The characteristics of Na +, K + and free proline distribution in several drought-resistant plants of the Alxa Desert, China

Abstract

The distribution characteristics of Na +, K + and free proline were investigated in succulent xerophytes— Haloxylon ammodendron and Zygophyllum xanthoxylum; xerophytes— Artemisia sphaerocephala and Caragana korshinskii; and mesophytes— Agriophyllum squarrosum and Corispermum mongolicum in the Alxa Desert of China. The results showed that mesophytes and xerophytes were salt excluding species, and the concentrations of Na + in these species were 1.5% to 3.8% of those in succulent xerophytes. Concentrations of K + in Agriophyllum squarrosum and Corispermum mongolicum were 1.3–2.7 times those in Artemisia sphaerocephala and Caragana korshinskii. Concentrations of K + in the stems of Agriophyllum squarrosum and Corispermum mongolicum were 1.8 and 2.2 times those in their roots, respectively. For mesophytes, accumulating large quantities of K + in their stems may facilitate water movement along a soil–plant gradient. The xerophytes accumulated large quantities of K + and free proline. Their proline concentrations in the whole plant were 6.0–16.0 times higher than those of mesophytes, and were 1.8–25.0 times higher than those of succulent xerophytes. In Artemisia sphaerocephala, the concentrations of proline increased by 3.1- and 10.5-fold from roots to stems, and from stems to leaves, respectively. A similar trend was observed in Caragana korshinskii. Therefore, accumulating K + and free proline may play a role in drought adaptation in xerophytes. Succulent xerophytes Haloxylon ammodendron and Zygophyllum xanthoxylum were identified as salt diluting species that absorbed much Na + by roots, and the Na + was transported to the leaves and photosynthesizing branches. The succulent xerophytes accumulated larger quantities of Na + than K + for osmotic adjustment even at low soil salinities, resulting in the lowest values of selective absorption and selective transport capacities of the root systems. Our data suggest that Na + accumulation rather than exclusion may be one of the most effective strategies for adaptation of succulent xerophytes to arid environments.