Linking Root System Architecture (RSA), Photosynthetic Efficiency, and Biomass Productivity in a Switchgrass (Panicum virgatum L.) Half-Sib Panel

Switchgrass ( Panicum virgatum L.) is a perennial forage crop for renewable fuel production. However, biomass yield is a complex trait shaped by physiological and root characteristics. We tested whether switchgrass families with contrasting multi-year biomass performance exhibit differences in leaf gas exchange parameters, chlorophyll fluorescence, and seedling root system architecture (RSA). Leaf gas exchange was assessed by comparing top- and low-yielding families using analysis of covariance (ANCOVA) of net CO ₂ assimilation (A) against stomatal conductance (g _sw ), including group × g _sw interaction. Chlorophyll a fluorescence was evaluated using rapid light curve (RLC)-derived OJIP induction metrics. Seedling RSA was quantified in rhizoboxes using RhizoVision analysis and complementary manual measurements of roots and shoots. Top-yielding families maintained higher A at comparable g _sw (group effect = 5.50, P = 0.00247), and showed a weaker dependence of A on g _sw (interaction estimate = − 18.685, P = 0.00703), consistent with greater photosynthetic capacity and tighter stomatal regulation. Intrinsic water-use efficiency (iWUE) varied, with the highest iWUE values confined to top-yielding families, and a quantile-regression upper envelope indicating a carbon-water trade-off at high A. In contrast, OJIP metrics such as electron transport rate (ETR), maximum quantum efficiency of photosystem II (Fv/Fm), and performance index (PIabs) were similar between yield groups (median initial slope, α = 0.081–0.082). Seedling RSA exhibited strong root-shoot coordination but showed weaker associations with field biomass within the top group (r = − 0.35 to − 0.41). Overall A–g _sw and iWUE patterns were informative of field biomass than seedling RSA or chlorophyll fluorescence alone.