EFFECTS OF AMBIENT AIR-POLLUTION IN OPEN-TOP CHAMBERS ON BEAN (PHASEOLUS-VULGARIS L) .2. EFFECTS ON PHOTOSYNTHESIS AND STOMATAL CONDUCTANCE

Field-grown bean plants (Phaseolus vulgaris L. cv. Taylor's Horticultu ral) were exposed to ambient air pollution in open-top chambers (OTCs) at two sites (urban and rural) in Northern Italy. At each site three OTCs received charcoal/Purafil filtered air (F) and three ambient non- filtered air (NF). Net photosynthesis, stomatal conductance and transp iration of selected leaves were measured from July to September 1989. The results showed that net photosynthesis of NF plants decreased prog ressively during the season in comparison with the F ones. The differe nce was greatest at September (-40% at urban site and -23% at rural si te). The maximum impairment of photosynthesis occurred during the ripe ning of the pods. For about 2 wk in August stomatal conductance of NF plants was greater than that of F plants at both sites. Later in Augus t and in September the situation was reversed and stomatal conductance of NF plants was significantly lower than that of F ones. By this tim e a significant negative correlation between net photosynthesis and su b-stomatal carbon dioxide concentration was found in NF plants at both sites, indicating an increased importance of non-stomatal limitation of photosynthesis. The measured leaves did not show any visible sympto m of damage at the time of the physiological measurements; however bro wn spots on the upper surface became evident later on 20% of total lea ves of NF plants. Ambient ozone is suspected to be the major air conta minant involved in causing the observed physiological effects.