Variability of Fv/Fm among the single leaf discs of one treatment
was again very high as in the previous experiments using filter
paper (data not shown). Hence, no clear differences between the
ranges of means of Fv/Fm after 14 dai were present between susceptible
(0.58–0.71) and resistant cultivars/lines (0.57–0.66) because
averaging levelled the specific effects on potential photosynthetic
competence (Fig. 5). In addition, differences between the different
cultivars/lines were not significant. Nevertheless, there were some
distinct reactions obvious, which were related to the resistance
versus susceptibility features of cultivars/lines. In highly infected
leaf discs of susceptible lettuce cultivars, only leaf-vein tissue
was partially physiologically active, while the intervascular region
showed clear symptoms of chlorophyll degradation and low Fv/Fm
(Fig. 6).