NON-DAMAGING AND POSITIVE EFFECTS OF UV RADIATION ON HIGHER PLANTS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 161

6. Cotyledon opening
When a dicotyledonous plant emerges from the soil, it is essential that the
cotyledons open quickly to maximise the absorption of PAR. Although phytochrome is
known to be essential for this de-etiolation process 70 , UV-B radiation also appears to
play a substantial role. Boccalandro et al 71 found that daily irradiation of etiolated
Arabidopsis seedlings with UV-B radiation for 2.5 h followed by a pulse of R light
resulted in substantial cotyledon opening. By contrast, a R pulse, or 2.5h of R light, or
2.5h of UV-B light followed by a far-red (FR) light pulse resulted in only slight
cotyledon opening. This implies a photoresponse controlled by a UV-B absorbing
photoreceptor, the effectiveness of which is determined by phytochrome. The response
to UV-B radiation increased with fluence rate up to 7.58 µmol m -2 s -1 in Boccalandro et
al's 71 study but decreased with higher fluence rates, thereby suggesting an initial
photomorphogenic effect followed by a damage effect at high fluence rates. 7.58 µmol
m -2 s -1 of UV-B radiation is a very high fluence rate for maximal response. However
from their experiments on phytochrome, cryptochrome and DNA repair mutants, the
authors convincingly conclude that cotyledon opening is greatly amplified by a specific
UV-B photoreceptor which is modulated by phytochrome B.
7. Cotyledon curling
Cotyledon curling is an alleged photomorphogenic effect of UV-B radiation.
Wilson and Greenberg 72 have provided evidence that cotyledon curling in Brassica
napus seedlings in response to UV-B radiation is not always a damage response.
8. Tendril curling
UV-B radiation induces coiling in broom sorghum first internodes 73 . The
response correlates well with pyrimidine dimer formation 74 in this species, thereby
indicating that this may be a DNA damage response. Application of jasmonic acid or
methyl jasmonate, which are associated with UV-B radiation effects 4,12 , to Bryonia
dioica plants results in tendril coiling 75 . Brosché and Strid 76 found that relatively low
levels of UV-B radiation (but not UV-A) added to broad-band white light (WL) caused
coiling in pea ( Pisum sativum L.) tendrils. The coiling response, which required
continuous irradiation, occurred within less than 8h in both attached and detached
tendrils and was reversible if the tendrils were returned to WL for 5d. In contrast to the
observations with B. dioica , methyljasmonate applied in the absence of UV-B did not
induce coiling. Although the evidence is not conclusive, it is unlikely that the reaction
of pea was a purely damage response at the low level of UV-B radiation used; it is also
possible, as with anthocyanin and flavonoid production, that different species use
different photoreceptors.
9. Fungal and viral diseases
Laboratory and glasshouse studies have been inconclusive. Four out of ten
studies recorded that added UV-B radiation resulted in lower fungal and viral infection;
however, six of these recorded that additional UV-B radiation resulted in higher