Effect of downy mildew disease on photosynthesis and chlorophyll fluorescence in Plantago ovata Forsk

Abstract

Plantago ovata is commercially grown in India for its dietary fibre. The crop is affected by downy mildew caused by Peronospora plantaginis Underwood leading to severe yield loss. A study was undertaken to assess the impact of downy mildew on host photosynthesis under field conditions. The results showed that primary photosynthetic pigments (total chlorophyll) were reduced by 34.74% and 62.11% in slight and severe chlorotic leaves, respectively, compared to healthy leaves. Net photosynthetic rate (Pn) was also significantly reduced in diseased leaves. The diurnal photosynthesis measurement showed that peak Pn in the diseased leaves was short lived as compared to healthy leaves. Infection caused an increase in dark respiration (Rd) and intracellular CO2 concentration (Ci), while stomatal conductance (gs) was similar to that of healthy leaves during peak photosynthetic period of the day. Infection caused a reduction in soluble sugar content accompanied by an increase in leaf starch content. Vitality index (Fv/Fm) of the diseased leaves was reduced by 24.39% in slight chlorotic and 44.90% in severe chlorotic leaves as compared to healthy leaves. Further, quantum yield of photosystem (PS) II (FPSII) showed severe reduction, which was correlated with the Pn. Our study suggests that carbon assimilation in the diseased leaves is mainly limited by PSII function. Disease induced reduction in chlorophyll content appears to be one of the causes for reduction in PSII yield. Increase in starch accumulation in infected leaves appears to be a cause for reduction in photosynthesis in infected leaves.