Nitrogen to phosphorus ratio of plant biomass versus soil solution in a tropical pioneer tree, Ficus insipida.
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Plant P concentration varied as a function of transpiration rate at constant nutrient solution P concentration, possibly due to transpiration-induced variation in the mass flow of P to root surfaces.The transpiration rate varied in response to nutrient solution N concentration, but not to nutrient solution P concentration, demonstrating nutritional control over transpiration by N but not P.Water-use efficiency varied as a function of N availability, but not as a function of P availability.
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PubMed Central - PubMed
Affiliation: Department of Geography, McGill University, Montréal, Québec H3A-1B1, Canada.
ABSTRACT
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It is commonly assumed that the nitrogen to phosphorus (N:P) ratio of a terrestrial plant reflects the relative availability of N and P in the soil in which the plant grows. Here, this was assessed for a tropical pioneer tree, Ficus insipida. Seedlings were grown in sand and irrigated with nutrient solutions containing N:P ratios ranging from <1 to >100. The experimental design further allowed investigation of physiological responses to N and P availability. Homeostatic control over N:P ratios was stronger in leaves than in stems or roots, suggesting that N:P ratios of stems and roots are more sensitive indicators of the relative availability of N and P at a site than N:P ratios of leaves. The leaf N:P ratio at which the largest plant dry mass and highest photosynthetic rates were achieved was approximately 11, whereas the corresponding whole-plant N:P ratio was approximately 6. Plant P concentration varied as a function of transpiration rate at constant nutrient solution P concentration, possibly due to transpiration-induced variation in the mass flow of P to root surfaces. The transpiration rate varied in response to nutrient solution N concentration, but not to nutrient solution P concentration, demonstrating nutritional control over transpiration by N but not P. Water-use efficiency varied as a function of N availability, but not as a function of P availability. |
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Mentions: Comparison of the N:P ratio of plant biomass with that of the nutrient solution is shown in Fig. 5. The plot of leaf N:P against nutrient solution N:P suggests that on a logarithmic scale the relationship between the two was approximately linear over most of the range of nutrient solution N:P ratios employed in the study. At the lowest nutrient solution N:P ratio, however, the relationship departed from linearity (Fig. 5A). For the N:P ratio of stem biomass, on the other hand, the relationship to the nutrient solution N:P ratio on a logarithmic scale was linear over the full range of values (Fig. 5B). The relationship between root N:P and nutrient solution N:P followed a similar pattern to that for stem N:P, but with slight departures from linearity at the highest and lowest nutrient solution N:P ratios (Fig. 5C). Given these patterns, geometric mean regressions were fitted to the log–log plots of plant biomass N:P against nutrient solution N:P excluding the lowest nutrient solution N:P treatment. With this treatment excluded, the nutrient solution N:P ratios ranged from 1 to 135. Across this range of nutrient solution N:P, the estimated H, describing the extent of homeostatic control over plant biomass N:P, was 6.3 for leaves, 2.5 for stems, and 2.8 for roots. Thus, regulatory control over N:P stoichiometry was considerably stronger in leaves than in stems or roots. For whole-plant N:P, the relationship to nutrient solution N:P was approximately linear over the full range of nutrient solution N:P (Fig. 5D). The H calculated for whole-plant N:P was 3.1. |
View Article: PubMed Central - PubMed
Affiliation: Department of Geography, McGill University, Montréal, Québec H3A-1B1, Canada.