Effect of aluminium supply on aluminium uptake, translocation and blueing of Hydrangea macrophylla (Thunb.) Ser. cultivars in a peatclay substrate

The aluminium (Al) accumulator Hydrangea macrophylla (H. m.) is extremely Al-resistant compared with other plant species. Sufficient transport of Al into the flowers is essential to achieve the desired colour change from original pink to blue by complexation of the anthocyanin delphinidine with Al. To overcome wide-spread difficulties in producing blue H. m., an improved understanding of the relationship between Al supply, Al uptake, translocation, and blueing of sepals is required. A strategy was previously developed that allows estimation of the Al supply needed for adjustment of a target pH at the beginning of forcing. After treatment with 0, 3, 6, 12 g Al-2(SO4)(3) x 18 H2O (1 substrate)(-1), determination of substrate pH, Al content and colour of the inflorescence proved that the strategy was suitable to achieve satisfying blue inflorescences in eight cultivars differing considerably in their easiness of blueing. These differences between cultivars could be attributed to cultivar-specific modifications of the substrate pH, translocation of Al in the xylem from the roots to the shoots and the inflorescences, and the Al contents of the sepals necessary to produce the desired blue colour. Measurement of Al and organic acid concentrations in the xylem sap and calculations with the speciation programme GEOCHEM-PC revealed a dominant role of citrate for Al transport particularly in the easy-to-blue cultivar 'Renate Steiniger'. The possible role of malonate and malate needs further experimental support.