Nutrients in Senesced Leaves: Keys to the Search for Potential Resorption and Resorption Proficiency

Abstract

Analyses of nitrogen and phosphorus in the senesced leaves of 89 species of deciduous and evergreen woody perennials were used (1) to discover the limits of ultimate potential resorption (maximal withdrawal of nutrients from senescing leaves), (2) to determine a means by which resorption can be categorized as complete or incomplete, (3) to develop the concept of resorption proficiency (measured as the levels to which nutrients have been reduced in senesced leaves), (4) to compare resorption in evergreen vs. deciduous species, (5) to assess the impact of phylogeny on resorption, (6) to compare resorption in actinorhizal vs. non—nitrogen—fixing species, and (7) to consider the efficacy of using multiple measures of resorption to answer questions regarding the function and evolution of this process, rather than relying solely on analyses of resorption efficiency (percentage reduction of nutrients between green and senesced leaves). Concentrations of 0.3% nitrogen and 0.01% phosphorus in senesced leaves represent ultimate potential resorption of these nutrients in woody perennials. Resorption proficiency and potential resorption were quantitatively defined in two models that describe both resorption that is maximal and biochemically complete, and that which is not. Resorption is highly proficient in plants that have reduced nitrogen and phosphorus in their senescing leaves to concentrations below 0.7% and 0.05%, respectively. An important feature of knowing the levels to which nutrients can be reduced in senescing leaves is that these values offer an objective gauge by which to measure the success of resorption as a nutrient conservation mechanism. Evergreens were significantly more proficient at resorbing phosphorus than were deciduous species (0.045% vs. 0.067% P in senesced leaves, respectively) and plants capable of symbiotic nitrogen fixation were significantly less proficient at resorbing nitrogen than were nonfixers (1.6% vs. 0.9% N in senesced leaves, respectively). Resorption proficiency appeared to parallel some phylogenic trends, yet the influence of phylogeny was not so significant as to overwhelm the effects of recent selection. The ability of plants to reduce nitrogen in senescing leaves was significantly correlated with their ability to reduce phosphorus. Measurement and analysis of resorption proficiency, when coupled with concurrent consideration of potential resorption and resorption efficiency, should facilitate and expedite the ongoing attempt to resolve complex questions regarding the environmental constraints that influence resorption, and the selection pressures that have directed the evolution of this process.