alpha-Pinene inhibits growth and induces oxidative stress in roots

Abstract

Background and aims: Determining the mode of action of allelochemicals is one of the challenging aspects in allelopathic studies. Recently, allelochemicals have been proposed to cause oxidative stress in target tissue and induce an antioxidant mechanism. alpha-Pinene, one of the common monoterpenoids emitted from several aromatic plants including forest trees, is known for its growth-inhibitory activity. However, its mechanism of action remains unexplored. The aim of the present study was to determine the inhibitory effect of alpha-pinene on root growth and generation of reactive oxygen species, as indicators of oxidative stress and changes in activities of antioxidant enzymes.

Methods: Effects of alpha-pinene on early root growth were studied in five test species, Cassia occidentalis, Amaranthus viridis, Triticum aestivum, Pisum sativum and Cicer arietinum. Electrolyte leakage, lipid peroxidation, hydrogen peroxide generation, proline accumulation, and activities of the enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), catalase (CAT) and glutathione reductase (GR) were studied in roots of C. occidentalis.

Key results: alpha-Pinene inhibited the radicle growth of all the test species. Exposure of C. occidentalis roots to alpha-pinene enhanced solute leakage, and increased levels of malondialdehyde, proline and hydrogen peroxide, indicating lipid peroxidation and induction of oxidative stress. Activities of the antioxidant enzymes SOD, CAT, GPX, APX and GR were significantly elevated, thereby indicating the enhanced generation of reactive oxygen species (ROS) upon alpha-pinene exposure. Increased levels of scavenging enzymes indicates their induction as a secondary defence mechanism in response to alpha-pinene.

Conclusions: It is concluded that alpha-pinene inhibits early root growth and causes oxidative damage in root tissue through enhanced generation of ROS, as indicated by increased lipid peroxidation, disruption of membrane integrity and elevated antioxidant enzyme levels.

Fig. 1

Effect of α-pinene on electrolyte leakage (measured as conductivity) of C. occidentalis roots determined one week after treatment. The dashed horizontal line indicates maximum ion leakage (432·05 μS) achieved after boiling samples; the vertical line at 20 h indicates the point of transition from dark to light conditions. Bars indicate s.e. (n = 5).

Fig. 2

Effect of α-pinene on (A) lipid peroxidation and (B) proline accumulation in C. occidentalis roots (n = 5) determined a week after treatment. Bars indicate s.e.; ^** indicates significance from control at P < 0·01 applying Dunnett's test.

Fig. 3

Effect of α-pinene on (A) activity of superoxide dismutase (SOD) and (B) hydrogen peroxide production in roots of C. occidentalis determined one week after treatment. Bars indicate s.e. (n = 5); ^** indicates significant difference from the control at P < 0·01, after applying Dunnett's test.

Fig. 4

Effect of α-pinene on activities of GPX, APX, CAT and GR in roots of C. occidentalis estimated one week after treatment. Bars indicate s.e. (n = 5); ^*, ^** indicate significant difference from the control at P < 0·05 and P < 0·01, respectively, after applying Dunnett's test.