Assessment of EDU stem injections as a technique to investigate the response of trees to ambient ozone in field conditions

The study of the effects of ambient air pollution on trees suffers from some practical difficulties. One of these is the limited space generally available in fumigation chambers in both the field and laboratory, which makes investigations on large trees extremely difficult. There is thus a need to develop experimental techniques which do not rely on fumigation chambers. In this paper we present the results of experiments aimed at evaluating the response of fast growing trees to ambient ozone by means of antioxidant stem injections. Tests were performed in greenhouse chambers and in the open field. Two clones of hybrid poplar were used: Populus X euramericana I-214' and Populus deltoides X maximowiczii 'Eridano'. In a greenhouse chamber experiment, 80-day-old (40-60 cm) plants of 'I-214' and 'Eridano' were injected with either water or a solution of N-[2-(oxo-1-imidazolidinyl)ethyl]-N' phenylurea (ethylene diurea; EDU) and exposed to 85 nl l(-1) O-3 for 8 h day(-1). The plants injected with EDU were significantly less injured than the water-injected controls. The foliar symptoms caused by O-3 fumigation of 'Eridano' were similar to those that this clone often shows under field conditions in the Po plain (interveinal brown stipples on the upper surface). The field experiment was performed at a rural location in the Po plain, northern Italy. Cuttings of both clones were planted in the ground in spring 1991. Every 2 or 3 weeks during summer for the following 2 years the plants were injected with either EDU solution or water. Leaf injury, gas exchange, chlorophyll content and increase in height and diameter were periodically assessed. At final harvest the aboveground biomass was measured. The results showed significantly less O-3 injury to the EDU-injected plants; they maintained their leaves for longer than the control and with higher chlorophyll content. However, no significant effects of the treatment on leaf gas exchange or on height and diameter growth were detected. The significance of these results in relation to the effectiveness of EDU application for assessment of ambient O-3 effects on trees is discussed.