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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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. Related in: MedlinePlus |
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Mentions: For plants grown at variable nutrient solution [N], photosynthesis expressed on a mass basis increased linearly with increasing leaf [N] (Fig. 11A). The regression equation relating the two was Am=9.0Nm+255 (R2=0.68, P <0.001, n=31), where Am is mass-based photosynthesis and Nm is mass-based leaf [N]. For plants grown at variable soil solution [P], photosynthesis also increased linearly with increasing leaf [P] (Fig. 11B), but the relationship was weaker than for leaf [N]. The relationship relating Am to leaf [P] on a mass basis (Pm) was Am=33Pm+500 (R2=0.14, P=0.03, n=32). A plot of mass-based photosynthesis against leaf N:P ratio showed that photosynthesis increased up to a leaf N:P of ∼12, and then decreased as leaf N:P increased further (Fig. 11C). |
View Article: PubMed Central - PubMed
Affiliation: Department of Geography, McGill University, Montréal, Québec H3A-1B1, Canada.