Abstract
Halophytes are salt-tolerant plants found exclusively in habitats with high levels of soil salinity. It is generally assumed that salt stress is the most important limiting factor for plant growth in natural saline environments, and that halophytes have developed specific adaptations to elevated salinity which make them unfitted to grow in the absence of salt, thus explaining their distribution in nature. To address experimentally this question, two halophytic species (Inula crithmoides L. and Plantago crassifolia Forssk.) and a maritime dune species (Medicago marina L.) were grown in the greenhouse for several weeks in different substrates: peat, vegetable garden soil, saline soil and sand from maritime dunes. Measurements of growth parameters-number of leaves, plant length, fresh and dry weights-showed that all three species grew much better on the salt-free and nutrient-rich substrates, peat and garden soil, than on saline soil and dune sand. These results indicate that salts are not compulsorily required for development of halophytic species, and suggest that limitation of water and nutrients, rather than soil salinity per se, are the most important restrictive factors for plant growth in saline habitats. The distribution of halophytes in nature is probably dependent on their limited ability to compete with glycophytes in non-saline areas, while remaining highly competitive under environmental conditions stressful for non-tolerant species.
Highlights
Halophilic (‘salt-loving’) plants, or halophytes, are salttolerant plants specific for saline environments
The peat substrate showed an acidic pH and low electrical conductivity (EC), the soil sampled from the salt marsh is alkaline with a high EC, whereas both garden soil and dune sand had similar, slightly basic pH and low EC
The evolution of leaf number (Fig. 1) and plant length (Fig. 2) during ten weeks of growth clearly showed that all species grew better on peat, followed by garden substrate; development was slower and by far less efficient on the other two substrates used, saline soil and dune sand
Summary
Halophilic (‘salt-loving’) plants, or halophytes, are salttolerant plants specific for saline environments. Halophytes have been defined as those plants able to grow and complete their life cycle in habitats with soil salinity higher than 200 mM NaCl (Flowers et al, 1986; Flowers and Colmer, 2008) This operational definition, which excludes 99% of all angiosperm species, may be useful but, obviously, this concentration threshold is rather arbitrary; there is a continuous spectrum of salt tolerance among plant species, from typical glycophytes (salt-sensitive plants) to extreme halophytes. Several definitions (Aronson and Le Floc’, 1996; Chapman, 1936; Dansereau, 1957) suggest that salts-especially NaCl-are compulsorily required during the life cycle of halophytes, due to their stimulating effect upon several biological processes in this type of plants To emphasize this requirement, a sub-category of halophytes has been described: euhalophytes, sometimes called ‘absolute halophytes’, ‘exclusive halophytes’, or ‘obligatory halophytes’; by extrapolation or by misinterpretation of the original definition, the term ‘euhalophytes’ is about to be used for all categories of salt tolerant plants
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