The aluminum source to produce toxicity in upland rice in nutrient solution experiments is not yet well established, althought the aluminum potassium sulfate has been utilized source to produce aluminum toxicity. However, in recent studies have used aluminum chloride. The aim of this study was to evaluate the capacity of aluminum sources and doses to produce toxicity in upland rice plants grown in nutrient solution. The experiment was arranged in a block randomized design, in a 2 x 5 factorial scheme and four repetitions. The treatments were two aluminum sources (aluminum potassium sulfate - AlK(SO4)2.12H2O and aluminum chloride - AlCl3.6H2O) and five aluminum doses in nutrient solution (0, 370, 740, 1100 and 1480 μmol L-1). The experiment was conducted in a greenhouse in Botucatu city, São Paulo state, Brazil, starting in April 2012, and was carried out for 56 days from transplanting of the seedlings. Using aluminum chloride, the rice plants show lower production of root and total dry weight, area and root volume, medium and thick root length, potassium and sulfur contents and accumulations. Using aluminum potassium sulfate, there are lower aluminum activity and availability, besides the formation of large amount of aluminum compounds non-toxic to the plants (aluminum sulfate) in the nutrient solution. The aluminum doses between 1100 to 1480 µmol L-1, corresponding to aluminum activity of 336.8 to 429.0 µmol L-1 of aluminum chloride as source, are more effective to produce aluminum toxicity in upland rice plants grown in nutrient solution.
Qualitative thermal tests and light and scanning electron microscopy of leaves, roots and trunks for opal-A (amorphous silica (Si)) as well as total leaf analysis for Si were performed on 11-to 12-yr old Hamlin sweet orange trees on Rangpur lime with and without blight symptoms and grown on soils classified as Oxisol. High amounts of silicified opaline plugs were found in the outermost active xylem in trunks of blight-affected trees, but very few in healthylooking trees. In the case of the leaves, much smaller amounts of opal plugs were found in a blighted tree in comparison with healthy looking trees. However, total Si contents of the Hamlin leaves did not show differences. Further microscopic observations indicated that leaves of a blight-affected sweet orange tree have more silica in the form of randomly distributed colloid bodies (silica-gel), whereas leaves from healthy trees had silica in the opal phytoliths form, which is known to be beneficial when it is located in the epidermis and between the cell walls.
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