Fig. 1.8.6. Excretion of malic acid by seedling of Al-sensitive and -resistant wheat varieties. Six-day-old seedlings were exposed for 24 h to different concentrations of an Al3+ salt solution. (After Delhaize et al. 1993 b)

binding to these acids, the Al3+ ion is no longer available to the uptake system.

Malate excretion is a specific reaction to Al3+, as it is not triggered by other trivalent cations (La3+, Sc3+), and also not by Al13. Citric acid chelates strongest, succinic acid weakest (Fig, 1.8.7).

Under aluminium stress, Al3+-sensitive culti-vars excrete malic acid, too, but much less than tolerant cultivars (see Table 1.8.3). Scavenging of Al3+ by excretion of malic acid by Al-sensitive cultivars could be boosted by the addition of malic acid to the nutrient solution (Fig. 1.8.8).

Resistance of wheat is mainly due to excretion of malic acid, whilst maize reacts to Al3+ stress by excreting citric acid (Kollmeier et al. 2001). Al3+-sensitive and -resistant cultivars of these crops likewise differ mainly in the amount of secreted acid. It is not the acidification of the rhizosphere

| Table 1.8.3. Excretion of organic acids by Al-tolerant and -sensitive wheat seedlings after 24-h exposure to Al (5 seedlings each in 20 ml nutrient solution, containing 50 |.iM Al3+). (After Delhaize et al. 1993b)

Organic acid

Excreted acid (nmol seedlinc. 1 h ')

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