To identify Cd- and Zn-accumulating plants exhibiting a high growth rate, seeds from the halophyte species Mediterranean saltbush (Atriplex halimus L.) were collected on a heavy-metal-contaminated site in southeastern Spain (Llano del Beal, Cartagena). Seedlings from this ecotype were exposed for 3 wk to 0.1 mM Cd or Zn in a nutrient solution in a fully controlled environment. All plants remained alive and no significant growth inhibition was recorded until the end of the experiment. Mean Cd and Zn accumulation in aerial parts was 830 and 440 mg kg(-1), respectively, and the rate of metal translocation even increased with the duration of stress exposure. Resistance to heavy metals in this species may be partly linked to precipitation of Cd in oxalate crystals in the stems. A Cd-induced decrease in glutathione concentration also suggests that phytochelatins overproduction may occur in these conditions. We conclude that Mediterranean saltbush, which is able to produce up to 5 Mg dry matter ha(-1) yr(-1), may be an effective species for phytoextraction and should be tested for this purpose in field conditions.

In order to characterize physiological modifications encountered by buckwheat plants exposed to both drought and lowlight stresses, seedlings (cv. La Harpe) were exposed under controlled environmental conditions, to a progressive decline in soil volumetric water content under two light regimes: low irradiance (80 mmol m À2 s À1 ) or moderate irradiance (160 mmol m À2 s À1 ). Phenological evolution of the whole plant until the macroscopic appearance of the reproductive structure and physiological properties of leaves in relation to their position on the main axis were quantified. Water stress reduced net assimilation rate (NAR) before specific leaf area (SLA) and induced a decrease in stomatal conductance (g l ) and carbon isotope discrimination (D). Water consumption by stressed plants was similar under both light treatments. Water-stressed plants under moderate irradiance exhibited higher growth, NAR, osmotic adjustment, and lower SLA than plants maintained under low irradiance. However, the former died after 27 days of treatment while the latter still remained alive until the experiment was discontinued (40 days). We concluded that the physiological strategy adopted by the water-stressed plants maintained under moderate irradiance did not afford a long-term advantage in terms of survival. The effects of a combination of low-light and water stress on chlorophyll concentration and carbon isotope discrimination (D) are discussed in relation to growth parameters.Abbreviations -A, photosynthetic rate indicated by the assimilation of CO 2 ; chl, chlorophyll; Ci/Ca, ratio of internal leaf CO 2 concentration to ambient CO 2 concentration; D, carbon, isotope discrimination; yv, soil volumetric water content; d 13 C: carbon isotope composition; FWt, fresh weight at full turgor; g l , leaf stomatal conductance; LA, leaf area; NAR, net assimilation rate; RWC, relative water content; SLA, specific leaf area; Cs, osmotic potential.

In order to analyze the adaptation potential of tomato shoots to a sudden increase in Cd concentration, tomato plants (Solanum lycopersicum L. var. Ailsa Craig) were exposed under controlled environmental conditions to a high dose of this heavy metal (250 microM CdCl2) in nutrient solution for 7 and 14 d. Both root and shoot growth was completely inhibited but all plants remained alive until the end of the treatment. Cell viability remained unaffected but the activity of the mitochondrial alternative pathway was stimulated by Cd stress at the expense of the cytochrome pathway. Cadmium concentration was higher in roots than in shoots and a decrease in the rate of net Cd translocation was noticed during the second week of stress. Cadmium decreased both leaf conductance (g(l)) and chlorophyll concentration. However, the effect on net CO2 assimilation remained limited and soluble sugars accumulated in leaves. Photochemical efficiency of PSII (Fv/Fm) was not affected despite a decrease in the number of reaction centers and an inhibition of electron transfer to acceptors of PSII. It is concluded that tomato shoot may sustain short term exposure to high doses of cadmium despite growth inhibition. This property implies several physiological strategies linked to both avoidance and tolerance mechanisms.