2.10. Salt resistance

Many areas of the world, including coastal ones, those with poorly drained soils in arid climates, and those with poorly designed irrigation systems, feature high concentrations of salt (>100–200 mM sodium chloride, NaCl). Only salt-resistant species, which exhibit strategies to reduce or avoid damaging effects of excess salt in their tissues, are able to maintain viable populations in such areas. Plants specialised for inhabiting saline soils, and often restricted to these, are termed halophytes.

Among members of the at least 139 plant families that include halophytes, evolution has yielded multiple solutions to the problem of excess salt in the environment, involving different biochemical, physiological, structural and/or phenological traits. Therefore, rather than a single recipe for assessing salt resistance, we give several traits and measurements that together help identify a species as salt-resistant, especially if these are accompanied by data on species distribution in saline areas. However, to positively classify a species as salt-sensitive would be problematic from these traits alone. Experimental testing of plant survival and growth under saline conditions is necessary, which would by no means be quick and easy for screening multiple species. Thus, the traits described below allow a qualitative rather than quantitative assessment of salt resistance, and do not allow the clear separation of more or less salt-resistant species from true halophytes. Hopefully, this text will stimulate research into novel approaches and protocols for testing salt resistance more efficiently and comprehensively.

Here, we simplify more extensive, previous classifications of mechanisms by which plants deal with excess environmental NaCl, focusing on three common strategies. Some salt-resistant plant species can limit the uptake of potentially damaging Na+ by their roots (NaCl ‘excluders’). However, many salt-resistant species cannot avoid significant NaCl uptake. These plants can either actively excrete excess salt or can accumulate NaCl in cell vacuoles, so as to prevent toxicity to the cytosol. The latter (‘salt-tolerant’) species are often succulent, with many characteristics of drought-tolerant species. Many salt-resistant species possess biochemical mechanisms to reduce salt stress or damage in the tissues, by accumulating compatible solutes (including secondary metabolites) in the cytosol. The salt-resistance traits detailed below fall into the foregoing categories, except special biochemical adaptations that are not covered here.

What and how to measure?

Selective root cation uptake. Roots of many salt-resistant plant species (particularly monocots) can discriminate against Na+, while maintaining uptake of essential potassium (K+). This selectivity for K+ over Na+ increases the K+ : Na+ ratio in the cytosol compared with that in the rooting medium. Because these ratios may vary with several environmental factors, including precipitation and evapotranspiration, we suggest sampling leaves and soil on at least three different days, at intervals of 2 weeks or more during the growing season, but not for 5 days after particularly heavy or prolonged rain. Collect leaves from five separate plants (Appendix 1), and a soil sample from the main fine-root zone below each. The Na+ and K+ concentrations of each sample are to be determined in the laboratory by a standard assay. Popular and convenient methods include atomic emission spectrometry (EAS), also called flame photometry, and atomic absorption spectrometry (AAS). Leaf samples are to be ground in an equal mass of water, which is then extracted from the homogenate by filtration. For soil, add water to a dry soil until it becomes water-saturated and then extract the liquid by suction or vacuum filtration. Na+ and K+ assays can be performed either on the water phase, or after evaporating it, depending on the Na+ and K+ assay method.

Calculate, for each plant and associated soil sample, the K : Na selectivity (S) as S = ([K+]/[Na+])plant / ([K+]/[Na+])soil. A mean S value for a species is calculated from the mean of all replicate S values per sampling date, by taking the average of these over all sampling dates.

Salt excretion. Salt-excreting species eject NaCl through special glands or bladders on the (usually lower) surfaces of their photosynthetic organs (usually leaves, but in some cases stems). These glands are often visible (especially under a hand lens) as small, irregularly shaped white spots that are excreted salt crystals on the surface of the gland. A salty taste, on licking one of these, will confirm this. Some species excrete salt from their roots. Although this is more difficult to observe, one may check for similar salt excretions on the surfaces of any roots uncovered during soil sampling. Note that salt excretions on shoots or roots will wash off during wet weather, so are best sought after a dry period.

Salt compartmentalisation. Salt compartmentalisation is indicated by clear succulence of the leaves or photosynthetic stems. Succulent green stems can be treated and measured as if they were leaves (see Special cases or extras in Section 3.1). Succulence leads to high leaf water content (LWC) and leaf thickness (Lth), and may be quantified as the product of these parameters (succulence (mm) = Lth × LWC) (see Section 3.3). Values >800–1000 mm indicate significant succulence.

Strong salt-related succulence is found almost exclusively in dicotyledonous species, although certain salt-tolerant monocots can be somewhat succulent, such as e.g. Elytrigia juncea on beach dunes. Salt-tolerant succulents show a high NaCl level in their leaves, which can distinguish them from crassulacean acid metabolism (CAM) succulents (see Section 3.12; some salt-tolerant succulents are actually also CAM plants). This could be detected by the Na assay on leaf or stem extracts noted above, or would be revealed very easily by measuring the electrical conductivity of such extracts (see Electrolyte leakage in Section 3.14), which requires only a simple, widely available conductivity meter (NaCl in solution gives a high conductivity). Qualitative evidence for this can be a combination of juiciness and noticeably salty taste when chewing the tissue. This property has made some halophytes popular as human food, e.g. Salicornia spp.

Special cases and extras

(1) Succulents and halophytes. Many salt-tolerant succulents are halophytes and occur only in saline environments; expression of the traits described above can depend on the actual salinity of the plants’ soil. We, therefore, suggest measuring soil salt concentrations (as described under Selective root cation uptake above, within the present Section) to accompany trait measurements. Several other salt-related habitat descriptors are also relevant, e.g. elevation and duration of daily marine inundation (if any) in salt marshes or on beaches, and location relative to the high tide mark visible as a litter belt, or white patches on the soil surface, indicating salt crystals in dry areas.
 
References on theory, significance and large datasets: Flowers et al. (1977, 1986); Yeo (1983); Rozema et al. (1985); Zhu (2001); Breckle (2002); Munns et al. (2002); Vendramini et al. (2002); Ashraf and Harris (2004); Flowers and Colmer (2008).

More on methods: Jennings (1976); Maas and Hoffman (1977); FAO (1999); Breckle (2002); Vendramini et al. (2002).