Uptake of heavy metal salt solutions by pollen grains ofMagnolia×soulangeana(Magnoliaceae)

Abstract

To investigate the role of the plasmalemmic cord in the pollen grains of members of the Magnoliaceae, anthers of Magnolia × soulangeana Soul.-Bod. were incubated for 1, 2.5, and 5 h in lead and lanthanum salt solutions. Ultrastructural analysis revealed differences in the labelling of the pollen grains with the time of exposure to the heavy metal salt, which is likely related to metal tolerance mechanisms operating in the pollen grains. Within 1 h of treatment, both tracers produced a fine precipitate lining the entire vegetative cell (VC) plasmalemma (including the plasmalemmic cord) as well as the generative cell (GC) plasmalemma. Heavy deposits were not found inside both the VC and the GC. At later times, the amount of heavy deposits increased on the pollen wall surface and, particularly, in the two outer intine layers. Also, heavy deposits were found in membrane-bound cell components of the VC and the GC, but never in the cytosol. In the VC, the cell components more frequently labelled were the protein storage vacuoles. In both pollen cells, multivesicular bodies, dictyosome cisternae, and small vesicles were also labelled. We hypothesize that the latter organelles participate in the metal ions accumulation (end of the trip), together with the protein vacuoles, or in the transport of metal ions to the vacuoles following the endocytotic uptake of these ions. The presence of both coated and uncoated pits and vesicles in the treated and the untreated pollen reinforces the latter hypothesis. Another significant result was that the tracers, after diffusing through the microchannels in the exine and crossing the intine layers, moved through the plasmalemmic cord to reach the GC periplasm. We suggest that the plasmalemmic cord may provide a system of communication for the flux of solutes from the anther loculus to both pollen cells and (or) from the VC to the GC.Key words: pollen grain, ultrastructure, metal uptake, metal tolerance, lead nitrate, lanthanum nitrate.