Photosynthetic response of tomato plants to vascular wilt diseases

Greenhouse experiments were conducted to study the chronology of effects on gas exchange and chlorophyll-a fluorescence, visible symptoms and hyphal colonization in plants of the susceptible tomato cultivar Bonny Best inoculated with tracheomycotic fungi Fusarium oxysporum f. sp. lycopersici or Verticillium albo-atrum. The net photosynthetic rates and related parameters of healthy (uncolonized and asymptomatic) leaves of infected plants were affected by both the parasites. In the first uncolonized leaf, net photosynthesis was depressed in different ways: in Fusarium-infected individuals, the maximum detrimental effect was observed a week after inoculation, while in Verticillium-infected plants the most severe depression was detected 21 days after inoculation. The behaviour of the physiological parameters investigated, together with the data relative to chlorophyll fluorescence measurements highlighted the fact that the depression in photosynthetic activity was caused by different concomitant factors in Verticillium-infected plants and was due mainly to drought stress in plants inoculated with Fusarium. (C) 1997 Elsevier Science Ireland Ltd.