Ecophysiological Responses To Drought Stress And Manganese Toxicity In Different Populations Of Section Tacamahaca Spach

Soil is the indispensable environment for human survival and important resource for agriculture development. Food and environmental problems facing the world are all closely related to soil and nowadays it is threatened by many factors, among which drought stress and heavy metal pollution are the most serious ones. Poplars (Populus spp.) are important components of ecosystem and suitable as a source of fuel, fiber and lumber due to their fast growth. In this study, different populations of Section Tacamahaca spach were used as model plants to investigate the adaptability to drought stress and manganese toxicity and differences between populations from dry and wet climate regions. Our results can provide theoretical evidence for the afforestation and prevention of desertification in the arid and semi-arid areas, and also can supply scientific direction for the reconstruction and rehalibitation of ecosystems contaminated by heavy metals. The results are as follows:1. Differences in ecophysiological responses to drought stress in two contrasting populations of Populus przewalskiiDrought stress not only significantly affected dry mass accumulation and allocation, but also significantly decreased chlorophyll pigment contents and accumulated free proline and total amino acids. On the other hand, drought also significantly increased the levels of abscisic acid and reactive oxygen species, as secondary messengers, to induce the entire set of antioxidative systems including the increase of reduced ascorbic acid content and the activities of superoxide dismutase, guaiacol peroxidase, ascorbate peroxidase and glutathione redutase. Thus the combination of drought avoidance and tolerance mechanisms conferred poplar a high degree of plasticity in response to drought stress. Compared with the wet climate population, the dry climate population showed lower dry matter accumulation and allocated more biomass to root systems, and accumulated more free proline and total amino acids for osmotic adjustment. The dry climate population also showed more efficient antioxidant systems with higher content of ascorbic acid and higher activities of ascorbate peroxidase and glutathione redutase than the wet climate population. All these made the dry climate population superior in adaptation to drought stress than the wet climate population.2. Effect of exogenous applied SNP on drought tolerance in Populus przewalskiiDrought stress significantly increased hydrogen peroxide content and caused oxidative stress to lipids and proteins assessed by the increase in malondialdehyde and total carbonyl contents, respectively. The cuttings of P. przewalskii accumulated proline and other amino acids for osmotic adjustment to lower water potential, and activated the antioxidant enzymes such as superoxide dismutase, guaiacol peroxidase and ascorbate peroxidase to maintain the balance of generation and quenching of reactive oxygen species. Moreover, exogenous SNP application significantly heightened the growth performance of P. przewalskii cuttings under drought treatment by promotion of proline accumulation and activation of antioxidant enzyme activities, while under well-watered treatment the effect of SNP application was very little.3. Morphological responses to manganese toxicity in the two contrasting populations of Populus cathayanaHigh concentration of manganese caused significant decrease in shoot height and biomass accumulation. The tolerance index of the dry climate population was significantly higher than that of the wet climate population, suggesting the superior Mn tolerance in the former and the existence of cross-tolerance of drought stress and high Mn toxicity. Injuries to the leaf anatomical features were also found as the reduced thickness in palisade and spongy parenchyma, the decreased density in the conducting tissue and the collapse and split in the meristematic tissue in the central vein. As for the Mn concentrations in the plant tissues, under 0, 0.1 and 0.5mM, most of the Mn accumulated in the roots, then leaves, and stem the least, while under 1mM, most of the Mn accumulated in the leaves. As far as the total amounts of Mn extraction are concerned, the leaf extracted more Mn than the root and stem in the two populations under various Mn concentrations. There is no difference between the two populations under control. Under 0.1mM, the wet climate population extracted higher Mn in the root than the dry climate population, while in the shoot, the dry climate population extracted much more Mn. Under 0.5 and 1 mM, the wet climate population translocated more Mn both in the root and the shoot than the dry climate population.4. Physiological and biochemical responses to manganese toxicity in the two contrasting populations of Populus cathayanaMn treatment resulted in oxidative stress indicated by the oxidation to lipids, proteins and DNA. A regulated network of defence strategies was employed for the chelation, detoxification and tolerance of Mn including the enhanced synthesis of ABA and polyamines, the accumulation of free amino acids, especially His and Pro, and the activation of the enzymes superoxide dismutase and guaiacol peroxidase. Contents of non-protein thiol, reduced glutathione, phytochelatins and phenolics compounds and activities of superoxide dismutase, guaiacol peroxidase and polyphenol oxidase also increased significantly for antioxidant or chelation functions. The wet climate population not only accumulated less abscisic acid, free amino acids, phytochelatins and phenolics compounds, but also exhibited lower activities of superoxide dismutase, guaiacol peroxidase and polyphenol oxidase thus resulting in more serious oxidative damage and more curtained growth.