Photophysiology of a Tomato Mutant Deficient in Labile Phytochrome

Summary Photomorphogenetic responses have been studied in a tomato ( Lycopersicon esculentum Mill.) aurea ( au ) mutant which has yellow-green leaves and elongated hypocotyls when grown in white light. Whereas etiolated tissue of the au mutant contains au mutant lacks labile P (IP) and that the P detectable in light-grown plants of the au mutant is light-stable P (sP). The deficiency of lP results in a loss of P control of hypocotyl elongation in etiolated seedlings of the au mutant. In blue light (BL) and UV-A, hypocotyl elongation of both the wild type and au mutant are inhibited, suggesting that under these light sources inhibition of elongation growth does not require the presence of P. The fluence-rate response curve for the au mutant in BL and UV-A is shifted to higher fluence rates and the difference between wild type and au mutant represents a quantitative estimate of the role played by BL and UV-A absorption by l P deficient in the mutant. Two P regulated responses of light-grown plants are shown to be exhibited by the au mutant: the end-of-day far-red light response and curvature of the hypocotyl as a result of covering one cotyledon and irradiating the seedling from above with white light (simulated phototropism). It is proposed that these responses are mediated by the low s P pool present in light-grown plants.