Modulation of antioxidant enzymes in Juniperus procera needles in relation to habitat environment and dieback incidence

Oxidative stress and the antioxidant enzymes' activity are higher in damaged than in healthy Juniperus procera trees, in summer than in winter, and in dry than in wet condition. Many of the small stands of Juniperus procera in Saudi Arabia, confined mainly to Aseer Mountains in the southern part of the country, are suffering from branch dieback. As a part of the project on the structural and functional responses of healthy and dieback-affected trees to local environmental conditions of Al-Ghalab, Al-Yazeed, and Saodah locations, this study quantifies the oxidative stress generated and the consequent modulation of proline accumulation and antioxidant enzymes' activity, as determined by chemical analysis of needle tissues from samples collected in summer and winter seasons. The level of TBARS, which indicated the extent of oxidative stress, was minimum (10.1 nM g(-1) f w) at Al-Ghalab and maximum (28.1 nM g(-1) f w) at Al-Yazeed, being relatively higher in summer than in winter. Healthy trees had a lower level of TBARS than those suffering from dieback. Proline content showed 147-54 A mu g g(-1) in healthy trees and 460-99 A mu g g(-1) f w in affected ones. Variation in the activity of superoxide dismutase, ascorbate peroxidase, glutathione reductase, and catalase was around 0.7-3.6, 0.01-0.09, 0.02-0.08, and 0.6-3.0 U mg(-1) min(-1), respectively, in healthy trees, whereas 2.3-6.1, 0.04-0.3, 0.04-0.3, and 2-5.8 U mg(-1) min(-1), respectively, in the dieback-affected trees of the different locations. Thus, the oxidative stress and the enzymatic stimulation were higher in damaged than in healthy trees and in summer than in winter season. Water-harvesting efforts at the collection sites showed ameliorative effects. Our observations suggest that J. procera tree can be made more tolerant toward stressful condition, and even the risk of dieback can be avoided or minimized by improving soil-water availability through adequate water-harvesting strategies in the drought-affected areas.