Ultrastructure, viability, and in vitro germination of the tricellular Sambucus nigra L. pollen

Mature pollen grains of Sambucus nigra L. are tricolporate, isopolar, 12.5 mm wide, 25 mm long, and tricellular. They have a tectate exine, a bilayered nexine, and a thin intine. The vegetative cell has abundant rough endoplasmic reticulum, many well-structured mitochondria, numerous apparently inactive dictyosomes, immature dividing, and starch-filled plastids and lipid bodies. The sperm cells have few and poorly structured organelles. They are linked by cytoplasmic bridges, indicating a persistent physiological unit, and are surrounded by fibrillar polysaccharide material. Freshly released pollen is 95% viable. Three- and 4-year-old pollen grains stored at 20C are 78.2% and 43% viable, respectively. In vitro germination requirements and pollen tube growth are more similar to those of bicellular pollen than to those of tricellular pollen.