Aluminium Effects on Pollen Germination and Tube Growth of Chamelaucium uncinatum. A Comparison with other Ca2+ Antagonists

An interaction between aluminium (Al) and calcium (Ca) may be a cause of Al toxicity in plants. The pollen tube is a suitable system to test the interaction between Al and Ca since Ca ions play a pivotal role in pollen germination and tube growth. We investigated how Al and other known blockers of Ca2+-permeable channels (trivalent cations, ruthenium red, verapamil and nifedipine) influence pollen of an Australian native species Geraldton waxflower (Chamelaucium uncinatum). Pollen germination was inhibited by micromolar concentrations of trivalent cations (La3+ > Al3+ > Gd3+) and ruthenium red, but it was relatively insensitive to a micromolar concentration of verapamil. Exposure of the growing pollen tubes to micromolar concentrations of Al3+ and La3+, and a millimolar concentration of Ca2+ chelator ethyleneglycol-bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) led to rapid tip bursting. In contrast, exposure to Gd3+ nifedipine, ruthenium red, verapamil and the organic trivalent cation tris (ethylenediamine)cobalt (TEC3+) caused only inhibition of pollen tube growth. The Al3+-related pollen tube bursting was reduced significantly by increasing either solution pH from 4.5 to 6 or activity of Ca2+ from 0.25 to 5 mh . In contrast, La3+-related pollen tube bursting was insensitive to changes in Ca2+ activity. The results are discussed in terms of Al interactions with cell wall Ca2+ and the plasma membrane Ca2+-permeable channels. (C) 1999 Annals of Botany Company.