Control of Stomatal Conductance

14.9 Control of Stomatal Conductance

We already mentioned the observation of Wong et al. who found that stomata tend to open or close to maintain a constant internal concentration. Stomata must therefore be sensitive to changes in environ- mental

concentration, opening when it decreases and closing when it increases. Others have observed that stomata also open or close in re- sponse to the vapor deficit of the air

et al. 1971). High vapor

T A BLE 14.1. Values of parameters and constants used in the photosyn- thesis model. The values of b,

are half those given by Collatz et al. (1991) because we assume the surface area of the leaf to be the total surface area, rather than the projected area.

and

Symbol Value

b 0.003 mol intercept, B-B model 0.08 maximum quantum efficiency

300 0.074 Michaelis constant for 300

0.01 8 inhibition constant for m

5.6 slope parameter for B-B model 1.5 0.088 day respiration

100 0.088 Rubisco capacity 340

ambient mole fraction 210

oxygen mole fraction 0.8 leaf absorptivity for PAR

0.98 colimitation factor

2.6 -0.056

specificity ratio 0.95 colimitation factor

Plants and Plant Communities

Temperature ( C )

F I G U R E 14.5. Temperature response of photosynthesis at three PAR levels for ymol

PAR

mol

F I G URE

14.6. Photosynthesis as a function of PAR at three leaf temperatures.

Optimum Leaf Form 243

Concentration

14.7. Photosynthesis as a function of concentration at three light levels.

F IG URE

deficits (low humidities) tend to close stomata. It is not, of course, the external environment that provides the direct response, but the internal environment of the leaf. This, in turn, is controlled by external environ- ment as well as the transpiration and assimilation rate of the leaf. Collatz et al. (1991) were able to combine all of these effects into an empirical model that looks relatively simple:

where and

concentration at the leaf surface, A, is the net assimilation rate, and m and b are constants de- termined from gas exchange studies. While Eq. (14.28)

are the humidity and

simple, it is in reality quite complex because the entire photosynthesis model de- termines the value of A,; the air vapor pressure, leaf temperature, and transpiration rate determine the value of the surface humidity, and the ambient

concentration, assimilation rate, and boundary layer con- ductance determine

Since conductance determines assimilation rate, and assimilation rate determines conductance, Eqs.

and (14.28) (with all of the equations that go into them) must be solved si- multaneously to determine the assimilation of the leaf. Again, this is not something that can be done easily by hand, but can be done with a com- puter. The interesting thing is that, after all of the work of solving these equations, the results are almost identical to those shown in Figs. 14.5 to

14.7. Using the values in Table 14.1, the internal concentration is controlled at 250

Plants and Plant Communities

Optimum Leaf Form

It is clear from Fig. 14.5 that there exists an optimum temperature for photosynthesis which appears to vary with irradiance. The optimum tem- perature varies from species to species and can even depend on the temperature under which the leaf is grown. For the particular leaf rep- resented by Fig. 14.5 the optimum temperature is a little above

C at low irradiance and increases to about

C at high irradiance. From the analysis we did earlier in this chapter, we know that leaf temper- ature is, to some extent, under the control of the plant. Narrow leaves tend to stay closer to air temperature than broad leaves and leaves with high evaporation rates can maintain temperatures well below air temper- ature when vapor deficits are high. Large leaves with high radiation loads and low evaporation rates can reach temperatures considerably above air temperature. With this range of possibilities available, the question arises whether plants evolve leaf shapes and responses to stress which maximize photosynthesis. In environments where water is a limiting resource for production, one could also ask whether plant design or behavior adapts to maximize production per unit water use, or water use efficiency.

We can not know whether plants maximize photosynthesis or water use efficiency, but we can investigate what size and orientation of leaves would give maximum photosynthesis or water use efficiency, and then see

if leaves in that environment have that size or orientation. A few cases appear to be straightforward. The alpine cushion plants, which would be

below the optimum temperature for photosynthesis most of the time were at air temperature, clearly benefit by radiative heating of the leaves. Their growth habit appears to be an adaptation to maximize temperature in the sun. Desert perennials, which maintain leaves throughout the summer with limited water supplies, in environments where air temperature is at or above the photosynthetic optimum, would benefit

minimizing daytime leaf temperature. Their small leaves appear to be an adaptation to keep leaves as close to air temperature as possible without evaporating large amounts of water. A number of these adaptations, and their energetic consequences, have been analyzed by Taylor (1975). At least for extreme environments, species native to those environments appear to evolve leaf shapes that tend to be optimum.

Leaf orientation is another interesting topic for investigation. Leaves of many species droop or roll when they are water stressed. This can reduce the radiation load on the leaf, decreasing leaf temperature and transpiration. On the other hand, leaves of sunflower, peanut, and many other species follow the sun, tending to increase the irradiance of the leaf. One interesting species, in this regard, is the prairie compass plant et al., 1990). This plant grows in hot, dry environments. Leaves grow so that the flat surfaces of leaf blades face east-west to maximize radiation interception in the morning and evening and

it during midday. Vapor deficits are maximum during midday and leaf temperatures are higher than optimum for photosynthesis. The main assimilation times

References 245

for compass grass are during morning and evening hours when vapor deficits are low, so the carbon gain per unit water loss is therefore high. Carbon gain was shown to be similar for all leaf orientations, but water use efficiency is higher for their preferred orientation.

The interactions between plants and their environment can be ex- tremely complex. We have shown that even the behavior of the leaf temperature and transpiration models is not always intuitive or straight- forward. When these are combined with the leaf photosynthesis model, and all of the interactions are in place, the result can be quite complex and difficult to predict. This appears, however, to be a fruitful area for research. The results of the work are not only useful for understanding plant adap- tations to particular environments, but also to design agricultural plants for optimum production in particular environments.

References

Allen, R. G., M. Smith, A. and L. S. Pereira (1994) An update for the definition of reference evapotranspiration.

Bulletin 43: Collatz, C. J., J. T. Ball, C. Grivet, and J. A. Berry (1991) Physiological

and environmental regulation of conductance, photosyn- thesis, and transpiration: a model that includes a laminar boundary layer. Agric. For. Meteorol. 54: 107-136.

Ehleringer, J.R., and H.A. Mooney (1978) Leaf Hairs: Effects on Phys-

Oecol, 37: T. W., H. Zhang, and J. M. Pleasants (1990) Ecophysiological consequences of nonrandom leaf orientation in the prairie compass plant,

iological Activity and Adaptive Value to a Desert

laciniatum. Oecologia 82: 180-1 86. Kelliher, F. M., R. Leuning, M. R. Raupach, and E. D. Schulze (1994) Maximum conductances for evaporation from global vegetation types. Agric. For. Meteorol. 73: 1-16.

Lange, O.L., R. Losch, E.-D. Schulze, and L. Kappen (1971) Responses of stomata to changes in humidity. Planta Monteith, J.L. (1965) Evaporation and Environment.

Symposia of the Society for Experimental Biology, University Press, Cambridge,

Monteith, J.L. (1977) Climate and the Efficiency of crop production in Britain. Phil. Trans. R.

Lond. B.

Norman, J.M. and F. Becker (1995) Terminology in thermal infrared remote sensing of natural surfaces. Agric. For. Meteorol. 77: 166.

Penman,

H. L. (1948) Natural evaporation from open water, bare soil, and grass. Proc. R. Tanner, C. B. and T. R. Sinclair (1983) Efficient water use in crop pro- duction: research or re-search? in Limitations to Efficient Water Use

246 Plants and Plant Communities

in Crop Production. ASA Special Publication. American Society of Agronomy, Madison,

Taylor, S. E. (1975) Optimal leaf form. Perspectives in Biophysi- cal Ecology (D. M. Gates and R. B. Schmerl, eds.) New York:

Taylor, S. E. and 0. J. Sexton (1972) Some implications of leaf tearing in Musaceae. Ecology 53 :

Wong, S.C., I.R.

Stornatal conductance correlates with photosynthetic capacity. Nature,

and G.D.

Problems

14.1. When

is 30" C, air vapor pressure is 1

and wind speed is

2 find the temperature of a 3 cm wide amphistomatous leaf with stomata1 conductance on each side of 0.3 mol

when absorbed radiation is

14.2. Find reference crop evapotranspiration on a clear day with total solar radiation of

= 30" C, = 3.5 and

0.3. Assume the albedo of the reference crop is 0.2.

14.3. Use Eq. (14.16) to compare dry matter production of crops in arid and humid environments A typical value of k for C3 crops is if assimilation is in kg dry mass per m 2 and transpiration is in kg water per m 2 . Assume that the humid and arid locations both have average maximum temperatures of

C, but the average mini- mum temperatures of the humid and arid locations are 20°C and 10" respectively. Using the maximum and minimum temperatures,

estimate the average maximum vapor deficit for each location.

D in Eq. (14.16) is the average daytime vapor deficit, which is around 0.7 times the maximum deficit. Assume

(500 of water is transpired for growing the crop in each location. How much dry matter could be produced in each? For a given invest- ment in water, is it most cost effective to irrigate crops in humid or arid environments?

14.4. How would reducing the wind speed by a factor of two affect leaf temperature compared to doubling the leaf size?

14.5. Under what conditions would you expect to leaf temperature near to air temperature?

14.6. Under what conditions of wind, radiation, air temperature, and hu- midity would you expect the leaf temperature to depart most from the air temperature in a positive direction (leaf hotter than the air) and negative direction (leaf cooler than the air)?

The Light Environment of