Carbon isotope labeling to detect source-sink relationships in grapevines upon drought stress and re-watering.

Kinetics of carbon allocation in the different plant sinks (root-shoot-fruit) competing in drought stressed and rehydrated grapevines have been investigated. A plant growth chamber for stable isotope labeling has been set in an environmental control system, basing on pulse-chasing isotopic strategy to trace carbon phloem flows on potted grapevines. In addition, an open-air plant/soil growth system consisting in twelve independent plant/pot balloons with computing-adjustable air flows allowing continuous gas exchange detection between plants / soil and atmosphere has been set. Water stress caused a drastic decrease in the photosynthesis rate and a decrease in the respiration rate of the soil by about 50%; after rehydration the plants fully recovered the photosynthetic capacity in the morning, while the photosynthetic capacity in the afternoon remained compromised. Sugar accumulation in berries decreased in plants subjected to continuous stress, while the acidity was higher for both plants subjected to continuous stress and rehydrated plants. Grape production was lower in plants subjected to continuous stress. Plants under water stress had a low and constant microbial biomass throughout the season, whereas irrigated and rehydrated plants remained similar in the first days of the experiment, and an explosion of microbial biomass was recorded in plants rehydrated 15 days after rehydration. This may indicate a higher contribution of carbon allocated by the rehydrated plant to the microbial mass of the rhizosphere. Water stress causes a greater diversion of the newly photosynthesized carbonaceous resources to the berry (about double compared to irrigation controls). The carbon accumulated in the berry is stored in a stable manner. The carbon diverted to the root over 30 days is mostly consumed. The plant in recovery diverts the same percentage of carbon marked to the berry of the plants in water stress although in absolute its photosynthesis is about double than under water stress (it is comparable or even higher than photosynthesis un irrigated control plants); therefore the total C sent to the berry is greater in recovery than in irrigation control. Through a daily respired / photosynthesized C balance we show that during the ripening of the berry 60% of the C assimilated in the irrigated condition is respired. Since the accumulation of neo-photosynthetate is stable at 27%, this amount does not affect the reserves accumulated in the pre-veraison root. In the condition of water stress 92% of the daily C balance is respired. Since 42% of neo-photosynthesized C is stored in a stable manner, we can conclude that the plant affects carbon root reserves accumulated to support the respiration rate. It is mostly the neo-photosynthetate that flows to the berry while respiration is guaranteed by the subtraction of previous reserves. Delivery of labeled carbon in different sinks is discussed in parallel with the expression of genes involved in carbohydrate transport.