8 Gas Exchange and Growth
J.S. Pereira
8.1 Introduction
Plant growth and ecosystem primary productivity are ultimately dependent
on photosynthesis. Although it has long been recognized that it is the
amount rather than the activity of the photosynthetic tissues that determines
plant productivity in most cases (Monteith 1977, 1981; Jarvis and Leverenz
1983; Kriedemann 1986; Osmond 1987), there has been a great interest in
the study of leaf photosynthetic rates during the last two decades because of
the improvements in the instrumentation for gas exchange measurements
and the discovery that the different biochemistry of photosynthesis in C3 and
C4 plants resulted in different growth rates. At the same time, the installation
of reductionist views in ecology and even in crop science led to the assumption that plant growth could be equated with leaf photosynthetic rates. The
recognition that this was often a misinterpretation led to disenchanting
contentions of the type, "photosynthetic rates play no role in crop yield".
Moreover, most of the recent increases in agronomic productivity of cereal
crops resulted largely from increases in the harvest index, sometimes even
accompanied by a decrease in A (Evans 1976). It is obvious that for the
purpose of growth and productivity studies, instantaneous rates of leaf
photosynthesis (A) must be inserted in the adequate time and biological
organization frameworks (Zelitch 1982; Osmond 1987). One solution for
this is the use of simulation models. However, even though there are
satisfactory models for canopy or whole-plant gas exchange (e.g., Norman
and Campbell 1983; Wang and Jarvis 1990), it is still difficult to "translate"
that into seasonal plant growth because of the gaps in our understanding of
the mechanisms controlling carbon allocation (Gifford et a1. 1984).
This chapter will review briefly the plant characteristics determining
growth, namely plant metabolic rates vs. plant morphology and the role of
assimilate partitioning in determining growth kinetics. In plant communities,
light interception by the canopy largely determines the productivity, but the
efficiency of conversion of solar radiation energy to biomass may vary with
species and environmental factors. The influence of different phenologies
and environmental factors on the relation between gas exchange rates and
growth will be addressed as well.
J.S. Pereira
8.1 Introduction
Plant growth and ecosystem primary productivity are ultimately dependent
on photosynthesis. Although it has long been recognized that it is the
amount rather than the activity of the photosynthetic tissues that determines
plant productivity in most cases (Monteith 1977, 1981; Jarvis and Leverenz
1983; Kriedemann 1986; Osmond 1987), there has been a great interest in
the study of leaf photosynthetic rates during the last two decades because of
the improvements in the instrumentation for gas exchange measurements
and the discovery that the different biochemistry of photosynthesis in C3 and
C4 plants resulted in different growth rates. At the same time, the installation
of reductionist views in ecology and even in crop science led to the assumption that plant growth could be equated with leaf photosynthetic rates. The
recognition that this was often a misinterpretation led to disenchanting
contentions of the type, "photosynthetic rates play no role in crop yield".
Moreover, most of the recent increases in agronomic productivity of cereal
crops resulted largely from increases in the harvest index, sometimes even
accompanied by a decrease in A (Evans 1976). It is obvious that for the
purpose of growth and productivity studies, instantaneous rates of leaf
photosynthesis (A) must be inserted in the adequate time and biological
organization frameworks (Zelitch 1982; Osmond 1987). One solution for
this is the use of simulation models. However, even though there are
satisfactory models for canopy or whole-plant gas exchange (e.g., Norman
and Campbell 1983; Wang and Jarvis 1990), it is still difficult to "translate"
that into seasonal plant growth because of the gaps in our understanding of
the mechanisms controlling carbon allocation (Gifford et a1. 1984).
This chapter will review briefly the plant characteristics determining
growth, namely plant metabolic rates vs. plant morphology and the role of
assimilate partitioning in determining growth kinetics. In plant communities,
light interception by the canopy largely determines the productivity, but the
efficiency of conversion of solar radiation energy to biomass may vary with
species and environmental factors. The influence of different phenologies
and environmental factors on the relation between gas exchange rates and
growth will be addressed as well.