Lecture 11 - Leaf Traits Flashcards
How do leaf shape and size affect resource acquisition and growth?
Recall: Functional markers are heritable plant traits that can be easily observed and measuredat the level of the indiviudal.
Leaf size and shape affect resource acquisition and growth: resources include both CO2 and light.
Resource Acquisition: getting CO2
**Most of the earth’s carbon is dissolved in water or locked up in stone, living, recently or long dead organisms.
**
With so little CO2 in the air, how can plants photosythesize enough to survive and sustain most of the animal kingdom?
Even more impressive: how do plants fix so much carbon without a mechanism for pulling air into the leaves.
In other words, how can plans fix so much with so little and by diffusion alone?
Diffusion of gasses, Fick’s Law and Optimal Stomatal Density
Fick’s law of diffusion
Rate of diffusion= k x A x P/D
Rate of diffusion is the number of molecules diffusing across a barrier per unit time.
K = diffusivity of the diffusing molecule. It is a constant.
P = the difference in the partial pressure of the diffusing molecule across the two sides of the barrier.
A = the total area available for gas exchange. In leaves this is the size of the stomatal pores
D = the thickness of the barrier. In leaves, this is the length of the stomatal pores.
What would happen if stomatal density increased?
Technically, the credit is not all for diffusion. Bulk flow (ie wind) comes to the rescue
**Bulk flow is the movement of a fluid powered by an external motive force. **Wind blowing over the surface of a leaf replenishes the supply of CO2. Therefore, plants don’t solely rely on diffusion.
But a moving fluid (say, air) moving across a solid surface (say, a leaf) is not the same as moving a solid across another solid.
*At the surface where a moving flowing makes contact with the substrate, the **fluid’s speed is zero. **This region of no flow is called the boundary region and it gets thicker the further along the surface the fluid moves. The thicker the region, the more difficult it gets for diffusion because D in the denominator gets bigger.
Now, since rates of diffusion decrease at the square of the distance molecules must travel, this places a constraint on leaf size.
A wind speed of 0.1 m/sec moving across a 5cm long leaf, will have a gradient region 8mm thick when the wind reaches the downstream edge of the leaf. A distance too large for diffusion to be effective.
**
Consequently, evolution favors narrower or deeply lobed leaves, or leaves interrupted with holes. But, of course, this interferes with photosynthesis!**
Now, since rates of diffusion decrease at the square of the distance molecules must travel, this places a constraint on leaf size.
A wind speed of 0.1 m/sec moving across a 5cm long leaf, will have a gradient region 8mm thick when the wind reaches the downstream edge of the leaf. A distance too large for diffusion to be effective.
**
Consequently, evolution favors narrower or deeply lobed leaves, or leaves interrupted with holes. But, of course, this interferes with photosynthesis!**
Diffusion however at high altitudes is not a problem. Diffusion in fact works better in low density air, why?
When the density of a fluid drops, molecules move further after a collision.
Thus, unlike animals, plants do not have a problem with high altitudes.
Thermal, water, cold and wind stress: Leaf morpho-anatomical adaptations
A broad leaf in direct sunlight and windless air can get as much as 20C degrees celsius above the ambient air temperature.
When local air temp reaches the mid-30s, leaf temp may climb to almost 50C. At this temp, proteins begin to denature.
Thermal Stress: Smaller and deeply lobed leaves are also better at dissipating heat
- Leaves at the top of the canopy are geerally smaller and more deeply lobed than leaves in the subcanopy.
- Smaller and more deeply lobbed leaves are more effective at dissipating heat. Heat dissipation is occuring through imporved conductive and convective heat transfer.
- *Conductive heat loss: **heat transfer across two stationary media with a temp gradient.
- Convective heat loss: *heat transfer between two media due to the movement of a fluid. In our case, cool wind blowing over the surafce of a warm leaf.
Heat and water stress: high-heat capacity of water lowers risk of overheating
Xerophytes are plants adapted to arid environments.
To reduce heat stress, xerophytes have thicker leaves to hold more water.
Water has a high capacity to hold heat without increasing in temperature.
Succulents are xerophytes.
Not all xerophytes are succulents. Leaves of non-succulent xerophytes have several layers of epidermal cells that hold a lot of water.
They also feature recessed stomata. The build up of humidity in the crypts reduces transpirational water loss.
Trichome are modified thread-like dermal cells that function to trap humidity within the crypts
How about cold stress?
Not sure how valid this is, but worth entertaining the argument:
Steven Vogl argued the following on cold stress and the curled-up leaves of tulip tree (Rhododendron).
“Leaves curl and drop to a near-vertical orientation. The colder the temperature, the tighter the roll. Air trapped within the curl will become colder than ambient air temperatures. This will make the air inside denser, causing it to sink, pulling the warmer outside air into the tube. This trick resembles an upside-down chimney and allows leaves to stay closer to ambient air temperatures.”
Wind stress
“Estimating the wind force or drag acting on tree crowns is central to understanding both the chronic effects of wind and the calculation of critical wind speed in windthrow prediction models. The classical drag equation is problematic for porous, flexible tree crowns whose frontal area declines as wind speeds increase and branches streamline.”
At 20 m/s, streamlining reduced the frontal area by:
* 54% for redcedar,
* 39% for hemlock
* 36% for lodgepole pine
Leaf fenestration is an adaptation to both heat and wind stress
Three reasons for fenestrated leaves:
- Reduces thermal stress by increasing capacity for conductive heat transfer
- Reduces the amount of drag
- But also… permits sunflecks to reach deeper into the sub-canopy and forest floor
Light, fisrt strike the cuticle.
Travels passed through the epidermal cell layer.
Then enters the mesophyll cells which contain the chloroplasts.
A single mesophyll cell has about 30-40 chlorplasts.
Plant Pigments Drive Photosynthesis
Leaves reject infrared –too little energy to drive photosynthesis. If absorbed, leaf tissue needlessly heats.
Leaves also reject ultra-violet radiation –too much energy; would cause pigments to break apart
Plant pigments only absorb light from the visible light portion of the electromagnetic spectrum.
* Chlorophyll a absorbs: blue light (430 nanometers) and red light (680nm) With only minor bands in between.
* Chlorophyll b and carotenoids are accessory pigments. Absorb blue, yellow, and, to some extent, green light.
* The energy absorbed from carotenoids is passed on to chlorophyll a. The accessory pigments therefore overcome the narrow range of absorption of chlorophyll a and widen the action spectrum of photosynthesis.
Rubisco drops the ball….
- Rubisco introduces carbon dioxide into the Calvin cycle. But when CO2 levels drop, rubisco picks up oxygen, leading to photorespiration (the production of CO2, without producing ATP or sugar).
- Rubisco is the “lazy bones”, “couch- potato” enzyme. Most enzymes catalyze a few thousand reactions per second. Rubisco, only a 2—12. The irony: it is the most abundant protein on the planet.
