Effect of Verticillium dahliae on photosynthesis, leaf expansion and senescence of field-grown sunflower

On the basis of known sunflower (Helianthus annuus L.) responses to soil water deficit, it is proposed that the effect of the fungus Verticillium dahliae Klebahn on plant leaf area precedes and is greater than its effect on leaf photosynthesis and stomatal conductance. To test this hypothesis, we measured shoot and leaf area growth, leaf photosynthetic rate, stomatal conductance and disease symptoms in a field experiment including hybrids of high (Sankol) and low (Dekasol 3900) susceptibility to V. dahliae. Plants inoculated with V. dahliae and controls were compared. We also investigated the effect of V. dahliae on key components of plant leaf area, leaf expansion and senescence, in inoculated and control plants of Sankol and Toba, a hybrid of intermediate susceptibility to V. dahliae. Reduction in plant leaf area caused by V. dahliae was first detected 31 d after inoculation (DAI), when visual symptoms of disease in inoculated plants were slight (Sankol) or absent (Dekasol 3900). Reduction in leaf photosynthesis was first observed 66 DAI: stomatal conductance and leaf dark respiration were both unaffected by V. dahliae during the whole experiment. In comparison with controls, V. dahliae reduced seasonal duration of plant leaf area by 25% in Dekalb 3900 and by 55% in Sankol, whereas the average reduction in leaf photosynthetic rate was 9%. In correspondence with the reduction in leaf area duration, inoculation reduced shoot dry matter of mature Sankol by 50%. In both experiments, less leaf expansion accounted for most of the early reduction in plant leaf area: as the disease progressed, increasing senescence also contributed to reduced plant leaf area. It is concluded that the response of sunflower to V. dahliae resembled the response of the plant to soil water deficit: (1) plant leaf area, rather than leaf photosynthetic rate, accounted for the reduction in growth in mass; and (2) reduced leaf expansion early in the season and faster leaf senescence in older plants accounted for the decrease in plant leaf area. (C) 2000 Annals of Botany Company.