Effects of neutral salt and alkali on ion distributions in the roots, shoots, and leaves of two alfalfa cultivars with differing degrees of salt tolerance

Abstract

The effects of neutral salt and alkali on the ion distribution were investigated in two alfalfa (Medicago sativa L.) cultivars, including Zhongmu 1, a high salt-tolerant cultivar, and Algonquin, a low salt-tolerant cultivar. The alkali stress expressed more serious growth inhibition than the neutral salt stress at the same Na + concentration. Compared with Algonquin, Zhongmu 1 did not exhibit a higher alkali tolerance under the Na 2 CO 3 -NaHCO 3 treatment with the low Na + concentration (50 mmol L -1 ). The alkali increased the accumulation of Na + , Ca 2+ , and Mg 2+ in the root and changed the Ca 2+ and Mg 2+ balance in the entire alfalfa plant. The salt and alkali stresses decreased the K + and Fe 3+ contents of the roots and leaves, the root Mn 2+ content, and the shoot Zn 2+ content, but they increased the Fe 3+ accumulation of the shoots, the shoot and leaf Cu 2+ contents, and the leaf Zn 2+ content in both alfalfa cultivars. Based on the results obtained under the conditions of this experiment, we found that the salt and alkali stresses reduced the plant growth in both alfalfa cultivars, while the alkali caused a stronger stress than the neutral salt in alfalfa. Thus, we conclude that under hydroponic conditions, the deleterious effects of the alkali on plants are due to the distribution change of some trophic ion balance in the roots, shoots, and leaves of the plants by causing of Na + , CO 3 2-, and/or HCO 3 stresses.

Ca2+/Na+ Ratio as a Critical Marker for Field Evaluation of Saline-Alkaline Tolerance in Alfalfa (Medicago sativa L.)

“…Interestingly, increased Na + , Ca 2+ , and Mg 2+ accumulation in plant roots and an altered Ca 2+ and Mg 2+ balance in entire alfalfa plants were observed under controlled alkaline solution conditions, indicating the critical roles of plant Na + , Ca 2+ , and Mg 2+ in conveying SA tolerance in alfalfa under alkaline salt-affected conditions [24]. Additionally, a high soil pH environment can cause soil Ca 2+ and Mg 2+ to precipitate (CaCO 3 and MgCO 3 ), leaving fewer ions available for plant uptake [7,58].…”

“…Given the growing demand for food, animal feed, and fiber to accommodate an expanding human population with limited new productive land, it will become necessary to utilize salt-affected lands [14,15]. 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; however, 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]. Alfalfa is commonly at risk of ion toxicity, osmotic stress, and even high pH stress [25,26].…”

“…To accomplish this objective, a crucial prerequisite is distinguishing the most informative indices for the salt tolerance of alfalfa [27,28]. Traditionally, the methods for identifying these important indices for plant SA tolerance are often derived from salinity tolerance [29,30] (Table S1) [17,19,20,[23][24][25]28,. This is especially true for NaCl tolerance, in which the underlying mechanism for the maintenance of adequate K + in plant tissues under salt stress seems to depend upon selective K + uptake and selective cellular K + and Na + compartmentation and distribution in the shoots [21,27,55].…”

“…This is especially true for NaCl tolerance, in which the underlying mechanism for the maintenance of adequate K + in plant tissues under salt stress seems to depend upon selective K + uptake and selective cellular K + and Na + compartmentation and distribution in the shoots [21,27,55]. Consequently, the conventional indices for SA tolerance in alfalfa, including its main growth indices such as dry biomass [17], root parameters [17,31,32], and relative growth rate [23] at the temporal scale of both days and weeks under controlled solutions and soil conditions as well as its major physiological indices, such as photosynthesis components [23], antioxidant enzyme activity and malondialdehyde (MDA) [31], proline [17,31], soluble sugar [31], soluble protein [31], and cation content [23,24,[31][32][33], are often relied upon to express SA tolerance under controlled laboratory conditions. Recent reports have illustrated that important indices for conveying saline tolerance or SA tolerance can be identified by quantifying the relationships between plant physiological indices and biomass [28,30,56], or between some physiological indices and comprehensive D-values based on statistical tools, such as linear regression analysis [57].…”

“…Similarly, a high pH environment outside the roots can lead to greater absorption of Na + , thus competing with the uptake of other nutrient cations, including Ca 2+ and Mg 2+ [58]. Wang et al [24] suggested that the contributions of Ca 2+ and Mg 2+ to the maintenance of plant ion balance and the response to SA stress should not be ignored under SA conditions. Thus, it is reasonable to infer that the roles of plant Ca 2+ and Mg 2+ in explaining the SA tolerance of alfalfa may be considerable under soil conditions vis-à-vis their traditional roles in terms of salinity tolerance and SA solution conditions (Table S1) [17,19,20,[23][24][25]28,.…”