Influence of aluminum at low pH on the rhizosphere processes of Masson pine (Pinus massoniana Lamb)

Abstract Trees in general are very tolerant of aluminum (Al, mainly Al3+ at pH ≦ 5.0), and the small effects seen in the contaminated soils may mislead people that the contamination is unimportant. We believe that the assessments with Al-sensitive Masson pine could have revealed a bigger difference. The key point of this study was to characterize the Al toxicity for Masson Pine. The objectives were to discover the specific eco-physiological relationship between pine roots and rhizosphere Al, and to investigate the Al effects on several parameters, measured in the rhizosphere of Masson pine. Masson pine seedlings were cultivated on a hydroponic setup. Through comprehensive laboratory dose-gradient experiments, Al-triggered composition of the root-released compounds and several rhizospheric parameters were determined by chromatography or spectroscopy. This study gives an important evidence of the Al-toxicity effects on the composition of root-released compounds and the root growth of Masson pine. Results showed that higher rhizospheric Al at pH 4.5 might contribute to increased release of sugars, and also could stimulate the release of oxalic acid and malic acid. The total of secreted amino acids were correlated with the rhizosphere Al. Zero additional Al induced no rhizosphere pH elevation, but Al-induced rhizosphere acidification (pH from 4.50 to 4.22) was observed at Al 100 µM. Greater additions of Al (>300 µM) suppressed the rhizosphere acidification at pH 3.92. Added Al had a negative effect on the dry weight of pine roots, but an opposite effect on Al accumulated in the roots was observed. The four endogenous hormones were also determined in the pine roots. Gibberellic acid (GA3) decreased, whereas abscisic acid (ABA) increased simultaneously with the addition of Al. Their inflexional concentrations were most frequently observed at 100 µM, which might be the threshold of Al toxicity for Masson pine. The secondary metabolites assayed have been studied in relation to the rhizospheric Al. The rhizosphere Al species at low pH can trigger pine roots to release the sugars (glucose, fructose + aldose), organic acids (oxalic acid, and malic acid), amino acids, secondary metabolites, and endogenous hormones during their growth. Meanwhile it also affected the growth of pine roots. This is an extensive study, which can help understanding the toxicity of Al to this important pioneer species of acid forest soils in south China.