Down-regulation of photosynthetic activity for field-grown grapevines

The adaptive responses of 'Sangiovese' and 'Montepulciano' grapevines to dry soil conditions were quantified in a study conducted in central Italy during the summer of 2003. 'Sangiovese' exhibited low stomatal sensitivity to dry conditions, maintaining higher photosynthetic activity and intrinsic water use efficiency. Changes in leaf inclination, lower leaf absorptance (due to lower chlorophyll and carotenoid contents) and greater leaf transmittance (due to less lamina thickness) by 'Sangiovese' may have helped in avoiding excessive light absorption. However, basal leaves of 'Sangiovese' grapevines exposed to direct sunlight and high temperatures and vapor pressure deficits displayed irreversible photoinhibition and chlorosis followed by necrosis. The Fv/Fm values measured on these leaves indicated clear damage to the PSII reaction center. This did not occur on younger leaves or those located in the shade under the same soil and environmental conditions. Basal leaves of 'Montepulciano' exposed to direct sunlight promptly closed their stomata, reduced photosynthetic activity and saved water during the hottest portion of the day. There was a slight decrease in Fv/Fm followed by complete recovering during the night. No photoinhibition was observed on a cloudy day for either cultivar. Results from this field study indicate that grapevine strategies in response to multiple and severe stress conditions were a function of genotype, leaf age and position along the shoot and within the canopy.