Plant Growth-Promoting Bacteria: An Emerging Tool for Sustainable Crop
2 Soil Salinity: The Technical Issues 1 Soil Salinization Process
Land is a limiting resource, considering the fact that there are only about 5 mil- lion km2 available for future sustainable land use. Although earth abounds in water,
an almost negligible portion (~2.5 % or 35 million km3) is fresh or with low salt
concentration (<1 dS/m), i.e. water that may be conditionally used for irrigation in crop production, whereas the rest is salty and therefore unsuitable for irrigation (Ondrasek et al. 2010). It has been estimated that irrigated agriculture consumes ~70 % (and >90 % in many developing countries) of total water withdrawal to pro- duce ~36 % of global food (Howell 2001). As a consequence, there is a continuous degradation of land resources (e.g. salt-affected soils), representing a large burden to natural ecosystems. According to FAO, Land and Plant Nutrition Management Service (2008), over 6 % of the world’s land is affected by either salinity or sodicity. The major causes of naturally induced salinity are salt water intrusion and wind- borne salt deposition in land. Another major cause for soil salinity is the deposition of oceanic salt carried in wind and rain. Salts also originate from mineral
105
weathering. The anthropogenic factors include crop irrigation with salt waters through which soil salinization gets dramatically exacerbated and accelerated. The other factors include inorganic fertilizers and soil amendments through gypsum, composts manures, etc.
Salinization is a natural or human-induced process that results in accumulation of dissolved salts in soil water to an extent that inhibits plant growth. The processes may be primary (natural) and secondary (anthropogenic) in nature. It involves accu- mulation of water-soluble salts in soil that includes potassium (K+), magnesium
(Mg2+), calcium (Ca2+), chloride (Cl−), sulphate (SO
42−), carbonate (CO32−), bicar-
bonate (HCO3−) and sodium (Na+) ions. Depending on soils, the extracted solutions
differ in content of dissolved salts. The salt concentration, with electrical conductiv- ity (ECse), exceeds 20 mM (~2 dSm−1) and is categorized as salt affected (Abrol et al. 1988). A saline soil thus is defined as soil having a high concentration of sol- uble salts (ECe of 4 dSm−1 or more) that are enough to affect plant growth. However,
many crops are affected by soil with an ECe less than 4 dSm−1. Excessive sodium
(Na+) accumulation from salt destroys soil structure, deteriorates soil hydraulic
properties, raises soil pH and reduces water infiltration and soil aeration, leading to soil compaction, increasing erosion and water run-off. Furthermore, sodium, being the most pronounced destructor of secondary clay minerals by dispersion, replaces calcium (Ca2+) and other coagulators like Mg2+ and gets adsorbed on the surface
and/or interlayers of soil aggregates (Ondrasek et al. 2010). Dispersed clay particles undergo leaching through the soil to accumulate and block pores, especially in fine- textured soil horizons. The soil becomes unsuitable for proper root growth and plant development. The secondary result are salinity-induced sodicity, where leaching either through natural or human-induced processes washes away the soluble salts into the subsoil and leaves negative charges of sodium bound to the clay.
2.2 Salinity Impacts on Rhizosphere-Associated Microbes
Bacteria are adsorbed onto soil particles by ion exchange, and a soil is considered to be naturally fertile when the soil organisms are releasing inorganic nutrients from the organic reserves at a rate sufficient to sustain rapid plant growth. Since the soil organic matter and consequently the biomass and microbial activity are generally more relevant in the first few centimetres at the surface of the soil, salinization close to the surface significantly affects a series of microbiologically mediated processes. Along with it disturbs the natural ecosystem functioning and plant health. For rhizo- bacteria, life in high salt concentrations is bioenergetically taxing because they must maintain an osmotic balance between their cytoplasm and the surrounding medium while excluding sodium ions from the cell interior, and as a result, sufficient energy is required for adaptation. Depletion of potassium ions by plants under saline condi- tions further reduces the ability of rhizobacteria to use potassium ions as a primary osmoregulator. Plant use of osmolytes under salt stress deprives rhizobacteria of osmolytes, which finally limits the bacterial growth. The salinity level above 5 % thus reduces the total count of bacteria and actinobacteria drastically. In addition, it
106
inhibits nitrogen fixation, root exudation and decomposition of organic matter. Significant negative correlations between soil electrical conductivity and total CO2
emission or microbial biomass C have also suggested that it has severe adverse effect on microbial biomass and activities. Naturally occurring soil organic matter decomposers thus become sensitive to salt-induced stress, and the effect is always more pronounced in the rhizosphere pursuant to increased water uptake by the plants due to transpiration. Alteration of proteins, exo-polysaccharide and lipopoly- saccharide composition of the bacterial cell surface, impairment of molecular signal exchange between bacteria and their plant host due to the alteration of membrane glucan contents and inhibition of bacterial mobility and chemotaxis towards plant roots significantly affect microbial diversity in the rhizosphere, under saline condi- tions. Overall, salinity has a negative impact on microbial abundance, diversity, composition and functions.
2.3 Soil Salinity Effects on Plant Growth and Development
During the onset and development of salt stress within a plant, all the major pro- cesses such as germination, cell division and elongation, leaf growth, leaf expansion, photosynthesis, protein synthesis and energy and lipid metabolism are adversely affected (Fig. 2). During the vegetative stages, salt stress induces stomatal closure, leading to reduction in CO2 assimilation and transpiration. The reduced turgor poten-tials affect the leaf expansion and leaf area, which in turn reduces the light
Fig. 2 Effect of salt stress on crop growth and development
107
interception and photosynthetic rates, coupled with spurt in respiration resulting into reduced biomass accumulation. Excessive salts reduce the water potential of soil, making the soil solution unavailable to the plants and creates physiological drought. Also, osmotic pressure in the rhizosphere solution exceeds in root cells which reduces water and nutrient uptake. Salinity further creates nutritional imbalance through increase in uptake of Na+ or decrease in uptake of Ca2+ and K+ in leaves.
Excess Na+ causes metabolic disturbances in processes where low Na+ and high K+
or Ca 2+ are required for optimum growth and developmental functions. Excess
sodium and more importantly chlorides affect plant enzymes and cause cell swelling, resulting in reduced energy production and other physiological changes. Uptake and accumulation of Cl− disrupts the photosynthetic function through inhibition of nitrate reductase activity. Under excessive Na+ and Cl− rhizosphere concentrations, com-
petitive interactions with other nutrient ions (K+, NO
3− and H2PO4−) occur for bind-
ing sites and transport proteins in root cells that have adverse effects on translocation, deposition and partitioning within the plant. Once the capacity of cells to store salts is exhausted, salt build-up in intercellular space leads to cell dehydration and death. Plants suffer from membrane destabilization and a general nutrient imbalance. All micro- and macronutrient contents decrease in roots and shoots with increasing NaCl concentration in the soil. Osmotic stress decreases cell growth and development, reduces leaf area and chlorophyll content, accelerates defoliation and senescence and reduces the yields. The primary salinity effects give rise to numerous secondary ones such as oxidative stress, characterized by accumulation of reactive oxygen spe- cies potentially harmful to bio-membranes, proteins, nucleic acids and enzymes. The plants with perturbed nutrient relations are more susceptible to invasion of different pathogenic microorganisms and physiological dysfunctions, whereas their edible parts have markedly less economic and nutritional value due to reduced fruit size and shelf life, non-uniform fruit shape and decreased vitamin contents.