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8.8 Environmental Stresses Change the Relationship
Between Photosynthesis and Growth
I.S. Pereira
A great deal of research has been devoted to the study of the effects of
environmental factors on plant growth and metabolism. The traditional
definition of environmental stress considers that stress conditions occur
whenever growth rate, survival, or gas exchange are lower than maximum.
It is virtually impossible to quantify these conditions. For example, in
normal circumstances plants regulate their metabolism according to the
resource or other limitations imposed, e.g., rates will slow down at temperatures lower than certain optimal values. On the other hand, different
processes will respond differently to environmental factors. Nevertheless, I
will use the term stress in this broad qualitative meaning. In terms of plant
response to the environment three aspects are of importance: short-term
responses, acclimation to new environmental conditions, and survival. Each
of these implies very different metabolic and regulatory responses and the
first two are most relevant for this study.
The econometric analogy sometimes used to study plant responses to
what we may define as "resources" (water, nutrients) has some limitations.
It often implies the assumption of optimization of resource use in relation to
some "return" that may be growth, survival, or reproduction. It is very
difficult to define goals at organism level, whereas at the ecological level the
number or interactions are virtually endless. For similar reasons, the socalled efficiency terms, i.e., the ratios of inputs (e.g., nutrient uptake or
water absorbed) vs. output (e.g., growth) should be interpreted with care.
For example, a greater efficiency in resource use may not increase the fitness
of the organism towards its environment. For example, in many cases ecological fitness may be more related with the efficacy of resource collection
than with the efficiency of its use, especially when competition is involved,
as argued for water use by Passioura (1982; see also Jones 1980).
We need to understand better the responses of physiological processes to
environmental factors (e.g., foliage growth and how the whole organism
integrates signals from the environment, or respiration) without being
biased by a priori concepts about how the plants use resources. Presumably
the most important response of plants when resources are limited is to slow
down growth and adjust size to resources. Still, plants may not respond to
resource availability by itself but to signals (hormonal or physical) that
trigger responses in organs away from the sites where resource availability
is sensed. The following sections will consider how water or nitrogen abundance and temperature affect growth and gas exchange. Most attention
will be given to situations where growth limitation occurs without major
deficiencies or tissue damage. Although of great importance, the interaction
between factors, such as between temperature and water or nutrients in the
soil, will be ignored.
8.8 Environmental Stresses Change the Relationship
Between Photosynthesis and Growth
I.S. Pereira
A great deal of research has been devoted to the study of the effects of
environmental factors on plant growth and metabolism. The traditional
definition of environmental stress considers that stress conditions occur
whenever growth rate, survival, or gas exchange are lower than maximum.
It is virtually impossible to quantify these conditions. For example, in
normal circumstances plants regulate their metabolism according to the
resource or other limitations imposed, e.g., rates will slow down at temperatures lower than certain optimal values. On the other hand, different
processes will respond differently to environmental factors. Nevertheless, I
will use the term stress in this broad qualitative meaning. In terms of plant
response to the environment three aspects are of importance: short-term
responses, acclimation to new environmental conditions, and survival. Each
of these implies very different metabolic and regulatory responses and the
first two are most relevant for this study.
The econometric analogy sometimes used to study plant responses to
what we may define as "resources" (water, nutrients) has some limitations.
It often implies the assumption of optimization of resource use in relation to
some "return" that may be growth, survival, or reproduction. It is very
difficult to define goals at organism level, whereas at the ecological level the
number or interactions are virtually endless. For similar reasons, the socalled efficiency terms, i.e., the ratios of inputs (e.g., nutrient uptake or
water absorbed) vs. output (e.g., growth) should be interpreted with care.
For example, a greater efficiency in resource use may not increase the fitness
of the organism towards its environment. For example, in many cases ecological fitness may be more related with the efficacy of resource collection
than with the efficiency of its use, especially when competition is involved,
as argued for water use by Passioura (1982; see also Jones 1980).
We need to understand better the responses of physiological processes to
environmental factors (e.g., foliage growth and how the whole organism
integrates signals from the environment, or respiration) without being
biased by a priori concepts about how the plants use resources. Presumably
the most important response of plants when resources are limited is to slow
down growth and adjust size to resources. Still, plants may not respond to
resource availability by itself but to signals (hormonal or physical) that
trigger responses in organs away from the sites where resource availability
is sensed. The following sections will consider how water or nitrogen abundance and temperature affect growth and gas exchange. Most attention
will be given to situations where growth limitation occurs without major
deficiencies or tissue damage. Although of great importance, the interaction
between factors, such as between temperature and water or nutrients in the
soil, will be ignored.