The photo-inhibition of Lycopersicon esculentum Mill, hypocotyl growth induced by UV radiation may be mediated by both phytochrome and UV-absorbing receptors. The inhibition of growth induced by continuous irradiation with high fluence rate UV radiation is similar in the au mutant, which is severely deficient in spectrophoto metrically and immunochemically detectable phytochrome, and in the isogenic wild type. Parallel irradiation with 692 nm light, which is equivalent to UV radiation for the phytochrome system in our experimental conditions, induced at high photon fluence rates a significant increase in hypocotyl growth in the au mutant. The same light treatments inhibited the hypocotyl growth of the wild type. The responses of water-grown seedlings and chlorophyll-free seedlings (streptomycin and norflurazon treated seedlings) were compared. Water-grown and chlorophyll-free seedlings responded similarly to UV radiation. The presence of chlorophyll correlates with a significant increase in hypocotyl growth of au mutants irradiated with 692 nm light. These results support the conclusion that UV-induced inhibition of growth in the au mutant is independent of phytochrome.
Photoinhibition of hypocotyl growth in white cabbage (Brassica oleracea L., cv "Bianco Brunswick") is controlled by UV absorbing receptor(s) and the phytochrome system, while in red cabbage (cv "Rosso Olandese tardivo invemale") phytochrome can act without any requirement for the action of a specific UV receptor. Similar results have been obtained for the photoregulation of anthocyanin production. Twenty-four hour preirradiations with UV light or 692 nanometers light lead to the same increase in responsiveness of the system toward Pfr in a following dark period, suggesting a phytochrome promotion of subsequent light induction for both.A considerable number of studies have been reported on the effects of UV radiation on higher plants (2,3,10,23). Nevertheless, the nature and the mode of interaction between the photoreceptors involved in the control of the photomorphogenic responses induced by UV irradiation, is still an open question. UV may induce both negative (damaging) and positive (nondamaging) effects in higher plants, and the same response can be differently affected (positively or negatively) depending on the fluence rate (1 1). UV light is absorbed by phytochrome, the red-far red reversible photoreceptor of plants, causing photoconversion of Pr to Pfr and vice versa, both in vitro (see 13 for the relevant literature) and in vivo (15). A specific blue-UV receptor known as cryptochrome, with one of its peaks at about 370 nm has been characterized by action spectroscopy (16). In addition, a little studied UV-
The photo-inhibition of Lycopersicon esculentum Mill, hypocotyl growth induced by UV radiation may be mediated by both phytochrome and UV-absorbing receptors. The inhibition of growth induced by continuous irradiation with high fluence rate UV radiation is similar in the au mutant, which is severely deficient in spectrophoto metrically and immunochemically detectable phytochrome, and in the isogenic wild type. Parallel irradiation with 692 nm light, which is equivalent to UV radiation for the phytochrome system in our experimental conditions, induced at high photon fluence rates a significant increase in hypocotyl growth in the au mutant. The same light treatments inhibited the hypocotyl growth of the wild type. The responses of water-grown seedlings and chlorophyll-free seedlings (streptomycin and norflurazon treated seedlings) were compared. Water-grown and chlorophyll-free seedlings responded similarly to UV radiation. The presence of chlorophyll correlates with a significant increase in hypocotyl growth of au mutants irradiated with 692 nm light. These results support the conclusion that UV-induced inhibition of growth in the au mutant is independent of phytochrome.
Phenylalanine ammonia-lyase (PAL; EC 4.3.1.5.) induction in cotyledons from 96-h dark-grown Lycopersicon esculentum Mill. was studied in response to continuous light and hourly light pulses (blue, red, far red). The increases of PAL promoted by blue and red pulses are reversed completely by immediately following 758 nm irradiations. The response to continuous red light could be substituted for by hourly 6-min red light pulses. The effect of continuous red treatments is mainly due to a multiple induction effect of phytochrome. In contrast to red light, hourly light pulses with far red and blue, light can only partially substitute for continuous irradiation. The continuous blue response could be due to a combination of a multiple induction response and of a high irradiance response of phytochrome. The continuous far red response, could represent a high irradiance response of phytochrome. Dichromatic irradiations indicate that phytochrome is the photoreceptor controlling the light response (PAL) in tomato seedlings.
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