The Essential Role of Calcium Ion in Pollen Germination and Pollen Tube Growth

pollen population effect occurs whenever pollen grains are grown in vitro. Small pollen populations germinate and grow poorly if at all, under conditions which support excellent growth of large pollen populations. The pollen population effect is overcome completely by a growth factor obtained in water extracts of many plant tissues. This factor is shown to be the calcium ion, and its action confirmed in 86 species representing 39 plant families. Other ions (K+, Mg++, Na+)serve in supporting roles to the uptake or binding of calcium. The high requirement of calcium (300-5000 ppm, as Ca (NO3)2 4H20, for optimum growth)and low calcium content of most pollen may conspire to give calcium a governing role in the growth of pollen tubes both in vitro and in situ. It is suspected that ramifications of this role extend to the self-incompatibilities of plants and to the curious types of arrested tube growth distinguishing, for example, the orchids. A culture medium which proved its merit in a wide variety of pollen growth studies included, in dis- tilled water, 10 percent sucrose, 100 ppm H3BO3, 300 ppm Ca (NO3)2. 4H20, 200 ppm MgSO4. 7H20 and 100 ppm KNO3.