Phytochrome-induced Germination, Endosperm Softening and Embryo Growth Potential in Datura ferox Seeds: Sensitivity to Low Water Potential and Time to Escape to FR Reversal

In Datura ferox seeds, the far-red absorbing form of phytochrome(Pfr) induces endosperm softening, larger embryo growthpotential,and germination. We investigated the effect of exposing theseeds to a range of water potentials in the presence of Pfronits induction of these responses. In addition, the escape timeto far-red-light (FR) reversal of the three responses wasdetermined. Low water potential inhibited Pfr action on endosperm softeningand germination in a similar way. In both cases, a 50% reductionin the response to a saturating red-light (R) irradiation wasobserved at a water potential of c. —0·5 MPa andtherewas very good correlation between the percentage numberof seeds with softened endosperm at 45 h after R and germinationat 72 h after R (R2=0·95). In contrast, the effect ofdecreasing the external water potential on Pfr induction ofa larger embryogrowth potential was more complex. Moderate decreasesin water potential (—0·3 to —0·5 MPa)enhanced Pfr action and thegrowth potential of the embryos waslarger (20—25%) than the water controls; water potentialsbelow —0·7 MPa inhibited the Pfr stimulus. The escape time to FR reversal of the R effect was shorter forthe increase in embryo growth potential than for endospermsoftening.Twenty-four h after R, the embryo response had escaped in morethan 80% of the population whereas endospermsoftening and germinationwere susceptible to FR inhibition in 100% of the seeds. These results indicate that in D. ferox seeds the increase inembryo growth potential is not sufficient for germination andthatendosperm softening is a necessary condition. Key words: Germination, dormancy, phytochrome, endosperm softening, water potential