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

Following an accidental spill, acute morphological and physiological effect s of chlorine gas exposure were evaluated on two conifer species, Pseudotsu ga 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 tha t was directly exposed to chlorine gas. Necrotic needles of both species de foliated during the months immediately following exposure. Buds of both spe cies within 50 m of the gas release were killed; this gave rise to secondar y shoot growth for Pseudotsuga menziesii. Cuticular injury was assessed by measuring droplet contact and retention angles on 1-year-old foliage (direc tly exposed) and current-year foliage (which flushed after the gas cloud ha d 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 lo ss and decreased total water content of needles in both age classes of Pseu dotsuga menziesii, and for 1-year-old needles of Pinus ponderosa (P < 0.000 1). On exposed trees, needles in both age classes had lower F-v/F-m ratios (P < 0.0001), suggesting reductions in photosynthetic efficiency. Thus, exp osure of needles to chlorine gas may lead to increased drought susceptibili ty 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 sto chastic movement of chlorine gas clouds. (C) 2001 Annals of Botany Company.