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Figure 3.21
Leaves of Simmondsia chinensis (jojoba, Simmondsiaceae), a desert shrub, in Joshua Tree National Park, in the Mojave Desert. The thick, leathery leaves are held at an upright angle to reduce radiant heat gain. (Photograph courtesy of P. Curtis.)
Some cacti have dense coverings of highly reflective spines; while these structures may also act to deter herbivores, their primary benefit is reducing radiant heat input. Encelia farinosa (brittlebush, Asteraceae) is a desert shrub that produces two different kinds of leaves, depending on the season. In summer, the plant puts out densely pubescent and highly reflective leaves. In spring, when temperatures are cooler and water is more available, it produces green, nonpubescent leaves, which absorb much more shortwave radiation and consequently have higher photosynthetic rates than the reflective summer leaves (Figure 3.22). Other plants also produce different types of leaves in different seasons, each of which has morphological or physiological adaptations for functioning optimally in its own season.
A common adaptation of many plants is to roll their leaves into narrow cylinders when facing water stress. This change serves to modify the leaf energy balance in several ways, minimizing both boundary layer resistance and radiant heat input. Also, rolling up the leaf directly reduces total water loss by reducing the amount of leaf surface exposed to the dry surrounding air. There are also many whole-plant adaptations for managing water and energy losses, including partial or total shedding of leaves in dry or cold seasons.
Water Relations and Energy Balance 61
Summary
Plants are descended from organisms that lived in the oceans. Their ability to live on land has required the evolution of anatomical, physiological, and other adaptations. The ability of a plant to obtain water, and the movement of water throughout the plant, depends on the water potential of plant tissues and of the surrounding soil and atmosphere. Water potential consists of various components, of which osmotic potential and pressure potential are generally most important within the plant. In any system, water tends to move from components with less negative water potential to components with more negative water potential, so we can predict the direction of water movement if the water potential of various components is known.
Plants lose much more water in transpiration than they use metabolically. Plant strategies for taking up and coping with the loss of water differ greatly depending on the species and the environment to which it is adapted. Mesophytes, aquatic plants, hygrophytes, xerophytes, halophytes, phreatophytes, and desert ephemerals all use different strategies to obtain water and restrict its loss.
Figure 3.22
Variation in the appearance of leaves of Encelia farinosa (brittlebush, Asteraceae). The leaves in the center of the plant are being produced as the environment is becoming warmer and drier; those surrounding it were produced earlier, when conditions were cooler and moister. The central leaves are thicker, smaller, and covered with a dense white pubescence (hairs) that reflects sunlight; those toward the outside are greener because they lack the thick pubescence. (Photograph courtesy of S. Schwinning.)
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There are a rich variety of physiological, morphological, and anatomical adaptations to the typical range of water conditions experienced in an environment.
Plants are continually exchanging energy with their surroundings. The temperature of leaves and other plant organs depends on their energy balance: the difference between the energy taken up and the energy lost. Radiant energy, sensible heat exchange (conduction and convection), and latent heat loss are the major components
Additional Readings
Classic References
Ehleringer, J., O. Björkman and H. A. Mooney. 1976. Leaf pubescence: Effects on absorptance and photosynthesis in a desert shrub. Science 192: 376–377.
Maximov, N. A. 1929. The Plant in Relation to Water. Allen and Unwin, London.
Odening, W. R., B. R. Strain and W. C. Oechel. 1974. The effect of decreasing water potential on net CO2 exchange of intact desert shrubs. Ecology 55: 1086–1094.
Contemporary Research
Borchert, R. 1994. Soil and stem water storage determine phenology and distribution of tropical dry forest trees. Ecology 75: 1437–1449.
of the leaf energy balance and together determine leaf temperature.
Plants are often perceived as passive receptors of environmental conditions. Yet, the long-term adaptations of plant species and the shorter-term functions of individuals modify the actual values they experience for many critical environmental factors affecting them, including temperature, light, and moisture.
Holbrook, N. M. and F. E. Putz. 1996. From epiphyte to tree: Differences in leaf structure and leaf water relations associated with the transition in growth form in eight species of hemiepiphytes. Plant, Cell, Environ. 19: 631–642.
Additional Resources
Campbell, G. S. 1977. An Introduction to Environmental Biophysics. Springer Verlag, New York.
Lambers, H., F. S. Chapin III and T. L. Pons. 1998. Plant Physiological Ecology. Springer-Verlag, New York.
Kramer, P. J. and J. S. Boyer. 1995. Water Relations of Plants and Soils. Academic Press, New York.
Nobel, P. S. 1983. Biophysical Plant Physiology and Ecology. W. H. Freeman, New York.