PHYTOCHROME-A, PHYTOCHROME-B AND HY4 ARE INVOLVED IN HYPOCOTYL GROWTH-RESPONSES TO NATURAL RADIATION IN ARABIDOPSIS - WEAK DE-ETIOLATION OFTHE PHYA MUTANT UNDER DENSE CANOPIES

The roles of phytochrome A (phyA), phytochrome B (phyB) and a putative blue-light (BL) photoreceptor (HY4) in the control of hypocotyl growt h by natural radiation were investigated using phyA, phyB and hy4 muta nts of Arabidopsis thaliana. Full sunlight inhibited hypocotyl growth to a larger extent in wild-type (WT) than in phyA, phyB and, particula rly, hy4 seedlings, In WT seedlings, hypocotyl growth was promoted by selectively lowering BL irradiance, lowering red-light (R) plus far-re d-light (FR) irradiance or lowering the R/FR ratio (which was achieved either by increasing FR or by reducing R). The effects of lowering BL were reduced in hy4 and exaggerated in phyA seedlings, The effects of lowering R+FR were reduced in phyA and exaggerated in hy4 seedlings, Neither phyB nor hy4 mutants responded to low R/FR ratios, Neighbourin g plants reflecting FR without shading caused subtle reductions of the R/FR ratio, This signal promoted hypocotyl growth in WT but not in ph yA, phyB or hy4 seedlings, Intermediate canopy shade produced similar effects in all genotypes, Under deep shade, de-etiolation was severely impaired in phyA seedlings, which died prematurely, Thus, the FR 'hig h-irradiance reaction' mediated by phyA could be important for seedlin g survival under dense canopies.