INCREASED NONPHOTOCHEMICAL QUENCHING IN LEAVES OF ALUMINUM-STRESSED WHEAT PLANTS IS DUE TO AL3-INDUCED ELEMENTAL LOSS()

Changes in elemental content and chlorophyll fluorescence parameters o f wheat (Triticum aestivum, cv. Nestos) grown in nutrient solution con taining 0, 37.1, 74.1 and 148 mu M Al at pH 4.6 were studied. At stead y-state photosynthesis, non-photochemical quenching (qN) was increased (14%) in 37.1 mu M Al but increased remarkably (45%) in 148 mu M Al. In the wheat leaves exposed to 148 mu M Al, Phi(PSII) was observed to drop to a low value corresponding to around 25% of its maximal value m easured in control leaves. The concentrations of the metals Ca2+, Mg2 and K+ in leaves decreased at all aluminium levels while the concentr ations of Ca2+ and Mg2+ under the high Al treatment were at the defici ency level. It is suggested that Al3+-induced chloroplast elemental lo ss and concomitant increase of the proton gradient across the thylakoi d membrane are primary indirect effects of Al3+-toxicity on the photos ynthetic process. The mechanism of aluminium-induced non-photochemical quenching of PSII excitation energy is thought to result from a steep proton gradient over the thylakoid membrane.