Effects of short-term, high exposure to chlorine gas on morphology and physiology of Pinus ponderosa and Pseudotsuga menziesii

Abstract

Following an accidental spill, acute morphological and physiological effects of chlorine gas exposure were evaluated on two conifer species, Pseudotsuga menziesii (Mirb.) Franco and Pinus ponderosa (L.), growing in a montane, coniferous forest in the Rocky Mountains, USA. Foliar injury, consisting of chlorosis, necrotic mottling and necrosis was observed only on foliage that was directly exposed to chlorine gas. Necrotic needles of both species defoliated during the months immediately following exposure. Buds of both species within 50 m of the gas release were killed; this gave rise to secondary shoot growth for Pseudotsuga menziesii. Cuticular injury was assessed by measuring droplet contact and retention angles on 1-year-old foliage (directly exposed) and current-year foliage (which flushed after the gas cloud had subsided). Chlorine gas exposure led to smaller droplet contact angles on needles in both age classes of Pseudotsuga menziesii (P < 0.0001), but not on Pinus ponderosa. Moreover, exposure led to increased cuticular water loss and decreased total water content of needles in both age classes of Pseudotsuga menziesii, and for 1-year-old needles of Pinus ponderosa (P < 0.0001). On exposed trees, needles in both age classes had lower Fv/Fm ratios (P < 0.0001), suggesting reductions in photosynthetic efficiency. Thus, exposure of needles to chlorine gas may lead to increased drought susceptibility and damage to chloroplast membranes in conifers, and may have a negative influence on tree growth. Importantly, plant responses to chlorine gas are species-specific and are influenced by variation between sites and the stochastic movement of chlorine gas clouds. © 2001 Annals of Botany Company.