Stable carbon and nitrogen isotope compositions change with leaf age in two mangrove ferns

摘要

After assimilation, plants often fractionate stable carbon and nitrogen isotopes among different organs depending on synthesis and transport of metabolites. We investigated stable carbon and nitrogen isotope compositions among leaves of different age (0 to 6 months) in two mangrove fern species (Acrostichum danaeifolium Langsd. & Fisch. and Acrosdchum aureum L.) from Mexico. Leaves of all ages were analysed for delta C-13, delta N-15, carbon and nitrogen concentrations and gas exchange parameters. In both species, delta C-13 slightly decreased with leaf age. Leaf salt concentration increased with leaf age, and thus did not decrease C-13 discrimination markedly. Enrichment in C-13 in younger versus older leaves can be explained by stage of development: Carbon is assimilated and incorporated into autotrophic leaves, but also transported as C-13-enriched carbohydrates into still expanding and more heterotrophic younger leaves-ndicated by lower rates of photosynthesis. Depletion in C-13 in old autotrophic leaves, which export photosynthetic assimilates, could mainly be explained by respiratory fractionation releasing C-13-enriched CO2. In contrast, delta N-15 values in A. danaeifolium increased with leaf age. This pattern could be related to the transport of N-15-depleted amino acids into younger leaves, and leaf construction with these compounds. In conclusion, C-13 depletion and N-15 enrichment with leaf age were described for other plant species earlier and were explained by different mechanisms of carbon and nitrogen assimilation and the export of these assimilates from older to younger leaves. These stable isotope patterns were approved for two mangrove fern species in this study. (C) Elsevier GmbH. All rights reserved.