IMPACT OF VERTICILLIUM WILT ON NET PHOTOSYNTHESIS, RESPIRATION AND PHOTORESPIRATION IN FIELD-GROWN COTTON (GOSSYPIUM-HIRSUTUM L)

Verticillium wilt systemically infects cotton (Gossypium hirsutum L.) and drastically limits crop productivity. Efforts to elucidate the physiological mechanisms responsible for growth and yield reductions have been inconclusive. Field experiments were conducted in 1987 and 1989 to document the influence of verticillium wilt on the photosynthetic and respiratory activity of cotton leaves. A portable gas-exchange measurement system was used to determine changes in net photosynthesis, photorespiration and dark respiration of lower-, middle- and upper canopy leaves of control and diseased plants. In addition, changes in stomatal behavior and chlorophyll content were also determined. Photosynthesis of diseased lower-canopy leaves was reduced by over 65% compared to controls, and the reduction was associated with chlorophyll loss, a more negative leaf water potential, and a greatly reduced stomatal conductance. Upper-canopy leaves of diseased plants, although showing no visible symptoms of verticillium wilt, exhibited a 20% decrease in photosynthetic activity which was independent of changes in chlorophyll or leaf water potential. Verticillium wilt reduced dark respiration of asymptomatic, upper-canopy leaves by 42%, photorespiration by 27% and increased mesophyll resistance by 46%. Our results suggest that the pathogen reduced photosynthesis initially through nonstomatal processes, which were not directly mediated by water-deficit stress. In contrast, leaves with symptoms exhibited decreased photosynthesis due to combined chlorosis, water stress, and stomatal closure. Disease progression and the resultant decrease in photosynthetic capacity apparently is the product of two distinct mechanisms of pathogenicity. © 1990.