A rapid increase in cytokinin levels and enhanced ethylene evolution precede Al³⁺-induced inhibition of root growth in bean seedlings (Phaseolus vulgaris L.)

The role of plant hormones in aluminium (Al3+)-induced inhibition of root growth was investigated in roots of Phaseolus vulgaris L. cv. Strike. Changes in ethylene evolution and changes in the content and composition of cytokinins (CKs), at intervals up to 150 min after treatment with Al3+, were determined by gas chromatography (GC) and combined gas chromatography-mass spectrometry (GC-MS), respectively. Seedlings were cultivated in a continuously aerated nutrient solution at pH 4.5 with and without Al3+. The growth rate of Al3+-treated roots was significantly decreased after 360 min. Ethylene evolution from excised root tips doubled after 15 min of Al3+ treatment and reached a maximum 30 min after treatment. Levels of CK nucleotides declined 60 to 80% after only 5 min of Al3+ treatment whereas the zeatin (Z) content increased six-fold. The increase in Z continued over the entire 150 min-sampling period and reached a level 80 times higher than that in roots not exposed to Al3+. These results show that Al3+-induced inhibition of root growth is preceded by significant changes in CK content and composition and enhanced ethylene evolution. Since CK can induce ethylene production, the rapid increase in CK, particularly Z and dihydrozeatin (dZ), may contribute to inhibition of root-growth either directly or indirectly by affecting plant hormone homeostasis.