Physiological responses of Gossypium hirsutum L. to infection by defoliating and nondefoliating pathotypes of Verticillium dahliae Kleb

Stems of cotton plants (Gossypium hirsutum L., cv. Acala SJ-2) growing in the greenhouse and field were inoculated with conidial suspensions of T9 (defoliating) and SS4 (nondefoliating) pathotypes of Verticillium dahliae Kleb. In the greenhouse, leaves of T9-inoculated plants had higher water potentials (ψ₁) and relative water contents (RWC) than the healthy controls until the occurrence of defoliation about 10–12 days after inoculation. In contrast, ψ₁ and RWC decreased in SS4-inoculated plants about 6–8 days after inoculation. Increases in stomatal resistance (R₁) and proline concentration were associated with the development of water stress in leaves of SS4-inoculated plants. Proline accumulation in T9-inoculated plants was much less than in SS4-inoculated plants, but it occurred about 2–4 days earlier and was accompanied by an increase in free amino-nitrogen and a decrease in the chlorophyll content of leaves. Stomatal resistance in T9-inoculated plants increased about 2–4 days earlier and remained higher than that of SS4-inoculated plants until the occurrence of defoliation. The higher water status of T9-inoculated plants compared with healthy controls suggests that, in contrast to SS4-inoculated plants, accelerated senescence rather than water stress is the major effect of infection.

Previous observations of enhanced ethylene production in cotton plants infected by the defoliating, T9 pathotype of V. dahliae were confirmed in the greenhouse. ACC (1-aminocyclopropane-l-carboxylic acid) appeared to be an intermediate in the pathway for ethylene production in diseased cotton plants.

In the field, midday ψ₁ of SS4-inoculated plants dropped rapidly about 5–8 days after inoculation, and the decrease in ψ₁ was accompanied by an increase of R₁ and proline concentration in leaf tissues. T9-inoculated plants had higher midday ψ₁ values than healthy plants up to the fourth day; however, by the eighth day after inoculation the ψ₁ of T9-inoculated plants had declined to about the same values as those of SS4-inoculated plants. Water stress in SS4-inoculated plants was associated with vascular dysfunction whereas water stress in T9-inoculated plants appeared to be associated more with accelerated senescence and defoliation.