Previous research concerning the physiological response of cotton to Verticillium wilt infection has dealt primarily with he consequent changes in plant-water status. Our preliminary sampling of field-grown cotton indicated that a significant decline in net photosynthesis (P(n)) preceded any decrease in leaf-water potential (YI (L)). These results suggested that a toxin, presumably of fungal origin, affected the photosynthetic capacity of the infected leaf. The objective of this study was to determine the impact of infection on P(n), and its components so to better understand the mechanism of infection.

Cotton (Gossypium hirsutum L. cv. Stoneville 506) was planted on 28 May 1989 in a field with a history of Verticillium infestation. Approximately 80 days postplanting, i.e. when flowering at main-stem (MS) node 11 occurred, plants were identified as being either noninfected or wilt-infected if leaves at MS nodes 6, 7, or 8 exhibited symptoms of localized chlorosis. Measurements Of P(n) stomatal conductance (g(s) CO₂) and internal CO₂ concentration (C(i)) were taken at MS node 8 with a Ll-6000 portable phothesis system in a paired-plant design. To corroborate the visual symptoms, leaf discs were taken for measurement of YI(L), ethylene evolution and chlorophyll concentrations. To observe the vertical progression of the infection, measurements were also taken on control and infected plants at MS 10 and 12 if these leaves did not exhibit symptoms. A:C(i) and P(n) vs ambient CO₂ concentration curves were constructed from P(n) measurements of the leaf at MS-node 12 at various ambient CO₂ levels. Dark respiration (R(d) ) and post-illumination CO₂ burst were determined by covering the cuvette of the photosynthesis system with an aluminum foil shroud to exclude light. Measurements of CO₂ compensation point were made by allowing the leaf enclosed in the cuvette to draw CO₂ levels down to a plateau and recording this concentration.

Results indicated that a 25% decrease in P(n) occurred prior to any measurable reduction in YI(L) or chlorophyll concentration. Furthermore, the decrease was not fully explained by decreases in g(s) CO₂, and the change in C(i) suggested an uncoupling of the P(n) vs. g(s) CO₂ relationship. This uncoupling was supported by the A:C(i) curves, further indicating a non-stomatal limitation to P(n). Wilt infection significantly reduced R(d) and photorespiration (R(p)) by 40 and 29%, respectively. Estimated gross photosynthesis (P(g)) was reduced by ca. 29%. Since the decrease in P(n), was parallelled by similar reductions in R(p) and P(g), i.e. similar reductions in carboxylase and oxygenase efficiency, these results suggest strongly that Verticil lium wilt infection affects RUBISCO activity directly at the protein level or indirectly by affecting chloroplast membranes and photophosphorylation.

The results of this study contributed to our understanding of the means by which Verticillium wilt infection affects cotton leaf physiology. Through better understanding of the mechanisms of pathogenicity, new and more resistant plants may be bred.