Effect of some mineral ions on pollen tube growth and release of proteins in culture

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Abstract
In the absence of cations, the release of proteins from pollen tubes ofNicotiana tabacum in culture is greatly dependent on boron concentration and inversely related to growth stimulation. The minimum of proteins in the medium occurs at 100 mg 1− of boric acid, which is the optimum concentration for growth. The shift of boron level to this optimum further increases the proportion of proteins bound to the insoluble pollen tube fraction; on the other hand the amount of soluble proteins is not affected inside pollen tubes, but greatly decreased in the medium. The loss of proteins into the medium is considerably reduced by oalcium and also at the optimal boron concentration and in the presence of K+ and Mg2+ ions. The rate of tube elongation, however, is slightly decreased and the duration of pollen growth activity is not extended. The release of proteins is not affected by potassium and is slightly reduced by magnesium. The possible mechanism of calcium and boron action on protein release is discussed in relation to the exocytotic function of secretory vesicles and it is suggested that boron supports the incorporation of proteins transported to the growing tip into the pollen tube wall structures.