Element uptake, accumulation, and resorption in leaves of mangrove species with different mechanisms of salt regulation

Abstract

Element uptake from substrate and resorption capacity of nutrients before leaf shedding are frequently species-specific and difficult to determine in natural settings. We sampled populations of Rhizophora mangle (salt-excluding species) and Laguncularia racemosa (salt-secreting species) in a coastal lagoon in the upper section of the Maracaibo strait in western Venezuela to estimate accumulation and resorption of mineral elements. Leaves collected fortnightly during 4 months within the rainy season were stratified as young, adult, old, and senescent. We measured changes in concentration of essential elements (N, P, S, K, Mg, Ca, Mn, Fe) and Na (elemental analyzer and plasma spectrometer), leaf succulence (water/area), and specific leaf area (area/mass) and calculated relative resorption or accumulation of elements in senescent leaves before abscission. Succulence was similar in young leaves of both species and increased with age, more abruptly in L. racemosa. Concentrations of N, K, and Mg were higher in R. mangle, whereas those of P, Na, Ca, and S were higher in L. racemosa. Concentration of K per unit leaf water decreased with age in both species; however, Na concentration in R. mangle remained at a similar level until increasing markedly in senescent leaves, whereas in L. racemosa it increased throughout the leaf lifespan. Relative changes based on leaf mass, leaf area, or whole leaf did not differ statistically. On a leaf mass basis both species showed resorption of C, N, P, and K and accumulation of S, Na, Mg, Ca, Mn, and Fe. However, R. mangle was more efficient restricting Na and S uptake, resorbing P, and accumulating Fe than L. racemosa. The P =N resorption ratio is > 1 in R. mangle and < 1 in L. racemosa. We conclude that those differences are related to higher root permeability to Na and S salts in the salt-secreting species and to higher P requirements of R. mangle compared to L. racemosa. Our results give a comprehensive picture of nutrient dynamics in the foliage of mangrove species with contrasting mechanisms of salt regulation.