Cytokinin levels in healthy and Verticillium-infected tomato plants

Abstract

Yellowing of leaves is often an early and persistent symptom of plants infected with vascular wilt fungi. This symptom is also characteristic of senescence and hence the cytokinin levels in healthy and Verticillium-infected tomato plants were investigated. Tomato plants (Lyeopersicon esculentum L. cv. Bonny Best) were inoculated with the fungus Verticillium dahliae Kleb. 49 days after seeding. At intervals after inoculation, cytokinin levels in root xylem exudate were determined in healthy and infected plants. Cytokinin concentrations (per plant and per millilitre of exudate) were similar in healthy and infected plants at days 50 and 55 but were lower in infected plants at days 61 and 65 owing to reduced concentrations of both free and bound forms of cytokinin. Infected plants first showed visible symptoms of chlorosis on older leaves at day 56. By day 61 the lower three to five leaves were obviously wilted and chlorotic and by day 65 wilt and chlorosis were apparent on most leaves on the lower half of the shoot. Whether the first chlorotic symptoms in infected plants were due to the concomitant reduction in cytokinin concentrations was not resolved. However, the consistently lower concentrations of cytokinin in infected plants after the onset of symptoms are presumed to contribute significantly to the continued rapid loss of chlorophyll measured in older leaves and the progressive development of visible chlorosis in younger leaves. Reduced water potentials in leaves and fungal activity in infected roots are likely to be among factors responsible for the reduction in cytokinin concentration in infected plants.