The effects of oxidative air pollutants on plant cuticles, cuticular transpiration, plant water balance, and growth (Pinus ponderosa, Pseudotsuga menziesii, Populus euramericana, Populus nigra)

Morphological, physiological and growth effects of acute chlorine gas (Cl₂) exposure were examined over three growing seasons in Western Montana, Rocky Mountains, USA, for two conifer species, Pinus ponderosa and Pseudotsuga menziesii. Acute damage symptoms after exposure consisted of chlorosis, necrosis, necrotic mottling, and defoliation. Cuticles of exposed P. menziesii needles and needles that flushed after Cl₂ exposure were more wettable. Moreover, foliage of both species had higher cuticular transpiration rates and lower total water content compared to control foliage, up to one year after exposure. Lower photosynthetic efficiencies, measured as F_v/F_m ratios, were observed for exposed foliage five months after exposure. Foliage on trees that flushed two months after Cl₂ exposure had higher foliar injury and lower needle longevity compared to controls, suggesting higher susceptibility to secondary stress factors. Exposed trees also had lower annual stem increment growth and cone production. Higher tree mortality was observed for P. menziesii but not for P. ponderosa.; Saplings of two poplar species, Populus Euramericana Robusta and Populus nigra Brandaris, and P. menziesii , were exposed to ozone (O₃) concentrations characteristic for growing seasons in urban areas and high elevations. Ozone exposure increased leaf wettability of poplars only temporarily, but increased cuticular transpiration of P. Euramericana. Both poplar species had lower foliar biomass in the ozone treatments compared to controls because of lower leaf growth and higher leaf abscission. Ozone effects on P. menziesii were limited to increased leaf wettability, which can affect ozone deposition and gas exchange. Leaf surface wetness, in the form of simulated dew, rain and mist, increased ozone deposition to poplar leaves 1.5–5 times. This increase was the result of lower ozone deposition due to stomata] closure, and ozone deposition to water present on leaf surfaces. Leaf surface wetness also decreased photosynthesis and CO₂ emissions in dark conditions.; This research has shown that acute Cl₂ exposure and chronic ozone exposure caused similar morphological, physiological and growth effects on trees, although ozone effects were less severe. Both pollutants may have long-term impacts on drought tolerance and growth of coniferous and deciduous trees. However, responses were highly dependent on species, both for conifers and deciduous trees.