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The decline of Fraxinus angustifolia Vahl in a Mediterranean salt meadow: Chlorophyll fluorescence measurements in long-term field experiment

Abstract : 15 The ecological transition zones also known as ecotones are specific ecosystems but how global change 16 will affect their biodiversity and functioning is poorly understood. In the case of the areas at the 17 interface between riparian forest and salt meadows the biodiversity conservation remains a major 18 issue. The aim of this work was to evaluate the effects of a salty environment on Fraxinus angustifolia 19 a non-halophyte species. For this, the photochemistry of PSII was in situ explored during the summer 20 between 2007 and 2016. The predawn maximum quantum efficiency of PSII (Fv/Fm) was always 21 greater than 0.8 which indicates an absence of chronic photoinhibition induced by salinity. During 22 these years, the light kinetic of the actual quantum yield of PSII (Φ PSII) was studied. It appears that 23 this yield is generally higher in salty environments than in the riparian forest. Multivariate analysis, 24 showed that the dissipation regulated light energy faction (Φ NPQ), the photochemical quenching (qP) 25 and the Fd/Fs ratio constitute the main explanatory variables of Φ PSII changes. The subsequent two-26 way-ANCOVA carried out demonstrated that Φ NPQ was weaker in saline environments, suggesting a 27 better efficiency of photosystem collecting antennas. Meanwhile, the analysis of photosynthetic 28 pigments did not exhibited a salt effect except the significant accumulation of chlorophyll a in 2016. 29 In order to address the question of the salt-related toxicity, sodium ion contents in the leaves were 30 found to be identical in the two sites, suggesting the establishment of an energy-dependent mechanism 31 of salt exclusion/retention in roots. The growth rings of F.angustifolia in saline areas highly follows 32 the annual water deficit changes. Between 2007 and 2016, the biomass in the salt meadow was higher 33 with a peak in 2011 representing 60% of the biomass of control individuals. But, since 2011 a 34 J o u r n a l P r e-p r o o f 2 continuous decline of growth was recorded. Water conditions seem to constitute a limiting factor even 35 if the photosynthetic apparatus generates the excess metabolic energy necessary to cope with the saline 36 toxicity. This study thus highlighted the plasticity of the photochemistry of the PSII depending to the 37 environmental conditions of the salt meadows. It also shows the vulnerability of an ecotone through 38 the decline in the productivity of a glycophyte with climate change. This augurs for changes in the 39 composition of plant communities which is in favor of strict halophytes. 40
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Jean-Philippe Mevy, Frédéric Guibal, Caroline Lecareux, Franco Miglietta. The decline of Fraxinus angustifolia Vahl in a Mediterranean salt meadow: Chlorophyll fluorescence measurements in long-term field experiment. Estuarine, Coastal and Shelf Science, Elsevier, 2020, pp.107068. ⟨10.1016/j.ecss.2020.107068⟩. ⟨hal-02980230⟩

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