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 Ca 2+ -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 (La 3+ >Al 3+ >Gd 3+ ) and ruthenium red, but it was relatively insensitive to a micromolar concentration of verapamil. Exposure of the growing pollen tubes to micromolar concentrations of Al 3+ and La 3+ , and a millimolar concentration of Ca 2+ chelator ethyleneglycol-bis(β-aminoethyl ether)-N,N′-tetraacetic acid (EGTA) led to rapid tip bursting. In contrast, exposure to Gd 3+ , nifedipine, ruthenium red, verapamil and the organic trivalent cation tris (ethylenediamine)cobalt (TEC 3+ ) caused only inhibition of pollen tube growth. The Al 3+ -related pollen tube bursting was reduced significantly by increasing either solution pH from 4.5 to 6 or activity of Ca 2+ from 0.25 to 5 m m . In contrast, La 3+ -related pollen tube bursting was insensitive to changes in Ca 2+ activity. The results are discussed in terms of Al interactions with cell wall Ca 2+ and the plasma membrane Ca 2+ -permeable channels.
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Wenhao Zhang (1999) studied this question.