EXPERIMENTS ON THE WATER-BALANCE OF INDIVIDUAL ATTACHED TWIGS OF PICEA-ABIES (L) KARST IN PURE AND OZONE-ENRICHED AIR

Rates of transpiration and xylem sap flow were continuously measured i n individual twigs in the upper crown of an 18-year-old spruce. Two ga s exchange chambers were run simultaneously under identical conditions . One of two equivalent twigs was exposed to pure air whereas the othe r received the ozone-enriched air of the site. A third gas exchange ch amber in mid-crown ran independently with normal outside air and was u sed for basic experiments. At certain times needles were sampled for w ater potential measurements. Chamber humidity was reduced step by step and the transpiration and xylem sap flow rates were permanently compa red. It turned out that sap flow keeps up with transpiration without l agging as long as chamber humidity is high and the twigs outside the c hamber are not subject to substantial evaporative demand. However, in warm summer weather and with high flow rates sap flow is no longer suf ficient. As the balance quotient (uptake/release) of 0.8 was reached t he stomata began to close and water balance improved. The flux quotien t increased far above 1.0 without water potential of the needles, whic h had decreased before, increasing significantly. The balance quotient of the twig in ozone-enriched air fell to relatively low values and o nly increased again correspondingly slowly due to lagging stomatal clo sure. Despite increased water uptake after the light phase, the ozone- treated twig ran into a water deficit in the daily balance during the course of an uninterrupted drought period. Water deficit increased fro m day to day and only disappeared with the next rainfall. For providin g insight into the ozone effect shown in this study it was thought nec essary to discuss comprehensively the controversial views of the stoma tal control mechanism. The results show that the stomata react to smal l scale changes in water status as expected for an effective negative feedback system. The less sensitive reaction of the stomata under the influence of ozone confirms earlier results. Increased severity and du ration of the daily drought stress loads were attributed to a lag in a djustment due to poor stomatal control. The demonstrated function mech anism shows that the detrimental effect of ozone is dependent on weath er conditions.