Biphasic inhibition of photosynthesis in powdery mildewed barley.

A careful restudy of the photosynthetic process of barley (Hordeum vulgare) infected with powdery mildew (Erysiphe graminis f. sp. hordei) is reported. Unlike previous reports, which indicated a stimulation in infected host tissue photosynthesis during early stages of the disease followed by a rapid decline in activity, this study observed no stimulation but instead a biphasic inhibition in infected host photosynthesis under physiological concentrations of CO(2). Under high CO(2) (1.0%) a stimulation was observed during early stages of infection. The inhibition in low CO(2) and stimulation in high CO(2) of infected host photosynthesis was shown to occur in healthy host tissue when in the presence of 10(-2)m alpha-hydroxy-2-pyridinemethanesulfonic acid. The possible significance of these similar results is discussed.