The hypothesis that water relations and growth of phreatophytic Tamarix ramosissima Ledeb. and Populus euphratica Oliv. on dunes of varying height in an extremely arid Chinese desert depend on vertical distance to a permanent water table was tested. Shoot diameter growth of P. euphratica was inversely correlated with groundwater depth (GD) of 7 to 23 m (adj. R 2 = = = = 0.69, P = = = = 0.025); growth of T. ramosissima varied independent of GD between 5 and 24 m ( P = = = = 0.385). Pre-dawn (pd) and midday (md) water potentials were lower in T. ramosissima (minimum pd − − − − 1.25 MPa, md − − − − 3.6 MPa at 24 m GD) than in P. euphratica (minimum pd − − − − 0.9 MPa, md − − − − 3.05 MPa at 23 m GD) and did not indicate physiologically significant drought stress for either species. Midday water potentials of P. euphratica closely corresponded to GD throughout the growing season, but those of T. ramosissima did not. In both species, stomatal conductance was significantly correlated with leaf water potential ( P. euphratica : adj. R 2 = = = = 0.84, P < < < < 0.0001; T. ramosissima : adj. R 2 = = = = 0.64, P = = = = 0.011) and with leaf-specific hydraulic conductance ( P. euphratica : adj. R 2 = = = = 0.79, P = = = = 0.001; T. ramosissima : adj. R 2 = = = = 0.56, P = = = = 0.019); the three variables decreased with increasing GD in P. euphratica . Stomatal conductance of P. euphratica was more strongly reduced ( > > > > 50% between − − − − 2 and − − − − 3 MPa) in response to decreasing leaf water potential than that of T. ramosissima (30% between − − − − 2 and − − − − 3 MPa). Tolerance of lower leaf water potentials due to higher concentrations of leaf osmotically active substances partially explains why leaf conductance, and probably leaf carbon gain and growth, of T. ramosissima was less severely affected by GD. Additionally, the complex below-ground structure of large clonal T. ramosissima shrub systems probably introduces variability into the assumed relationship of xylem path length with GD.

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Phytosiderophore release in relation to micronutrient metal deficiencies in barleyD i r k Gries, S y l k e B r u n n , D a v i d E. C r o w l e y and D a v i d R. P a r k e r AbstractPhytosiderophore release occurs under both iron and zinc deficiencies in representative Poaceae and has been speculated to be a general adaptive response to enhance the acquisition of micronutrient metals. To test this hypothesis, phytosiderophore (PS) release rates from barley (Hordeum vulgare cv. CM72) subjected to deficiencies of Fe, Zn, Mn, and Cu were compared using chelator-buffered nutrient solutions. PS release rates were determined at two day intervals during onset and development of deficiency symptoms. Plant dry matter yields and nutrient concentrations, measured at three time points were used to construct growth curves for calculation of PS release per unit root mass and estimation of critical internal nutrient levels associated with PS release. In comparison to trace metal-sufficient control plants, dry matter production was markedly reduced in the Zn, Mn, and Cu deficiency treatments, with final relative yields of 49, 61, and 34%, respectively. Relative yields for Fe-deficient plants grown at three suboptimal Fe levels ranged from 95 to 33% of control, and provided a basis for comparison of PS release rates by Zn-, Mn-, and Cu-deficient plants at similar levels of growth inhibition. Under Fe deficiency, PS release increased with severity of the deficiency as measured by foliar Fe concentration, yield reduction, and chlorosis. Changes in PS release rates over time suggested a cyclical pattern that may be regulated by Fe concentration in the plant shoot. The highest rate of PS release (35 ~mol g-1 root dw 2 h -l) was measured after 10 days of growth at pFe 19, whereas control plants adapted for growth at pFe 17 released only 2 to 3 ~umol g-~ root dw 2 h -I . In a second experiment, maximum PS release rates for barley subjected to Zn, Mn, and Cu deficiencies were only 2.6, 2.5 and 1/lmol g-l 2 h -1, respectively, and were only slightly elevated over those of the control plants (ca. ! ~mol g-z root dw 2 h -l) grown at pFe 16.5. Moreover, enhanced PS release under Zn deficiency occurred much later, after the deficiency had already caused severely reduced growth. The results suggest that phytosiderophore release in this barley cultivar is a specific response to Fe deficiency and is not significantly induced in response to deficiencies of other trace metals.

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