Abstract
Current indices of saline-alkaline (SA) tolerance are mainly based on the traditional growth and physiological indices for salinity tolerance and likely affect the accuracy of alfalfa tolerance predictions. We determined whether the inclusion of soil alkalinity-affected indices, particularly Ca2+, Mg2+, and their ratios to Na+ in plants, based on the traditional method could improve the prediction accuracy of SA tolerance in alfalfa, determine important indices for SA tolerance, and identify suitable alfalfa cultivars in alkaline salt-affected soils. Fifty alfalfa cultivars were evaluated for their SA tolerance under SA and non-SA field conditions. The SA-tolerance coefficient (SATC) for each investigated index of the alfalfa shoot was calculated as the ratio of SA to non-SA field conditions, and the contribution of SATC under different growth and physiological indices to SA tolerance was quantified based on the inclusion/exclusion of special alkalinity-affected indices. The traditional method, excluding the special alkalinity-affected indices, explained nearly all of the variation in alfalfa SA tolerance, and the most important predictor was the SATC of stem length. The new method, which included these special alkalinity-affected indices, had similar explanatory power but instead identified the SATC of shoot Ca2+/Na+ ratio, followed by that of stem length, as key markers for the field evaluation of SA tolerance. Ca2+, Mg2+, and their ratios to Na+ hold promise for enhancing the robustness of SA-tolerance predictions in alfalfa. These results encourage further investigation into the involvement of Ca2+ in such predictions in other plant species and soil types under more alkaline salt-affected conditions.
Highlights
It is estimated that ca. 955 × 106 ha of land suffers from salinity and sodicity [1,2] with 60% of salt-affected soils primarily categorized as sodic/saline soils [3]
Coupled with the significant relationship identified between stem length (SL) and Shoot dry mass (SDM) (Table S5), these results provide supporting evidence for the selection of the saline-alkaline-tolerance coefficient (SATC) of shoot Ca2+ /Na+ ratio and SATC of SL as important markers for predicting the SA tolerance of alfalfa crops
The results revealed that the inclusion of shoot Ca2+, Mg2+, and their ratios to Na+ holds potential for enhancing SA-tolerance evaluations of alfalfa plants
Summary
It is estimated that ca. 955 × 106 ha of land suffers from salinity and sodicity (alkalinity) [1,2] with 60% of salt-affected soils primarily categorized as sodic/saline soils [3]. As one of China’s five largest salt-affected soil regions, the Songnen Plain encompasses 3.42 × 106 ha of salt-affected soils characterized mainly by NaHCO3 and Na2 CO3 salts [4,5] Such alkaline salt-affected soils often exhibit unique structural problems as a result of certain physical processes (slaking, swelling, and dispersion of clay) and specific conditions (i.e., surface crusting and hardsetting) due to the Agronomy 2020, 10, 191; doi:10.3390/agronomy10020191 www.mdpi.com/journal/agronomy. As one of the most vital perennial legume forages cultivated worldwide, alfalfa (Medicago sativa L.) is a prominent feature of salt-affected lands, such as saline soil [16], mainly because of its good nutritional forage quality for livestock husbandry and its ability to fix nitrogen; it is considered moderately sensitive to salinity stress and alkalinity stress [17] Different from saline soils, where the focus is on ionic osmotic stress due to high salinity [18,19,20,21,22], alfalfa grown in such SA conditions often experiences alkaline stress, which inhibits plant growth and reduces yield [23,24]
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