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Grape PhysiologySection 3Stomata Photosynthesis
The Aerial Structure Of The Vine •A leaf requires between 30-40 days to
become fully expanded•The leaves begin to senesce about 4-5
months after unfolding in full sunlight
The Aerial Structure Of The Vine •The pallisade cells consist of one layer of
cells containing many chloroplasts •The spongy mesophyll cells also contain
many chloroplasts and numerous air spaces, to allow gases and water to pass between the cells and the air spaces
The Aerial Structure Of The Vine •Stomata are small openings in the leaves
and are generally found on the underside of the leaf
•Stomata allow gas and water exchange with the atmosphere
The Aerial Structure Of The Vine
Figure 3.5: Dicotyledon leaf showing tissue arrangement in cross-section
(Source Weaver, (1976)
The Aerial Structure Of The Vine •3.3.2 Stomata •Open and close in response to sunlight
and internal CO2 concentration
The Aerial Structure Of The Vine
Figure 3.7: The Mechanism of Stomatal Opening and Closing(Source: Berg L R. (1997) Introductory Botany. Plants, People and the
Environment)
The Aerial Structure Of The Vine How Stomata Function •Consists of two guard cells and associated
cells •Open and close in response to
photosynthesis•Light strikes the leaf, photosynthesis
begins and reduces the CO2 concentration within the leaf cells
The Aerial Structure Of The Vine •Potassium and other solids are
transported into the guard cells•As potassium increases in the guard cells,
water moves into the cells by osmosis•The guard cells then open as the cell
becomes turgid, the cells become curved, creating an opening
The Aerial Structure Of The Vine •Stomata close in response to decreasing
light (ie reduced photosynthesis) and•When water supply is limited (ie reduced
turgidity)
The Aerial Structure Of The Vine •Stomatal closure results in:•an increase in leaf temperature•an increase in leaf O2•a decrease in leaf CO2•Photosynthesis will decrease and
respiration will increase, resulting in a decrease in sugar and carbohydrate production
The Aerial Structure Of The Vine3.4 Photosynthesis
The Aerial Structure Of The VinePhotosynthesis •Photosynthesis is the process whereby
sugars are produced primarily in the leaves of the grapevine
The Aerial Structure Of The Vine 3.4.1 Environmental Factors affecting
Photosynthesis •Photosynthesis is fundamental to the
vines function, therefore the rate of photosynthesis affects the rate of growth and the crop production
•The rate of photosynthesis is dependent on many factors
The Aerial Structure Of The Vine
Sunlight 1. Sunlight Quantity •Photosynthesis will not occur between 1-
5% full sunlight •The stomata are closed preventing CO2
movement into the leaf.
The Aerial Structure Of The Vine •The amount of sunlight received by the
vine depends on many factors including: • latitude•Season•time of day•cloud cover•the depth of the canopy•row orientation
The Aerial Structure Of The Vine•As the light intensity increases the rate of
photosynthesis increases•If environmental conditions are optimal
and the leaf is in full sunlight then the leaf will photosynthesise at the optimum capacity at one third to one half of full sunlight
•An increase in light levels will not increase the rate of photosynthesis in that leaf
The Aerial Structure Of The Vine •The grower can use techniques which
increase the sunlight availability to the vines leaves, and therefore increase the vines photosynthetic capacity, increasing the potential yield
• What are some?
The Aerial Structure Of The Vine 2. Sunlight Quality •Grapevine leaves strongly absorb light •In full sunlight leaves absorb
approximately 80% of full light, transmit 10% to the interior and reflect 10% to the atmosphere
•Therefore a leaf behind the first leaf will only absorb approximately 8% of the original light
The Aerial Structure Of The Vine
•Leaves absorb only part of the colour spectrum of sunlight which is in the visible range from 400-700 nm wavelength
The Aerial Structure Of The Vine •As sunlight passes through the canopy,
the ratio of light changes, with an increase in other wavelengths compared to the visible wavelengths
The Aerial Structure Of The Vine •Red light at 660 nm is absorbed by leaves
whereas far red light at 730 nm is not•As light passes through the canopy the
amount of far red increases compared to red light
The Aerial Structure Of The Vine •This reduces the rate of photosynthesis in
leaves deeper in the canopy •Shaded leaves have a lower light
compensation point than leaves in full sun
The Aerial Structure Of The Vine •The change in red/far red ratio also affect
fruit colour and shoot growth•Therefore, leaves need to be well exposed
to achieve their potential rate of photosynthesis
The Aerial Structure Of The Vine 3. Diffuse light •Diffuse light is light reflected from the
clouds, soil surface, impurities in the air and other objects
•Most light reaching the canopy surface is diffuse light
The Aerial Structure Of The Vine 4. Sunflecks •As the sun moves across the sky and wind
moves the leaves, they become illuminated
•Grapevine leaves are efficient at utilizing sunflecks
•Experiments have shown that only 1% of the leaf needs to be illuminated for the photosynthetic rate to be greater than the compensation point
The Aerial Structure Of The Vine
Figure 3.10: Angle of light in relation to leaf photosynthesis
The Aerial Structure Of The Vine Temperature •Photosynthesis involves reactions
involving enzymes which are temperature responsive
•The rate of photosynthesis at temperatures below 20OC is less than at 25-30OC, the optimum
The Aerial Structure Of The Vine
Figure 3.11: Affect of Temperature on the Rate of Photosynthesis
The Aerial Structure Of The Vine Temperature •However, 25-30OC is not necessarily the
optimum temperature for vine development and growth
•For example, anthocyanin development is greatest at 15-20OC rather than at 25-30OC
The Aerial Structure Of The Vine •Above 30OC, the photosynthetic rate
declines and may almost cease at 45OC •Increased temperatures affect enzymes,
desiccate tissue and close the stomatal pores, all reducing photosynthesis
•The rate of respiration is also increased at higher temperatures, therefore net gain of sugars may be reduced.
•Generally leaves are 0.5-5OC warmer than air temperature
The Aerial Structure Of The Vine •If there is insufficient water for
transpiration then a moisture deficit will result in reduced transpiration increasing the leaf temperature further
The Aerial Structure Of The Vine
Figure 3.12: Affect of Temperature on Photosynthesis
The Aerial Structure Of The Vine Water Availability •Water influences photosynthesis in 2 ways•It is required as a reactant in the
production of sugars•It influences stomatal opening
The Aerial Structure Of The Vine •Therefore water availability does not
impact on the photosynthesis reaction itself.
•The effect of water is greater if evaporative demand exceeds the water supply
The Aerial Structure Of The Vine •The leaves then become moisture
stressed•This will cause the stomata to close, due
to loss of turgor as water moves out of the guard cells
•CO2 levels are then reduced, inside the leaf
The Aerial Structure Of The Vine •If leaf water status remains low then the
leaves will wilt•Stomatal reopening is slowed and
consequently photosynthesis will be reduced
•The leaves may take several days to regain their full photosynthesis potential once sufficient water is available
The Aerial Structure Of The Vine Carbon Dioxide •As the level of carbon dioxide (CO2) increases
the rate of photosynthesis also increases•This has been shown experimentally using
potted vines in controlled environments•CO2 concentration in the atmosphere is now
around 387 ppm, up from about 280 ppm at the start of the industrial revolution. ▫Intergovernmental Panel on Climate Change
predict 400ppm by the end of the century
The Aerial Structure Of The Vine •At high levels of light the availability of
CO2 may limit the rate of photosynthesis
•CO2 will not diffuse into the cells fast enough to replace that used in photosynthesis.
•If stomata close, for example due to water stress then CO2 is also limited
The Aerial Structure Of The Vine •A light wind helps to keep CO2 levels
balanced as a constant supply is near the stomata
The Aerial Structure Of The Vine
Figure 3.13: Effect of Temperature and CO2 on Rate of
Photosynthesis (Adapted from Winkler, et al (1974))
The Aerial Structure Of The Vine 3.3.2 Vine Factors affecting
Photosynthesis •Leaf Age •Young rapidly unfolding leaves require
carbohydrates•Young leaves begin to export
carbohydrates when they reach approximately 30% of full size but, they continue to import carbohydrates until they reach 50-75% of their full size
The Aerial Structure Of The Vine
Figure: 4.12: Effect of Leaf Age on Photosynthesis of Thompson Seedless Vines
NetPhotosynthesis
Leaf Age – days since unfolding
The Aerial Structure Of The VineCrop Load•Sugars produced by photosynthesis are
essential for ripening fruit•Crop load and canopy size can affect the
photosynthesis rate
The Aerial Structure Of The Vine •If leaves are removed to increase light
exposure to fruit, the remaining leaves can increase their photosynthetic rate to compensate, at least partially, for the removed leaves
The Aerial Structure Of The Vine •However, excessive defoliation will
reduce the ability of the vine to produce sufficient sugars to ripen the fruit
•The practice of fruit thinning is used to balance the crop load to ensure sufficient ripening