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K. Fichtner et a1.
a strong effect on biomass partitioning (Bastow-Wilson 1988). Recently,
genetic engineering offered a new approach for the production of genetically
altered well-defined near-isogenic plants. These transgenic plants can be
used to address specific questions such as about the role of phytochrome
(Sharkey et al. 1991) or carbon metabolism (Stitt et al. 1991) in plants. In
our analysis here we utilize findings on transgenic tobacco plants (Nicotiana
tabacum L.) which were transformed with antisense DNA-sequences to
rbcS, the small subunit for ribulose-1,5-bisphosphate carboxylase-oxygenase
(Rubisco, the enzyme responsible for the primary CO2 fixation in C 3 plants)
(Rodermel et al. 1988). Individuals of these plants have decreased amounts
of Rubisco and therefore also decreased rates of photosynthesis (Quick
et al. 1991a,b, 1992). By changing Rubisco genetically, photosynthesis is
altered as directly as possible with minimal secondary or indirect effects.
Thus, these transgenic tobacco plants offer the unique opportunity to investigate plants grown under uniform environmental conditions with the
individuals having a wide range of different photosynthetic rates. In the data
presented here interactions of photosynthesis and N availability are also
explored.
7.2.1 Photosynthesis and Growth
When the rate of photosynthesis was altered genetically, the relative growth
rate (RGR) increased linearly with photosynthesis over its entire range (r =
0.83) when the plants were grown under high N supply (HN; 5.0mM
NH4N03) (Fig. 7.1A). RGR was much lower and independent of the rate
of photosynthesis (r2 = 0.23) when the plants were grown under low N
supply (LN; O.lmM NH4N03). At an intermediate N supply (MN; 0.7mM
NH4N03) RGR increased linearly with photosynthesis at low rates (but with
a smaller slope than at HN) and was constant at higher rates of photosynthesis. The linear increase of RGR with photosynthesis at HN resulted in an
exponential increase in biomass in these plants, because the biomass B at a
given time is given by B = Binitial . eRGR-AT. This exponential increase demonstrates the multiplying effect of increased carbon availability for growth
if nutrient supply is ample. At LN, N deficiency inhibited further growth
when photosynthesis increased (compare also the low N content in these
plants in Fig. 7.2D). In this case a high portion of photosynthetic products
was stored as starch (Fig. 7.2A) instead of being used for formation of new
leaves and roots. Similar interactions between carbon and N availability
have been seen qualitatively when plants were grown under varying light
(sun and shade plants, Corre 1983; birch, Ingestad and McDonald 1989) or
CO 2 (soybean, Cure et al. 1988; tobacco, K. Fichtner, unpubl. results)
conditions. Thus, enhanced carbon availability promotes growth only it
nutrients are sufficiently supplied. The use of transgenic plants with their
broad range of photosynthetic rates allows us to describe this dependency in
a more quantitative way.
K. Fichtner et a1.
a strong effect on biomass partitioning (Bastow-Wilson 1988). Recently,
genetic engineering offered a new approach for the production of genetically
altered well-defined near-isogenic plants. These transgenic plants can be
used to address specific questions such as about the role of phytochrome
(Sharkey et al. 1991) or carbon metabolism (Stitt et al. 1991) in plants. In
our analysis here we utilize findings on transgenic tobacco plants (Nicotiana
tabacum L.) which were transformed with antisense DNA-sequences to
rbcS, the small subunit for ribulose-1,5-bisphosphate carboxylase-oxygenase
(Rubisco, the enzyme responsible for the primary CO2 fixation in C 3 plants)
(Rodermel et al. 1988). Individuals of these plants have decreased amounts
of Rubisco and therefore also decreased rates of photosynthesis (Quick
et al. 1991a,b, 1992). By changing Rubisco genetically, photosynthesis is
altered as directly as possible with minimal secondary or indirect effects.
Thus, these transgenic tobacco plants offer the unique opportunity to investigate plants grown under uniform environmental conditions with the
individuals having a wide range of different photosynthetic rates. In the data
presented here interactions of photosynthesis and N availability are also
explored.
7.2.1 Photosynthesis and Growth
When the rate of photosynthesis was altered genetically, the relative growth
rate (RGR) increased linearly with photosynthesis over its entire range (r =
0.83) when the plants were grown under high N supply (HN; 5.0mM
NH4N03) (Fig. 7.1A). RGR was much lower and independent of the rate
of photosynthesis (r2 = 0.23) when the plants were grown under low N
supply (LN; O.lmM NH4N03). At an intermediate N supply (MN; 0.7mM
NH4N03) RGR increased linearly with photosynthesis at low rates (but with
a smaller slope than at HN) and was constant at higher rates of photosynthesis. The linear increase of RGR with photosynthesis at HN resulted in an
exponential increase in biomass in these plants, because the biomass B at a
given time is given by B = Binitial . eRGR-AT. This exponential increase demonstrates the multiplying effect of increased carbon availability for growth
if nutrient supply is ample. At LN, N deficiency inhibited further growth
when photosynthesis increased (compare also the low N content in these
plants in Fig. 7.2D). In this case a high portion of photosynthetic products
was stored as starch (Fig. 7.2A) instead of being used for formation of new
leaves and roots. Similar interactions between carbon and N availability
have been seen qualitatively when plants were grown under varying light
(sun and shade plants, Corre 1983; birch, Ingestad and McDonald 1989) or
CO 2 (soybean, Cure et al. 1988; tobacco, K. Fichtner, unpubl. results)
conditions. Thus, enhanced carbon availability promotes growth only it
nutrients are sufficiently supplied. The use of transgenic plants with their
broad range of photosynthetic rates allows us to describe this dependency in
a more quantitative way.