GERMINATION RESPONSES TO LIGHT AND ALTERNATING TEMPERATURES IN EUROPEAN POPULATIONS OF DACTYLIS GLOMERATA L.: V. THE PRINCIPLE COMPONENTS OF THE ALTERNATING TEMPERATURE REQUIREMENT.

Detailed germination responses to temperature and light in two populations of Dactylis glomerata L. originating from S.E. Denmark and W. France were defined using thermogradient bars. Despite quantitative differences patterns of response were broadly similar. In both cases seeds were sensitive to the relative periods spent at the warm and cool phases of alternating temperature cycles with germination markedly reduced in diurnal cycles involving a 16 h warm phase and an 8 h cool phase. Whereas the proportion of seeds capable of germination increased sharply as the amplitude of temperature alternation increased the optimum amplitude was dependent on both the period spent at each phase and the presence or absence of light. Even under optimum temperature conditions there was an overriding requirement for light in a high proportion of individuals. When five 21/11 °C cycles of reduced duration were applied at 24 h intervals, 1 h/2 h alternations were as effective in stimulating germination as diurnal cycles. Germination response was directly related to the logarithm of the number of diurnal 21/11 °C cycles applied in three different seed samples. Sensitivity was not related to inherent dormancy status (between populations) or to a degree of after-ripening.