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This research was conducted to determine the environmental conditions necessary for development and expression of very low fluence (VLFR) germination sensitivity, and to determine the roles of type I phytochrome, gibberellic acid (GA) biosynthesis, and nitrate in this process. Redroot pigweed seed was subjected to pregermination incubation regimes of 5, 12, and 23 C for up to 21 d in a −1.2 MPa polyethylene glycol solution in the dark. Development of VLFR sensitivity was most pronounced at 23 C, and induction of germination by the lowest fluence tested (3 μmol m –2 ) was greater at a germination temperature of 30 than 20 C. In dormant smooth pigweed seed, development of VLFR sensitivity occurred only when seed was subjected to a prechilling treatment to overcome primary dormancy. The instability of the fluence response of seed subjected to pregermination conditions favorable for development of VLFR was consistent with type I phytochrome accumulation and degradation after photoconversion. In redroot pigweed seed, inhibition of GA biosynthesis during pregermination incubation resulted in the partial attenuation of VLFR sensitivity. The addition of 75 ppm nitrate to pregermination and germination media enhanced VLFR sensitivity. Our results suggest that development of VLFR sensitivity is due to cooperative action between type I phytochrome and phytochrome-independent GA biosynthesis. In the soil seedbank, this sensitivity may be augmented in elevated soil nitrate environments.
Gallagher et al. (Sun,) studied this question.