Responses of transgenic poplar (Populus tremula x P-alba) overexpressing glutathione synthetase or glutathione reductase to acute ozone stress: visible injury and leaf gas exchange

Untransformed hybrid poplar (Populus tremula x P. alba) and transgenic line s overexpressing glutathione synthetase (GshS) in the cytosol (200-300-fold ) or glutathione reductase (GR) either in the cytosol (5-fold) or in the ch loroplast (150-200-fold) were exposed to 0 (control), 100, 200 or 300 nl l( -1) ozone for 3 d for 7 h d(-1). Following acute ozone stress treatments, w ildtype and transgenic poplar suffered from visible foliar injury consistin g of dark brown necrotic lesions on the laminae. Necrotic lesions were shar ply separated from photosynthetically active cells by a band of red-violet discoloured cell lines showing yellow autofluorescence by blue light, and b lue autofluorescence by UV-light excitation. When plants were exposed to 10 0 nl l(-1) ozone, leaf injury was in general negligible, but when 200 and 3 00 nl l(-1) ozone was applied, in both untransformed poplar and transgenic lines overexpressing GshS or GR up to 60% and 80%, respectively, visible in jury developed on mature leaves. The mean percentage of injured leaf area a mounted to 20-30% (200 nl l(-1)) and 50-60% (300 nl l(-1)). Irrespective of transformation, young leaves of poplar trees were only slightly affected b y ozone treatments. In support of these observations, net CO2 assimilation rates of mature leaves were decreased by up to 75% (300 nl l(-1) ozone) in wild-type and transformed poplar, whereas net photosynthesis of young leave s remained unaffected even under severe stress conditions. Leaf conductance was significantly decreased by all ozone treatments, but was in the same r ange in young and mature leaves, and in wild-type and transformed poplar, p re- and post-exposure to ozone. It can therefore be assumed that the ozone doses effectively taken up into the leaf tissue were not dependent on leaf development and that the strength of the ozone stress exerted was similar i n all types of poplar trees investigated in this study. From these data it is concluded that: (i) elevated foliar activities of glu tathione synthetase or glutathione reductase alone are not sufficient to im prove tolerance of hybrid poplar to acute ozone stress, and (ii) the sensit ivity of poplar leaves to acute ozone stress is controlled by unknown facto rs closely related to leaf development rather than by foliar activities of glutathione synthetase and glutathione reductase, or leaf conductance.