Gas Exchange and Growth
151
tosynthesis in C4 than in C3 plants. There are other examples of positive
correlations between fast growth rates and high A (poplar clones, Ceulemans
and Impens 1983) or low maintenance respiration rates (Lotium perenne
genotypes, Wilson 1975). However, the relationship between A and Rw
is quite variable and the correlation between the two is rather poor, as in
Fig. 8.1.
It is unlikely that leaf photosynthesis or respiratory rates per se really
explain most differences between fast and slow growers. Poorter (1989)
reviewed growth and gas exchange data and concluded that only in a few
experiments was photosynthetic rate on leaf area basis found to explain any
significant part of the variation in Rw. In three Eucalyptus globulus clones
with different inherent growth rates, photosynthetic rates were practically
the same. The only important difference among these clones was a greater
capacity to produce leaves in the fastgrowing plants when compared to the
slow growers (Osorio and Pereira, unpublished). On the other hand, there is
no real ecological advantage to plants to have a high photosynthetic rate per
se, whereas fast growth rates may be advantageous for ruderals as well as
for some successful competitors (Grime 1979).
If A and Rw are poorly related, what are the differences between fast and
slow growers? Even though Poorter (1989) found that A on a leaf area basis
was unrelated with Rw , he also found that photosynthetic rates expressed on
a leaf mass basis correlated well with Rw as a result of differences in cr. Slow
growers had higher amounts of tissue "packed" per unit of leaf area, i.e.,
lower cr, than fast growers. Similar results have been reported in the literature, e.g., as in a comparison of 11 half-sib families of Robinia pseudoacacia
showing a negative correlation coefficient between A and growth (Mebrahtu
and Hanover 1991). Total dry weight accumulated was positively related to
cr. Similar results are illustrated in Fig. 8.2 with data of the eucalypt trial
shown in Table 8.1.
Because high gs is often related with high photosynthesis, it has been
assumed that high transpiration should be related with high growth rates.
However, because the limitation of photosynthesis caused by stomata may
vary widely with genotypes and environmental conditions, this relationship
is weak. Typically, the gas phase (mainly stomatal) limitation represents ca.
30% of the total limitation to photosynthesis in C3 plants at ambient CO2
(Jones 1983). The gas phase limitation is considerably higher in C 4 plants.
Stomatal conductance varies diurnally according to well-described patterns
(Tenhunen et al. 1986). This poses the methodological problem of defining
the most meaningful A rate during a day. There is a great variety of criteria
in the literature. The daily integral of A would provide the most useful
information for comparison with growth [see Eqs. (6) and (7)]. However,
for many porposes the daily maximum (Amax) is quite useful (Schulze and
Hall 1982). Lack of standardization in these measures is certainly a source of
noise in the type of relationships illustrated in Fig. 8.1. Nevertheless, in
many circumstances, e.g., low soil water or high water vapor pressure deficit
151
tosynthesis in C4 than in C3 plants. There are other examples of positive
correlations between fast growth rates and high A (poplar clones, Ceulemans
and Impens 1983) or low maintenance respiration rates (Lotium perenne
genotypes, Wilson 1975). However, the relationship between A and Rw
is quite variable and the correlation between the two is rather poor, as in
Fig. 8.1.
It is unlikely that leaf photosynthesis or respiratory rates per se really
explain most differences between fast and slow growers. Poorter (1989)
reviewed growth and gas exchange data and concluded that only in a few
experiments was photosynthetic rate on leaf area basis found to explain any
significant part of the variation in Rw. In three Eucalyptus globulus clones
with different inherent growth rates, photosynthetic rates were practically
the same. The only important difference among these clones was a greater
capacity to produce leaves in the fastgrowing plants when compared to the
slow growers (Osorio and Pereira, unpublished). On the other hand, there is
no real ecological advantage to plants to have a high photosynthetic rate per
se, whereas fast growth rates may be advantageous for ruderals as well as
for some successful competitors (Grime 1979).
If A and Rw are poorly related, what are the differences between fast and
slow growers? Even though Poorter (1989) found that A on a leaf area basis
was unrelated with Rw , he also found that photosynthetic rates expressed on
a leaf mass basis correlated well with Rw as a result of differences in cr. Slow
growers had higher amounts of tissue "packed" per unit of leaf area, i.e.,
lower cr, than fast growers. Similar results have been reported in the literature, e.g., as in a comparison of 11 half-sib families of Robinia pseudoacacia
showing a negative correlation coefficient between A and growth (Mebrahtu
and Hanover 1991). Total dry weight accumulated was positively related to
cr. Similar results are illustrated in Fig. 8.2 with data of the eucalypt trial
shown in Table 8.1.
Because high gs is often related with high photosynthesis, it has been
assumed that high transpiration should be related with high growth rates.
However, because the limitation of photosynthesis caused by stomata may
vary widely with genotypes and environmental conditions, this relationship
is weak. Typically, the gas phase (mainly stomatal) limitation represents ca.
30% of the total limitation to photosynthesis in C3 plants at ambient CO2
(Jones 1983). The gas phase limitation is considerably higher in C 4 plants.
Stomatal conductance varies diurnally according to well-described patterns
(Tenhunen et al. 1986). This poses the methodological problem of defining
the most meaningful A rate during a day. There is a great variety of criteria
in the literature. The daily integral of A would provide the most useful
information for comparison with growth [see Eqs. (6) and (7)]. However,
for many porposes the daily maximum (Amax) is quite useful (Schulze and
Hall 1982). Lack of standardization in these measures is certainly a source of
noise in the type of relationships illustrated in Fig. 8.1. Nevertheless, in
many circumstances, e.g., low soil water or high water vapor pressure deficit