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I.S. Pereira
(photosynthetic), only respiratory, or passive (accumulation) compartments.
This helps to explain the great variability in the relationship between growth
and photosynthetic rates on a leaf area basis. In this context the regulation
of photo assimilate partitioning and the formation of new organs (sources or
sinks for assimilation of carbon) are points of major ignorance.
Whenever the rate of assimilation exceeds the rate of incorporation of
carbon in plant structure or its use in respiration, carbon compounds are
stored. The same may happen with macronutrients, namely nitrogen. Some
of these stored compounds may be utilized at times when growth may occur
at rates higher than those that could be supported by current photosynthetic
rates. The lack of phase between growth and carbon assimilation (either on
a daily or on a seasonal basis) is bound to make the relationship between
growth and gas exchange rather complex. Furthermore, active growth may
be occurring in some plant organs at the same time that storage occurs in
other plant parts. The use of adequate simulation models and a better
understanding of the mechanisms determining carbon and nitrogen allocation
and internal cycling are essential tools to improve our understanding of the
relationship with growth.
Environmental factors also modify the relationship between growth and
gas exchange. Growth is normally reduced during acclimation to low
nutrients, to water deficits, or to temperatures different from optimum. This
may occur because of direct effects in metabolic rates (e.g., temperature) or
through the whole-plant growth regulation (water or nitrogen) even without
any negative effect on photosynthetic capacity. The end result of acclimation
is the tendency towards the adjustment of plant size and development rate
to resources available. In any case, carbon balance is negatively influenced
by environmental stresses as a result of changes in plant structure (e.g.,
increase in nonphotosynthetic biomass relative to foliage) and stomatal
closure, decreases in photosynthesis in part of the canopy, or increases in
respiration rates. The changes in the rates of development resulting from the
effects of environmental factors (e.g., leaf aging, flowering) may strongly
modify the relationship between gas exchange and growth.
In recent years, our understanding of the biochemistry and biophysics
of photosynthesis under different environmental conditions has progressed
steadily, accompanied by the development of gas exchange equipment and
chlorophyll a fluorometers usable in the field. However, the recognition of
the importance of canopy expansion and carbon and nitrogen allocation in
determining growth has not been accompanied by a research effort to
understand the controling mechanisms, at least comparable to that which
has been devoted to photosynthesis research.
I.S. Pereira
(photosynthetic), only respiratory, or passive (accumulation) compartments.
This helps to explain the great variability in the relationship between growth
and photosynthetic rates on a leaf area basis. In this context the regulation
of photo assimilate partitioning and the formation of new organs (sources or
sinks for assimilation of carbon) are points of major ignorance.
Whenever the rate of assimilation exceeds the rate of incorporation of
carbon in plant structure or its use in respiration, carbon compounds are
stored. The same may happen with macronutrients, namely nitrogen. Some
of these stored compounds may be utilized at times when growth may occur
at rates higher than those that could be supported by current photosynthetic
rates. The lack of phase between growth and carbon assimilation (either on
a daily or on a seasonal basis) is bound to make the relationship between
growth and gas exchange rather complex. Furthermore, active growth may
be occurring in some plant organs at the same time that storage occurs in
other plant parts. The use of adequate simulation models and a better
understanding of the mechanisms determining carbon and nitrogen allocation
and internal cycling are essential tools to improve our understanding of the
relationship with growth.
Environmental factors also modify the relationship between growth and
gas exchange. Growth is normally reduced during acclimation to low
nutrients, to water deficits, or to temperatures different from optimum. This
may occur because of direct effects in metabolic rates (e.g., temperature) or
through the whole-plant growth regulation (water or nitrogen) even without
any negative effect on photosynthetic capacity. The end result of acclimation
is the tendency towards the adjustment of plant size and development rate
to resources available. In any case, carbon balance is negatively influenced
by environmental stresses as a result of changes in plant structure (e.g.,
increase in nonphotosynthetic biomass relative to foliage) and stomatal
closure, decreases in photosynthesis in part of the canopy, or increases in
respiration rates. The changes in the rates of development resulting from the
effects of environmental factors (e.g., leaf aging, flowering) may strongly
modify the relationship between gas exchange and growth.
In recent years, our understanding of the biochemistry and biophysics
of photosynthesis under different environmental conditions has progressed
steadily, accompanied by the development of gas exchange equipment and
chlorophyll a fluorometers usable in the field. However, the recognition of
the importance of canopy expansion and carbon and nitrogen allocation in
determining growth has not been accompanied by a research effort to
understand the controling mechanisms, at least comparable to that which
has been devoted to photosynthesis research.