We investigate observational constraints on a specific one-parameter extension to the minimal quintessence model , where the quintessence field acquires a quadratic coupling to the scalar curvature through a coupling constant \xi . The value of \xi is highly suppressed in typical tracker models if the late-time cosmic acceleration is driven at some field values near the Planck scale . We test \xi in a second class of models in which the field value today becomes a free model parameter . We use the combined data from type-Ia supernovae , cosmic microwave background , baryon acoustic oscillations and matter power spectrum , to weak lensing measurements and find a best-fit value \xi > 0.289 where \xi = 0 is excluded outside the 95 per cent confidence region . The effective gravitational constant G _ { eff } subject to the hint of a non-zero \xi is constrained to -0.003 < 1 - G _ { eff } / G < 0.033 at the same confidence level on cosmological scales , and can be narrowed down to 1 - G _ { eff } / G < 2.2 \times 10 ^ { -5 } when combining with Solar System tests .