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The uptake of rubidium by excised roots of barley, perennial ryegrass, mung beans, and subterranean clover has been studied.All four root systems possess similar double mechanisms. Below about 1.0 mM rubidium concentration, a single mechanism operates according to Michaelis-Menten kinetic theory. Between 1.0 and 2.0 mM rubidium, a second mechanism begins to make an effective contribution to uptake.The capacities (Vmax) of the first mechanism operating below I mM rubidium are much greater for barley and ryegrass roots than for the roots of the two legumes. Differences between the capacities of the second mechanism of the four plants are relatively much smaller.The first mechanism is sensitive to changes in pH and calcium concentration, but the second mechanism is relatively insensitive under some circumstances. It is argued that there is a fundamental difference between the two mechanisms.An experiment using the roots of whole plants showed that the activity of the first mechanism is greatly determined by the previous history of the plant, but this factor has little effect on the activity of the second mechanism.Conditions for the valid comparison of different roots and the relevance of the findings to the problem of inter-plant competition for nUhients in mixed pastures are briefly discussed.

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