INHIBITION OF HYPOCOTYL ELONGATION BY ULTRAVIOLET-B RADIATION IN DE-ETIOLATING TOMATO SEEDLINGS .1. THE PHOTORECEPTOR

Broad-band UV-B radiation inhibited hypocotyl elongation in etiolated tomato (Lycopersicon esculentum Mill. cv. Alisa Craig) seedlings. This inhibition could be elicited by <3 mu mol m(-2) s(-1) of UV-B radiation provided against a background of white light (> 620 mu mol m(-2) s(-1) between 320 and 800 nm), and was similar in wild-type and phytochrome-I-deficient aurea mutant seedlings. These observations suggest that the effect of UV-B radiation is not mediated by phytochrome. An activity spectrum obtained by delivering 1 mu mol m(-2) s(-1) of monochromatic UV radiation against a white light background (63 mu mol m(-2) s(-1)) showed maximum effectiveness around 300 nm, which suggests that DNA or aromatic residues in proteins are not the chromophores mediating UV-B induced inhibition of elongation. Chemicals that affect the normal (photo)chemistry of flavins and possibly pterins (KI, NaN3 and phenylacetic acid) largely abolished the inhibitory effect of broad-band UV-B radiation when applied to the root zone before irradiation. KI was effective at concentrations < 10(-4) M, which have been shown in vitro to be effective in quenching the tripler excited states of flavins but not fluorescence from pterine or singlet states of flavins. Elimination of blue light or reduction of UV-A, two sources of flavin excitation, promoted hypocotyl elongation, but did not affect the inhibition of elongation exerted by UV-B. KI applied after W-B irradiation had no effect on the inhibition response. Taken together these findings suggest that the chromophore of the photoreceptor system involved in W-B perception by tomato seedlings during de-etiolation may be a flavin.