At concentrations of potassium dihydrogen phosphate in the external medium below 10−3 M, the greater proportion of phosphate which enters beech mycorrhizal roots is incorporated into bound form. Above 10−3 M, an increasing proportion is accumulated unchanged as orthophos-phate. Studies using 32P confirm, in greater detail, the observation of Harley and Loughman (1963) that phosphate in the external medium rapidly equilibrates with a small pool of ortho-phosphate within the fungal sheath, from which the ion is incorporated into bound form or transferred either to the host core or to a larger pool in the sheath itself. Ammonium chloride at 10−2 M increases the total uptake of phosphate through an increase in the rate of both the incorporation into bound form and the primary absorption process. Ammonium chloride over the range 10−4 to 10−1 M brings about an increase in the size of the small pool. However, ammonium chloride has no detectable effect on phosphate fractions in the absence of phosphate in the external medium. Sodium azide, on the other hand, causes a breakdown of bound phosphate, although it is only at high concentration that this inhibitor has any detectable effect on the level of orthophosphate within the roots. The results suggest that the size of the small pool may be an important determinant of phosphate uptake at low concentration and that the role of potassium in the primary absorption process must not be overlooked.
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