The effect of sunlight interception by sooty mold on chlorophyll content and photosynthesis in orange leaves (Citrus sinensis L.)

Sooty molds are a lineage of follicolous fungi that cover the upper surface of leaves with black mycelia. Sooty molds do not infect plants, but grow on surfaces where honeydew deposits accumulate. It causes a reduction of incident sunlight by physical obstruction and in some species it interferes with photosynthesis. However, there are no studies proving that light interception by the sooty mold mycelia affects photosynthesis in orange plants. The aim of this study was to experimentally evaluate changes in the interception of sunlight caused by the black coating of sooty mold formed on orange leaves and to investigate its effects on the leaf chlorophyll content, stomatal conductance and photosynthetic rate. To facilitate the measurements, orange leaves with and without sooty mold colonies were selected. On a clear day, the sooty mold mycelia intercepted between 44 and 74 % of the total incident photosynthetic photon flux density (PPFD). However, even on leaves covered by the sooty mold mycelia, the measured PPFD was sufficient to saturate maximum net photosynthesis rate (Amax) for much of the day. No differences were found in Amax or leaf conductance, but there were increases in chlorophyll content and quantum yield in leaves infested by sooty mold, revealing a clear acclimation response. This study is the first to experimentally assess the direct effects of sunlight interception by sooty mold on chlorophyll content and net photosynthesis in orange leaves.