Inuence of Aluminum at Low pH on the Rhizosphere Processes of Masson Pine (Pinus Massoniana Lamb)

Trees in general are very tolerant of aluminum (Al, mainly Al 3+ at pH 5.0), and the small effects seen in the contaminated soils may mislead people that the contamination is 10 unimportant. We believe that the assessments with Al-sensitive Masson pine could have 11 revealed a bigger difference. The key point of this study was to characterize the Al 12 toxicity for Masson Pine. The objectives were to discover the specific eco-physiological 13 relationship between pine roots and rhizosphere Al, and to investigate the Al effects on 14 several parameters, measured in the rhizosphere of Masson pine. Masson pine seedlings 15 were cultivated on a hydroponic setup. Through comprehensive laboratory dose-gradient 16 experiments, Al-triggered composition of the root-released compounds and several 17 rhizospheric parameters were determined by chromatography or spectroscopy. This study 18 gives an important evidence of the Al-toxicity effects on the composition of root-released 19 compounds and the root growth of Masson pine. Results showed that higher rhizospheric 20 Al at pH 4.5 might contribute to increased release of sugars, and also could stimulate the 21 release of oxalic acid and malic acid. The total of secreted amino acids were correlated 22 with the rhizosphere Al. Zero additional Al induced no rhizosphere pH elevation, but 23 Al-induced rhizosphere acidification (pH from 4.50 to 4.22) was observed at Al 100 µM. 24 Greater additions of Al (>300 µM) suppressed the rhizosphere acidification at pH 3.92. 25 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 27 determined in the pine roots. Gibberellic acid (GA 3 ) decreased, whereas abscisic acid 28 (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 31 rhizospheric Al. The rhizosphere Al species at low pH can trigger pine roots to release 32 the sugars (glucose, fructose + aldose), organic acids (oxalic acid, and malic acid), amino 33 acids, secondary metabolites, and endogenous hormones during their growth. Meanwhile 34 it also affected the growth of pine roots. This is an extensive study, which can help 35 understanding the toxicity of Al to this important pioneer species of acid forest soils in 36 south China.

associated with root-released compounds. The root system, especially root apex, is the 53 critical site for Al toxicity. Kidd et al. (2001) and Tolrà et al. (2005) wrote about maize 54 and Rumex acetosa L., respectively. Nowak and Friend (2005) also observed Al 55 resistance in the root tips of slash pine. Normally, plant roots excrete low molecular 56 weight organic molecules into plant rhizosphere to adapt various stressful circumstances. 57 Study of the root-released compounds holds great promise for revealing the effects of Al 58 on tree rhizosphere. 59 Rhizosphere is an important root-soil interface for releasing organic compounds, 60 intense nutrients exchange and microbial activity, and also a gateway for potentially toxic 61 pollutants such as Al, in which normal root physiological action is greatly influenced by 62 root-released compounds (Hinsinger et al. 2005). Currently, the association between Al 63 chemistry of tree rhizosphere and soil acidification is an important concern (Rehmus et al. AlCl 3 were added to each part to reach concentrations of 0, 7.5, 15, 30, 45, 60, 75, 100, 101 150, 200, 250, 300, 350, 400, 450, 600 and 750 µM. Then the pH values of these 102 solutions were initially adjusted to 4.5 using HCl or NaOH and checked and adjusted 103 again three times a week. The nutrient solution without Al was used for control plants. 104 The solutions were aerated by pumps, which connected the containers with pump lines. 105 The nutrient solutions were changed regularly every 10 days. The seedlings from each 106 treatment were harvested after 100 d of incubation for Al determination. To have Masson 107 pine seedlings in a hydroponic and non-sterile system for 100 days, it is likely that 108 contaminants such as fungi or bacteria can grow as well and establish on the root surface. 109 Ultraviolet radiation was used to disinfect the growth chamber and the nutrient solutions 110 prior to use.

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The seedlings were transplanted into a 25-L pot containing nutrient solutions (

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With increasing rhizosphere Al 3+ , the eight secondary metabolites gradually disappeared. where Wt is the dry weight of pine root (mg g -1 dw), and C Al is the Al-treated 283 concentration (μM).

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Meanwhile, added Al had a positive effect on the Al contents accumulated in the 285 pine roots (Fig. 4b). When Al-treated concentration was more than 300 µM, the Al 286 accumulated contents increased gradually from 2.5 to 8.3 µ g g -1 dw. where Q Alac is the Al content accumulated in the pine roots (µg g -1 dw), and C Al is the 290 Al-treated concentration (μM).  were electrolytes, H + , sugar, organic acids, amino acids and other secondary metabolites. 304 We really want to perform a comprehensive analysis of the root-released compounds.

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Due to the limitation of instrument sensitivity, we could only detect these compounds at 306 present, although we try to figure out every peak detected via our analytical instruments