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The cup that cheers--a new approach to vintage problem
As I write this in August 2003, many European vine growers are suggesting that the very fine summer will lead to an excellent year for wine--in their terms, one of the best vintages ever. Nevertheless there are concerns in some regions that water deficiency may yet have deleterious effects on final yield and quality. It is thus interesting that Zhen-Ping Wang and colleagues (Montpellier, France and Ningxia, China, pp. 523-528) have devised a very neat experimental system to study in vivo phloem unloading into ripening grapes under well-watered and water-deficient conditions. The basis of the technique is the ?berry cup?, a perspex cup in which an individual grape, still attached to the vine, may be immersed in buffer solution. Prior to immersion, the grape is carefully peeled without damage to the vascular tissue or to the pulp, thus exposing the dorsal vascular bundles. The buffer solution in the cup is sampled at intervals and its sugar content (mainly sucrose and fructose) is determined. It takes about 60 min to purge the sugars already present in the apoplast, after which the kinetics of the unloading process itself may be studied. Initial unloading from the phloem is into the apoplast from where the sugar solution is normally taken up into the vacuoles. The effects of inhibitors suggest that unloading is dependent on respiratory energy and on sugar transporters. Throughout the day, the unloading rate is also related to the rate of photosynthesis. And what about the effects of water deficit? In vines growing under water-stress (-0.5 MPa) phloem unloading rates are certainly slower than in control vines (-0.2 MPa). This is mostly attributable to a reduction in rates of photosynthesis, but in the later hours of daylight there is some evidence for a more direct effect. Perhaps those vine growers should be just a little concerned.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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