| Lead(Pb) is one of toxic heavy metals. For soils with p H > 6.5, 500 mg kg-1 Pb may have toxicity on growth, physiological processes and propagation of plants. Phragmites australis is a perennial plant, whose developed and extended rhizomes confer it strong ability of survival and propagation. This species has been used widely in the construction and restoration of wetlands, to treat toxical wastes, such as heavy metals pollutants. Wet and wet plants often suffer from extreme water changes, such as flooding and drought caused by climate changes. Given each plant species’ tolerance of Pb can be best assessed with toxicity assays across a range of concentrations. Thus, we designed pot experiments to imitate moist and drought environments along a gradient of five Pb concentrations(0, 500, 1500, 3000 and 4500 mg kg-1), to more systematically state the growth and development response and tolerant mechanism of reeds to Pb contamination under different habitats, from growth, physiology and clonal reproduction. Our experiments represent the first study of the clonal propagative tolerance of reeds to heavy metal contamination, which will provide scientific basis for applying reeds to phytoremediation and ecological restoration of wet contaminated by heavy metals. The main results and conclusions are as follows:(1) Withered leaves and death of mother shoots occurred along with stress progressed, while daughter shoot biomass was increased and reached 60-90% of aboveground biomass at the late stage of stress. So, the aboveground productivity is from daughter shoots, which have more adaptability and tolerance. More biomasses were assigned to aboveground parts to obtain more light resource. Simultaneously, more belowground biomasses were allocated to rhizomes. However, the synergistic interaction effect of drought and Pb decreased significantly biomasses of mother shoots, daughter shoots and belowground parts, leading to each part biomass under drought was less than those under well-watered condition.(2) The photosynthetic parameters of daughter shoots were greater than that of mother shoots, resulting in more biomass accumulation and tolerance of daughter shoots, which ensure the main source of aboveground biomass in well-watered environment. At the same Pb level, the photosynthetic parameters and biomasses of mother and daughter shoots in well-watered environment were greater than those in drought treatment, indicating the reeds in well-watered environment had more tolerance to Pb contamination. In the whole stress period, the stomatal closure was one of the reasons for decreased photosynthesis of mother and daughter shoots, while, at the late of stress, the non-stomatal limitation may be the main reason for reduced photosynthesis of mother and daughter shoots treated by 4500 mg kg-1 and >500 mg kg-1 Pb under well-watered and drought condition respectively.(3) At the middle stage of stress, the activity of CAT, POD and SOD of reeds under Pb contamination were greater than controls under well-watered condition, which indicated that reeds had the effective resistance and defense to oxidic damage caused by Pb and water stresses. At the late stage of stress, the activities of POD and SOD under drought condition were less than under well-watered condition due to the synergetic effect of drought and Pb, while their activities at Pb levels were greater versus controls. The results showed that POD and SOD played critical role in resistance to oxidic stess caused by Pb contamination, and reeds had stronger resistant ability under well-watered condition. In a word, antioxidant enzymes can be the effective resistance mechanism to deal with oxidic stress caused by Pb.(4) Our results showed that the axillary rhizome buds mainly grew horizontally to secondary rhizomes rather than emerged to shoots. The number of axillary shoot buds was increased by Pb additions, exhibiting a phalanx growth pattern under well-watered condition. The increase percentages of number of axillary rhizome buds, apical buds, daughter axillary and apical rhizome shoots and rhizome elongation at the same Pb level at the late stage of stress were greater than that at middle stage of stress. These indicated that the buds, daughter shoots and rhizomes showed compensatory growth. The aboveground population density and productivity were maintained by the two strategies(phalanx and compensatory growth). However, the synergetic effect of drought and Pb inhibited significantly the growth and propagation of clonal modules, which would be adverse to reproduction and maintenance of reed population. The number of each type of buds and daughter shoots and rhizome growth were inhibited significantly by 4500 mg kg-1 under well-watered and by 500 mg kg-1 under drought condition. Thus, the two levels of Pb(4500 and 500 mg kg-1) in soils might meet or exceed the Pb tolerance threshold of clonal propagation in wet and dry environments respectively.(3) The decrease order of Pb concentrations in different modules was root> mother shoots > daughter shoots or rhizomes. The vast majority of Pb was accumulated in roots, resulting in decreased root biomass. More amount of Pb in aboveground parts was allocated in mother shoots, and more amount of Pb in mother shoots was allocated in leaves, which caused toxic effects on mother shoots leading to reduction in mother biomass and withered leaves. The Pb concentration in daughter shoots was stem > leaf under drought, while the reverse occurred under well-watered condition, which resulted to daughter shoot biomass under well-watered condition was greater than under drought condition. Therefore, more allocation of Pb transported aboveground parts to mother shoots, less allocation of Pb to daughter shoots and rhizomes might be a strategy, adopted by reeds in high concentration of Pb contaminated environment, to protect clonal propagation, daughter shoot growth and aboveground productivity.In short, Pb allocation strategy, phalanx growth, compensatory growth and antioxidant enzymes confer reeds more tolerance and clonal propagation abilities to resist Pb contamination. Inversely, the growth and propagation of clonal modules were inhibited significantly by the synergetic effect of drought and Pb, which might lead to reeds population depression. |
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