grape physiology section 3 stomata photosynthesis

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

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The Aerial Structure Of The Vine •The cuticle•Protect the leaf and conserve water

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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 

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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 

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The Aerial Structure Of The Vine

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Figure 3.5: Dicotyledon leaf showing tissue arrangement in cross-section 

(Source Weaver, (1976)

The Aerial Structure Of The Vine 

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The Aerial Structure Of The Vine •3.3.2 Stomata •Open and close in response to sunlight

and internal CO2 concentration

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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)       

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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

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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 

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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) 

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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 

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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

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Photosynthesis

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carbon dioxide water sugar oxygen

sunlight

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

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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. 

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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 

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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

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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?

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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

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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

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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

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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

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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 

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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 

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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

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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

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Figure 3.9: Light Absorption 

The Aerial Structure Of The Vine 

Figure 3.10: Angle of light in relation to leaf photosynthesis

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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

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The Aerial Structure Of The Vine

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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

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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

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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 

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The Aerial Structure Of The Vine

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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 

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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

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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

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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 

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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

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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 

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The Aerial Structure Of The Vine •A light wind helps to keep CO2 levels

balanced as a constant supply is near the stomata

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The Aerial Structure Of The Vine 

Figure 3.13: Effect of Temperature and CO2 on Rate of

Photosynthesis (Adapted from Winkler, et al (1974)) 

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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

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The Aerial Structure Of The Vine 

Figure: 4.12: Effect of Leaf Age on Photosynthesis of Thompson Seedless Vines

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NetPhotosynthesis

Leaf Age – days since unfolding

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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

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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

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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

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