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 halepensis) forests has been observed in mediterranean areas. In fact, the combination of various environmental factors (photochemical oxidants, drought, high light,...) was suspected to cause this decline. In this study, three year-old Aleppo pines were exposed during 3 months to ozone fumigation (100 ppb) combined or not with mild drought to study the effects of these combined factors on some sequences linked to carbon partitioning and primary carbon metabolism within the tree. After a cumulative ozone exposure of 132 ppm . h, ozone induced a significant decrease in specific activity of the whole-plant (-38%) combined with a disequilibrium of the carbon transfer between root and shoot in favour of the shoots (non significant). Moreover, while the same cumulative dose of ozone had no effect on total Rubisco activity in one year-old needles, mitochondrial NAD malic enzyme activity increased significantly (+32%). By combining ozone with mild drought, the ozone-induced responses of all the parameters were significantly amplified and Rubisco activity was significantly decreased (by 44%). These results allowed us to conclude that at 132 ppm . h, ozone alone led to an increase in dark respiration. Moreover, by the combination of ozone and mild drought, a decrease carbon fixation capacity was associated to a decrease of the carbon transfered to the roots, leading to a reduced root growth. Thus, 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.