Anatomical, developmental and physiological aspects of silica in wheat
The major aims of this thesis were to study silica deposits in cereal inflorescences and wheat roots and investigate soluble/deposited silica partitioning and uptake of silica by wheat. The morphology of a particular phytolith (the papilla) from inflorescence bracts of 70 species from Triticum (wheat). Hordeum, Aegilops families and Secale cereale was analyzed. Using papilla pit number and/or diameter it is possible to distinguish between Hordeum sp. and Triticum sp., and between wheats of different ploidy levels. The root growth of T. aestivum was reduced by silica in the nutrient solution (Si+) during the first six days of growth, compared to plants grown in solution not supplemented by silica (Si-), After a further four days Si+ plants had significantly longer roots than Si- plants. At the light microscope level, a time course for silicification of the inner tangential wall of the root endodermis of T. aestivum was elucidated over ten days. Very few walls were silicified after 24 hours exposure to Si+ solution, but by day 10 up to 80% of endodermal cells in the basal area of the root were silicified. The percentage of silicified cells increased from the apex to the base of the root. Silica in T. aestivum was measured quantitatively using a molybdate blue detection method. Typically, 99% of the total silica in the seedlings was in the deposited form. The average concentration of soluble silica within the xylem exudate was 3.6 mM. The uptake of silica was affected by the silica concentration and pH of the nutrient solution. By measuring transpirational water loss and the silica content of the plants, it was concluded that silica uptake was active in T. aestivum. Uptake mechanisms were investigated by growing seedlings in nutrient solutions containing 2 mM Si and an inhibitor for 24 hours. The inhibitors used were ATPase inhibitors ( sodium orthovanadate, diethylstilbestrol, erythrosin B) and ionophores (nigericin, FCCP). The data suggests that functional ATPases and a proton gradient are required for silica uptake. Transport into the xylem and uptake at the root surface may be differentially affected by the inhibitors.
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