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 CO₂. Under high CO₂ (1.0%) a stimulation was observed during early stages of infection. The inhibition in low CO₂ and stimulation in high CO₂ of infected host photosynthesis was shown to occur in healthy host tissue when in the presence of 10⁻²m α-hydroxy-2-pyridinemethanesulfonic acid. The possible significance of these similar results is discussed.