Improved sapflow methodology reveals considerable night-time ozone uptake by Mediterranean species

Due to the evident tropospheric ozone impact on\r\nplant productivity, an accurate ozone risk assessment for the\r\nvegetation has become an issue. There is a growing evidence\r\nthat ozone stomatal uptake may also take place at night and\r\nthat the night-time uptake may be more damaging than diurnal\r\nuptake. Estimation of night-time uptake in the field is\r\ncomplicated because of instrumental difficulties. Eddy covariance\r\ntechnology is not always reliable because of the\r\nlow turbulence at night. Leaf level porometry is defective\r\nat relative humidity above 70% which often takes place at\r\nnight. Improved sap flow technology allows to estimate also\r\nslow flows that usually take place at night and hence may\r\nbe, at present, the most trustworthy technology to measure\r\nnight-time transpiration and hence to derive canopy stomatal\r\nconductance and ozone uptake at night. Based on micrometeorological\r\ndata and the sap flow of three Mediterranean\r\nwoody species, the night-time ozone uptake of these species\r\nwas evaluated during a summer season as drought increased.\r\nNight-time ozone uptake was from 10% to 18% of the total\r\ndaily uptake when plants were exposed to a weak drought,\r\nbut increased up to 24% as the drought became more pronounced.\r\nThe percentage increase is due to a stronger reduction\r\nof diurnal stomatal conductance than night-time stomatal\r\nconductance.