Cold tolerance in grain sorghum requires vigorous growth under chilling (< 20 ^oC) conditions. Assimilation capacity (e.g. A+R_d, Φ_PSII:Φ_CO2*, A v Q, A v Ci; at 35 ^oC and 12 ^oC) and components of relative growth rates (ULR, SLA, LMF) were determined for Shan Qui Red (SQR), a Chinese kaoliang and TX430, a tropical breeding line.  Similar analyses were applied to a selection of F₆ recombinant inbred lines (RIL) resulting from a TX430 x SQR cross. Parental lines exhibited similar assimilation capacity with characteristics similar to reports of C₄ grasses utilizing the NADP-ME pathway. Chilling temperatures resulted in 50% reduced photosynthesis and 40% reduced photosynthetic quenching for both parent lines.  The ratio of Φ_PSII:Φ_CO2* was 13.4 (35 ^oC) and 13.1 (12 ^oC). However, allometric studies indicated the mid-vegetative relative growth rate of SQR (26.2 g m^-2 day^-1) exceeded that of TX430 (16.8 g m^-2 day^-1) despite 24% lower chlorophyll content for SQR. Some RIL exhibited evidence of CO₂ leakage from bundle sheath cells [a linear decrease in d¹³C discrimination (–13.1 to –14.5 d¹³C (%)) with increased ratio of Φ_PSII:Φ_CO2* (13.1 to 18.1)]. Reduced photosynthetic efficiency in RIL, but not parent lines indicate genetic recombination altered significant assimilation processes; likely increasing bundle sheath leakage of CO₂. The similarity in assimilation characteristics (under near-optimal and chilling temperatures) of the kaoliang and tropical lines suggests a significant role for post-assimilatory processes in differential growth rates.