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Wide variations in total aluminum (AL) concentration reported to have a toxic effect on plants arc attributed to imprecise definition of the chemical environment at the root interface. Because AL can complex with natural ligands and form time-dependent metastable species in water, the requirements of a fixed AL speciation in the nutrient medium can only be met by simultaneously holding constant the pH, the AL/OH ratio and the renewal time of the solution. It is also important that the root be constantly perfused by a fresh solution so that the root interface is as chemically close to the bulk solution as possible. The rhizostat presented here uses a process controller to constantly replenish four tanks containing different combinations of nutrients and AL. The solution is pumped from each tank to a tray where it is continuously delivered to seedlings growing on artificial medium. This rhizostat provides an accurate control for AL speciation. Preliminary experiments on white spruce seedlings show that levels of 10 micromolal or less total AL are sufficient to cause toxic effects at pH of 4.5 and without the formation of AL complexes.

A new and simple technique for the measurement of nitrogenase activity in plant material using an infrared gas analyzer equipped with an ethylene filter is described. The technique requires less manipulation than conventional gas chromatography and is particularly suitable for continuous analysis in field studies.