L. Vanrensburg et al., A PHYTOCENTRIC PERSPECTIVE OF ASTEROLECANIUM-QUERCICOLA BOUCHE INFESTATION ON QUERCUS-ROBUR L TREES ALONG AN URBANIZATION GRADIENT, South African journal of botany, 63(1), 1997, pp. 25-31
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.