Gas Exchange and Growth
169
This impact was rather small because in spring days (when photosynthetic
rates are highest) the amount of carbon fixed by the heterogeneous canopy
was only 5 to 10% better than the homogeneous canopy (Pereira et al.
1992b). Hirose and Werger (1987), using a model parameterized for the
actual distribution of leaf N in the canopy of Solidago altissima, which was
more uniform than the "optimal" distribution, found that the heterogeneous
canopy realized over 20% more Ap than the uniform N distribution and
4.7% less Ap than the "optimal" distribution. They also found that redeployment of N to the leaves at the top of the canopy with aging should be
more effective in increasing Ap with high L than with low L values. This
may also be the case of the modest variation in N concentration in E.
globulus canopies which have typically low L values and little mutual shading
of leaves. A decrease in light intensity or a modification in spectral composition as characterized by red/far-red ratio due to shade seem to be
necessary to bring about an important nitrogen retranslocation and therefore
a gradient in photosynthetic capacity in the canopy.
A greater nitrogen concentration in the foliage results normally in a
decrease in the "photosynthetic nitrogen use efficiency" at leaf level
(maximum A per unit of leaf nitrogen content per unit of area) as found, for
example, in sunflower (Fredeen et al. 1991). In E. globulus there was a 16%
decrease in "photosynthetic nitrogen use efficiency" in plants grown with
high N supply (Table 8.2, differences significant P < 0.05). The same
happened with the nitrogen use efficiency (NUE) calculated as the ratio of
biomass produced/N uptake during a given period. In potted E. globulus
plants, high N supply reduced NUE to 114 of the corresponding value in the
low N plants. This reflects the observation that in many species an increase
in nitrogen use efficiency is a phenotypic response to low nutrient availability
(Millard and Proe 1991). In this case a greater proportion of N from older to
new leaves in low N than in high N plants may explain these differences in
part (Pereira et al. 1992a). On the other hand, in the field trial shown in
Table 8.1, the NUE was not substantially different in high nutrition (IL) and
the rain-fed without fertilizers added control (C). The same is shown for
the instantaneous rates of carbon assimilation per unit of nitrogen mass in
Table 8.3. This may be explained by the decrease in photosynthetic and
growth efficacy due to summer water stress in the rain-fed control in comparison with the irrigated IL plots.
In conclusion, abundant nitrogen in the rooting medium leads to fast
growth as a result of increases in F and/or in A. In crops, Sinclair and Horie
(1989) showed that increases in N concentration on a leaf area basis resulting
in higher photosynthetic rate have a limited impact in E unless the plants
have deficiency symptoms. The value of N concentration that maximizes E is
lower than the optimal concentration for net photosynthesis. Before canopy
closure (or at low L as in eucalypt stands), nitrogen abundance seems to be
most important in canopy development bringing about increases in foliage
area.
169
This impact was rather small because in spring days (when photosynthetic
rates are highest) the amount of carbon fixed by the heterogeneous canopy
was only 5 to 10% better than the homogeneous canopy (Pereira et al.
1992b). Hirose and Werger (1987), using a model parameterized for the
actual distribution of leaf N in the canopy of Solidago altissima, which was
more uniform than the "optimal" distribution, found that the heterogeneous
canopy realized over 20% more Ap than the uniform N distribution and
4.7% less Ap than the "optimal" distribution. They also found that redeployment of N to the leaves at the top of the canopy with aging should be
more effective in increasing Ap with high L than with low L values. This
may also be the case of the modest variation in N concentration in E.
globulus canopies which have typically low L values and little mutual shading
of leaves. A decrease in light intensity or a modification in spectral composition as characterized by red/far-red ratio due to shade seem to be
necessary to bring about an important nitrogen retranslocation and therefore
a gradient in photosynthetic capacity in the canopy.
A greater nitrogen concentration in the foliage results normally in a
decrease in the "photosynthetic nitrogen use efficiency" at leaf level
(maximum A per unit of leaf nitrogen content per unit of area) as found, for
example, in sunflower (Fredeen et al. 1991). In E. globulus there was a 16%
decrease in "photosynthetic nitrogen use efficiency" in plants grown with
high N supply (Table 8.2, differences significant P < 0.05). The same
happened with the nitrogen use efficiency (NUE) calculated as the ratio of
biomass produced/N uptake during a given period. In potted E. globulus
plants, high N supply reduced NUE to 114 of the corresponding value in the
low N plants. This reflects the observation that in many species an increase
in nitrogen use efficiency is a phenotypic response to low nutrient availability
(Millard and Proe 1991). In this case a greater proportion of N from older to
new leaves in low N than in high N plants may explain these differences in
part (Pereira et al. 1992a). On the other hand, in the field trial shown in
Table 8.1, the NUE was not substantially different in high nutrition (IL) and
the rain-fed without fertilizers added control (C). The same is shown for
the instantaneous rates of carbon assimilation per unit of nitrogen mass in
Table 8.3. This may be explained by the decrease in photosynthetic and
growth efficacy due to summer water stress in the rain-fed control in comparison with the irrigated IL plots.
In conclusion, abundant nitrogen in the rooting medium leads to fast
growth as a result of increases in F and/or in A. In crops, Sinclair and Horie
(1989) showed that increases in N concentration on a leaf area basis resulting
in higher photosynthetic rate have a limited impact in E unless the plants
have deficiency symptoms. The value of N concentration that maximizes E is
lower than the optimal concentration for net photosynthesis. Before canopy
closure (or at low L as in eucalypt stands), nitrogen abundance seems to be
most important in canopy development bringing about increases in foliage
area.