Effect of cadmium on pollen germination and tube growth in Lilium longiflorum and Nicotiana tabacum

Cadmium had a highly toxic effect on pollen germination and tube growth, which were greatly inhibited as metal concentrations increased. Cadmium concentrations up to 10(-2) M completely stopped pollen germination and pollen showed an increasing tendency to burst within 1 h. At low concentrations, the metal caused a slight stimulation of pollen germination, growth rate and tube elongation at the initial stages of tube development. Comparing the two plants studied, cadmium was more toxic for Nicotiana tabacum than for Lilium longiflorum pollen. Pollen tubes showed a range of strong morphological abnormalities, characterized by uneven or aberrant growth, including apical branching or swelling at the tip of the pollen tube. Cell wall intrusions at or near the tip were evident on the inner side, whereas a loose network formed from fibrillar material was observed on the outer layers. After prolonged cadmium exposure, round (ball-like) aggregates were embedded in a fine fibrillar network. Increased cadmium concentrations (10(-3)-10(-2) M) decreased or completely paralyzed cytoplasmic streaming. No typical cytoplasmic zonation existed, while cell organelles (plastids, lipid droplets) were relocated toward the tip. The vesicular apical zone was drastically reduced, with vesicles dispersed into the subapical region. Mitochondria were distributed throughout the subapical region and among the vesicles of the tube apex. Visible ultrastructural changes in cell organelles were not observed.