Topic 4 - An introduction to sunlight and the vine Flashcards
(52 cards)
How much can light intensity vary in a grapevine canopy?
Exterior leaves can receive ~2000 μmol·m-2·s-1 PAR, while interior leaves in dense canopies may get less than 10 μmol·m-2·s-1.
Why does light intensity drop so dramatically within grapevine canopies?
Because grapevine leaves absorb most sunlight, transmitting only about 6%, causing rapid decline through leaf layers.
What is the light saturation point for grapevine photosynthesis?
Between 700 and 1000 μmol·m-2·s-1 PAR.
Why is PAR not usually a limiting factor in viticulture?
Because ambient PAR is generally above 700 μmol·m-2·s-1 during most of the growing season.
What is the phytochrome system in grapevines?
A light-sensing mechanism that responds to red (660 nm) and far-red (730 nm) light, affecting growth and berry composition.
How does the red:far red light ratio affect grapevine physiology?
Lower ratios in dense canopies reduce active phytochrome, slowing enzymes involved in ripening.
Which grapevine enzymes are influenced by phytochrome activity?
PEP-carboxylase, malate dehydrogenase, phenylalanine ammonia lyase, and invertase.
What is stomatal conductance?
A measure of stomatal opening, influenced by light, humidity, and CO2 concentration.
When are grapevine stomata fully open?
Around PAR 200 μmol·m-2·s-1, typically late morning to early afternoon.
What is transpiration in grapevines?
Evaporation of water from leaves through stomata, influenced by sunlight, temperature, humidity, and wind.
Why do exterior leaves transpire more than interior leaves?
They receive more sunlight and heat, increasing stomatal opening and evaporation.
What light intensity is required to initiate photosynthesis in grapevines?
At least 30 μmol·m-2·s-1 PAR.
Where does most photosynthesis occur in grapevines?
In exterior canopy leaves exposed to high light levels.
Why do interior leaves in dense canopies contribute little to photosynthesis?
They receive too little light, resulting in low photosynthetic activity or chlorophyll loss.
What is the difference between photorespiration and dark respiration?
Photorespiration requires light and competes with photosynthesis; dark respiration occurs without light and is temperature-dependent.
How does temperature affect dark respiration in grapevines?
It roughly doubles with every 10°C increase, up to the damage threshold.
How does respiration affect malic acid levels in ripening berries?
Malic acid is respired during ripening, leading to a decline in its concentration, especially in warm climates.
Why do exterior leaves experience more water stress during soil water deficit?
They transpire more due to higher light, temperature, and wind exposure.
How does canopy density affect water use in grapevines?
Open canopies use more water and show more water stress symptoms than dense, shaded canopies.
What is an inflorescence primordium in grapevines?
A microscopic tissue in compound buds that develops into a bunch in the following season.
How does light affect inflorescence primordia development?
Higher light intensity increases the number and size of inflorescence primordia, especially on shoots to be pruned.
What influences budburst percentage in grapevines?
The number of nodes retained at pruning—fewer nodes often increase budburst percentage.
How does canopy shading affect budburst?
Shaded canopies in the current season reduce budburst the next season.
What are water shoots and when are they more common?
Shoots from old wood that often bear few or no bunches; common when few nodes are retained on high-capacity vines.
