Plant tolerance mechanism to aluminum (Al) toxicity can be studied by comparing the Al-tolerant and Al-sensitive species. Setaria splendida is an Al tolerant forage, while Chloris gayana is a Al sensitive one. This research was aimed to reveal the main Al-tolerant mechanism in S. splendida. The first experiment was arranged in completely randomized design with two factors and two replications to study the organic acid and Al accumulation in the S. splendida and C. gayana. The first factor was Al concentration (0 and 2 mM Al), while the second factor was Al tolerant (S. splendida) and Al sensitive (C. gayana) species. The second experiment was arranged in completely randomized design with two factors and three replications to study the effect of Al stress on plant growth. The first factor was Al tolerant (S. splendida) and Al sensitive (C. gayana) species, while the second factor was four levels of Al concentration in the growth medium (28.19, 27.37, 13.74, and 0.13 me Al3+). The results showed that the Al accumulation in the root tissue of S. splendida was similar to C. gayana. Although S. splendida accumulated higher Al in the shoots in comparison to C. gayana, they were more tolerant to Al. S. splendida tolerated Al-toxicity by secreting oxalic acids and citric acid from roots to the external solution, and by accumulating oxalic acid and malic acid in their roots and shoots. Keywords: aluminum, Chloris gayana, organic acid, Setaria splendida, tolerance.
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