Protein synthesis of binucleate and trinucleate pollen and its relationship to tube emergence and growth

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Abstract
Under humid conditions, both bi- and trinucleate pollen species incorporate, on the average, very low amounts of leucine, e.g., 0.4 pmol min-1mg pollen-1. During germination in vitro, however, the two types of pollens greatly differ in their capacity for protein synthesis. Binucleate pollen species such as Typha, which are characterized by slow respiration in humid air and prolonged lag periods during germination in vitro, contain large amounts of monoribosomes at dehiscence. Polyribosomes are formed soon after the pollen is wetted in the germination medium, and a considerable incorporation of leucine is initiated after 10–15 min. More rapidly respiring, binucleate pollen showing a short lag period, such as Tradescantia, may already contain many polysomes at dehiscence and incorporate leucine within 2 min of germination. However, rapidly respiring, trinucleate Compositae pollen contains very limited amounts of ribosomal material and never attains any substantial level of incorporation. Cycloheximide completely inhibits both protein synthesis and tube emergence and growth in the slowly respiring, binucleate pollen species. The more rapidly respiring types are less dependent on protein synthesis, while germination of the phylogenetically advanced, trinucleate Compositae pollen proceeds completely independently. It is concluded that the level of phylogenetic advancement of the male gametophyte is characterized by its overall state of metabolic development at dehiscence rather than by the number of its generative cells.