Integration of biophysical photosynthetic parameters into one photochemical index for early detection of Tobacco Mosaic Virus infection in pepper plants

Photosynthesis in host plants is significantly reduced by many virus families. The early detection of viral infection before the onset of visual symptoms in both directly and systemically infected leaves is critical in crop protection. Viral pathogens cause a variety of symptoms through modifications of chloroplast structure and function and the response of the photochemistry process is immediate. Therefore, chlorophyll fluorescence monitoring has been extensively investigated the last two decades as a tool for timely assessment of pathogenic threats. Alternatively, the analysis of Chla fluorescence transients offers several interlinked parameters which describe the fate of excitation energy round and through the photosystems. Additionally, OJIP fluorescence transients and leaf reflectance spectra methodologies serve for rapid screening of large number of samples. The objective of the present study was to achieve early detection of viral infection, integrating the multiparametric information of the Chla fluorescence transients and of the leaf reflectance spectra into one photochemical performance index. Infection decreased the maximum quantum yield of PSII (FV/FM), the effective quantum yield of PSII (ΦPSII), the CO₂ assimilation rate (A) and the stomatal conductance (gₛ) in the studied TMV-pepper plant pathosystem, while non-photochemical quenching (NPQ) increased. Some parameters from the OJIP transients and the leaf reflectance spectra were significantly affected 24 h after infection, while others modified three to five days later. Similar results were obtained from systemically infected leaves but with one to three days hysteresis compared to inoculated leaves. Differences between healthy and infected leaves were marginal during the first 24 h post infection. The Integrated Biomarker Response tool was used to create a photochemical infection index (PINFI) which integrates the partial effects of infection on each fluorescence and reflectance index. The PINFI, which to the best of our knowledge is the first photochemical infection index created by the IBR method, discriminated reliably between the infected and healthy leaves of pepper plants from the first 24 h after infection with the TMV.