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Plant Ecophysiology
Plant growth is the net result of several processes. At the ecosystem scale, we can define
Gross Primary Production (GPP) as the rate of carbon fixation per unit time. Carbon
fixation is the formation of reduced carbon compounds (such as sugars) from CO2 by the
photosynthetic process. However, a significant fraction of the carbon fixed by
photosynthesis is metabolized to provide the energy for cellular processes, including but
not limited to photosynthesis. This is primarily via respiration, RP. Net Primary
Production (NPP) is the net gain in fixed carbon by the system in question, where
1.
NPP = GPP – RP
NPP is most simply determined by measuring the gain in biomass of a system over a
period of time.
Plant production is a function of several things, including availability of CO2, water,
light, and essential nutrients. At the plant level, carbon uptake and fixation is referred to
as “assimilation” (A). The key substrates are CO2 and Ribulose-1,5-biposphate (RuBP),
and the key enzyme is Rubisco. Rubisco catalyzes the reaction of CO2 with RuBP
(carboxylation) to produce 3-carbon phosphoglyceric acid (PGA). PGA is futher reduced
to a triose-P compound using ATP and NADPH, and eventually converted to glucose or
other compounds.. But it is the Rubisco-catalyzed carboxylation (“CO2-adding”) reaction
that is most often limiting. In most plants, the carboxylation reactions takes pace in
chloroplasts that are embedded in the mesophyll cells. Rubisco also catalyzes
oxygenation of RuBP, and some fraction of RuBP is oxygenated instead of carboxylated,
which releases CO2. This process is known as “photorespiration”, and is suppressed at
high CO2/O2 ratios. With pCO2 = 350 µatm and pO2 = .21 atm, the ratio of carboxylation
to oxygenation is roughly 4 for many plants. We can think of photorespiration as a “tax”
on net assimilation. This somewhat unfavorable ratio (who wants to pay a 25% tax?)
may be the result of the evolution of Rubisco under a much higher CO2/O2 ratio in the
geological past, where photorespiration would have been very minor.
Photosynthesis as a function of CO2 availability
From a chemical point of view, we can consider limitation of the carboxlytion reaction
either by the availability of CO2 or the availability of RuBP. CO2 supply is limited by
diffusion, which depends primarily on the difference in partial pressures of CO2 between
the atmosphere (pa) and the intercellular space (pi). The rate of CO2 assimilation under
diffusion-limited conditions can be written:
2.
A = gc(pa – pi)/P
where P is the total atmospheric pressure and gc is the leaf conductance for CO2. The leaf
conductance is the sum of several terms – the internal conductance for transport from the
intercellular space to the chloroplast, the stomatal conductance for transport from the
immediate environment into the intercellular space, and the boundary layer conductance,
for transport across the region of “stagnant” air adjacent to the leaf surface. In most cases
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the stomatal conductance, controlled by size and density of stomata, is the most important
term (i.e. has the smallest value), and may have values up to about 1 mol m-2 s-1.
Clearly consumption of CO2 within the leaf maintains pi < pa in order to drive
diffusion. For many studied plants, the rate of CO2 supply appears to be limiting up to
about pi = 25 Pa (pascals), equivalent to about 250 ppmv of CO2. Consequently,
intercellular CO2 levels are not uncommonly near this value – any lower would decrease
the efficiency of carbon assimilation, A, while any higher would reduce the partial
pressure difference (pa – pi) that drives CO2 diffusion.
At higher levels of intercellular CO2 (pi > 25 Pa) assimilation is often limited by the
plant’s ability to regenerate RuBP fast enough. Thus increasing pCO2 to higher levels
has a more limited impact on A. Figure 1 illustrates this relationship.
The compensation point, Γ, is where the rate of CO2 fixation equals the rate of CO2
production by respiration associated with the photosynthetic process. Below this, more
CO2 is produced than consumed, and so A = 0. The compensation point is in the vicinity
of pi = 5 Pa. Above Γ, as pi rises A responds roughly in a linear fashion, indicating that it
is indeed the availability of CO2 that is limiting the rate of assimilation. At around pi =
25 Pa the response of A to increasing CO2 starts to flattens out, as the availability of
RuBP becomes limiting.
1. CO2 assimilation rate (A) as a function of intracellular pCO2 (Pi). At lower pi
assimilation is limited by the availability of CO2 (blue dashed curve). At higher pi
assimilation rate is limited by the plant’s ability to regenerate RuBP (red dotted curve).
The overall response is shown in the solid orange line. These curves are calculated for a
“typical C3 plant”. Values of pi from 20-25 Pa (black bar) are also typical under nonwater stressed conditions. Water stress causes plants to close their stomata, which
reduces the transport efficiency for CO2, and results in lower pi.
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It is interesting to consider the constraints on plant carbon assimilation under conditions
of low atmospheric pCO2 such as during the recent glacial intervals, when pa ≈ 18 Pa
(180 ppmv). In order to drive diffusion of CO2 into the intercellular space, pi must have
been even lower. If the results from modern studies are applicable to glacial-era plants
(and recall, less than 20,000 years has elapsed to allow evolutionary change since the
Last Glacial Maximum – LGM - with low pCO2), we would expect CO2 limitation to
have been widespread.
C4 photosynthesis
We have so far discussed the more common C3 pathway for carbon assimilation.
A variant on the carbon uptake process is the C4 pathway, so named because an enzyme
known as PEP-carboxylase catalyzes the reaction of bicarbonate (HCO3-) to form 4carbon sugars such as malate. This carboxylation takes place in the mesophyll cells, but
the mesophyll cells in C4 plants do not contain Rubisco. The C4 compounds are
transported to bundle sheath cells (in structures known as the Kranz anatomy) that do
contain chloroplasts where they are decarboxylated (i.e. give up a CO2), creating high
pCO2 in the chloroplasts. Consequences of the C4 pathway include a value for the
compensation point, Γ, that is a factor of 10 lower than in C3 (< 0.5 Pa), pi values near 10
Pa, and almost complete suppression of photorespiration. There are several variants of
the C4 pathway, and the most common representative of C4 plants are grasses. These
include sugar cane, bamboo, and corn, among others. Note that not all grasses are C4.
Because C4 plants create an “artificially” high pCO2 for Rubisco in the sheath cells, they
do not need to open their stomata as much as similar C3 plants. Consequently they have
high “water use efficiency”, and are usually found in semi-arid to arid environments
where water limitation is important. We may view the evolution of the C4 pathway as a
response to some combination of falling atmospheric pCO2 and drying climate
conditions. C4 plants appear to have radiated mostly in the last 20 million years, and
become especially widespread in the last 7-8 million years.
Photosynthesis as a function of light availability
This is a surprisingly intricate subject which we will discuss only very briefly.
Carbon assimilation rates depend on light in a similar way to the dependence of A on CO2
(Figure 2). At very low light levels, there is no net assimilation, until the Light
Compensation Point (LCP) is reached. If sufficient CO2 is available, further increases in
irradiance (usually given in units of µmol of quanta m-2 s-1) show a linear relationship
between irradiance and A, with a slope that describes the efficiency of conversion of
absorbed light energy to fixed carbon, or quantum yield. The quantum yield Φ is on the
order of 0.05 moles of CO2 fixed per mole of quanta absorbed. At high light levels, the
response flattens out, as assimilation is limited by the plant’s biochemical ability to
carboxylate RuBP (either from lack of CO2 or RuBP or both).
Interestingly, leaves on the same plant that have developed in the shade have
lower maximum rates of A, and lower LCPs than do leaves that developed in full sun.
This means that they can have net positive A at lower light levels but cannot effectively
harvest high light levels. There are various anatomical and biochemical differences
between shade and sun leaves on the same plant, or leaves on shade-adapted versus sun-
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adapted species that suggest that shade leaves are specialized for maximizing light
absorption, while sun leaves are specialized to minimize the negative effects of too much
radiation (heating, photodegradation).
2. Assimilation curve as a function of light level for a plant leaf. The LCP is the
minimum light level where A exceeds respiration demand. The shape of this curve and
both the maximum Amax and LCP often varies among leaves on the same plant,
depending on whether they developed in full sun or in shade.
Photosynthesis as a function of water availability
Water from precipitation can have several fates. Figure 3 below (from Chapin et al,
2002) illustrates a schematic water balance in a forest.
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Plants need to maintain both a high water content and water flow. The need to open
stomata to admit CO2 from the atmosphere promotes water loss. Water loss from the
stomata is known as transpiration. The air within the stomatal cavity is typically
saturated with water vapor, i.e. at 100% relative humidity (RH). The external air is
almost always undersaturated (RH < 100%, sometimes much less) , so water vapor will
be lost from the leaf to the environment. A fundamental challenge for land plants is to
optimize their CO2 uptake while controlling their water loss.
As a rough rule of thumb, we may consider that for each gram of C fixed, a plant
transpires 1 liter of water. This ratio of 1:1000 is only a “ballpark” figure, as there is
considerable variation between species and environmental conditions. Water use
efficient (WUE) differs significantly among plant types.
Plant type
WUE, mmol mol-1
CAM (include cacti)
4 - 20
C4
(C4 grasses)
4 - 12
C3 woody
(“trees”)
2 - 10
C3 herbaceous (grasses, sedges, non-woody)
2-5
In molar units, 100 – 500 moles of H2O are transpired per mole of C fixed, with C4 and
CAM plants generally having high WUE.
Transpiration serves several functions: it maintains water and solute flow through the
plant, and provides cooling for the leaf surface. Evaporative cooling is one way plants
maintain moderate leaf temperatures. Heat loss due to evaporation may be written
analogously to equation 2;
3.
4
∆H = -L•E
where L is the latent heat of vaporization and E is the rate
of evaporation.
E = gw(ei – ea)/P
where gw is the leaf conductance for water, ei is the vapor
pressure of water in the leaf, ea the vapor pressure of water
in the surrounding air, and P is the total atmospheric
pressure.
Values for (ei – ea) range from a few Pa to around 40 Pa, while gw may vary from around
100 to 1000 mmol m-2 s-1. L has a value of 2450 J g-1 at 20 ˚C. Clearly evaporation can
be important for cooling leaves, but plants are limited in how much water they can afford
to lose in this way. Small leaves (or compound leaves) which lose heat to passing air
more readily than large leaves, and rotating the angle of leaves to minimize irradiance
also minimize leaf temperature buildup.
What forces supply water to plant canopies?
The loss of water from the leaf “pulls” water up from the roots via capillary forces. As
an example, consider what we could accomplish if we used a vacuum pump to pump
water up from below. If we could evacuate a chamber above the ground to zero pressure
and let a tube down to a water source, the maximum pressure difference we could
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generate is 1 bar. (∆P = 1 – 0). Recall that pressure is force per unit area. What height
water column could a pressure drop of 1 bar support? Hydrostatic pressure (the pressure
produced by a column of liquid in under gravitational forces) is:
P = ρgh
where ρ is density, g is gravitational acceleration, and h is the
column height. Solving for h we find:
h = P/(ρg)
For P = 1 bar = 105 Pa (1 Pa is 1 N m-2), ρ = 1000 kg m-3, and g = 9.8 m s-1 we can
support a water column of :
h = (105 N m-2)/(103 kg m-3 • 9.8 m s-2) = 10.2 m
Ten meters is reasonably high, but still lower than many tree canopies. So we need
something other than vacuum “suction” to bring water up trees. Capillary effects are
mostly responsible. The very polar nature of water molecules results in a high value of
surface tension, and produces large forces at the air-water interface, and produce
“negative pressures” in the xylem and in soil water, which drive water toward roots and
up the xylem vessels. Osmotic pressure, the pressure induced when water moves across a
semi-permeable membrane from a region of low solute concentration to one of higher
solute concentration, also is important. These negative pressures are usually expressed in
as potentials, with units of MPa (megapascals, where 1 MPa = 10 bars).
In a leaf, the water potential is a measure of how much upward force is applied to
the xylem water column. During the day, the leaf water potential will drop (i.e. become
more negative) as water is lost to transpiration. Thus the “pull” on stem water will
increase. In a soil, matric potential is a measure of how “hard” a plant must pull to access
water adsorbed onto particle surface and in pore spaces. More negative matric potentials
imply that the remaining water in soils is increasingly tightly bound.
4. Water potential illustrated for an arid scrub system (Chapin et al., 2002).
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Water in soils
Water in soils is held both adsorbed to particle surface and in pore spaces of a range of
sizes. The gravimetric water content is usually determined by weighing a soil before and
after oven-drying at 105 ˚C, and is a measure of the total water content of the soil. The
field capacity is determined by letting a saturated soil drain under the influence of
gravity. Water at field capacity has matric potential of just below that of pure water
(defined to be zero), usually around -0.02 MPa, so at least some of this water is readily
available to plants. Not all of the water at field capacity is available to plants, however.
Some is too tightly bound to surfaces or small pores. It is common to define the
permanent wilting point as -1.5 MPa, or – 15 bars. Water held this tightly is unavailable
to many plants, although some drought tolerant species can access water held several
times more tightly (i.e. down to -6 or -8 MPa). The available water capacity (AWC) is
the water that can be removed between the field capacity and the permanent wilting
point. To measure this, a soil is brought to field capacity, and then placed in a press with
a porous plate. The applied pressure is increased to + 1.5 MPa and the water yield
measured.
Fine-grained soils have higher water contents, primarily because they have high
surface area on their many particles and many small pores. Sandy (coarse-grained) soils
do not hold as much water. The water holding capacity of a soil is often strongly
influenced by its organic matter content. Organic matter usually increases the AWC
significantly.
As long as water potential in soils doesn’t fall too low, transpiration rates are mostly
independent of soil moisture. They are instead mostly regulated by the carbon
assimilation rate. In other words, plants are trying to maximize their carbon assimilation,
and will adjust transpiration accordingly under most circumstances. If soil moisture falls
enough, however, plants will close up stomata and reduce assimilation and transpiration
in order to avoid wilting. If most plants wilt too severely they cannot recover, so it is to
their advantage to limit transpiration under dry conditions.
The strong potential plants can apply to soil water makes them very efficient water
pumps from the soil to the atmosphere. A medium sized tree may transpire tens of liters
of water per hour or more during midday.
References and further reading:
Chapin, F. S. III, P. A. Matson, H. A. Mooney, 2002. Principles of Terrestrial Ecosystem
Ecology, Springer, New York, 436 pp.
Lambert, H., F. S. Chapin III, T. L. Pons, 1998. Plant Physiological Ecology, Springer,
New York, 540 pp.