The Response of Seeds of Bromus Sterilis L. And Bromus Mollis L. To White Light of Varying Photon Flux Density and Photoperiod

S ummary The germination response of seeds of Brnmus mollis L. and B. sterilis L. to white fluorescent light was studied over a wide range of photon flux densities and photoperiods. Photon doses below 3 × 10 −3 mol m −8 d −1 had little effect on the percentage germination of either species, although in B. sterilis photon doses as low as 10 −4 mol m −2 d −1 depressed the rate of germination. An increase in photon dose above 10 −1 mol m −2 d −1 in B. mollis at 15 ° C C, 3 × 10 −3 mol m −2 d −1 in B. sterilis at 15 °C, and 3 × 10 −2 mol m −2 d −1 in B sterilis at 25 °C prevented the germination of an increasing number of seeds. The proportion of seeds which were inhibited showed a cumulative normal distribution with respect to the logarithm of photon dose. For a given photon dose, however, there was evidence that longer photoperiods were more inhibitory in both species: 24 h (continuous light) more than 8 h, and in fi sterilis 8 h more than 1 h. We conclude that the high irradiance reaction is the major cause of inhibition by white light in both species, but the sensitivity amongst the seeds within a population to photon dose varies by a factor of up to 10 5 . Amendments to seed‐testing procedures for B mollis are suggested.