138
K. Fichtner et al.
The carbon content in the leaves increased with the rate of photosynthesis, no matter whether it was altered genetically or by N nutrition (Fig.
7.2C). An elemental analysis revealed that this was accompanied by a
decrease of the macro elements Nand K at all levels of N supply, and S at
MN and LN (cf. Fig. 7.2D and see Fichtner et al. 1993). This demonstrates
a changed balance between the incorporation of carbon, taken up by the
leaves, vs. nutrients, taken up by the roots, into biomass when photosynthesis declined. The decreased carbon content at low rates of photosynthesis
will also reduce the tissue construction costs (Williams et al. 1987). This
contributes further to a compensation of declining growth when carbon gain
is reduced.
In the tobacco plants investigated, the leaf nitrate content increased when
photosynthesis was inhibited genetically, but to a much lower degree when
the external supply of ammonium nitrate was low as in MN and LN (Fig.
7.2B). The enhanced nitrate levels were accompanied by a decrease in
nitrate reductase measured in vivo (data not shown). In these transgenic
plants grown under uniform light conditions, there is no energy limitation
even at low rates of photosynthesis (Quick et al. 1991b). Thus, the cause of
the low nitrate assimilation might be the lower availability of photosynthates
itself. In agreement with these results, Kaiser and Forster (1989) and Kaiser
and Brendle-Behnisch (1991) found that leaf nitrate levels are higher and
nitrate reductase activity is lower when plants are kept under conditions
limiting photosynthesis, e.g., low CO2,
Total leaf N concentration increased when photosynthesis was inhibited
genetically and when N supply increased (Fig. 7.2D). The decrease of N
with photosynthesis can be interpreted as a N dilution effect due to high
CO2 uptake. N dilution in biomass due to promotion of photosynthesis or
growth can also be seen with increasing light (Waring et al. 1985), CO 2
(Tremblay et al. 1988; Hocking and Meyer 1991), or phosphate availability
(Tremblay et al. 1988). Thus, a decrease in the N rich enzyme, Rubisco, was
not followed by a decline in N allocation to leaves. Instead, the content of
other proteins (Quick et al. 1991a,b, 1992) and marginally the content of
amino acids (K. Fichtner, unpubl. results) increased. Additionally, when N
and nitrate values are compared, it is obvious that especially at HN there is
an increasing partitioning of N to nitrate when photosynthesis is inhibited.
7.2.4 The Tobacco System: Conclusions
The effect of the photosynthetic rate on growth depends on the N supply.
While photosynthesis promotes growth in well-fertilized plants, there is no
effect when N is deficient. When photosynthesis is decreased genetically in
plants grown under uniform environmental conditions there is a strong
decrease in biomass investment per unit leaf area, resembling changes in the
light or CO2 environment. The partitioning between above- and below-
K. Fichtner et al.
The carbon content in the leaves increased with the rate of photosynthesis, no matter whether it was altered genetically or by N nutrition (Fig.
7.2C). An elemental analysis revealed that this was accompanied by a
decrease of the macro elements Nand K at all levels of N supply, and S at
MN and LN (cf. Fig. 7.2D and see Fichtner et al. 1993). This demonstrates
a changed balance between the incorporation of carbon, taken up by the
leaves, vs. nutrients, taken up by the roots, into biomass when photosynthesis declined. The decreased carbon content at low rates of photosynthesis
will also reduce the tissue construction costs (Williams et al. 1987). This
contributes further to a compensation of declining growth when carbon gain
is reduced.
In the tobacco plants investigated, the leaf nitrate content increased when
photosynthesis was inhibited genetically, but to a much lower degree when
the external supply of ammonium nitrate was low as in MN and LN (Fig.
7.2B). The enhanced nitrate levels were accompanied by a decrease in
nitrate reductase measured in vivo (data not shown). In these transgenic
plants grown under uniform light conditions, there is no energy limitation
even at low rates of photosynthesis (Quick et al. 1991b). Thus, the cause of
the low nitrate assimilation might be the lower availability of photosynthates
itself. In agreement with these results, Kaiser and Forster (1989) and Kaiser
and Brendle-Behnisch (1991) found that leaf nitrate levels are higher and
nitrate reductase activity is lower when plants are kept under conditions
limiting photosynthesis, e.g., low CO2,
Total leaf N concentration increased when photosynthesis was inhibited
genetically and when N supply increased (Fig. 7.2D). The decrease of N
with photosynthesis can be interpreted as a N dilution effect due to high
CO2 uptake. N dilution in biomass due to promotion of photosynthesis or
growth can also be seen with increasing light (Waring et al. 1985), CO 2
(Tremblay et al. 1988; Hocking and Meyer 1991), or phosphate availability
(Tremblay et al. 1988). Thus, a decrease in the N rich enzyme, Rubisco, was
not followed by a decline in N allocation to leaves. Instead, the content of
other proteins (Quick et al. 1991a,b, 1992) and marginally the content of
amino acids (K. Fichtner, unpubl. results) increased. Additionally, when N
and nitrate values are compared, it is obvious that especially at HN there is
an increasing partitioning of N to nitrate when photosynthesis is inhibited.
7.2.4 The Tobacco System: Conclusions
The effect of the photosynthetic rate on growth depends on the N supply.
While photosynthesis promotes growth in well-fertilized plants, there is no
effect when N is deficient. When photosynthesis is decreased genetically in
plants grown under uniform environmental conditions there is a strong
decrease in biomass investment per unit leaf area, resembling changes in the
light or CO2 environment. The partitioning between above- and below-