Wild-type Arabidopsis L. leaves exposed to low ultraviolet-B (UVB) conditio
ns contained predominantly kaempferol glycosides, with low levels of querce
tin glycosides. The flavonoid level doubled on treatment with UVB and an in
crease in the ratio of quercetin: kaempferol was observed. These results su
ggest that flavonols protect Arabidopsis plants from UVB damage, and indica
te that the flavonoid 3'-hydroxylase (F3'H) enzyme, which converts dihydrok
aempferol to dihydroquercetin, may play a crucial role. The tt7 mutant lack
s this gene and, after treatment with sub-ambient U-VB, contained kaempfero
l glycosides exclusively, to a level of total flavonols similar to that in
wild-type Arabidopsis. Total flavonols after enhanced UVB treatment were hi
gher in tt7 than in similarly treated wild-type plants, and only kaempferol
glycosides were detected. Despite this high level, tt7 plants were less to
lerant of UVB radiation than wild-type plants. These observations suggests
that kaempferol is a less effective photoprotectant than quercetin. The cha
lcone isomerase (CHI) mutant (tt5) surprisingly did not accumulate naringen
in chalcone, and this suggests that the mutation may not be restricted to t
he CHI gene alone. The concentration of hydroxycinnamic. acid derivatives d
id not change with UVB treatment in most varieties indicating that their ro
le in UV photoprotection may be subordinate to that of the flavonoids.