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By permission: Copyright 1987 by the American Institute of Biological Sciences
Recent advances are helping to determine the biochemistry and
physiology behind plant performance under natural conditions
During the past few decades plant physiological ecology has expanded
tremendously. This growth has come partly from substantial technological
advances that now make it possible to quantify precisely, under
natural conditions, the microenvironment of plants and plant tissues
as well as their metabolic responses. In addition, accompanying
theoretical developments have provided a conceptual framework
for relating environmental factors to plant mass and energy exchanges.
Such information has been incorporated into simulation and optimization
models of both morphological characteristics (e.g., leaf color,
size, angle) and physiological properties (e.g., photosynthesis,
transpiration, stomatal conductance). Plant physiological ecology
is thus becoming increasingly predictive and is providing management
tools in a number of areas, including forestry and pollution control.
It is also providing a new understanding of community function
and evolutionary development.
To summarize past progress and set priorities for future research
in this field, the National Science Foundation sponsored a symposium
at Asilomar, California, in December 1985. We previously discussed
these priorities (Ehleringer et al. 1986). Here, in a series
of five articles broadly encompassing the field of plant physiological
ecology, we review recent accomplishments. In this introduction
we sketch some of the important events of the past two decades.
Billings (1985) provides a comprehensive review of earlier influences.
In the mid to late 1950s, a number of developments that initiated
the consolidation of modern plant physiological ecology were Monsi
and Saeki's (1953) theoretical work on the light climate within
plant communities; Gaastra's (1959) work on the transport resistances
to the movement of gases in and out of leaves; and Gates' (1962)
and Raschke's (1956) studies of leaf energy balance. These pioneering
studies, each performed in a different nation, provided a quantitative
framework for relating environmental influences to plant metabolism.
Given the physical and physiological input, researchers could
predict exchange rates of carbon dioxide, water, or energy between
a plant and its environment. This energy-balance approach provided,
for example, the means for predicting transpirational water loss
of leaves. Scientists could answer such questions as: "If
a leaf were a different size and shape, what would be its temperature
and rate of water loss under given environmental conditions?"
These studies also laid the foundation for the plant growth models
of the late 1960s (Brouwer and de Wit 1969). Developed initially
for crops, and soon thereafter extended to natural communities
(Miller and Tieszen 1972), these models integrated environmental
conditions and plant metabolism to allow researchers to predict
biomass accumulation rates under various scenarios including,
for example, elevated CO2 concentrations. An important conceptual
advance in growth modeling was the theory estimating biomass maintenance
and conversion efficiencies from tissue analysis (Penning de Vries
et al. 1974, Penning de Vries 1975). More recently, researchers
have developed photosynthesis models based on the underlying biochemical
reactions (Farquhar et al. 1980), and optimization theory models
to explain stomatal behavior (Cowan and Farquhar 1977).
Stimulating and interacting with these theoretical advances was
the development of instrumentation to measure accurately, under
field conditions, plant microenvironment and metabolism. Probably
most important was the availability in the 1950s of portable infrared
gas analyzers for measuring photosynthesis (Bosian 1960) and pressure
chambers for measuring plant water potential (Scholander et al.
1964). Instrumentation and conceptual advances in microclimatology,
stimulated by Geiger's (1957) masterful synthesis, were equally
important.
Along with new tools and theories, the unique working philosophy
that now characterizes research in this field developed during
the 1960s and 1970s. This philosophy brought a vertical integration
to the study of plant adaptive traits by leading investigators
to seek the biochemical and physiological mechanisms underlying
adaptive features and to demonstrate the relevance of these mechanisms
to performance under natural conditions. This powerful approach
is best illustrated by the studies of Björkman and coworkers
(1972a) on sun and shade leaves and on C3 and C4 metabolism (Björkman
et al. 1970).
An additional important dimension was the incorporation of a
strong evolutionary approach, which stemmed in part from studies
on species evolution (Clausen et al. 1940). These studies led
to important comparisons of the physiological behavior of ecotypes,
or closely related species, from contrasting environments. The
increasingly popular tools of cost-analysis and optimization also
have their basis in evolutionary theory.
The recent development of plant physiological ecology thus has
multiple roots. The Germans have contributed almost continuously
since Schimper (1898), particularly in analyzing the physiological
basis for plant distribution (Lang 1957, Stocker 1935, Walter
1964). The English laid the foundations for examining soil-plant
interrelationships in natural environments (Rorison 1969) and
for rigorous microclimatic analysis (Monteith 1973). The Scandinavians
pioneered studies of plants' carbon economy (Boysen-Jensen 1932)
and ecotypic differentiation (Turesson 1922). The French have
contributed heavily to the development of instrumentation (Eckardt
1966), and the Australians to our understanding of plant-water
relations (Slatyer 1967), US scientists, following the early lead
of the Carnegie Institution desert group (Billings 1980), examined
adaptive traits in a variety of habitats, initially severe ones
such as deserts and tundra.
Certain study sites and research programs have played particularly
important roles. Austrian timberline studies of Tranquillini
(1957); tundra studies of Billings and students (Billings 1973);
and desert studies in Death Valley, California (Björkman
et al. 1972b), and Avdat, Israel (Lange et al. 1969). These studies
demonstrated that precise physiological and microenvironmental
measurements could be made, even under adverse conditions, on
plants growing in their natural environments. The results of
these measurements in turn provided the basis for meaningful experiments
in controlled-environment growth facilities.
As a result of these multiple approaches, several major but closely
interrelated research focuses have emerged. Physiological ecologists
have long studied how plants acquire carbon, water, and nutrients.
Advances in the biochemistry of leaf CO2 exchange are now allowing
detailed understanding of the metabolic limitations to photosynthesis
and how these interact with environmental limitations (see Chapin
et al., page 49, and Pearcy et al, page 21, this issue). At the
same time, researchers are increasingly aware that the investments
of acquired carbon and nutrients in new structure and the losses
due to respiration and herbivores are also critical to plant performance
in natural environments. In addition, plant architecture influences
the capture of light aboveground (see Pearcy et al., page 21,
this issue) and water and nutrients belowground (see Chapin et
al., page 49, and Schulze et al., page 30, this issue).
One new focus in physiological ecology concerns the interactions
of multiple resource limitations and stresses. Although earlier
research usually concentrated on single factors, field studies
made it clear that plants are often subjected to multiple limitations
and stresses and that studies of their interactions provide new
insights. Chapin et al. (page 49, this issue) discuss these interactions
and their consequences for efficient resource use and maximal
plant performance. Studies of the relationship between water
loss and carbon gain have been stimulated by new techniques measuring
carbon isotope ratios (see Schulze et al., page 30, this issue).
These techniques promise novel approaches to understanding the
nature and significance of efficient water use by plants in natural
environments. Osmond et al. (page 38, this issue) consider the
interaction of stresses, such as high light, temperature and drought,
that often occur together. Studies of these interactions may
reveal the underlying mechansims of stress damage and tolerance,
as well as indicate how genetic manipulation could lead to improved
crop or forest productivity.
Analysis of energetic costs has gained considerable momentum
in plant physiological ecology in recent years. These studies
focusing on various compounds and structures have made possible
cost-benefit analyses of the production of any plant trait --
such as herbivore protection and leaf longevity (see Bazzaz et
al., page 58, this issue). These approaches are particularly
important because they focus on whole-plant performance and the
trade-offs among various developmental or allocation patterns.
Such information is crucial in, for example, analyzing the overall
benefit of moving a single trait by genetic manipulation. It
also provides important linkages between physiological ecology
and ecosystem and evolutionary ecology.
H.A. Mooney is a professor in the Department of Biological Sciences,
Stanford University, Stanford, CA 94305. R.W. Pearcy is a professor
in the Department of Botany, University of California, Davis,
CA 95616. J. Ehleringer is a professor in the Department of
Biology, University of Utah, Salt Lake City, UT 84112. 1987
American Institute of Biological Sciences.
Acknowledgments
This report was supported by Grant BSR 84-15520 from the Population
Biology and Physiological Ecology Program at NSF.
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By permission: Copyright 1987 by the American Institute of Biological Sciences
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