Gas Exchange and Growth
149
where Y G in the "growth conversion efficiency" given as Y G = AST/(ASR +
AS T ) , where ST and SR refer to the amounts of carbon substrate used as
carbon skeletons for biomass accumulation and for respiration to provide
energy for growth, respectively, Ap is the whole plant carbon assimilation,
and m is the maintenance respiration coefficient (Amthor 1989 see also
Chap. 4, this Vol.).
The so-called structural component of relative growth rate, i.e., F, is
not directly related to gas exchange rates but gives information on the
"leafiness" of the plant, i.e., the photosynthetic area per unit of plant dry
weight. The value of F is, therefore, dependent on the partition of biomass
between leaves and the total respiring biomass (leaf weight ratio, LWR) and
on a foliage structure parameter, i.e., specific leaf area (SLA or cr).
Empirical data show that in many cases EA decreases with increasing cr
(Konings 1989). EA will often decrease with increasing LWR. This means a
compensation of any increase in F by a decrease in EA, resulting in Rw being
more conservative than either EA or F.
In trying to relate growth and metabolic rates, we face the inherent
difficulties of differences in scale. In fact, the instantaneous rates of gas
exchange vary with leaf age and position and with time (of day, season),
whereas growth integrates over space and time. In addition, the relationship
is intrinsically complex because of the interplay of metabolic processes with
structural characteristics in determining growth. The following equation is
an attempt to relate growth with photosynthetic rate on a leaf area basis (A)
and respiration (R) in simple herbaceous plants (Masle and Farquhar 1988;
Farquhar et al. 1989),
dW/dt = (LaA - Rr) * d - (R/ + Rs') * n,
(6)
where the prime superscript denotes nighttime rates, d and n for the hours
of light and dark and rand s for root and shoot, respectively. Equation (6)
may be rewritten as dW /dt = LaAI * (1 - is the proportion of
carbon fixed that is respired and 1 = d/(d + n), i.e., light period as a
proportion of the 24 h. The relative growth rate is then
Rw = dW/dt* 1IW = AI(l - (7)
where p is the mass of carbon in the plant per unit leaf area, W/(CrLa),
where Cr is the conversion factor indicating the biomass/carbon ratio of
recently produced dry matter. Notice that p is proportional to lIF in a given
plant or species. On the other hand, p is inversely related to Cr. Therefore
high carbon concentration in the biomass (low Cr, reflecting, e.g., high
lignin concentration) will influence growth rates negatively, other conditions
being equal. A low lignin concentration is typical of fast-growing plants
(Poorter 1989).
The ratio of carbon isotopes (13C/ 12 C) in plant tissues is related to photosynthesis, namely the ratio of Pi (intercellular CO2 partial pressure) and
149
where Y G in the "growth conversion efficiency" given as Y G = AST/(ASR +
AS T ) , where ST and SR refer to the amounts of carbon substrate used as
carbon skeletons for biomass accumulation and for respiration to provide
energy for growth, respectively, Ap is the whole plant carbon assimilation,
and m is the maintenance respiration coefficient (Amthor 1989 see also
Chap. 4, this Vol.).
The so-called structural component of relative growth rate, i.e., F, is
not directly related to gas exchange rates but gives information on the
"leafiness" of the plant, i.e., the photosynthetic area per unit of plant dry
weight. The value of F is, therefore, dependent on the partition of biomass
between leaves and the total respiring biomass (leaf weight ratio, LWR) and
on a foliage structure parameter, i.e., specific leaf area (SLA or cr).
Empirical data show that in many cases EA decreases with increasing cr
(Konings 1989). EA will often decrease with increasing LWR. This means a
compensation of any increase in F by a decrease in EA, resulting in Rw being
more conservative than either EA or F.
In trying to relate growth and metabolic rates, we face the inherent
difficulties of differences in scale. In fact, the instantaneous rates of gas
exchange vary with leaf age and position and with time (of day, season),
whereas growth integrates over space and time. In addition, the relationship
is intrinsically complex because of the interplay of metabolic processes with
structural characteristics in determining growth. The following equation is
an attempt to relate growth with photosynthetic rate on a leaf area basis (A)
and respiration (R) in simple herbaceous plants (Masle and Farquhar 1988;
Farquhar et al. 1989),
dW/dt = (LaA - Rr) * d - (R/ + Rs') * n,
(6)
where the prime superscript denotes nighttime rates, d and n for the hours
of light and dark and rand s for root and shoot, respectively. Equation (6)
may be rewritten as dW /dt = LaAI * (1 - is the proportion of
carbon fixed that is respired and 1 = d/(d + n), i.e., light period as a
proportion of the 24 h. The relative growth rate is then
Rw = dW/dt* 1IW = AI(l - (7)
where p is the mass of carbon in the plant per unit leaf area, W/(CrLa),
where Cr is the conversion factor indicating the biomass/carbon ratio of
recently produced dry matter. Notice that p is proportional to lIF in a given
plant or species. On the other hand, p is inversely related to Cr. Therefore
high carbon concentration in the biomass (low Cr, reflecting, e.g., high
lignin concentration) will influence growth rates negatively, other conditions
being equal. A low lignin concentration is typical of fast-growing plants
(Poorter 1989).
The ratio of carbon isotopes (13C/ 12 C) in plant tissues is related to photosynthesis, namely the ratio of Pi (intercellular CO2 partial pressure) and