Plant morphogenesis is influenced greatly by the irradiance, quality, direction and periodicity of the ambient light. At least three different photomorphogenic photoreceptors have been distinguished: (i) the red light (R)- and far-red light (FR)- absorbing phytochromes; (ii) the UV-A and blue light (B)-absorbing cryptochromes; and (iii) the UV-B photoreceptor. The phytochromes, which are the best characterized photosensory photoreceptors, are encoded by a small multigene family. In tomato (Lycopersicon esculentum Mill.) five phytochrome genes have been cloned: PHYA, PHYB1, PHYB2, PHYE and PHYF. In this thesis a genetic approach is used to assign functions to the different phytochrome types in tomato. Two classes of phytochrome mutants in tomato were analyzed both molecularly and physiologically: (i) phytochrome photoreceptor mutants: f ar- r ed light- i nsensitive (fri) mutants, deficient in phytochrome A (phyA); t emporarily r ed light- i nsensitive (tri) mutants, deficient in phytochrome B1 (phyB1) and a phytochrome chromophore biosynthesis mutant aurea (au); (ii) signal transduction chain mutants: h igh- p igment- 1(hp-1),h igh- p igment- 2(hp-2), a tro v iolacea (atv) and I ntensive p igmentation ( Ip ). In adult plant stages fri mutants are hardly phenotypically distinguishable from wild type (WT) in white light (WL). The phyB1 -deficient tri mutants are only insensitive during the first two days upon transition from darkness to R. The tri mutants are slightly taller than the WT when grown in WL. The kinetics of stem elongation rate of these mutants were determined very precisely using a custom-built plant growth-measuring apparatus as well as their response to vegetational shade light. The immature fruits of hp-1 and hp-2 mutants have higher chlorophyll levels and are darker-green in colour than WT. The signal transduction chain mutants all exhibit exaggerated phytochrome responses, i.e. high anthocyanin synthesis and short hypocotyl length compared to WT. Anthocyanin biosynthesis that accumulated during a 24-h period of different monochromatic irradiations was determined. At 660 nm the fluence rate-response relationships for induction of anthocyanin in WT are complex, showing a low fluence rate response (LFRR) and a fluence rate dependent high irradiance response (HIR), which have been attributed to phyA and phyB 1, respectively. The hp-1 mutant exhibits a strong amplification of both the LFRR and HIR. The atv mutant shows strongest amplification of the HIR component. The Ip mutant exhibits an exaggerated anthocyanin response in B. The results are discussed in relationship to the published work on photomorphogenesis.
