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 wa ter deficit, it is proposed that the effect of the fungus Verticillium dahl iae Klebahn on plant leaf area precedes and is greater than its effect on l eaf photosynthesis and stomatal conductance. To test this hypothesis, we me asured shoot and leaf area growth, leaf photosynthetic rate, stomatal condu ctance and disease symptoms in a field experiment including hybrids of high (Sankol) and low (Dekasol 3900) susceptibility to V. dahliae. Plants inocu lated with V. dahliae and controls were compared. We also investigated the effect of V. dahliae on key components of plant leaf area, leaf expansion a nd senescence, in inoculated and control plants of Sankol and Toba, a hybri d of intermediate susceptibility to V. dahliae. Reduction in plant leaf are a caused by V. dahliae was first detected 31 d after inoculation (DAI), whe n visual symptoms of disease in inoculated plants were slight (Sankol) or a bsent (Dekasol 3900). Reduction in leaf photosynthesis was first observed 6 6 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, in oculation reduced shoot dry matter of mature Sankol by 50%. In both experim ents, less leaf expansion accounted for most of the early reduction in plan t leaf area: as the disease progressed, increasing senescence also contribu ted to reduced plant leaf area. It is concluded that the response of sunflo wer 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 th e season and faster leaf senescence in older plants accounted for the decre ase in plant leaf area. (C) 2000 Annals of Botany Company.