Nitrogen to phosphorus ratio of plant biomass versus soil solution in a tropical pioneer tree, «Ficus insipida»

It is commonly assumed that the N:P ratio of a terrestrial plant reflects the relative availability of N and P in the soil in which the plant grows. Here it 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 greatest plant dry mass and highest photosynthetic rates were achieved was about 11, whereas the corresponding whole-plant N:P ratio was about 6. Plant P concentration varied as a function of the transpiration rate at constant nutrient solution P concentration, possibly due to transpiration-induced variation in the mass flow of P to root surfaces. 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.Il est couramment admis que le rapport N:P pour les plantes terrestres reflète la disponibilité relative d'azote et de phosphore dans sol où croît la plante. Ici, le rapport N:P a été évalué pour un arbre tropical pionnier, le Ficus insipida. Les semis ont été plantés dans le sable et irrigués avec des solutions nutritives contenant des rapports N:P allant de 1 a 100. Ce concept expérimental a permis une évaluation des réponses physiologiques à la disponibilité d'azote et de phosphore. Le contrôle homéostatique sur le rapport N:P était plus élevé dans les feuilles que dans les tiges ou les racines, ce qui peut signifier que le rapport N:P des tiges et des racines est un indicateur plus sensible à la disponibilité relative d'azote et de phosphore dans un site donné que celui des feuilles. Le rapport N:P de feuilles auquel les taux les plus élevés de masse sèche et de photosynthèse ont été obtenus était d'environ 11 alors que le rapport N:P pour l'ensemble de la plante était d'environ 6. La concentration de phosphore dans les plantes variait en fonction du taux de transpiration à des niveaux constants de concentration de phosphore dans la solution nutritive, ce qui est possiblement dû à des variations dans le débit de phosphore à la surface des racines provoquées par la transpiration. Le taux de transpiration variait en fonction du niveau de concentration de l'azote dans la solution nutritive, mais pas à celui du phosphore, indiquant un contrôle nutritionnel sur la transpiration par l'azote mais pas par le phosphore. L'efficacité de l'utilisation de l'eau variait en fonction de la disponibilité de l'azote mais pas en fonction de celle du phosphore

Nitrogen to phosphorus ratio of plant biomass versus soil solution in a tropical pioneer tree, Ficus insipida

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 ∼11, whereas the corresponding whole-plant N:P ratio was ∼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