Abstract
A 15-min exposure with UV-A light (320–400 nm) significantly increased the extractable activity of phenylalanine ammonia-lyase (PAL) 4 hr after the onset of the light treatment. The effect produced by a 15-min UV-A exposure was completelt reversed by a consecutive brief exposure to far red light. The UV-A- and red-mediated increases in PAL activity appear to be very similar with continuous exposure. Seedlings grown under 5 W/m 2 UV-A supplemented with blue, far red, and white light showed less PAL activity than control seedlings (i.e. exposed to blue, far red, and white light alone). Seedlings grown under 5W/m 2 UV-A supplemented with red light showed the same PAL activity as control seedlings exposed to UV-A and red light alone. It appears that prolonged UV-A exposure does not affect the response to continuous red light, but it is able to decrease significantly PAL activity in the blue and far red part of the spectrum. Furthermore, the positive and the inhibitory effects of prolonged UV-A irradiation demonstrate a different fluence rate requirement. The data indicated that phytochrome and a separate UV-A photoreceptor are involved in the photoregulation of PAL activity in cotyledons of tomato plants.
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