Phytochrome A-mediated inhibition of seed germination in tomato

Abstract.

Far-red light (FR) inhibition of seed germination of tomato (Solanum lycopersicum L.) was studied with the phytochrome (phy)-hypersensitive mutants, hp-1 ^w, hp-1 ^w,fri ¹, a phyA-deficient double mutant, and hp-1 ^w,tri ¹, a phyB1-deficient double mutant. Seeds of all mutants germinated readily in the dark at 25 °C, and the germination was retarded by a single 100-s FR pulse given 1–3 h after sowing. The effect of an FR pulse was red-light reversible in all mutants used. After 24 h where a single FR pulse was no longer effective, prolonged FR exposure or hourly FR pulses suppressed germination in hp-1 ^w and hp-1 ^w,tri ¹, whereas in hp-1 ^w,fri ¹ the suppressive effect of FR was almost absent. The effect of the prolonged FR was greater than that of the hourly 3-min FR pulses having equal photon fluence, and was fluence-rate dependent. Thus we conclude that the germination inhibition by FR in tomato seed consists of a low-fluence response and a high irradiance response (HIR); the latter is controlled by phyA, but not phyB1. This is the first indication of phyA being involved in the HIR of seed germination inhibition.