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

Abstract

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 the seeds to a range of water potentials in the presence of Pfron its induction of these responses. In addition, the escape time to far-red-light (FR) reversal of the three responses wasdetermined. Low water potential inhibited Pfr action on endosperm softening and germination in a similar way. In both cases, a 50% reduction in the response to a saturating red-light (R) irradiation was observed at a water potential of c. —0·5 MPa and therewas very good correlation between the percentage number of seeds with softened endosperm at 45 h after R and germination at 72 h after R (R2=0·95). In contrast, the effect of decreasing the external water potential on Pfr induction of a larger embryogrowth potential was more complex. Moderate decreases in water potential (—0·3 to —0·5 MPa) enhanced Pfr action and thegrowth potential of the embryos was larger (20—25%) than the water controls; water potentials below —0·7 MPa inhibited the Pfr stimulus. The escape time to FR reversal of the R effect was shorter for the increase in embryo growth potential than for endospermsoftening. Twenty-four h after R, the embryo response had escaped in more than 80% of the population whereas endospermsoftening and germination were susceptible to FR inhibition in 100% of the seeds. These results indicate that in D. ferox seeds the increase in embryo growth potential is not sufficient for germination and thatendosperm softening is a necessary condition.