The effect of humidity during darkness on the transfer of assimilates from cucumber leaves to sporangia of Pseudoperonospora cubensis

Abstract

Cucumbers infected by Pseudoperonospora cubensis were labelled with 14CO 2 during a light period and exposed for up to 36 h to darkness in either dry or wet conditions. Under dry and dark conditions the level of photosynthetic assimilates previously accumulated in lesions remained more or less constant, while the level of assimilates in the surrounding green tissue decreased. During exposure under these conditions, assimilates in lesions changed from high to low-molecular-size, hexose-like compounds, which were found essential for formation of sporangia. The level of assimilates in leaves kept under dark and wet conditions was lower than in the dry and dark treatment, and fractionation into the high-molecular-size and low-molecular-size compounds was somewhat less clear. In both treatments assimilates from both the lesions and the surrounding green tissues participated in sporulation. It was concluded that under wet and dark conditions the advantage of an early start of sporulation is offset by a comparatively low level of metabolites. Under the dry and dark conditions, formation of sporangia is inhibited until the subsequent commencement of wetness, but more nutrients are available for sporulation.