A PHYTOCENTRIC PERSPECTIVE OF ASTEROLECANIUM-QUERCICOLA BOUCHE INFESTATION ON QUERCUS-ROBUR L TREES ALONG AN URBANIZATION GRADIENT

Diurnal courses in gas exchange, photochemical efficiency and water re lations were monitored during two late summers in three groups of adul t Quercus robur L. trees, planted along an urbanization gradient that correlated positively with the degree of die-back exhibited by the tre es. Leaf carbon:nitrogen ratios, proline and polyphenol levels were mo nitored to explain why the intermediate group of trees were more sever ely infested (p less than or equal to 0.01) with Asterolecanium querci cola (Bouche). All three groups of trees showed a significant correlat ion of net photosynthesis (A) with photon flux density (PPFD), but A c orrelated more positively with the pre-dawn leaf water potential psi(p d) Of the moderately (trees of group b, i.e. at the edge of town) and severely (trees of group c, i.e. urban) water-stressed trees. A of the rural trees and stomatal conductance (g) of the three groups of trees showed little correlation psi(pd) values. Possibly due to the long-te rm effect of stress, g, as reflected by changes in the transpiration r ate (a, showed a significantly (p less than or equal to 0.01) higher s ensitivity to relative ambient humidity (RH) in the trees of groups b and c. Photochemically, a close coupling was found to exist between A, psi(pd), RH, the time needed to reach the maximum fluorescence level, i.e. FTm, and S, i.e. the complementary area normalized to the variab le fluorescence, indicating that the trees were also affected at this level of organization. Proline accumulation occurred in the trees of g roup c but not in the trees of group b, as opposed to the polyphenolic concentrations which were significantly (p less than or equal to 0.05 ) higher in the trees of group b than in the trees of group c. A possi ble explanation for the higher infestation of A. quercicola on the tre es in group b is given in terms of their host specificity and changes in these trees' nitrogen levels.