ECOPHYSIOLOGY OF SALT EXCRETION IN THE XERO-HALOPHYTE REAUMURIA-HIRTELLA

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 pa ttern, which gradually decreased toward midday, showed a negative corr elation with the daily transpiration pattern. The relative excretion, which is the ratio between the absolute excretion of Na+ and the chang e in its internal content, was maximal at sites with low salinity, and decreased when the concentration of NaCl in the root environment incr eased. By contrast, the absolute excretion increased from 253 to 323 m mol 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 eith er by increasing soil salinity or by dryness of the atmosphere, greatl y influenced the efficiency of excretion process. Six ions: Na+, K+, C a2+, Mg2+, Cl- and SO42- constituted 96.8%, of the d. wt of the field- collected secreted salts. Na+ and Cl-, which were the predominating io ns in the soil solution of the root environment, were also the predomi nantly secreted, constituting c. 89% of salts secreted. The proportion of Na+ which was prevented from entry into the roots increased with i ncreasing Naf concentration in the soil solution. More than 67% of the absorbed NaCl was secreted by leaves during the day. However, the acc umulated salts contributed to the osmotic potential (psi(s)) and impro ved 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 fracti on of osmotic potential decreased during the day (Delta psi(s)) was po sitively correlated with the soil salinity. These retained salts are e xpelled from the leaves by increased efficiency of extraction during t he night and early morning, so that the plant begins the day with low internal salt content, and the leaf reservoirs become suitable for a n ew supply. The rejection mechanism at the plant roots and the secretio n mechanism at shoots allow the plant to maintain its internal salt co ntent at an approximately constant level despite the great variation i n soil salinity.