Effects Of Aluminum Toxicity On Growth Of Hevea Rubber Tree Seedlings

Soil aluminum toxicity to plants has been a great interests to scientists ever since1980s. Most rubber plantations are established on acidic soils that are apt to aluminum toxicity stress. The present study aimed at determining the possible effects of soil aluminum toxicity on the growth and some physiological parameters of young seedlings of rubber tree (Hevea brasiliensis (Willd. Ex A.Juss) Muell.-Arg.). Some young plants of Clone CATAS7-33-97from somatic cell culture were used as plant materials, which were grown in pots containing river sands. The potted seedlings received aluminum treatments at five concentrations, i.e.,0mmol/L,50mmol/L,100mmol/L,200mmol/L and400mmol/L.Observations and measurements were made on variations of root morphology and relative plant height, photosynthetic capacity, chlorophyll fluorescence parameters and chlorophyll content, malondialdehyde (MDA) content, peroxidase content, superoxide dismutase (SOD) content and membrane permeability. Determinations were also made of total nitrogen content, total phosphorus content, total potassium content, calcium content, and magnesium content of the rubber seedlings. The major findings are as follows:(1) At50mmol/L treatment level, aluminum could promote the growth of rubber tree saplings while at higher concentrations aluminum inhibited the gowth of the rubber tree saplings. Even at50mmol/L concentration, the root length and root volume decreased significantly.It is commonly believed that root growth inhibition is the first phenomenon of aluminum toxicity to plants. Estimation made on the basis of the toxicity severity established the threshold concentration of aluminum toxicity to root system of rubber tree saplings at<4mg/g, while to relative plant height at200mmol/L, signifying that the root system is the most sensitive organ to aluminum toxicity.(2) It was found that at50mmol/L treatment level, leaf net photosynthesis (Pn) and stomatal conductance(Gs) showed a trend of rising before declining; while a trend of decreasing all along at other concentrations with the increase of stress time. However, no matter at any aluminum concentrations, the intercellular CO2concentration (Ci) tended to rise before declining.No significant correlation was recorded between Pn, Gs and Ci. Meanwhile, the coincidence of leaf chlorophyll content decrease with the increase of the leaf original florescence (FO) suggested that the net photosynthetic rate decrease was the result of the decrease of chlorophyll content and the destruction of photosynthetic reaction center.(3) Significant liner correlation was found between aluminum content and water content of rubber seedlings (R=0.99). It is believed that the decrease of plant water content was due to the increase of roots resistance. However, plant transpiration rate did not reduce with the decrease of the water content, but increased somewhat at the beginning of the falling of plant water content. It is assumed that the aluminum tolerance of the rubber seedlings was kicked off.(4) In consideration of the leaf MDA content, POD activity, SOD activity and membrane permeability of rubber seedlings at different aluminum concentration treatments, it was presumed that the maximum tolerance of aluminum concentration was200mmol/L, while the maximum aluminum content at6.2mg/g for root and2.9mg/g for the aboveground part of rubber seedlings.(5) The aluminum content of rubber tree seedlings were found to have liner relationship with total nitrogen, total phosphorus, total potassium, calcium and magnesium contents. The significant level was in the order of:total nitrogen (R=0.87)> calcium (R=0.79)> total phosphorus (R=0.72)> magnesium (R=0.68)> total potassium (R=0.52). The aluminum is therefore considered to inhibit the absorption and transportation of various nutrients, so as to hinder plant growth.It is therefore concluded that in the present experiment aluminum toxicity stress hindered the normal growth and physiological activity of rubber seedlings at higher concentrations while promoted the growth of the rubber tree saplings at low concentration of50mmol/L, and that the threshold concentration of aluminum stress for rubber seedlings is200mmol/L. The present findings contributed new knowledge to stress ecology of rubber plantation forest and will be referential for future soil amelioration study in rubber plantations.