Ecophysiology of salt excretion in the xero-halophyte Reaumuria hirtella

Abstract

Different aspects of salt excretion from leaves of the xero-halophyte Reaumuria hirtella Jaub. et Sp. (Tamaricaceae) were investigated under the extremely arid conditions of the desert. The diurnal excretion pattern, which gradually decreased toward midday, showed a negative correlation with the daily transpiration pattern. The relative excretion, which is the ratio between the absolute excretion of Na⁺ and the change in its internal content, was maximal at sites with low salinity, and decreased when the concentration of NaCl in the root environment increased. By contrast, the absolute excretion increased from 253 to 323 mmol Na⁺ g⁻¹ f. wt per 12 h with increase in the total soil salinity from 0·24 to 1·23%, respectively. Water stress conditions, caused either by increasing soil salinity or by dryness of the atmosphere, greatly influenced the efficiency of excretion process.

Six ions: Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻ and SO₄²⁻ constituted 96·8% of the d. wt of the field-collected secreted salts. Na⁺ and Cl⁻, which were the predominating ions in the soil solution of the root environment, were also the predominantly secreted, constituting c. 89% of salts secreted. The proportion of Na⁺ which was prevented from entry into the roots increased with increasing Na⁺ concentration in the soil solution. More than 67% of the absorbed NaCl was secreted by leaves during the day. However, the accumulated salts contributed to the osmotic potential (ψ_s) and improved the plant water status. The osmotic potential of the plant sap at the end of the day or during periods of low excretion rates was shown to be decreased −7·3 to −13·7 bar from that in the morning. The fraction of osmotic potential decreased during the day (Δψ_s) was positively correlated with the soil salinity. These retained salts are expelled from the leaves by increased efficiency of extraction during the night and early morning, so that the plant begins the day with low internal salt content, and the leaf reservoirs become suitable for a new supply. The rejection mechanism at the plant roots and the secretion mechanism at shoots allow the plant to maintain its internal salt content at an approximately constant level despite the great variation in soil salinity.