Janusz J. Zwiazek

Ectomycorrhizas and water relations of trees: a review.

There is plenty of evidence for improved nutrient acquisition by ectomycorrhizas in trees; however, their role in water uptake is much less clear. In addition to experiments showing improved performance during drought by mycorrhizal plants, there are several studies showing reduced root hydraulic conductivity and reduced water uptake in mycorrhizal roots. The clearest direct mechanism for increased water uptake is the increased extension growth and absorbing surface area, particularly in fungal species with external mycelium of the long-distance exploration type. Some studies have found increased aquaporin function and, consequently, increased root hydraulic conductivity in ectomycorrhizal plants while other studies showed no effect of ectomycorrhizal associations on root water flow properties. The aquaporin function of the fungal hyphae is also likely to be important for the uptake of water by the ectomycorrhizal plant, but more work needs to be done in this area. The best-known indirect mechanism for mycorrhizal effects on water relations is improved nutrient status of the host. Others include altered carbohydrate assimilation via stomatal function, possibly mediated by changes in growth regulator balance; increased sink strength in mycorrhizal roots; antioxidant metabolism; and changes in osmotic adjustment. None of these possibilities has been sufficiently explored. The mycorrhizal structure may also reduce water movement because of different fine root architecture (thickness), cell wall hydrophobicity or the larger number of membranes that water has to cross on the way from the soil to the xylem. In future studies, pot experiments comparing mycorrhizal and nonmycorrhizal plants will still be useful in studying well-defined physiological details. However, the quantitative importance of ectomycorrhizas for tree water uptake and water relations can only be assessed by field studies using innovative approaches. Hydraulic redistribution can support nutrient uptake during prolonged dry periods. In large trees with deep root systems, it may turn out that the most important function of mycorrhizas during drought is to facilitate nutrient acquisition.

Aquaporins in poplar: What a difference a symbiont makes!

The formation of ectomycorrhizas, a tight association between fine roots of trees and certain soil fungi, improves plant nutrition in a nutrient-limited environment and may increase plant survival under water stress conditions. To investigate the impact of mycorrhiza formation on plant water uptake, seven genes coding for putative water channel proteins (aquaporins) were isolated from a poplar ectomycorrhizal cDNA library. Four out of the seven genes were preferentially expressed in roots. Mycorrhiza formation resulted in an increased transcript level for three of these genes, two of which are the most prominently expressed aquaporins in roots. When expressed in Xenopus laevis oocytes, the corresponding proteins of both genes were able to transport water. Together, these data indicate, that the water transport capacity of the plasma membrane of root cells is strongly increased in mycorrhized plants. Measurements of the hydraulic conductance of intact root systems revealed an increased water transport capacity of mycorrhized poplar roots. These data, however, also indicate that changes in the properties of the plasma membrane as well as those of the apoplast are responsible for the increased root hydraulic conductance in ectomycorrhizal symbiosis.

Effect of High Salinity Tailings Waters Produced From Gypsum Treatment of Oil Sands Tailings on Plants of the Boreal Forest

Bitumen extraction methods currently in use in the operating oil sands plants produce large volumes of fluid tailings. Ions leached from the ore and added by process chemicals during the extraction process result in tailings waters containing elevated ionic content relative to the non-process-affected waters of the area, in particular the sodium, sulfate, and chloride ions. It is anticipated that the areas requiring reclamation will be affected by this high salinity of the process waters. The objectives of this study were to test the impact of a tailings alternative (consolidated tailings process, based on gypsum treatment of extraction tailings) on the viability of plant species of the northern boreal forest and to determine the relative salt tolerance and suitability of selected plant species for land reclamation. Seedlings were grown for 4 weeks in a greenhouse in solution culture containing mineral nutrients and various dilutions of consolidated tailings water and with Na2SO4 additions (1 g L−1 and 3 g L−1). Of all examined plant species, raspberry and strawberry seedlings were the most susceptible to damage, while the seedlings of white spruce, black spruce and lodgepole pine survived, but showed some effects. In the willow and aspen seedlings, there was a rapid loss of leaves, which were quickly replaced by new, morphologically different leaves. Dogwood and hybrid poplar showed high tolerance to all treatments.

Functional characterization of drought-responsive aquaporins in Populus balsamifera and Populus simonii×balsamifera clones with different drought resistance strategies

We have characterized poplar aquaporins (AQPs) to investigate their possible functions in differential drought responses of Populus balsamifera and Populus simonii×balsamifera leaves. Plants were exposed to mild and severe levels of drought stress and to drought stress recovery treatment, and their responses were compared with well-watered controls. Compared with P. balsamifera, P. simonii×balsamifera used drought avoidance as the main drought resistance strategy, and rapidly reduced stomatal conductance in response to stress. This strategy is correlated with growth rate reductions. Eleven AQPs were transcriptionally profiled in leaves from these experiments and five were functionally characterized for water channel activity. PIP1;3 and PIP2;5 were among the most highly expressed leaf AQPs that were responsive to drought. Expression of PIP1;3 and five other AQPs increased in response to drought in the leaves of P. simonii×balsamifera but not in P. balsamifera, suggesting a possible role of these AQPs in water redistribution in the leaf tissues. PIP2;5 was upregulated in P. balsamifera, but not in P. simonii×balsamifera, suggesting that this AQP supports the transpiration-driven water flow. Functional characterization of five drought-responsive plasma membrane intrinsic proteins (PIPs) demonstrated that three PIP2 AQPs (PIP2;2, PIP2;5, PIP2;7) functioned as water transporters in Xenopus laevis oocytes, while the two PIP1 AQPs (PIP1;2 and PIP1;3) did not, consistent with the notion that they may be functional only as heterotetramers.

Ethylene Enhances Water Transport in Hypoxic Aspen

Water transport was examined in solution culture grown seedlings of aspen (Populus tremuloides) after short-term exposures of roots to exogenous ethylene. Ethylene significantly increased stomatal conductance, root hydraulic conductivity (L p), and root oxygen uptake in hypoxic seedlings. Aerated roots that were exposed to ethylene also showed enhanced L p. An ethylene action inhibitor, silver thiosulphate, significantly reversed the enhancement of L p by ethylene. A short-term exposure of excised roots to ethylene significantly enhanced the root water flow (Q v), measured by pressurizing the roots at 0.3 MPa. The Q v values in ethylene-treated roots declined significantly when 50 μm HgCl2was added to the root medium and this decline was reversed by the addition of 20 mm 2-mercaptoethanol. The results suggest that the response of Q v to ethylene involves mercury-sensitive water channels and that root-absorbed ethylene enhanced water permeation through roots, resulting in an increase in root water transport and stomatal opening in hypoxic seedlings.

Effects of water deficit stress and recovery on the root water relations of trembling aspen (Populus tremuloides) seedlings

Abstract Aspen (Populus tremuloides Michx.) seedlings were grown in sand culture and subjected to mild and severe water deficit stress by withholding watering. Severely-stressed seedlings were also rewatered for 24 h to determine the effects of water deficit stress and stress recovery on root water flow properties. Both stress levels and stress recovery treatment reduced leaf stomatal conductance and shoot water potentials. However, root volume flux density and hydraulic conductivity were inhibited only by the severe water deficit stress and did not recover within 24 h following rewatering. The inhibition of root hydraulic conductivity in severely-stressed plants was accompanied by an increase in the proportion of apoplastic root water flow, as determined by the trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate fluorescent tracer dye. In all treatments and control, the mean activation energy values for root water flow were lower than 6 kcal mol−1, pointing to the presence of AQP-mediated transport. However, compared with earlier studies using solution culture-grown aspen seedlings, mercuric chloride had relatively little effect on root volume flux density. We interpreted this result as likely due to limited penetration of the root cortex by mercuric chloride through the exodermal layer.

Colonization with Hebeloma crustuliniforme increases water conductance and limits shoot sodium uptake in white spruce (Picea glauca) seedlings

White spruce [Picea glauca (Moench) Voss] seedlings were inoculated with Hebeloma crustuliniforme and treated with 25 mM NaCl to examine the effects of salinized soil and mycorrhizae on root hydraulic conductance and growth. Mycorrhizal seedlings had significantly greater shoot and root dry weights, number of lateral branches and chlorophyll content than non-mycorrhizal seedlings. Salt treatment reduced seedling growth in both non-mycorrhizal and mycorrhizal seedlings. However, needles of salt-treated mycorrhizal seedlings had several-fold higher needle chlorophyll content than that in non-mycorrhizal seedlings treated with salt. Mycorrhizae increased N and P concentrations in seedlings. Na levels in shoots and roots of salt-treated mycorrhizal seedlings were significantly lower and root hydraulic conductance was several-fold higher than in non-mycorrhizal seedlings. A reduction of about 50% in root hydraulic conductance of mycorrhizal seedlings was observed after removal of the fungal hyphal sheath. Transpiration and root respiration rates were reduced by salt treatments in both groups of seedlings compared with the controls, however, both transpiration and respiration rates of salt-treated mycorrhizal seedlings were as high as those in the non-mycorrhizal seedlings that had not been subjected to salt treatment. The reduction of shoot Na uptake while increasing N and P absorption and maintaining high transpiration rates and root hydraulic conductance may be important resistance mechanisms in ectomycorrhizal plants growing in salinized soil.

Effects of NaCl and Na2SO4 on red-osier dogwood (Cornus stolonifera Michx) seedlings

Sodium chloride and sodium sulfate are commonly present in extraction tailings waters produced as a result of surface mining and affect plants on reclaimed areas. Red-osier dogwood (Cornus stolonifera Michx) seedlings were demonstrated to be relatively resistant to these high salinity oil sands tailings waters. The objectives of this study were to compare the effects of Na2SO4 and NaCl, on growth, tissue ion content, water relations and gas exchange in red-osier dogwood (Cornus stolonifera Michx) seedlings. In the present study, red-osier dogwood seedlings were grown in aerated half-strength modified Hoaglands mineral solution containing 0, 25, 50 or 100 mM of NaCl or Na2SO4. After four weeks of treatment, plant dry weights decreased and the amount of Na+ in plant tissues increased with increasing salt concentration. Na+ tissue content was higher in plants treated with NaCl than Na2SO4 and it was greater in roots than shoots. However, Cl− concentration in the NaCl treated plants was higher in shoots than in roots. The decrease in stomatal conductance and photosynthetic rates observed in presence of salts is likely to contribute to the growth reduction. Our results suggest that red-osier dogwood is able to control the transport of Na+ from roots to shoots when external concentrations are 50 mM or less.

The effect of salinity on the emergence and seedling growth of Picea mariana, Picea glauca, and Pinus banksiana

Mining operations in areas of the boreal forest have caused salinity issues to be a major concern for reclamation. One of the factors determining successful reclamation is the ability of species to self-propagate. The effects of salinity on the seedling emergence and early growth of three boreal forest conifers: Picea mariana, Picea glauca, and Pinus banksiana were determined. Seeds were planted in sand moistened with solutions of various concentrations of sodium chloride or sodium sulfate. Seedling emergence was monitored on a daily basis and growth parameters assessed after 6 weeks. The emergence of Pinus banksiana seedlings was least affected by salinity, and at certain concentrations, emergence even appeared to be stimulated by the presence of salt. Picea glauca was the most sensitive of the species studied. Hypertrophia was observed in all species at high concentrations of Na2SO4, and an increase in salt levels caused a corresponding reduction in seedling height and weight, root length and number of lateral roots.

Overexpression of PIP2;5 Aquaporin Alleviates Effects of Low Root Temperature on Cell Hydraulic Conductivity and Growth in Arabidopsis

The effects of low root temperature on growth and root cell water transport were compared between wild-type Arabidopsis (Arabidopsis thaliana) and plants overexpressing plasma membrane intrinsic protein 1;4 (PIP1;4) and PIP2;5. Descending root temperature from 25°C to 10°C quickly reduced cell hydraulic conductivity (Lp) in wild-type plants but did not affect Lp in plants overexpressing PIP1;4 and PIP2;5. Similarly, when the roots of wild-type plants were exposed to 10°C for 1 d, Lp was lower compared with 25°C. However, there was no effect of low root temperature on Lp in PIP1;4- and PIP2;5-overexpressing plants after 1 d of treatment. When the roots were exposed to 10°C for 5 d, Lp was reduced in wild-type plants and in plants overexpressing PIP1;4, whereas there was still no effect in PIP2;5-overexpressing plants. These results suggest that the gating mechanism in PIP1;4 may be more sensitive to prolonged low temperature compared with PIP2;5. The reduction of Lp at 10°C in roots of wild-type plants was partly restored to the preexposure level by 5 mm Ca(NO3)2 and protein phosphatase inhibitors (75 nm okadaic acid or 1 μm Na3VO4), suggesting that aquaporin phosphorylation/dephosphorylation processes were involved in this response. The temperature sensitivity of cell water transport in roots was reflected by a reduction in shoot and root growth rates in the wild-type and PIP1;4-overexpressing plants exposed to 10°C root temperature for 5 d. However, low root temperature had no effect on growth in plants overexpressing PIP2;5. These results provide strong evidence for a link between growth at low root temperature and aquaporin-mediated root water transport in Arabidopsis.