Update on Vine Water Relations

(from the 9/10/2002 Lake Erie Regional Grape Program Crop Update)

Despite the hefty shot of rain we received on August 24, the warm and dry weather this summer has maintained a certain level of water stress on the Concord vines in the belt. In recent crop updates, I have illustrated the effect of water stress on vine photosynthesis (crop updates from 8/6/02 and 8/13/02) and the interaction of sunlight and water stress on photosynthesis (crop update from 9/3/02). The attached figure shows the interaction of water stress and the time of day on leaf photosynthesis. As with my previous water stress articles, I compare Concord grafted to a deep rooted rootstock (C3309) with own rooted Concord with and without weed control (cultivation). This time, on a cloud free day, I measured leaf photosynthesis from 9AM to 4PM.

The figure shows that most of the treatments start off in the morning with relatively high and similar leaf photosynthesis rates. At night, the vines have a chance to recover from the previous day and become pumped up with water before dawn. The only treatment that does not match the other three is the own rooted vines in sod. Presumably, those vines have been water stressed so much this summer that they have lost the capacity to become fully hydrated in the overnight period. Some researchers prefer to record pre-dawn stem potentials to determine plant water status. Severely water stressed plants will have lower pre-dawn stem potentials than well watered plants.

The figure also shows how quickly and how severely photosynthesis drops as the sun climbs higher in the sky and the vines' demand for water increases. By 11:00 AM, leaf photosynthesis on own rooted vines with weed control drops nearly 50% before leveling off for the remainder of the day. The drop off for grafted vines is less and the drop off for own rooted/weedy vines is more than 50%. By mid-day, all the treatments have settled into a photosynthetic rate that is reflective of their current water status. This is why I prefer to use mid-day photosynthesis and stem potentials to determine vine water status and to make decisions on floor management or irrigation.

What is the effect of drought on grapevines?
During the dry summer of 1999, Bob Pool and Alan Lakso wrote an excellent article on drought stress in vineyards. It appeared in the September issue of the Lake Erie Vineyard Notes and in the August issue of the Finger Lakes Newsletter (go to http://www.cce.cornell.edu/programs/finger-lakes-grape/No8%20082499.pdf to see the article - this is in pdf format and will take a while to download so be patient). I would urge you to re-read the article because it is very applicable to the 2001 season and the measurements that we are taking in the field.

Current Berry Growth Information

(from the 9/10/2002 Lake Erie Regional Grape Program Crop Update)

The attached chart shows the current berry growth information for balanced (20+20) and minimal pruned Concord vines at the Fredonia vineyard lab.

For a good article on Understanding Grape Berry Development by James Kennedy with a really cool picture explaining the berry curve, go to: http://www.practicalvineyard.com/JulyAugust02/julaug02p14.htm.

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