Growth of Incompatible Pollen Tubes in Oenothera organensis

The characteristic features of self-sterility in Oenothera organensis are discussed. The genetic basis of self-sterility is a series of some forty-five allelomorphs of one gene. The reaction of the pollen is a gametophytic character, governed solely by the genetic constitution of the pollen itself, with no influence from the second allelomorph carried by the sporophyte plant on which it is borne. Incompatible pollen tubes differ from compatible in extent of growth, and in no other discernible morphological character. Self-sterility is apparently completely effective in preventing seed formation in this species: there is no bud fertility or end-season fertility and all usual methods of obtaining self-pollinated seeds have failed. The incompatibility response is due to an inhibition of pollen-tube growth, which is not governed by diffusible substances but seems to be a reaction between the pollen tube and the tissue in which it is growing. 2. The extent of growth of incompatible pollen tubes varies with the particular sterility allelomorph present in the pollen, with other undetermined genetic factors, and with certain environmental conditions. The most pronounced effect of changes in environment is that resulting from prolonged absence of light. When plants have been kept in the dark for 3 days or longer, their styles permit much longer incompatible tube growth than occurs in flowers from plants kept in the light. The maximum effect is noticed after 5 days, after which the plant develops signs of general weakness and usually stops flowering after the 7th or 8th day. When parts of the plants are covered and the remainder left exposed to light, flowers from branches kept in the dark do not permit increased growth of incompatible pollen tubes. It has not been determined that the diffusion from branches in the light to those in the dark involves substances specifically connected with the incompatibility reaction. It is equally likely that the greater growth of incompatible pollen tubes in flowers from the dark is due to a general physiological debility of the plant, which can be overcome by leaving part of the plant exposed to light.