UV Effect On Grapes???

In the latest Decanter, PeterMolnar, grower of ObsidianRidge in LakeCnty, comparing to NapaVlly, made the statement:

The UV light here is 50% higher here than in Napa and it ripens grapes quicker and sets the stage for flavor development at the top of the hillside and tannic structure at the bottom.

First, the 50% difference between Napa & ObsidianRidge strikes me as a little high.
Second, I'd never heard of UV being more efficient than visible wavelength light in ripening grapes. Any botanist types out there who can confirm that??
Tom

I think I have some literature at home on the effect of UV on ripening. If I do it was Australian research. I have no idea about the comment on 50% higher UV. Not even sure if how higher is measured. I assume it is a measure of intensity but could it also be related to the wavelength being measured at the site?

Interesting global perspective on UV from NASA http://visibleearth.nasa.gov/view_set.php?categoryId=2360&p=1 seems to show why if I did read something about UV it would be from the Souther Hemisphere.

Tom,
I've read (from non-technical sources like maybe Oswaldo) that UV is responsible for the development of polyphenolics in the skins of the grapes, which does make some sense since the polyphenolics are an effective UV chromophore that might protect sensitive contents of the grapes.

Mark Lipton

I haven't heard that UV is more effective for ripening, although there may be some element of truth to that. More importently, UV can damage cells so grapes produce carotenoid compounds which have a sunscreen-like effect. In grapes like Riesling, the carotenoids are thought to be precursors to TDN, which is one of the molecules responsible for the 'petrol' aromas in aged wines. See this link for some more details: http://www.lenndevours.com/2010/05/at-t ... verti.html

Hi Tom,

I am a research scientist who does research on plant pigments in leaves. UV light is absorbed by pigments and one of the theories as to why certain trees turn red or purple in the fall is that the pigments responsible for these colors (anthocyanins - the same as in red grapes) are a sun screen that allow the leaves to recycle nutrients (especially Nitrogen) before the sunlight destroys the leaves as the green pigment (chlorophyll) is gradually destroyed. So the pigment development in red grapes could definitely be related to the amount of UV absorbed, although I am not familiar with the grape ripening scientific literature on this subject.

From Climate and Australian Viticulture by J. S. Gladstones

“Described effects of UV-B radiation include the presence (or increase) of phenolic and other UV-absorbing compounds in leaves and grape skins; reduced carotenoids and certain of the amino acids important in fermentation; less chlorophyll, and damage to photosynthetic enzymes, hence reduced growth; and thicker and more waxy leaves, enhancing disease resistance. Some plant compounds affected are important for wine flavour. All these factors influence the optimum sunlight/UV-B exposure of leaves and especially the fruit, and hence optimum canopy management across environments."