Pre-symptomatic biophysical profiling as a candidate approach for early screening of cowpea mild mottle virus tolerance in common bean

Cowpea mild mottle virus (Carlavirus vignae, CPMMV) is an emerging pathogen impacting common bean (Phaseolus vulgaris L.) production. While genetic tolerance protects yield, it allows systemic viral replication, demanding rapid, non-destructive tools for early diagnostic screening. This study characterized the chronological biophysical alterations in the photosynthetic apparatus of near-isogenic common bean lines under early CPMMV stress. Two cultivars with contrasting responses to CPMMV — BRSFC401RMD (susceptible) and BRSFC420RMD (tolerant) — were evaluated. High-sensitivity RT-qPCR detected systemic viral loads in stems and roots of the susceptible cultivar by 3 days after inoculation (DAI), long before any macroscopic symptoms emerged. Simultaneously, OJIP chlorophyll a fluorescence profiling captured immediate pre-symptomatic photosynthetic disruptions by 1 DAI. The susceptible cultivar exhibited a marked and consistent reduction in maximum fluorescence (Fm) as early as 1 DAI, accompanied by more variable changes in electron acceptor pool size (Area) and thermal dissipation yield (ΦD0). Conversely, the tolerant cultivar preserved photosystem II structural integrity (Fm/Fo and Fv/Fo) ratios) across evaluation seasons. Correlation matrix analysis at 10 DAI suggested that these early biophysical shifts reflect internal kinetic realignments that precede visual cell damage or biomass reduction. This rapid screening approach is a promising, non-destructive candidate tool for early-stage phenotyping in breeding programs under controlled or partially controlled environments and warrants further validation across a broader panel of genotypes.