The Blue Hydrangea: its Al in the soil
Many plants are known to be intolerant of acid soils, but the acidity is not the problem. A major component of soil is aluminium in the form of aluminosilicates. In alkaline or neutral conditions the aluminium remains insoluble but in acid soils it becomes soluble and available for uptake by plant roots. The actual process by which aluminium exerts its toxicity remains obscure although it leads to stunted roots in wheat seedlings. The problem is significant as some 40% of the earth’s arable soils are acidic, reducing the range of plants which can be grown and the yields which can be achieved. However, varying degrees of tolerance to aluminium are shown by plants which have evolved a variety of ways of overcoming the problem. Tolerant varieties of wheat, for instance, protect themselves by restricting uptake of aluminium. In soils rich in organic matter soluble aluminium binds to organic molecules and is unable to affect the plant. In poor soils, with limited organic content, this is unable to occur increasing the risk of aluminium toxicity. Tolerant varieties of wheat mimic this by secreting organic acids which bind soluble aluminium in the vicinity of the root. In an alternative approach the roots absorb aluminium which is then transported to some part of the plant where it can be stored and where it is unable to interfere with the plant’s growth. Tea, for instance, stores huge quantities of aluminium in the older leaves (it is the current year’s growth which is used for the beverage). The native Brazilian tree, Vochysia tucanorum, accumulates aluminium in the seeds with as much as 40g per Kg being reported. And hydrangea? As early as 1937 EM Chenery showed that the normally red flowers of Hydrangea macrophylla become blue in acid soils due to aluminium being taken up by the roots and transported to the sepals where it forms a stable, blue, complex with a pigment called delphinidin -3-glucoside. This same pigment gives delphinium flowers their blue colour and hydrangea flower s, in less acid soils, their red colour.
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