Responses of transgenic poplar (Populus tremula × P. alba) overexpressing glutathione synthetase or glutathione reductase to acute ozone stress: visible injury and leaf gas exchange
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Media type:
E-Article
Title:
Responses of transgenic poplar (Populus tremula × P. alba) overexpressing glutathione synthetase or glutathione reductase to acute ozone stress: visible injury and leaf gas exchange
Description:
<p>Untransformed hybrid poplar (Populus tremula × P. alba) and transgenic lines overexpressing glutathione synthetase (GshS) in the cytosol (200—300-fold) or glutathione reductase (GR) either in the cytosol (5-fold) or in the chloroplast (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, wild-type and transgenic poplar suffered from visible foliar injury consisting of dark brown necrotic lesions on the laminae. Necrotic lesions were sharply separated from photosynthetically active cells by a band of red-violet discoloured cell lines showing yellow autofluorescence by blue light, and blue autofluorescence by UV-light excitation. When plants were exposed to 100 nl l-1 ozone, leaf injury was in general negligible, but when 200 and 300 nl l-1 ozone was applied, in both untransformed poplar and transgenic lines over-expressing GshS or GR up to 60% and 80%, respectively, visible injury developed on mature leaves. The mean percentage of injured leaf area amounted 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 by 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 leaves remained unaffected even under severe stress conditions. Leaf conductance was significantly decreased by all ozone treatments, but was in the same range in young and mature leaves, and in wild-type and transformed poplar, pre- 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 in all types of poplar trees investigated in this study. From these data it is concluded that: (i) elevated foliar activities of glutathione synthetase or glutathione reductase alone are not sufficient to improve tolerance of hybrid poplar to acute ozone stress, and (ii) the sensitivity of poplar leaves to acute ozone stress is controlled by unknown factors closely related to leaf development rather than by foliar activities of glutathione synthetase and glutathione reductase, or leaf conductance.</p>