Measuring ultraviolet radiation exposure of individual grape bunches

Measuring ultraviolet radiation exposure of individual grape bunches

In recent years many researchers have considered the influence of light and heat loads on wine grape quality. Such studies revealed negative influences of excessive bunch sunlight exposure and high fruit zone temperatures, impacting on colour development, accumulation of secondary metabolites and skin integrity, and in severe cases and causing berry sunburn. Berry ultraviolet radiation (especially UVB) exposure in moderate doses may have positive impacts, for example enhancing the colour development and accumulation of flavonols.

Sophisticated equipment is available for measurement of temperature and visible light in very small spaces; however
measurement of UV radiation has until now required cumbersome light meters. To measure UVB exposure we utilised
small films of polysulphone, sensitive to wavelengths shorter than 330 nm. The 3 cm x 3cm films were applied to
Cabernet Sauvignon bunches in a VSP canopy from véraison to harvest over the 2010 season in Queensland, Australia.
Bunches were situated in the shaded canopy interior, the west facing side and the east facing side of a N-S running row, and films positioned at the bunch top, bottom and north, east, south and west sides.

We found differences in UV radiation exposure on the six faces of the bunch, the side exposed to the sun obviously
receiving the highest proportion of UV radiation. Interestingly the northerly faces of exposed bunches had greater UV exposure than the southerly faces; likely due to the more northerly orientation of the sun in the Southern Hemisphere sky.

While shaded bunches had lower overall UV exposure, their UV exposure was greater at the base of the bunch, possibly
due to albedo reflection from the cordon. It is unlikely that the vineyard floor impacted on UV exposure of the bottom of bunches, since the floor management comprised a mid row sward and undervine wood chip mulch.

The findings of this project clearly illustrated the differences in the amount of UV radiation impacting upon different parts of the bunch, and indicate that dosimetry has valid application for viticultural research. Future work on this project will employ dosimetry to explore the impacts on the UV environment of the bunch of vineyard factors such as trellis type and row orientation, canopy management and vineyard floor management, and to further investigate relationships between UV radiation exposure and fruit quality.