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V. O. SADRAS, F. QUIROZ, L. ECHARTE, A. ESCANDE and V. R.
PEREYRA
Annals of Botany 86: 1007-1015, 2000
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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 sun¯ower 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. |
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