The pollution of soils by heavy metals resulting from mining activities is one of the major environmental problems in North Africa. Mycorrhizoremediation using mycorrhizal fungi and adapted plant species is emerging as one of the most innovative methods to remediate heavy metal pollution. This study aims to assess the growth and the nutritional status of ectomycorrhizal Pinus halepensis seedlings subjected to high concentrations of Pb, Zn, and Cd for possible integration in the restoration of heavy metals contaminated sites. Ectomycorrhizal and non-ectomycorrhizal P. halepensis seedlings were grown in uncontaminated (control) and contaminated soils for 12 months. Growth, mineral nutrition, and heavy metal content were assessed. Results showed that ectomycorrhizae significantly improved shoot and roots dry masses of P. halepensis seedlings, as well as nitrogen shoot content. The absorption of Pb, Zn, and Cd was much higher in the roots than in the shoots, and significantly more pronounced in ectomycorrhizal seedlings—especially for Zn and Cd. The presence of ectomycorrhizae significantly reduced the translocation factor of Zn and Cd and bioaccumulation factor of Pb and Cd, which enhanced the phytostabilizing potential of P. halepensis seedlings. These results support the use of ectomycorrhizal P. halepensis in the remediation of heavy metal contaminated sites.
Container-grown black spruce (Picea mariana (Mill.) B.S.P.) seedlings were planted in trays containing a sand and peat mixture, and placed in a climate-controlled greenhouse. One group of seedlings was kept well-watered, and another group was subjected to three cycles of drought. Gas exchange analysis showed that mesophyll photosynthetic function was largely unimpaired by drought. In contrast, stomatal conductance was sensitive to drought, although it became less sensitive with each drought cycle. Both stomatal and mesophyll conductances increased with time in control and drought-stressed seedlings, but mesophyll conductance increased with time more rapidly than did stomatal conductance. Limitation of photosynthetic rate was dominated by the mesophyll. In control seedlings, relative stomatal limitation increased from 6 to 16% by the end of the experiment. In drought-stressed seedlings, relative stomatal limitation of photosynthesis reached 40% during the first drought, but decreased to near control values immediately after rewatering. Because the third, most severe drought had only a minor effect on stomatal conductance, relative stomatal limitation of photosynthesis was similar to that in control seedlings by the end of the experiment. Inhibition of ontogenetic change during drought stress may be responsible for the apparent acclimation of mesophyll photosynthetic processes. We conclude that it would be more effective to select for high photosynthetic capacity than for reduced stomatal sensitivity when breeding for increased drought resistance in black spruce seedlings.
In a study of the seasonal variation of water-relation parameters in four black spruce (Picea mariana (Mill) BSP) populations, the weekly derivation of 32 pressure–volume curves with the sap-expression method between May and October required the simultaneous use of four pressure chambers and variable sample rehydration periods. Pressure chamber had a significant effect on three of the five characteristics studied: symplastic water fraction, maximum modulus of elasticity, and osmotic potential at full turgor. These effects were constant over the growing season. On average, rehydration-time effects were significant for all water-relation parameters except osmotic potential at turgor loss point, but they were not constant over sampling dates. Variations in water-relation parameters due to chambers and rehydration times were controlled and estimated through an experimental design based on the Latin square, which allowed precise comparisons among populations despite the increase in variability due to these two factors. The efficiency of the design varied between 110 and 142% relative to a completely randomized design in which neither chambers nor rehydration times would have been controlled for. Key words: black spruce, populations, experimental design.
1994. Water stress preconditioning of black spruce seedlings from lowland and upland sites. Can. J. Bot. 72: 1511 -1518.One-year-old seedlings from two pairs of upland-lowland populations were subjected to three cycles of drought preconditioning. During a subsequent fourth cycle, preconditioned seedlings and seedlings that had been maintained under wellwatered conditions were subjected to a final soil drought. During the drought periods, small but significant differences in gas exchanges and water relations were observed between upland and lowland seedlings but the differences were not consistent either within each lowland-upland pair or over the four cycles. During the final drought period, preconditioned seedlings maintained significantly higher net photosynthesis rate, stomata1 conductance, and transpiration rate than unconditioned seedlings. In contrast, no significant differences were found in midday water potential, total sugar content, or water relations parameters between preconditioned and unconditioned seedlings. The results suggest an absence of ecotypic variation with respect to drought tolerance in black spruce. They also suggest that preconditioning in black spruce seedlings occurs mostly through the acclimation of stomata1 and photosynthetic mechanisms to drought, without active osmotic adjustment. . 1994. Water stress preconditioning of black spruce seedlings from lowland and upland sites. Can. J. Bot. 72 : 15 11 -15 18. Nous avons soumis des semis d'un an de quatre populations originant de milieux humides et secs pairCes deux a deux par sites contrastants a trois cycles d'asskchement. Au cours d'un quatrikme cycle, les semis ainsi prCconditionnCs et d'autres semis jusque la bien irriguCs ont Ct C soumis a une dernikre pCriode d'asskchement. Au cours des pCriodes d'asskchement, nous avons observC des diffCrences faibles mais significatives dans les Cchanges gazeux et les relations hydriques entre les semis des milieux humide et sec, mais ces diffkrences n'Ctaient pas consistentes dans les paires de sites contrastants, ou entre les pCriodes d'asskchement. Au cours de la dernikre pCriode d'asskchement, les semis prCconditionnCs au stress hydrique ont maintenus des niveaux de photosynthkse, de conductance stomatique et de transpiration supirieurs a ceux non prCcondition-nCs. Par contre, aucune diffkrence n'a pu Ctre observCe dans les mesures de potentiel hydrique du xylkme de midi, de contenu en sucres totaux ou de paramktres des relations hydriques. Les rCsultats suggkrent l'absence de variabiliti Ccotypique en ce qui a trait a la tolCrance au stress hydrique chez 1'Cpinette noire. I1 semble aussi que l'acclimatation au stress hydrique se produit principalement par l'acclimatation des mCcanismes stomatique et photosynthCtique, sans qu'il n'y ait d'ajustement osmotique actif.
The success of mine site restoration programs in arid and semi-arid areas poses a significant challenge and requires the use of high-quality seedlings capable of tolerating heavy metal stresses. The effect of ectomycorrhizal fungi on different physiological traits was investigated in Pinus halepensis seedlings grown in soil contaminated with heavy metals (Pb-Zn-Cd). Ectomycorrhizal (M) and non-ectomycorrhizal (NM) seedlings were subjected to heavy metals stress (C: contaminated, NC: control or non-contaminated) soils conditions for 12 months. Gas exchange, chlorophyll fluorescence, water relations parameters derived from pressure–volume curves and electrolyte leakage were evaluated at 4, 8 and 12 months. Ectomycorrhizal symbiosis promoted stronger resistance to heavy metals and improved gas exchange parameters and water-use efficiency compared to the non-ectomycorrhizal seedlings. The decrease in leaf osmotic potentials (Ψπ100: osmotic potential at saturation and Ψπ0: osmotic potential with loss of turgor) was higher for M-C seedling than NM-C ones, indicating that the ectomycorrhizal symbiosis promotes cellular osmotic adjustment and protects leaf membrane cell against leakage induced by Pb, Zn and Cd. Our results suggest that the use of ectomycorrhizal symbiosis is among the promising practices to improve the morphophysiological quality of seedlings produced in forest nurseries, their performance and their tolerance to multi-heavy metal stresses.
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