title: Salinity stress constrains photosynthesis in Fraxinus ornus more when growing in partial shading than in full sunlight: consequences for the antioxidant defence system.
abstract: OBJECTIVE
  A major challenge in plant ecophysiology is understanding the effects of multiple sub-optimal environmental conditions on plant performance. In most Mediterranean areas soil salinity builds up during the summer because of low availability of soil water coupled with hot temperatures. Although sunlight and soil salinity may strongly interact in determining a plant's performance, this has received relatively little attention.
  
  
  METHODS
  Two-year-old seedlings of Fraxinus ornus were grown outdoors in pots during a Mediterranean summer in either 45 % (shaded plants) or 100 % (sun plants) sunlight irradiance and were supplied with either deionized water or deionized water plus 75 mm NaCl. Morpho-anatomical traits, water and ionic relations, gas exchange and photosystem II performance, concentrations of individual carotenoids, activity of antioxidant enzymes, concentrations of ascorbic acid and individual polyphenols were measured in leaves. Leaf oxidative stress and damage were estimated by in vivo analysis of stable free radicals and ultrastructural analyses.
  
  
  RESULTS
  Leaf concentrations of potentially toxic ions did not markedly differ in shaded or sun plants in response to salinity. Leaves of sun plants displayed superior water use efficiency compared with leaves of shaded plants, irrespective of salinity treatment, and had both better stomatal control and higher CO2 carboxylation efficiency than leaves of shaded plants. In the salt-treated groups, the adverse effects of excess midday irradiance were greater in shade than in sun plants. The activity of enzymes responsible for detoxifying hydrogen peroxide decreased in shaded plants and increased in sun plants as a result of salinity stress. In contrast, the activity of guaiacol peroxidase and the concentration of phenylpropanoids increased steeply in response to salinity in shaded plants but were unaffected in sun plants.
  
  
  CONCLUSIONS
  It is concluded that salinity may constrain the performance of plants growing under partial shading more severely than that of plants growing under full sun during summer. The results suggest co-ordination within the antioxidant defence network aimed at detoxifying salt-induced generation of reactive oxygen species.
authors.affiliation: Department of Agri-Food and Environmental Science, University of Florence, Viale delle Idee 30, I-50019, Sesto Fiorentino, Florence, Italy.
authors.forename: Alessio
authors.initials: A
authors.lastname: Fini
authors.affiliation: Department of Agricultural, Food and Agro-Environmental Sciences, University of Pisa, Via del Borghetto 80, I-56124, Pisa, Italy.
authors.forename: Lucia
authors.initials: L
authors.lastname: Guidi
authors.affiliation: The National Research Council of Italy, Centre for Electron Microscopy (Ce.M.E.), Via Madonna del Piano 10, I-50019, Sesto Fiorentino, Florence Italy.
authors.forename: Cristiana
authors.initials: C
authors.lastname: Giordano
authors.affiliation: Department of Biotechnology, Chemistry and Pharmacy, University of Siena, Via Aldo Moro 2, I-53100 Siena, Italy.
authors.forename: Maria Camilla
authors.initials: MC
authors.lastname: Baratto
authors.affiliation: Department of Agri-Food and Environmental Science, University of Florence, Viale delle Idee 30, I-50019, Sesto Fiorentino, Florence, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Ferrini
authors.affiliation: Department of Agri-Food and Environmental Science, University of Florence, Viale delle Idee 30, I-50019, Sesto Fiorentino, Florence, Italy The National Research Council of Italy, Department of Biology, Agriculture and Food Sciences, Institute for Plant Protection, Via Madonna del Piano 10, I-50019, Sesto Fiorentino, Florence Italy.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Brunetti
authors.affiliation: Department of Agri-Food and Environmental Science, University of Florence, Viale delle Idee 30, I-50019, Sesto Fiorentino, Florence, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Calamai
authors.affiliation: The National Research Council of Italy, Department of Biology, Agriculture and Food Sciences, Institute for Plant Protection, Via Madonna del Piano 10, I-50019, Sesto Fiorentino, Florence Italy tattini@ipp.cnr.it.
authors.forename: Massimiliano
authors.initials: M
authors.lastname: Tattini
mesh_terms: D000975:Antioxidants / Q000378:metabolism*; D031661:Fraxinus / Q000033:anatomy & histology / Q000187:drug effects / Q000378:metabolism*; D010788:Photosynthesis / Q000187:drug effects*; D018515:Plant Leaves / Q000502:physiology; D018526:Plant Transpiration / Q000187:drug effects; D036226:Seedlings / Q000033:anatomy & histology / Q000187:drug effects / Q000378:metabolism; D012965:Sodium Chloride / Q000494:pharmacology*; D013472:Sunlight
chemical_list: D000975:Antioxidants; D012965:Sodium Chloride
keywords: Antioxidant enzymes; Fraxinus ornus; electron spin resonance (ESR); gas exchange performance; ionic relations; leaf morpho-anatomy; oxidative damage; photosynthesis; salinity; shade versus sun; transmission electron microscopy (TEM); water relations