Gas Exchange and Growth
155
taking into consideration that assimilation, phloem transport, and sink activity are not in phase even in the short term.
The kinetics of growth is largely dictated by how new photo assimilates
are partitioned into "productive" (i.e., photosynthetic) or "support" (roots,
shoots) biomass. A merely econometric approach would emphasize the
importance of the rate of partitioning of photo assimilates into new leaf area
as the component that would accelerate growth. For a given amount of net
carbon increment, a plant will tend to grow relatively more in the following
interval if partition of photoassimilates to new leaves is high, therefore
"investing" more in new photosynthetic biomass, than plants "investing"
less new carbon in leaves. In a number of cases Rw has been found well
correlated with "leaf area partitioning", i.e., the relative increment in leaf
area per unit increment in biomass (Potter and Jones 1977; Patterson et al.
1978).
Normally, the relative growth rates of two different components of the
whole plant, are proportional. The constant of proportionality is called the
"allometric constant". The allometric constants are often different from 1
and therefore the biomass ratios between plant parts change with time. For
example, if the allometric constant of the root to shoot quotient is greater
than 1, root biomass will increase faster than the shoot and, thus, wholeplant Rw will tend to decrease.
8.5 Do Growth Rates Influence Carbon Assimilation?
I have been discussing the influence that metabolic rates may have on
growth. However, the inverse, i.e., to what extent growth activity may
modify plant metabolism, is also pertinent. For example, fast growth rates
increase respiration rates (Charles-Edwards et al. 1986; Amthor 1989). The
answer is more complex for photosynthesis and it raises the question as to a
possible feedback between "sink" and "source" of photoassimilates. This
topic has been studied for some time (see Neales and Incoll 1968) and
evidence for such a relationship between sink demand and photosynthesis
has accumulated (King et al. 1967; Azc6n-Bieto 1983; Lauer and Shibles
1987; Foyer 1988), although with some contradictory results (Geiger and
Servaites 1991). Even though the results of some experiments to evaluate
constraints on photosynthesis due to the accumulation of assimilates in
leaves when growth or phloem transport has been restricted remain obscure,
the biochemical evidence suggests the likelihood of a feedback relation
between sink and source despite considerable variation among plants. This
variation seems to be related in part with the capacity of leaves for starch
synthesis and in part to metabolic regulation of sucrose synthesis (Huber
1981; Foyer 1987), which vary with genotype and environmental conditions. It has been shown that during periods when sucrose is produced
155
taking into consideration that assimilation, phloem transport, and sink activity are not in phase even in the short term.
The kinetics of growth is largely dictated by how new photo assimilates
are partitioned into "productive" (i.e., photosynthetic) or "support" (roots,
shoots) biomass. A merely econometric approach would emphasize the
importance of the rate of partitioning of photo assimilates into new leaf area
as the component that would accelerate growth. For a given amount of net
carbon increment, a plant will tend to grow relatively more in the following
interval if partition of photoassimilates to new leaves is high, therefore
"investing" more in new photosynthetic biomass, than plants "investing"
less new carbon in leaves. In a number of cases Rw has been found well
correlated with "leaf area partitioning", i.e., the relative increment in leaf
area per unit increment in biomass (Potter and Jones 1977; Patterson et al.
1978).
Normally, the relative growth rates of two different components of the
whole plant, are proportional. The constant of proportionality is called the
"allometric constant". The allometric constants are often different from 1
and therefore the biomass ratios between plant parts change with time. For
example, if the allometric constant of the root to shoot quotient is greater
than 1, root biomass will increase faster than the shoot and, thus, wholeplant Rw will tend to decrease.
8.5 Do Growth Rates Influence Carbon Assimilation?
I have been discussing the influence that metabolic rates may have on
growth. However, the inverse, i.e., to what extent growth activity may
modify plant metabolism, is also pertinent. For example, fast growth rates
increase respiration rates (Charles-Edwards et al. 1986; Amthor 1989). The
answer is more complex for photosynthesis and it raises the question as to a
possible feedback between "sink" and "source" of photoassimilates. This
topic has been studied for some time (see Neales and Incoll 1968) and
evidence for such a relationship between sink demand and photosynthesis
has accumulated (King et al. 1967; Azc6n-Bieto 1983; Lauer and Shibles
1987; Foyer 1988), although with some contradictory results (Geiger and
Servaites 1991). Even though the results of some experiments to evaluate
constraints on photosynthesis due to the accumulation of assimilates in
leaves when growth or phloem transport has been restricted remain obscure,
the biochemical evidence suggests the likelihood of a feedback relation
between sink and source despite considerable variation among plants. This
variation seems to be related in part with the capacity of leaves for starch
synthesis and in part to metabolic regulation of sucrose synthesis (Huber
1981; Foyer 1987), which vary with genotype and environmental conditions. It has been shown that during periods when sucrose is produced