Wines & Vines

June 2013 Enology & Viticulture Issue

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WineEast 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. Figure 3 Cuticle Epidermis Palisade mesophyll Air space Vein Stomata Spongy mesophyll 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. 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. Each one of the football-shaped structures in the leaf cell is a chloroplast. They are arranged to capture as much incident radiation as possible. The interstitial space in the leaf is for the gas exchange for CO2 coming in and the excess O2 coming out, as regulated by the stomata. 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 Win es & V i n es JU NE 20 13 77

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