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.