UVB region : 280-320 nm
( UVA region: 320-400 nm )
DIRECT EFFECTS OF UV-B ON PLANTS
Studies have shown that plants exhibit a tremendousvariability in their sensitivity to UV-B radiation (Musil,1995; Musil et al.,
2002; Zuk-Golaszweska et al.,2003;Ryan et al., 199
. Responses that occur includechanges in leaf secondary chemistry (flavonoid accumu-lation), alterations in leaf anatomy and morphology,reductions in net carbon assimilation capacity (photosyn-thesis) and changes in biomass allocation and growth(Musil, 1996). The direct UV-B action on plants thatresults in changes in form or function of plants appears tooccur more often through altered gene activity rather thannon-specific damage to DNA (Britt, 1997). These includeboth mechanistic damage to the photosynthetic appara-tus and changes in growth and morphology which mayreduce light interception and competitiveness. Directeffects include radiation-induced changes in photosyn-thesis, cell division and other life processes of directimportance to growth and development such as altera-tions in plant hormones or nucleic acids. These effectsare observed after relatively short periods of irradiation,which could be hours or days (Bornman and Sundby-Emmanuelsson, 1995).
FLAVONOIDS BIOSYNTHESIS AND THEIR ROLE INPLANT DNA PROTECTION
Rapid protective response to the damaging effects of UVirradiation is paradigmatic of active defence mechanismsin plants. In many cases, protection is thought to derivefrom the induced accumulation of strongly UV-absorbingflavonoid compounds in the outer tissue layers, pre-ferentially in epidermal layers, which presumably protectsensitive targets from UV-damage (Schulze-Lefert et al.,989). Most higher plants accumulate UV-B absorbing pigments in their leaves particularly phenylpropanoids such as cinnamoyl esters, flavones, flavonols, andanthocyanins esterified with cinnamic acids after irradia-tion with UV-B (Wellmann, 1983). In addition to phenylpropanoids, other important products of the shikimic acidpathway such as furanocoumarins, and polyketides
and terpenoids such as canabinoids,
also accumulate under increased UV-B radiation. Several researchers havefound key enzymes in the biosynthetic pathways of thesecompounds to be specifically induced by UV-B irradiation(Schulze-Lefert et al., 1989; Stapleton, 1992; Middletonand Teramura, 1993; Kootstra, 1994).
(...)This increase in UV-B has been found to cause both
photomorphogenic as well as
genetic changes in plants. Photoreceptors acting through signal transduction pathways are responsible for sensing this ultraviolet radiation. Several components of the photosynthetic apparatus have been found to be affected by UV-B, with
nuclear encoded genes
being more sensitive to UV-B than
chloroplast encoded genes. However, long-term effects of UV-Bradiation in plants are still not well understood, therefore,more research need to be carried out over longer timeperiods to provide definitive answers to questions such as
cumulative effects of UV-B, effects of UV-B at ecosystem level, and interactions of
elevated UV-B with other stress factors. Currently not enough is known about the reasons for the
large UV-B response differences among cultivars observed by a number of researchers (Musil, 1994; Mpoloka, 2001; Chimphango etal.,2007). Ideally it is necessary to understand the gene-tic basis (heritability) of UV-tolerance and sensitivity.Once this is known, an estimate of the possibility of using conventional breeding practices to minimise the potential effects of UV-damage could be made. At present plant breeders have not yet considered UV-sensitivity as a selective factor.
http://www.academia.edu/227934/Effects_of_prolonged_UV-B_exposure_in_plants
. Check here too if you haven't.
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And yes ,deep reds 660 nm do prolong transition to flowering somewhat ...Depends strongly on plant genetics.
