F. Role of Transpiration
Shoot ion concentrations are a product of transpiration rate, xylem ion concentrations, and growth rate
[93]. Under high evapotranspirational demands, transpiration increases, while K/Na selectivity decreases,
resulting in increased Na and Cl uptake [94,95]. Alternatively, a reduction in transpiration can decrease
ion (Na and Cl) uptake [7,39,72].
A number of hypotheses have been proposed to explain increased xylem sap Na and Cl levels under
high evapotranspiration rates in saline growth media. Enhanced water flow interacts with ion flow across
membranes of root cells at more than one site, thus interfering with processes that regulate the balance
between ion accumulation in the root cell vacuole and transport to the shoot [7]. Increased water flow due
to transpiration promotes passive ion movements where there is no active transport barrier [96].
Water flow can promote the ion flow across the cortex toward a pump that secretes ions into xylem
vessels [92]. Ions could either be moved by water along an apoplastic pathway at high concentrations or
be coupled to water flow during symplastic passage across the root [97]. High transpiration rates in-
creased Na transport more than K, thus shifting the selectivity toward Na [98,99]. Potassium ions ab-
sorbed in roots may be released through Na/K exchange, mainly from vacuoles, for transport to the shoot
at times of high evaporative demand [7].
In halophytes, the entry of ions such as Na or Cl into the roots or their release to the xylem sap is
tightly regulated at high evapotranspirative demand under saline conditions, thus regulating ion supply to
GENETIC IMPROVEMENT OF SALINITY TOLERANCE IN CROP PLANTS 863
Figure 2 Relationship between decrease in dry matter production and (a) Na retranslocation and (b) efflux
of Na from roots in species differing in their tolerance to salinity. (From Ref. 87.)