Variability of Photosynthetic Performances in Tree Seedlings Under Ozone and Water Stress

Analysis of the variability in photosynthetic properties and responses (within and among individuals) is important in assessing the impact of stress factors in plant population and communities, as well as in supporting remote sensing techniques. Photosynthesis parameters, namely the Performance Index (PIABS), assessed by direct fluorescence techniques, were measured during an open-top chamber experiment at Curno (North Italy) in 2005. This experiment consisted in treatments of seedlings belonging to different tree species, in non filtered (NF, i.e. treated with ambient air) and charcoal-filtered (CF, i.e. treated with air with 50% of ambient air ozone) air, in watered (W) and not watered (D) conditions. The variability of PIABS was tested by means of the coefficient of variation (CV% = standard deviation/mean expressed as percent). CV% changed over time: it is higher at the beginning of the season and lower in the central months of the summer. CV% was usually higher in stressful conditions and correlated negatively with PIABS. This finding indicates that moderate stress factors act selectively on different individuals, and individual differences within population are enhanced.