CARBON METABOLISM ENZYME-ACTIVITIES AND CARBON PARTITIONING IN PINUS-HALEPENSIS MILL EXPOSED TO MILD DROUGHT AND OZONE

Since several years, accelerated decline of Aleppo pine (Pinus halepen sis) forests has been observed in mediterranean areas. In fact, the co mbination of various environmental factors (photochemical oxidants, dr ought, high light,...) was suspected to cause this decline. In this st udy, three year-old Aleppo pines were exposed during 3 months to ozone fumigation (100 ppb) combined or not with mild drought to study the e ffects of these combined factors on some sequences linked to carbon pa rtitioning and primary carbon metabolism within the tree. After a cumu lative ozone exposure of 132 ppm . h, ozone induced a significant decr ease in specific activity of the whole-plant (-38%) combined with a di sequilibrium of the carbon transfer between root and shoot in favour o f the shoots (non significant). Moreover, while the same cumulative do se of ozone had no effect on total Rubisco activity in one year-old ne edles, mitochondrial NAD malic enzyme activity increased significantly (+32%). By combining ozone with mild drought, the ozone-induced respo nses of all the parameters were significantly amplified and Rubisco ac tivity was significantly decreased (by 44%). These results allowed us to conclude that at 132 ppm . h, ozone alone led to an increase in dar k respiration. Moreover, by the combination of ozone and mild drought, a decrease carbon fixation capacity was associated to a decrease of t he carbon transfered to the roots, leading to a reduced root growth. T hus, there are indications that high levels of ozone during the summer months may impair the ability of Pinus halepensis to withstand severe water stress in its natural environment.