**5.1. Salinisation and wider (agro)ecological impact**

stress. Salt stress encompasses wide range of physiological dysfunctions as a consequence of primary salinity effects, that is, osmotic and ionic disorder. Primary salinity effects, depending on the salinity level/duration, crop/genotype type, development stage, and so on, very often cause different secondary salinity-induced effects such as reduced cell expansion and assimilate production (i.e. growth and yield reduction), production of reactive oxygen metabolites and even plant mortality [75]. The general salinity effects are quite visible and assume reduce biomass growth (shoot/root height and weight, leaf area) and changes in root and shoots colour (e.g. presence of leaf tip burns, scorching/firing of leaves) [76]. The extent to which growth and yield will be reduced under salt stress mostly depends on the salinity level

Electrical conductivity (EC) is commonly used as an expression of the total dissolved salt concentration in an aqueous sample (e.g. water, soil solution) and usually express soil salin-

Therefore, ECe threshold level (i.e. ECt), as a numeric value at which crop growth and yield start to decline (more or less intensive under certain slope) can be very useful for categorisation of plants from salt tolerant (halophytes) to salt-sensitive (glycophytes) (**Figure 2**). In general, wheat is categorised as moderately tolerant to soil salinity (e.g. a threshold EC of 6.0 dS/m) [78] although existing significant differences among genotypes that it is difficult to make a categorical statement [79]. The relative effects of salt stress on wheat vegetative

**Figure 2.** Simplified presence of salt tolerance in some cereals (adapted according to Maas [81]). ECt represents the threshold in soil EC that is expected to cause the initial significant reduction in the maximum expected yield, whereas

the slope is the percentage of yield expected to be reduced for each soil salinity unit above the ECt [82].

) (e.g. Rhoades et al. [77]).

ity level based on measured EC of saturated soil paste extracts (ECe

and plant (crop) species (**Figure 2**).

36 Global Wheat Production

Environmental salinisation process represents an increasing environmental issue especially in intensive agroecosystems such as (fert)irrigated areas [83] but also in less intensive rain feed (semi)arid regions [84]. Salt-affected areas are often overlapping with numerous other physical, chemical and/or biological pedosphere constrains such as sandy soils with low water retention capacity, non-structured/dispersed (waterlogged) soils, organically depleted soils with diminished microbial activity/diversity and excessive alkalinity, specific ionic (Al, B) toxicity, and many other [84, 85] (**Figure 2**). Saline or alkaline (sodic) soils due to increased concentration of particular slats (Na+ , Cl<sup>−</sup> , Ca2+, Mg2+ etc) and ionic interrelations (e.g. Na+ /Ca2+; Na+ /Mg2+) can be recognised and visually by crystallised (precipitated) salts on the soil surface (forming a brighter salt forms on the soil surface; **Figure 3a**) or at later (developed) stages by topsoil crusting, as a consequence of dispersed clay minerals and soil aggregates (**Figure 3b**).

A constitute of structurally dispersed soils (e.g. clay particles, minerals, organics) undergo leaching through the soil profile, accumulating and blocking deeper macro/micro pores, especially in textured-heavier soil layers, and finally causing waterlogging (e.g. Burrow et al. [86]. Thus, salt-affected soils (profiles) depleted in adsorption matrices (organic matter and clay content notably) might be more prone to mobility and transfer of certain pollutants (e.g. toxic trace elements) on the soil/crop/groundwater routes, although certain genotypic differences have to be considered.

For instance, it was shown that raised soil solution salinity can significantly impact mobility of toxic Cd in the rhizosphere and enhance its uptake and root/shoot accumulation in different wheat cultivars [87, 88]. Also, sublimating the results of studies conducted by Norvell et al. [89], Khoshgoftarmanesh et al. [88] and Ozkutlu et al. [90] their outcomes suggest that

**Figure 3.** Topsoil (a) crystallisation of soluble salts (dotted brighter areas) in a wheat paddock, depleted with soil organic matter, and (b) crusting in an adjacent saline plot with disturbed soil structure (Esperance area, Western Australia).

durum *vs*. bread wheat genotypes could be more effective, not only in Cd root extraction, but also in Cd root to shoot (leaf/grain) translocation and deposition under excessive Cl salinity. Such genotypic differences should be considered also in wheat breeding programs related to salt resistance (next section).

Application of (in)organic soil amendments, such as mineral/organic fertilisers, lime, gypsum phospho-gypsum, and so on to salt-affected pedosphere has multi-beneficial impact [75]. Introduction of Ca-/Mg-enriched amendments enhances to maintain soil micro-aggregate structure in the soil profile, and consequently improves physical pedovariables such as improved flocculation, reduced spontaneous dispersion (air-dry aggregates) and dispersion of remoulded aggregates, increased hydraulic conductivity and soil aeration [92]. Furthermore, it was confirmed that soil salinity/alkalinity is frequently associated with microelement Zn deficiency, and that under such conditions, application of certain inorganic Zn-based fertilisers is able to improve salt tolerance but also and nutritional value of wheat. Namely, ~40% of the soils used for wheat production in Iran are Zn-deficient [93] and comparing to some other widely cropped cereals, wheat genotypes are especially very sensitive to Zn deficiency which markedly reduce wheat grain yield [94]. However, one of the biggest issues with soil amendments (Ca-/Mg-/Zn-based) application and their beneficial impact to crops in saline conditions is often lacking of their dissolution (i.e. phytoavailability of specific element/substance)

Wheat Sensitivity to Nitrogen Supply under Different Climatic Conditions

http://dx.doi.org/10.5772/intechopen.76154

39

Another promising strategy to enhance wheat salt tolerance might be introduction of salt more tolerant root-associated microbes that enhance plant growth under excessive salinity. Namely, it was widely discussed how spatial rhizosphere adaptation of plants is also driven by genetic differentiation in closely associated microbe populations such as: (i) arbuscular mycorrhizal fungi (whose hyphal networks ramify throughout the soil and within the plant cells) then (ii) ectomycorrhizal fungi (over a fungal layer around the root system and root intercellular spaces) and (iii) root-associated plant growth-promoting rhizobacteria (see reviews by Rodriguez and Redman [95]; Dodd and Perez-Alfocea [96]). Alleviation of salt stress on yield and mineral nutrition (e.g. increased K/Na ratio) exploiting the arbuscular mycorrhizal fungi was confirmed in certain wheat varieties under field saline conditions [97]. For instance, the mycorrhizal colonisation enhanced grain wheat yield up to >31% in Kavir (spring cultivar), up to >32% in Roshan (spring and semi-early maturing cultivar) and even up to >38% in Tabasi (mutated salt tolerant line) [97]. Furthermore, Sadeghi et al. [76] applying the isolate of *Streptomyces* in cultivated soil with wheat (cul. Chamran) observed: (i) increased the growth/ development and shoot concentration of N, P, Fe and Mn in both saline and non-saline conditions and (ii) significant increases in germination rate, percentage and uniformity, shoot length and dry weight of salt-exposed plant (*vs*. non saline control). Also, studying the effect of inoculation of the five halotolerant bacterial strains in alleviation of NaCl-induced stress (80–320 mM) in wheat (var. HD 2733) Ramadoss et al. [98] observed an increase in root elongation (by >90%) and root dry weight (by >17%) in comparison with control (uninoculated) plants. Such beneficial effects of salt-tolerant microbes to (wheat) crops exposed to salinity are explained by improved plant water relations (e.g. due to enhanced accumulation of specific osmolytes), then regulating plant homeostasis and improved phytonutrients (e.g. N, P, K, Zn,

due to (semi)arid conditions and/or not implemented irrigation practice.

Cu, Mn, Fe) uptake as well by enhanced germination rate [96, 97, 99].

Breeding programs to salt tolerance (as relatively long-term approach) are expecting that might have crucial role in (wheat) cropping under saline conditions in the near future (see down). Relatively little work has been done on breeding programs of wheat cultivars for saline conditions [80] given on polygenic character of salt tolerance, but continuous progress
