142
K. Fichtner et al.
7.3.3 The Radish System: Conclusions
The ratio of shoot to root activity appears to be the best predictor of
allocation to shoots, roots, and specific storage compounds, there being
no simple relationship between photosynthesis or root activity and allocation
across different light and N environments. Variation in the relative availabilities of light and N can encompass conditions ranging from N acquisition
being largely supply-limited at low N/light ratios to largely demand-driven
at high N/light ratios. Whether N acquisition is demand-driven or supplylimited will vary with the plant factors (e.g., inherent growth potential,
developmental stage) and environmental conditions (e.g., light, COz, or
temperature) that determine photosynthesis and relative growth rate, and
thus, plant N demand (Clarkson 1986; Garnier et al. 1989).
7.4 Storage as Related to Resource Availability
A further example of the tight coupling between N and carbon availabilities
and stores, as demonstrated for tobacco and radish, is shown by recent experiments on Phaseolus. Specifically, these latter experiments were directed
toward learning more about the role of N and carbon reserves versus stores.
Reserve formation is the metabolically regulated synthesis of storage compounds, in competition for growth, in contrast to accumulation of stores that
occurs when resource supply exceeds demands for growth and reproduction
(Chapin et al. 1990).
These experiments utilized two genotypes of Phaseolus lunatus, the lima
bean. One of these was a perennial wild type from the deciduous forests of
western Mexico (Chamela, Jalisco). The second genotype was the lima bean
cultivar, Burpee's Best. This genotype is annual, as are all cultivars of P.
lunatus. The rationale for utilizing these two different genotypes was that it
might be expected that the perennial would accumulate reserves, even at a
loss of growth potential, since it must utilize stores to endure a long drought
period and to regrow after its cessation. The annual, on the other hand,
would not have such demands for stores, and hence, might not have to
regulate reserve formation away from competitive growth potential.
We grew these two genotypes under differing supply rates of carbon (by
altering the rate of photosynthesis by different PFD) and N. They were
grown for 3 weeks under low (100 Ilmol m-Zs- 1 ) or high (400 Ilmolm-z S-l)
PFD and under either low (O.S mmol) or high (S.Ommol) ammonium nitrate
with other nutrients nonlimiting.
The results of these analyses (Mooney et al., submitted) indicate a strong
and direct relationship between resource availability and growth, with storage
playing an important role in buffering the effects of imbalances of resource
availability and use. Specifically, it was shown that the concentration of N in
K. Fichtner et al.
7.3.3 The Radish System: Conclusions
The ratio of shoot to root activity appears to be the best predictor of
allocation to shoots, roots, and specific storage compounds, there being
no simple relationship between photosynthesis or root activity and allocation
across different light and N environments. Variation in the relative availabilities of light and N can encompass conditions ranging from N acquisition
being largely supply-limited at low N/light ratios to largely demand-driven
at high N/light ratios. Whether N acquisition is demand-driven or supplylimited will vary with the plant factors (e.g., inherent growth potential,
developmental stage) and environmental conditions (e.g., light, COz, or
temperature) that determine photosynthesis and relative growth rate, and
thus, plant N demand (Clarkson 1986; Garnier et al. 1989).
7.4 Storage as Related to Resource Availability
A further example of the tight coupling between N and carbon availabilities
and stores, as demonstrated for tobacco and radish, is shown by recent experiments on Phaseolus. Specifically, these latter experiments were directed
toward learning more about the role of N and carbon reserves versus stores.
Reserve formation is the metabolically regulated synthesis of storage compounds, in competition for growth, in contrast to accumulation of stores that
occurs when resource supply exceeds demands for growth and reproduction
(Chapin et al. 1990).
These experiments utilized two genotypes of Phaseolus lunatus, the lima
bean. One of these was a perennial wild type from the deciduous forests of
western Mexico (Chamela, Jalisco). The second genotype was the lima bean
cultivar, Burpee's Best. This genotype is annual, as are all cultivars of P.
lunatus. The rationale for utilizing these two different genotypes was that it
might be expected that the perennial would accumulate reserves, even at a
loss of growth potential, since it must utilize stores to endure a long drought
period and to regrow after its cessation. The annual, on the other hand,
would not have such demands for stores, and hence, might not have to
regulate reserve formation away from competitive growth potential.
We grew these two genotypes under differing supply rates of carbon (by
altering the rate of photosynthesis by different PFD) and N. They were
grown for 3 weeks under low (100 Ilmol m-Zs- 1 ) or high (400 Ilmolm-z S-l)
PFD and under either low (O.S mmol) or high (S.Ommol) ammonium nitrate
with other nutrients nonlimiting.
The results of these analyses (Mooney et al., submitted) indicate a strong
and direct relationship between resource availability and growth, with storage
playing an important role in buffering the effects of imbalances of resource
availability and use. Specifically, it was shown that the concentration of N in