Water constraints on the photoinduction of weed seed germination during tillage

Germination of light-requiring seeds may be induced by very brief exposure to sunlight during soil disturbance through the very-low fluence (VLF) mode of phytochrome action. We studied the effect of soil water availability af ter cultivation on the photoinduction of seed germination in two important weed species, Datura ferox and Chenopodium album. In daily-irrigated plots, seedling density was 1- to 4-fold greater in plots cultivated during dayti me than in those tilled at night. In contrast, when plots were not irrigate d soon after tillage and rainfall was excluded, no significant differences were observed between seed germination in daytime vs night-time cultivated plots, although seedling emergence in night-time cultivated plots was highe r than in non-cultivated controls. The average critical value of soil water potential required for the expression of VLF-induced germination was highe r than -0.5 MPa (at 3-cm depth during the 6 d following cultivation). Dark germination was less sensitive to decreasing soil moisture than light-induc ed seed germination. The promotive effect of the light signal perceived by the seeds during daytime cultivation is maintained for several days (ca 6) in drying soil, even though laboratory data suggest that the far-red-light absorbing form of the phytochrome inducing the VLF photoresponse is unstabl e, disappearing in less than 24 h. These results reveal the complexity of i nteractions between the light signal and other environmental factors that c ontrol seed germination under natural conditions.