THE INFLUENCE OF ELECTROLYTES ON POLLEN TUBE DEVELOPMENT

These experiments serve to show that neutral salts in amounts considerably below those commonly employed in culture solutions may be very injurious to pollen. It has been found, for example, that NaCl, one of the least toxic salts tried, excepting CaCl(2), added to a sucrose solution in a concentration of 0.0002 M, or about 11 parts per million, reduces the growth of sweet pea pollen tubes 15 per cent. When it is considered that MgCl(2) and BaCl(2) are about fifteen times as toxic as NaCl it becomes evident that the susceptibility of pollen tubes to injury by these substances amounts virtually to hypersensitiveness. On the other hand calcium salts in concentrations ranging from 0.02 to 0.002 M markedly enhance the growth of sweet pea pollen tubes. MgCl(2) has a similar action in the case of Nicotiana. Calcium, moreover, exerts a strong protective action in the presence of the injurious monovalent cations Na and K. So far as can be determined by microchemical means these salts do not alter the wall of the pollen tube; presumably, their effect is on the protoplast itself. In the light of recent experimentation (Osterhout) with other forms better adapted to precise investigation of these phenomena it seems probable that the explanation of the facts presented here lies in changes brought about in the permeability of the cells. Since several gaps exist in our evidence, however, conclusions drawn at this time must necessarily be provisional. The highly injurious action manifested by the cations of several of the salts used indicates that they penetrate the protoplast very rapidly. Possibly in pure sucrose cultures, exosmosis is a limiting factor in pollen tube growth. The addition of salts of calcium or magnesium may favor development by retarding or preventing this outward diffusion. The protective effect of calcium in the presence of the toxic cations K and Na is best interpreted on the assumption that the entry of these latter into the protoplast is retarded by the calcium. The mode by which hydrogen ion concentration affects pollen tube growth is largely a matter of speculation. It has previously been been shown by Brink that the time relations of the growth process simulate those of an autocatalytic reaction. It has been demonstrated also that elongation of the tubes in artificial media is related to the digestion of the reserve food materials contributed by the pollen grain. In the case of the sweet pea these stored substances are largely fats and their hydrolysis may constitute the most important chemical reaction in growth. If, as seems not improbable, the other reactions involved wait upon this one, it is the "master reaction" according to Robertson's hypothesis. If this conception really applies to the case in hand as outlined, the effect of the concentration of hydrogen ions on growth may be a direct one. It is known that the action of the fat-splitting enzyme lipase is favored by a certain amount of free acid. The maximum rate of germination of the pollen and the greatest amount of growth of the pollen tubes occur at pH 6.0. This may be due in large part to the immediate effect of this concentration of hydrogen ions upon the digestion of the reserve food.