Abstract
Knowing how acid soils and aluminum in soils may limit the growth of Eucalyptus trees in plantations is important because these plantations grow in many tropical and subtropical regions. Seedlings of four vegetatively propagated Eucalyptus clones, E. grandis × E. urophylla ‘GLGU9’(G9), E. grandis × E. urophylla ‘GLGU12’ (G12), E. urophylla × E. camaldulensis ‘GLUC3’ (G3) and E. urophylla ‘GLU4’(G4), were subjected to liquid culture with Hoagland nutrient solution for 40 days, then treated with four different treatments of acid and aluminum for 1 day. The four treatments used either pH 3.0 or 4.0 with or without added aluminum (4.4 mM) in all possible combinations; a control used no added aluminum at pH 4.8. Subsequently, the photosynthetic parameters and morphology of leaves from eucalypt seedlings were determined and observed. The results showed that the tested chlorophyll content, net photosynthetic rate, transpiration rate and water use efficiency were apparently inhibited by aluminum. Under uniform Al concentration (4.4 mM), the Al-induced limitation to photosynthetic parameters increased with pH, indicating acid stimulation to Al toxicity. Among all treatments, the most significant reduction was found in the combination of pH 3.0 and 4.4 mM Al. The photosynthetic and transpiration rates showed similar trends with G9 > G12 > G3 > G4, suggesting that G9 and G12 had higher Al-tolerance than other two clones. Microscopic observation revealed changes in leaf morphology when exposed to Al stress; for example, a reduced thickness of leaf epidermis and palisade tissue, the descendant palisade tissue/spongy tissue ratio and leaf tissue looseness. Overall, the acid and aluminum stress exerted negative effects on the photosynthetic activity of eucalypt seedlings, but the differences in tolerance to Al toxicity between the clones were favorable, offering potential to improve Eucalyptus plantation productivity by selecting Al tolerant clones.
Highlights
Aluminum is the most abundant metal element in the earth’s crust and bound aluminum will dissolve in acidic soils
Researches related to the aluminum tolerance of different genotypes of a single crop species revealed that high concentrations of aluminum limited root growth, causing the accumulation of reactive oxygen species [11, 12], and causing damage to DNA [12] in aluminumsensitive genotypes
Aluminum concentrations, increased acidity resulted in lower chlorophyll content (K0-3 < K0-4, K4.4–3 < K4.4–4)
Summary
Aluminum is the most abundant metal element in the earth’s crust and bound aluminum will dissolve in acidic soils. Aluminum resistance mechanisms are classified into exclusion and tolerance [3], both mechanisms are related to mitochondrial activity [7] as well as organic acid transport [8, 9]. The organic acid anions (malate, citrate and oxalate) are released by plants to chelate the toxic Al3+ in the rhizosphere, such resistance mechanism to Al toxicity has been observed in crops, such as wheat, barley, rye, rice and maize. For woody plants, both coniferous (Picea, Cryptomeria and Pinus) and broad-leaved (Populus, Eucalyptus, Citrus and Melaleuca) trees have this mechanism [8]. Researches related to the aluminum tolerance of different genotypes of a single crop species revealed that high concentrations of aluminum limited root growth, causing the accumulation of reactive oxygen species [11, 12], and causing damage to DNA [12] in aluminumsensitive genotypes
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