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When thinking of overall vine productivity, I like to look at total sunlight interception (big vines vs. small vines - single curtain vs. double curtain), carbon assimilation (photosynthesis), and carbon partitioning (are photosynthates going into the fruit or into the wood). At this point in the season, the grapevine canopies have filled out so potential light interception is taken care of. Likewise, the crop has set and possibly has been adjusted for through thinning to give a desirable leaf area to fruit ratio that balances the partitioning of photosynthates into ripening fruit and wood. The last piece of the puzzle in vine productivity is carbon assimilation. Of course, the quantity of photosynthetically active radiation (PAR) has the largest influence on leaf photosynthetic rate. As clouds pass in front of the sun, leaf photosynthesis decreases because PAR decreases. Unfortunately, we cannot control clouds so we move on to the other environmental factors that influence carbon assimilation. The factor running second to PAR in influencing carbon assimilation is vine water status. When vines start to run out of water, pores in the leaves that control gas exchange start to close to conserve water, resulting in less total photosynthesis. The tricky thing about grapevine water stress is that it can reduce photosynthesis by 50% or more before we start to notice a difference, such as flagging leaves or a droopy canopy.
So how are the vines doing now? The attached figure shows the rate of leaf photosynthesis of vines from the West Tier and Soil pH experiments at the Fredonia Vineyard Lab taken last Thursday afternoon (8/1/2002). As expected, Concord vines grafted to C3309 rootstock with good weed control showed the best vine water status and highest leaf photosynthesis. Leaf photosynthesis of Ownrooted Concord with good weed control was about 30% lower than the maximum. Ownrooted Concord with weeds in the row centers had the lowest photosynthesis (about 50% lower than the max - ouch!).
What does this mean for vine productivity? On a cloudy day, all of the vines in the West Tier are photosynthesizing about the same because PAR is the limiting factor. However, on sunny days when PAR is not limiting and the vines can work at peak performance, the ownrooted vines are falling short of their potential because they are running into water stress. Under these mild water stress years what we typically see is that fruit ripening in Concord proceeds as normal but wood development is retarded, leading to lower pruning weights (vine size) and lower crop potential in year two.
The second half of the attached figure shows leaf photosynthesis in the soil pH experiment. Vines in a soil pH of 5.5 to 7.5 (with good weed control) have photosynthesis equal to that of ownrooted vines with good weed control in the West Tier. Vines in a soil pH of 4.5 have significantly lower photosynthesis. It is possible that a nutrient deficiency in the '4.5' vines is causing lower leaf photosynthesis. However, it is more likely that the 4.5 soil pH is restricting root growth and causing water stress in those vines.
Think basics for productivity!! Good weed control to conserve soil moisture, good nutrient availability to promote root growth, good crop control to maintain vine balance, and (as always) grow big vines to promote a full canopy and maximize light interception.