Relationship between leaf life-span and photosynthetic activity of Quercus ilex in polluted urban areas (Rome)

Anatomical, morphological and physiological leaf traits of Quercus ilex in response to different traffic levels (high traffic level, type A sites; average traffic level, type B sites; control sites, type C sites) were analysed in Rome. Superficial leaf deposits were analysed comparing unwashed and washed leaf samples. Washing lowered Pb 61% in A, 54% in B and 27% in C. Sr, Fe, Cu, Zn and Al showed the same trend as Pb. The higher photosynthetic activity of 1-year-old leaves (Pn = 7.0 +/- 2.9 mu mol m(-2) s(-1), average value) in A sites with respect to B sites (6.7 +/- 2.4 mu mol m(-2) s(-1)) and C sites (6.7 +/- 1.8 mu mol m(-2) s(-1)) seems to be related to higher stomatal conductance (g(s) = 0.13 +/- 0.06 mol m(-2) s(-1)), higher total chlorophyll content (Chl = 1.57 mg g(-1)) and higher leaf thickness (L-T = 218.9 mu m), particularly palisade parenchyma thickness (109.4 mu m) Q. ilex showed, on average, 95% of 1-year-old leaves and rarely 2-year-old leaves in A and B sites; 77% 1-year leaves, 20% previous-year leaves and sporadic 3-year leaves in C sites. The enhanced leaf senescence in A sites is compensated by a stimulated shoot production (18% higher with respect to C sites); 25% increased specific leaf area seems to be compensatory growth occurring in order to increase the size of the assimilatory area. The inverse trend of leaf life-span and Pn seems to be Q. ilex' adaptive strategy in polluted areas. (C) 2000 Elsevier Science Ltd. All rights reserved.