Antagonistic action of low-fluence and high-irradiance modes of response of phytochrome on germination and beta-mannanase activity in Datura ferox seeds

Seed germination is often induced by a pulse of red light perceived by phyt ochrome and cancelled by a subsequent pulse of far-red light, When the puls e of red light is followed by several hours of darkness, a pulse of far-red light is no longer effective and prolonged far-red is necessary to block g ermination. The aim was to investigate whether the red light pulse and prol onged far-red light act on the same or different processes during germinati on of Datura ferox seeds. Forty-five hours after the inductive red light pu lse, germination could not be blocked by one pulse or six hourly pulses of far-red light, but was significantly reduced by 6 h of continuous far-red l ight. The pulse of red light increased embryo growth potential and the acti vities Of beta-mannanase and beta-mannosidase extracted from the micropylar region of the endosperm. Continuous far-red light had no effect on embryo growth potential or beta-mannosidase activity, but severely reduced the act ivity of beta-mannanase, The effect of far-red light had the features of a high-irradiance response of phytochrome, Both germination and beta-mannanas e activity were restored by a pulse of red light given after the end of the continuous far-red treatment. It is concluded that the low-fluence respons e and the high-irradiance response modes of phytochrome have antagonistic e ffects on seed germination and that the control of beta-mannanase activity is one process where this antagonism is established.