17.2.3  Heritability of salt tolerance

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Put simply, salt tolerance in vascular plants considered so far (non-halophytes) is achieved by a combination of root exclusion and shoot compartmentation. Both aspects of salt tolerance vary according to genetic makeup with strong interspecific contrasts in their relative importance. Pistachio and quandong showed remarkable tolerance of leaf salt compared with grapevines and citrus; lupins had a meagre capacity for salt exclusion compared to wheat and especially barley (Section 17.2.1).

Such variation in salt tolerance between species within a commercially important genus is always of interest to applied science, and offers promise of achieving even further gains by judicious breeding and selection for improved traits. Significantly, Sykes (1992) has shown that Cl ion exclusion is a heritable trait, and in crosses between Vitis champini (a strong excluder) and V. vinifera (a weak excluder) some offspring transgressed the Cl-excluding capacity of either parent (Sykes 1985). Field data on petiolar Cl in these hybrids revealed a 16-fold range in Cl-excluding capacity, skewed towards exclusion, implying dominance effects. Again, in crosses and backcrosses involving V. berlandieri and V. vinifera, hybrids segregated as either strong or weak Cl excluders, pointing to a dominant gene for Cl exclusion from V. berlandieri (Sykes 1987). Hybridisations between highly heterozygous species of grapevine thus offer good prospects for further improvement in Cl-excluding capacity of root-stocks for future irrigation viticulture.

In citrus, exclusion of both Cl and Na+ ions are quantitative characters (controlled by many genes), but are inherited independently. Consequently, a strong Cl excluder is not necessarily a strong Na+ excluder or vice versa, but some modest positive correlations do apply in progeny from some crosses (Clementine mandarin × trifoliate orange, Clementine mandarin × Rangpur lime, and Rangpur lime × trifoliate orange; Table 1 in Sykes 1992).

Rangpur lime (Citrus reticulata var. austera hybrid?) and trifoliate orange (Poncirus trifoliata) are of special relevance to genetic improvement of rootstocks because Rangpur lime is well established as a strong Cl excluder while trifoliate orange is recognised as a strong Na+ excluder (Table 17.5). Hybrid individuals which inherit both traits would offer promise of significant gains for citriculture, and their detection warrants strenuous efforts. To this end, Sykes’s screening trial (Figure 17.16) showed continuous variation in Cl ion accumulation by leaves that ranged between two extremes: 0.5% to 7.0% Cl (% of dry mass). Encouraged by the extent of this genotypic variation as well as by a general correlation with Na+ ion exclusion, Sykes (1992) identified some contrasting genotypes for propagation and further testing (hatched bars in Figure 17.16). New salt-excluding citrus hybrids were sub-sequently selected that performed better than parent cultivars when grafted to a common scion and established in salinised field plots.



Figure 17.16 Hybrid progeny (numbers along abscissa) from Rangpur lime × trifoliate orange show wide variation in Cl- ion exclusion that is broadly correlated with Na+ ion excluding ability. Variation in Cl- ion exclusion shown here spans a range that exceeded differences between the original parents and offered promise of genetic improvement in salt-excluding rootstocks. Dark green bars correspond to progeny that were selected for field trials and confirmed results from this initial screening. (Based on Sykes 1992; reproduced with permission of Kluwer Academic Publishers)

Next to wheat, rice (Oryza sativa) would rank as the world’s most important cereal, and the International Rice Research Institute (IRRI) in the Philippines has an ongoing program of genetic improvement for salt tolerance. Significantly, rice is less salt tolerant than wheat, presumably because selection pressure for salt tolerance was not a major part of its evolutionary history, but rice lands in southern and southeast Asia are becoming more saline so that development of tolerant cultivars is a matter of some urgency.

Garcia et al. (1997) report significant progress in this quest, as well as providing a neat explanation for interspecific differ-ences between wheat and rice with respect to transpirational flow and salt tolerance. Progeny/parent regressions of two segregating breeding populations were used to estimate narrow-sense heritability of both Na+ transport and K+ transport, while an apoplasmic tracer (a fluorescent dye) was used to estimate bypass flow (apoplasmic leakage).



Figure 17.17 When Na+ content of rice shoots is expressed as a function of apoplasmic flow (inferred from accumulation of a fluorescent dye), variation is clearly correlated with apoplasmic flow, and implies a functional relationship. By contrast, wheat shoot Na+ is unrelated to apoplasmic flow, and confirms a qualitative difference between these two species in their mode of salt exclusion. (Based on Garcia et al. 1997)

Transpirational flow via a plant’s apoplasm bypasses mecha-nisms that exclude solutes so that NaCl is swept into transpiring shoots. By plotting shoot Na+ content as a function of bypass flow (Figure 17.17) a positive correlation in rice explains variation between genotypes in salt tolerance in terms of varying degrees of bypass flow. An absence of correlation in wheat shows that bypass flow does not explain genetic variation in shoot Na+ content, and points to a major difference from rice. Overall, bypass flow was estimated to be about 10 times greater in rice than in wheat, and this explains this major interspecific contrast in salt tolerance.

In summary, salt tolerance in wheat hinges on Na+ exclusion by roots and a compensatory accumulation of K+, especially in young leaves; a lower order of salt tolerance in rice is a consequence of an inherently lower water use efficiency, plus an extensive transpirational bypass flow that avoids root exclusion mechanisms.

Notwithstanding potentially large gains for salt tolerance in cereals such as rice, genetic variation within a species is generally more muted than variation between species. Such interspecific differences are most notable in present-day crop plants such as sugar beet with halophytic ancestors that carry highly specialised features. Such adaptations are considered in Sections 17.3 and 17.4.