Different mechanisms of ion homeostasis are dominant in the recretohalophyte Tamarix ramosissima under different soil salinity

Abstract

Total ion (Na+, K+, Ca2+, SO4 2− and Cl−) accumulation by plants, ion contents in plant tissues and ion secretion by salt glands on the surface of shoots of Tamarix ramosissima adapted to different soil salinity, namely low (0.06 mmol Na+/g soil), moderate (3.14–4.85 mmol Na+/g soil) and strong (7.56 mmol Na+/g soil) were analyzed. There are two stages of interrelated and complementary regulation of ion homeostasis in whole T. ramosissima plants: (1) regulation of ion influx into the plant from the soil and (2) changing the secretion efficiency of salt glands on shoots. The secretion efficiency of salt glands was appraised by the ratio of ion secretion to tissue ion content. Independent of soil salinity, the accumulation of K+ and Ca2+ was higher than the contents of these ions in the soil. Furthermore, the accumulation of K+, Ca2+ and SO4 2− ions by plants was maintained within a narrow range of values. Under low soil salinity, Na+ was accumulated, whereas under moderate and strong salinity, the influxes of Na+ were limited. However, under strong salinity, the accumulation of Na+ was threefold higher than that under low soil salinity. This led to a change in the Na+/K+ ratio (tenfold), an increase in the activity of salt glands (tenfold) and a reduction in plant growth (fivefold). An apparently high Na+/K+ ratio was the main factor determining over-active functioning of salt glands under strong salinity. Principal component analysis showed that K+ ions played a key role in ion homeostasis at all levels of salinity. Ca2+ played a significant role at low salinity, whereas Cl− and interrelated regulatory components (K+ and proline) played a role under strong salinity. Proline, despite its low concentration under strong salinity, was involved in the regulation of secretion by salt glands. Different stages and mechanisms of ion homeostasis were dominant in T. ramosissima plants adapted to different levels of salinity. These mechanisms facilitated the accumulation of Na+ in plants under low soil salinity, the limitation of Na+ under moderate salinity and the over-activation of Na+ secretion by salt glands under strong salinity, which are all necessary for maintaining ion homeostasis and water potential in the whole plant.