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.

N availability and mechanisms of N conservation in deciduous and semideciduous tropical forest legume trees

“…An increase in the availability of N for the plant, due to SNF, leads to a decrease in N-resorption proficiency (NRP), that is, there is an increase in the concentration of N in the senescent leaves (Killingbeck 1996;Killingbeck & Whitford 2001). The data from the present study agree with these observations, given that the N concentrations found in senescent leaves for species without SNF were much lower than those with SNF (Fig.…”

“…Resorption proficiency is considered a more stable indicator of the plant capacity to reuse nutrients than resorption efficiency (Killingbeck 1996). An increase in the availability of N for the plant, due to SNF, leads to a decrease in N-resorption proficiency (NRP), that is, there is an increase in the concentration of N in the senescent leaves (Killingbeck 1996;Killingbeck & Whitford 2001).…”

“…Subsamples of leaves weighing 100 mg per plant were dried at 65 ºC for 7 days and total N determined according to Nelson & Sommers (1973). The N concentration in senescent leaves was used as an indicator of NRP (Killingbeck 1996). NRE was calculated by: NRE = (N mature green -N senescent ) / N mature green × 100%, where: N mature green = N in mature green leaves, N senescent = N in senescent leaves (Pugnaire & Chapin 1993).…”

“…However, consensus has not been reached in the literature comparing these two groups of plants (Aerts 1996;Lal et al 2001) and attempts to relate nutrient resorption efficiency to soil fertility have led to controversy (Aerts 1996;Eckstein et al 1999;Cordell et al 2001;Carrera et al 2003). Killingbeck (1996) concluded that plants which perform symbiotic N fixation (SNF) presented lower N-resorption proficiency (NRP), and N-resorption efficiency (NRE) than those which do not. Resorption efficiency is the difference between the nutrient concentration in green leaves and senescent leaves, given as a percentage (Distel et al 2003), whilst resorption proficiency is the absolute value by which nutrients are reduced in senescent leaves.…”

“…Resorption efficiency is the difference between the nutrient concentration in green leaves and senescent leaves, given as a percentage (Distel et al 2003), whilst resorption proficiency is the absolute value by which nutrients are reduced in senescent leaves. Thus, the lower the concentration of a nutrient in senescent leaves, the greater the resorption proficiency (Killingbeck 1996).…”