Environmental Engineering Reference
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Figure 3 . (a) Response spectrum for the inhibition of the elongation of etiolated tomato hypocotyls
by UV radiation (4h exposure to 1.0 µmol m -2 s -1 monochromatic radiation against a background of
63 µmol m -2 s -1 PAR). Redrawn after Ballaré 53 . (b) Time course for the inhibition of elongation and
the formation of UV absorbing pigments in etiolated tomato hypocotyls in response to
3.1 µmol m -2 s -1 UV-B radiation. Control plants received no UV-B radiation. Redrawn after Ballaré
et al 54 .
The possible adaptive significance of a UV-B photomorphogenic response is
unclear. It has been suggested 37 that the UV-B inhibitory effect may be a mechanism by
which plants delay cell division and thereby reduce the potentially damaging effects of
UV-B radiation on their DNA before protective measures such as protective pigment
formation have been fully induced. This hypothesis may be supported by the
observations of Ballaré et al 54 on tomato which showed that the inhibition of hypocotyl
elongation rate in response to UV-B radiation had a lag time which was much shorter
than that for the formation of bulk UV-B absorbing pigments (Fig. 3b).
Faster production of UV-B absorbing compounds than those found in tomato
have been observed, for example in etiolated rye 55 , but comparative growth
measurements were not made. Ballaré et al 54 have suggested that the rapid inhibition of
growth response may delay the emergence of the seedling from the soil until
photoprotective mechanisms are established. However, rapid inhibition of growth can
also be induced by the B, R and FR wavebands and much lower fluence rates of these
wavelengths are required.
5. Stomatal opening
Stomata play an essential role in gas exchange and the regulation of their
aperture is a critical factor in optimising CO2 uptake and water loss. Light plays a
predominant role in controlling stomatal opening and action spectra have demonstrated
peaks of activity in both the B and R wavebands 56 . At least two photoreceptors are
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