Physiologia Plantarum 146: 26–38. 2012 Copyright © Physiologia Plantarum 2012, ISSN 0031-9317 Oxidative stress protection and stomatal patterning as components of salinity tolerance mechanism in quinoa (Chenopodium quinoa) Lana Shabala a,† , Alex Mackay a,† , YuTian b , Sven-Erik Jacobsen c , Daowei Zhou d and Sergey Shabala b,∗ a School of Agricultural Science, University of Tasmania, Hobart, Tas 7001, Australia b Key Laboratory of Vegetation Ecology, Institute of Grassland Science, Northeast Normal University, Changchun 130024, China c Faculty of Life Sciences, University of Copenhagen, DK-2630 Taastrup., Denmark d Northeast Institute of Geography and Agro Ecology, Chinese Academy of Sciences, Changchun 130012, China Correspondence *Corresponding author, e-mail: sergey.shabala@utas.edu.au Received 3 December 2011 doi:10.1111/j.1399-3054.2012.01599.x Two components of salinity stress are a reduction in water availability to plants and the formation of reactive oxygen species. In this work, we have used quinoa (Chenopodium quinoa), a dicotyledonous C3 halophyte species displaying optimal growth at approximately 150 mM NaCl, to study mecha- nisms by which halophytes cope with the afore-mentioned components of salt stress. The relative contribution of organic and inorganic osmolytes in leaves of different physiological ages (e.g. positions on the stem) was quantified and linked with the osmoprotective function of organic osmolytes. We show that the extent of the oxidative stress (UV-B irradiation) damage to photosynthetic machinery in young leaves is much less when compared with old leaves, and attribute this difference to the difference in the size of the organic osmolyte pool (1.5-fold difference under control conditions; sixfold difference in plants grown at 400 mM NaCl). Consistent with this, salt-grown plants showed higher Fv/Fm values compared with control plants after UV-B exposure. Exogenous application of physiologically relevant concentrations of glycine betaine substantially mitigated oxidative stress damage to PSII, in a dose- dependent manner. We also show that salt-grown plants showed a significant (approximately 30%) reduction in stomatal density observed in all leaves. It is concluded that accumulation of organic osmolytes plays a dual role providing, in addition to osmotic adjustment, protection of photosynthetic machinery against oxidative stress in developing leaves. It is also suggested that salinity- induced reduction in stomatal density represents a fundamental mechanism by which plants optimize water use efficiency under saline conditions. Introduction Elevated NaCl levels in the soil solution affect the ability of plants to take up water (Munns 2002). This osmotic effect has a flow-on affect, via internal signals, to reduce the rate of cell expansion in growing tissues, and the Abbreviations – EBC, epidermal bladder cell; GB, glycine betaine; NSCC, non-selective cation channels; ROS, reactive oxygen species; WUE, water use efficiency. † These authors have made an equal contribution to this work. degree of stomatal aperture in leaves. Both osmoti- cally induced stomatal closure and accumulation of high levels of toxic Na + species in the cell’s cytosol under saline conditions impair photosynthetic machin- ery and reduce a plant’s capacity to fully utilize light absorbed by the photosynthetic pigments (Shabala et al. 26 Physiol. Plant. 146, 2012