Dormant barnyardgrass seeds imbibed in darkness for 8 days at 38 C displayed a very low fluence response to phytochrome, and very little germination occurred in darkness. However, if seeds were imbibed at temperatures >42 C or exposed to 46 C for only 0.5 h, considerable germination was evoked in the absence of a light stimulus. It is proposed that the high temperatures caused a transition in dormancy from a light-requiring to a light-independent state. The finding may have relevance to field behavior of barnyardgrass seeds where burial could prevent reception of the required light stimulus.

Soil tillage operations stimulate germination of buried seeds in cultivated lands, allowing them to perceive light as a germination-promoting factor. The time of burial and the effect of changing environmental factors affect the physiological state of the seeds, which may lead to an extreme light-sensitivity and very low fluence response (VLFR) through phytochrome A. This paper describes the influence of the progressive process of dormancy breakage, which is accompanied by the acquisition of extreme light-sensitivity, on processes associated with endosperm weakening and embryo growth potential in the VLFR-mediated promotion of Datura ferox seed germination. Our results show that endosperm weakening is mainly limited by β-mannosidase enzyme activity after far-red light stimulation, which is highly dependent on the dormancy level of the seeds. In addition, stimulation of the embryo growth potential by far-red irradiation did not require an extreme light-sensitivity to very low fluence of photons to reach its maximum response, and it was not completely correlated with expansin gene expression in the embryo. Our work indicates that responses of endosperm weakening and embryo growth potential to far-red irradiation, dependent on dormancy level, have different requirements for stimulation by the signalling network initiated by phytochrome A during the course of the very low fluence response in Datura ferox seeds.

Amaranthus albus L. seeds able to germinate in darkness can be prevented from germinating by prolonged irradiation with incandescent light (CIL). The germination in darkness is controlled by pre-existent Pfr. Allowing the seeds to remain in darkness for about 16 h allows an escape from the inhibition by CIL, about comparable with the time required to escape from pre-existent Pfr control. By examining escape from low fluence far-red (FR), it was found that no escape occurred in CIL. It was concluded that the mode of action of CIL in inhibiting A. albus seed germination involved prevention of the action of the pre-existent Pfr and a subsequent dormancy which does not allow a response to the Pfr generated by the incandescent light. Other actions of CIL were found but were considered less likely as causative for the inhibition of germination. One was a direct inhibition of radicle elongation and another involved prevention of an increased responsiveness to Pfr induced by treatment at 40°C.