temperature has important effects on germination itself. These may be divided, conveniently but rather arbitrarily, into effects of constant and alternating temperatures. The latter are considered later. In seasonal climates, temperature is of course a good indicator of the time of year and is therefore implicated strongly in determining the timing of germination. Washitani and Masuda (1990) conducted a remarkably detailed study of germination in a Japanese grassland, in which germination of almost all species was confined to the spring–early summer period. They found that the temperature at which seeds began to germinate, when subjected to gradually increasing temperatures in a standardized screening programme, was linked closely to the observed timing of emergence in the field (Fig. 6.1). Interestingly, emergence timing was not correlated at all with presence of dormancy or requirements for dormancy breaking, illustrating the important point that dormancy normally plays little part in determining germination timing.