Current Vine Water Status

(from the 8/13/2002 Lake Erie Regional Grape Program Crop Update)

The Fredonia Lab crew and I took more pressure bomb (mid-day stem potentials) and photosynthesis measurements last Thursday afternoon to record the water status of Concord vines. The attached figure shows the relationship between leaf photosynthesis (micromoles CO2/meter2/sec) and mid-day stem potential (-bars) of Concord vines in the West Tier experiment. All measurements were taken on vines pruned to 100 nodes and fertilized with 50 pounds of Nitrogen.

The figure demonstrates several aspects of vine water relations. First, there is a direct relationship between stem potential and photosynthesis which confirms that the lower photosynthetic rates are because of water stress and not some other factor like mineral nutrition or mildew. Second, the figure shows the effect of management practices on vine water relations. All of the Concord vines grafted to C3309 rootstock are in the upper right of the figure, meaning that they have good vine water status and high photosynthesis. Among the grafted vines, the ones with reduced weed competition through cultivation are the least water stressed. Ownrooted Concord vines tend to be in the lower left of the figure, meaning they are under more water stress with lower photosynthetic rates (25% to 50% lower than the max). Again, vines with weed free row middles have higher photosynthetic rates than ones with heavy weed competition.

Why does photosynthesis drop with increasing water stress?
To understand the relationship between water stress and leaf gas exchange, we have to talk about tiny leaf structures that we cannot see with the naked eye, guard cells and stomatal pores (see attached photo from S. Assman website). Stomatal pores are tiny holes on the underside of your grape leaves where gases like carbon dioxide can enter the plant and where water vapor can leave the plant. Guard cells surround the stomatal pore and regulate the size of the stomatal pore. Under good water conditions, the guard cells are tugid and keep the stomatal pore wide open and maximize the exchange of gas and water vapor between the plant and surrounding atmosphere. When water becomes limiting the guard cells loose turgor and collapse around the stomatal pore. This helps the plant conserve water by decreasing transpiration but also decreases gas exchange and potential photosynthesis.

In the wine industry, researchers are using this relationship as a management tool in regulated deficit irrigation to control canopy growth and berry size for wine quality. In the Concord industry, we are shooting for big berries, high brix, and (in many cases) increased vine size. Therefore, it makes sense to keep vine water status and photosynthesis rates as high as economically possible for Concord grape production.

For more information on guard cell research, visit the research lab web site of Sarah Assmann of Penn State at http://www.bio.psu.edu/People/Faculty/Assmann/Lab/ . Dr. Assmann is one of the leading plant physiologists in the world on the subject of guard cell physiology. This photo showing an open and closed stomatal pore is from her web site.