In the March 2013 issue of Wines & Vines, I looked at how growers could use high-tunnel technology to control environmental factors affecting fruit quality, described how a tunnel works and reported the results of a research project that compared vines grown inside a tunnel and vines outside its protection. My article in the April 2013 issue examined the economic impact of using three-season tunnels and discussed factors that should be considered when constructing a tunnel and how they impact grapegrowing.

This article will address the question of how tunnels can enable grapevines to provide high yields of high-quality fruit. Many in the grape- and wine-producing communities have seen high yields and superior quality as a difficult prospect to execute in the vineyard, while others such as John Gladstones, Richard Smart and Nick Dokoozian have offered hope that we can have both. From the work presented here, the physiology of the grapevine actually sets up conditions whereby the plant takes advantage of the controlled tunnel environment and allows quality fruit to be produced in larger quantities.

The wine industry usually thinks of California’s Mediterranean climate, with its warm days and cool nights, as being ideal for growing grapes. Later in this article we will look at what really happens in the grapevine and see that California’s cool nights are not really an advantage due to relative reaction rates that take place in the vines’ photosynthetic processes. It is possible that a climate with less diurnal variation is preferable for achieving full ripeness in grapes. With less variation between day and night temperatures, the East may be able to produce superior fruit quality at higher yields—especially if other problematic issues can be taken care of by utilizing innovative systems such as high tunnels.

If a plant is going to have the energy to produce fruit, it starts with the leaf. Remember the structures involved for light capture and their ability to capture that energy and efficiently transfer it to the enzymatic motor that drives plant growth. That structure in the leaf builds everything a grape plant needs to produce its final product: grapes.

The impact of irradiance
There has been a considerable amount of research investigating the relationship between irradiance intensity on the leaf and production of carbohydrates. One interesting factor about this relationship has been the discovery that for a whole group of plants known as broad-leafed plants (C3 plants), as opposed to grasses (C4 plants), the photosynthetic rate for C3 plants reaches its maximum production of carbohydrates at about 75% of full sun incident radiation. C4 plants do not have this limitation. Thus, for C3 plants, more radiation does not mean there is more carbohydrate production. Above this irradiance, the electrons that are generated cannot transfer their energy to the site of compound production fast enough to make use of that energy. Much like a solar panel that can’t deliver its energy to the power grid, it “grounds” out and is lost to thermal increases in the leaf’s temperature, and at times the demise of the leaf due to heat.

On a bright sunny day, light in the field falls directly on a leaf, and the light impacts the chloroplasts to generate the electrons for energy production. Leaves not directly exposed to this light can still produce energy, but they lose a vast amount of light’s direct impact on the leaf that is in a direct line from the sun.

Grapes grown in a tunnel that has the correct plastic barrier receive reflected light that is bouncing around the inside of the tunnel. On a clear sunny day I measured the direct incident radiation outside the tunnel at 1 watt per square centimeter. This is a normal energy of noonday sun—perpendicular to the radiometer. Inside the tunnel, 75% of the direct sun measurement was averaged when measuring from perpendicular to the sun to as much as 30o declination from perpendicular. That means that much of the useful light is being scattered in the tunnel and is available to be captured by leaves that are not in line with the sun’s angle. It also means that the leaves that are receiving this light are efficiently using that light and not losing energy by grounding out the electrons captured.

This diffusion of light does not necessarily affect the final efficiency of the leaf. What it does do is make better use of the available photons because it is possible for more leaves of the plant to utilize those photons.

The next part of the photosynthetic process is conversion into energy-producing compounds. The light reaction is essentially instantaneous. The light-independent reaction takes considerably longer to occur because it involves migration of the components from the site of production to the site of use. Each species has its own maximal level of energy conversion. If there is ADP present at the reaction center, it will be energized to ATP and for NADP+ to be converted into NADPH to make glucose.

However, the plant does not have all these important structures “hanging out” exposed to the outside atmosphere. As you can see in Figure 3 at left, the chloroplasts are protected inside the cuticle of the leaf, and the cuticle has openings called stomata (pl. stomates). These are the openings where carbon dioxide (CO₂) and oxygen (O₂) enter and exit the grape leaf. The guard cells regulate the opening of the stomates, and the guard cells are controlled by the water potential of the leaf.

The impact of temperature and water
In hot, dry conditions, the guard cells become progressively more restrictive in their opening to protect the leaf from losing too much water vapor and thus jeopardizing the chloroplasts’ survival. If the stomata are closed, then no CO₂ gets in and the O₂ builds up on the inside of the leaf in the spongy mesophyll. CO₂ consumption changes the pH of the water vapor, and that change has a feedback loop that slows the production of glucose. The backup of this process sends the light reaction into its ground state discharge of the electrons. Thus the chloroplasts are protected from overproduction of their light-capturing function when water becomes a limiting constituent in a leaf’s survival.

This limitation starts happening as the temperature in the vineyard climbs above 85^oF, with photosynthesis completely shut off above about 95^oF. The dryer the air, the higher the evapotranspiration rate in the leaves and the lower the water potential of the leaf; thus, the shutdown can be at lower temperatures in dry climates than in more humid climates.

If, on any given day during the growing season, a particular leaf has a localized water stress, either from being in direct sun or some other more global effect on the plant (such as a lack of water from the soil environment), then that is going to affect the overall efficiency of the plant’s energy production.

Through this knowledge of the leaf functioning during the day, it is reasonable to expect that in plain daylight growing conditions, a grape leaf can have reduced energy production efficiencies in what we have thought is a perfectly good environment: long, warm, sunny days. The effect on grapevines growing in a classic Mediterranean climate is to have carbohydrates produced during early and later times of the day, with significant parts of the middle not performing at a maximal rate. In a climate like the eastern United States, energy production may not operate at the maximal rate for longer times of the day than Mediterranean climates, but with higher humidity, the slightly lesser intensity of the sun should let the net carbohydrate production be at least close, if not slightly ahead, of vineyards in a Mediterranean climate.

The next element in the process is to examine the light-independent reactions. These reactions create the motor that constructs the vine. Whether the sun is out or not, respiration continues as long as there is “juice” in the “battery”. All of these reactions are enzymatically controlled and as such are rate-limited by the local temperature of the reaction sites.

It is common wisdom in grapegrowing circles that warm days and cool nights make the best wines. If in Mediterranean climates you have warm days that may have some rate-limiting carbohydrate production during the day, coupled with cool nights that slow the rate of respiration used in carbohydrate production and other plant physiological developments, then the average rate of development is slowed down for every hour that the plant is at less than optimal temperature for respiratory reactions during the growth season.

In grapegrowing environments in the eastern U.S., the average temperature is lower than California’s Mediterranean climate, but the diurnal fluctuation is much narrower due to higher nighttime temperatures. Growing grapes in a tunnel environment further moderates the highs and the lows of the ambient environment.

In the tunnel, scattered light enters the canopy and gives a larger number of leaves more sunlight throughout the day. When warm nights provide a higher rate of respiration during the growing season, the likely outcome will be to have physiological maturity coincide more closely with carbohydrate production. Thus, vines can reach full maturity at lower carbohydrate production, and the grapes will make wines that are better balanced than what is currently happening in many western Mediterranean growing regions.

In conclusion, from every person who has seen these plants and every person who has tasted the wines produced, these vines are not over-cropped and have not showed signs of stressful growth. The pruning weight seems to be increasing with each year and is in balance with the crop load. The only way a vine can support this amount of production is if the systems are in balance for the grape. Otherwise, flavors and aromas would not be produced, and/or color would not be available. This is new territory to examine, and I will report back when there is more information.

Author’s note: I used many different sources to validate my thoughts and conclusions for the information collected for this article. If anyone wishes to have the citations of the original research that helped me come to my understandings of the results and conclusions presented here, please email me via edit@winesandvines.com, and I will provide that information.