Photosynthesis, Storage, and Allocation
139
ground biomass is much more strongly influenced by N nutrition than by
photosynthesis when equivalent changes in photosynthetic rate are compared. Photosynthesis strongly affects biomass composition. With decreasing
photosynthesis, carbon, and particularly starch, stores decline while N, and
especially nitrate, stores increase.
7.3 Allocation in Relation to Shoot and Root Activity
The studies with tobacco indicated that the relative levels of carbon and N
availability predicted carbon and N allocation to shoots, roots, and storage
compounds better than did absolute levels of either resource. This is consistent with conceptual models that view allocation responses as tending
to compensate for changes in resource availability ratios (Brouwer 1963;
Davidson 1969; Schulze 1982). From this it would be expected that the ratio
of shoot to root activity should predict shoot/root allocation better than
photosynthesis or nitrogen acquisition rate alone. Here we test this idea,
drawing on results obtained in studies examining relationships among photosynthesis, nitrate (N03 -) uptake, and allocation in wild radish plants
(Raphanus sativus x raphanistrum) grown under different combinations of
N and light availability (Koch et al. 1987, 1988; Kiippers et al. 1988; G.W.
Koch, unpubl. data).
7.3.1 Resource Ratios, Growth, and Allocation
Different light regimes and N0 3 - availabilities produced a similar range
of RGR in radish, but through influences on different aspects of plant
physiology. Whereas light primarily affected photosynthesis, N supply had a
large influence on shoot/root allocation (expressed as the shoot fraction in
Table 7.1; shoot fraction = 1 - root fraction). For a given N environment,
daily light availability had only a modest influence on shoot allocation, while
for a given light environment, varied N availability elicited a comparatively
large change in allocation. The extremes of allocation occurred at extreme
ratios of light to N availability; the greatest shoot allocation was obtained at
low light and high N (8.6mol quanta m- 2 d- 1 and 100mmol N0 3 - m- 3 ),
while shoot allocation was lowest at high light and low N (25.2 mol quanta
m- 2 d- 1 and O.01X N0 3 - treatment).
7.3.2 Photosynthesis, Specific Absorption Rate, and Allocation
The relationship between photosynthesis and shoot/root allocation in radish
apparently depends on the cause of variation in photosynthesis. When
139
ground biomass is much more strongly influenced by N nutrition than by
photosynthesis when equivalent changes in photosynthetic rate are compared. Photosynthesis strongly affects biomass composition. With decreasing
photosynthesis, carbon, and particularly starch, stores decline while N, and
especially nitrate, stores increase.
7.3 Allocation in Relation to Shoot and Root Activity
The studies with tobacco indicated that the relative levels of carbon and N
availability predicted carbon and N allocation to shoots, roots, and storage
compounds better than did absolute levels of either resource. This is consistent with conceptual models that view allocation responses as tending
to compensate for changes in resource availability ratios (Brouwer 1963;
Davidson 1969; Schulze 1982). From this it would be expected that the ratio
of shoot to root activity should predict shoot/root allocation better than
photosynthesis or nitrogen acquisition rate alone. Here we test this idea,
drawing on results obtained in studies examining relationships among photosynthesis, nitrate (N03 -) uptake, and allocation in wild radish plants
(Raphanus sativus x raphanistrum) grown under different combinations of
N and light availability (Koch et al. 1987, 1988; Kiippers et al. 1988; G.W.
Koch, unpubl. data).
7.3.1 Resource Ratios, Growth, and Allocation
Different light regimes and N0 3 - availabilities produced a similar range
of RGR in radish, but through influences on different aspects of plant
physiology. Whereas light primarily affected photosynthesis, N supply had a
large influence on shoot/root allocation (expressed as the shoot fraction in
Table 7.1; shoot fraction = 1 - root fraction). For a given N environment,
daily light availability had only a modest influence on shoot allocation, while
for a given light environment, varied N availability elicited a comparatively
large change in allocation. The extremes of allocation occurred at extreme
ratios of light to N availability; the greatest shoot allocation was obtained at
low light and high N (8.6mol quanta m- 2 d- 1 and 100mmol N0 3 - m- 3 ),
while shoot allocation was lowest at high light and low N (25.2 mol quanta
m- 2 d- 1 and O.01X N0 3 - treatment).
7.3.2 Photosynthesis, Specific Absorption Rate, and Allocation
The relationship between photosynthesis and shoot/root allocation in radish
apparently depends on the cause of variation in photosynthesis. When