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J.S. Pereira
8.2 How Plants Grow
Mathematical growth analysis has been the preferred methodology for more
than 70 years to understand how plants grow and to study differences in
growth among genotypes or in different environments. The key variable in
this analysis is the growth rate expressed per unit of plant mass (W) or
relative growth rate (Rw or RGR):
dW/dt* lIW = Rw.
(1)
Of course, the relative growth rate is not constant throughout the plant
life cycle, but decreases as maximum plant mass is reached. Accumulated
growth can be expressed by any of a family of curves belonging to a
generalized asymptotic function (Hunt 1982). This is important because
growth comparisons must take into consideration development and how far
a plant is from maximum Rw.
Rw is traditionally decomposed into two components, one more related to
the rate of carbon assimilation and the second expressing plant structure.
The first component is net assimilation rate (EA or NAR):
(2)
with La as leaf area of the plant and W as whole plant mass. The second
component of Rw is called leaf area ratio (F or LAR):
F = LalW.
(3)
The value of Rw is then
Rw = EA*F.
(4)
It is worth noting that the time-averaged Rw is not equal to the product of
averaged F e EA over the same period because each of these variables
follows different patterns of change with time. The variable EA is essentially
the balance between the daily integrals of photosynthesis and whole plant
respiration expressed per unit of leaf area. It is not surprising, therefore,
that EA has been often correlated with net photosynthesis in a number of
species (Konings 1989). However, the relationship between EA and photosynthetic rate may be modified by changes in the proportion of respiration
relative to photosynthesis. Respiration may account for up to 30 to 50% of
the carbon assimilated daily (Lambers et al. 1990). Total respiration rates
per unit of plant mass normally increase with Rw (Amthor 1989; CharlesEdwards et al. 1986). As discussed in Chapter 4, (this Vol), respiration is
traditionally separated in growth respiration and maintenance respiration,
but empirical quantification of these two components is difficult. The absolute growth rate, dW IdT, may be rewritten in terms of respiration as:
dW/dT = YG(Ap - mW),
(5)
J.S. Pereira
8.2 How Plants Grow
Mathematical growth analysis has been the preferred methodology for more
than 70 years to understand how plants grow and to study differences in
growth among genotypes or in different environments. The key variable in
this analysis is the growth rate expressed per unit of plant mass (W) or
relative growth rate (Rw or RGR):
dW/dt* lIW = Rw.
(1)
Of course, the relative growth rate is not constant throughout the plant
life cycle, but decreases as maximum plant mass is reached. Accumulated
growth can be expressed by any of a family of curves belonging to a
generalized asymptotic function (Hunt 1982). This is important because
growth comparisons must take into consideration development and how far
a plant is from maximum Rw.
Rw is traditionally decomposed into two components, one more related to
the rate of carbon assimilation and the second expressing plant structure.
The first component is net assimilation rate (EA or NAR):
(2)
with La as leaf area of the plant and W as whole plant mass. The second
component of Rw is called leaf area ratio (F or LAR):
F = LalW.
(3)
The value of Rw is then
Rw = EA*F.
(4)
It is worth noting that the time-averaged Rw is not equal to the product of
averaged F e EA over the same period because each of these variables
follows different patterns of change with time. The variable EA is essentially
the balance between the daily integrals of photosynthesis and whole plant
respiration expressed per unit of leaf area. It is not surprising, therefore,
that EA has been often correlated with net photosynthesis in a number of
species (Konings 1989). However, the relationship between EA and photosynthetic rate may be modified by changes in the proportion of respiration
relative to photosynthesis. Respiration may account for up to 30 to 50% of
the carbon assimilated daily (Lambers et al. 1990). Total respiration rates
per unit of plant mass normally increase with Rw (Amthor 1989; CharlesEdwards et al. 1986). As discussed in Chapter 4, (this Vol), respiration is
traditionally separated in growth respiration and maintenance respiration,
but empirical quantification of these two components is difficult. The absolute growth rate, dW IdT, may be rewritten in terms of respiration as:
dW/dT = YG(Ap - mW),
(5)