Sequential staining to assess viability and starch content in individual pollen grains.

Individual pollen grains may be assessed for viability and starch content by dusting a sample onto drops of an aqueous medium containing fluorescein diacetate and potassium iodide, and allowing them to accumulate free fluorescein for ten minutes. They are then illuminated with ultraviolet or blue light and photographed to record the proportion that fluoresce, as an index of viability. The preparation is exposed to iodine vapor and the same field of view rephotographed in bright field illumination to record starch content. Iodine vapor avoids disturbing the grains by adding further liquid, so that the same pollen grains may be classified by fluorescence and starch content. The method makes it possible to test whether viability and starch content are associated or depend on other variables, such as pollen-grain diameter. Iodine-potassium iodide is shown to be inadequate as a test for pollen viability. The method is quick and easy and provides data not otherwise available.