Evidence against the involvement of phytochrome in UVB-induced inhibition of stem growth in green tomato plants

The effects of UVB on the kinetics of stem elongation of wild type (WT) and photomorphogenic mutants of tomato were studied by using linear voltage tr ansducers connected to a computer. Twenty-one or twenty-six-day-old plants, grown in 12 h white light (150 mu mol m(-2) s(-1) PAR)/12 h dark cycles, w ere first transferred to 200 mu mol m(-2) s(-1) monochromatic yellow light for 12 h, then irradiated with 0.1 or 4.5 mu mol m(-2) s(-1) UVB for 12 h a nd finally kept in darkness for another 24 h. The measurements of the kinet ics of stem elongation started after 4 h under yellow light. Significant di fferences in stem growth during the irradiation with yellow light, as well as during the dark period, were found between the genotypes. In darkness, t he magnitude of stem growth followed the order: tri > AC = fri > MMau > hp1 . Two factors determined the large differences of growth in darkness: 1) th e different stem elongation rate (SER) and 2) the different duration of the growing phase among the genotypes. In darkness the stem growth of au and h p1 mutants lasted for about 18 h, whereas it continued for the whole experi mental period (36 h) in the other genotypes. UVB irradiation substantially reduced elongation growth of all genotypes (4.5 mu mol m(-2) s(-1) being mo re effective than 0.1 mu mol m(-2) s(-1)). Both fluence rates of UVB induce d a detectable reduction of SER already after 15 min of irradiation. Red li ght inhibited, while far red light promoted stem growth of all the genotype s tested. fri (phyA null), tri (phyB1 null), hp1 (exhibiting exaggerated ph ytochrome responses) mutants and WT tomato showed similar levels of UVB-ind uced inhibition of growth, while the aurea mutant showed the largest growth inhibition during the 12 h of irradiation. These results indicate that phy tochrome is not directly involved in UVB control of stem elongation. The re sults of dichromatic irradiations UVB + red or UVB + far red indicate the p resence of distinct and additive action of UVB photoreceptor and of the phy tochrome system in the photoregulation of stem growth.