Occurrence of endogenous hormones in the roots of Masson pine (Pinus massoniana Lamb.) seedlings subjected to aluminum stress under the influence of acid deposition

Abstract

Aluminum (Al) toxicity is perceived as a potential threat to the integrity of forest ecosystems. The detrimental effects of acidic deposition on trees are caused by mobilization of Al in soils. To better understand how roots adapt to acidic deposition releasing toxic Al including the endogenous hormones {{abscisic acid (ABA), gibberellic acid (GA3), indole-3-acetic acid (IAA) and zeatin riboside (ZR)}}, chemistry {{Ca/Al ratio, and base saturation (BS)}}, and growth rate, with the 100-day-old Masson pine (Pinus massoniana Lamb.) seedlings as research target, we studied the responses of the hormones to rhizospheric Al concentrations and their relationships between pine growth rates and Ca/Al ratios in the simulated soil solutions and forested soils differing in BS. In a hydroponic culture, after the seedlings were exposed to the simulated soil solutions containing 0 (control), 50, 100, 150, 200 µM Al at pH 4.0, ABA formed in the roots increased significantly with the time and intensity of Al exposure. ABA concentrations were positively correlated with rhizospheric Al concentrations (r = 0.92), whereas the opposite trend was seen for GA3 (r = −0.77). Simultaneously, IAA and ZR did not change with the increasing Al concentrations. Increasing of either Al concentration or exposure time caused an inhibition of root elongation and seedling growth. Moreover, in a field soil culture, the seedlings were grown for 60 days (d) in either A horizon soils with a BS above 17% or B horizon soils with a BS below 7%. Evident regional variation of ABA concentrations was observed in the pine roots, which declined in the order: Jin Yun Shan > Nan Shan > Tie Shan Ping, as well as B horizon soil > A horizon soil. Also, ABA concentrations were negatively correlated with soil Ca/Al ratios (r = −0.67). The opposite trends were simultaneously observed for GA3, IAA and ZR (r = 0.45, 0.25 and 0.18, respectively). Based on the laboratory and field results, under the influence of acid deposition, ABA, and GA3 may play an important synergic role in regulating Al resistance of Masson pine as an Al-stress signal.