Fig. 5. The effect of photosynthetic photon flux (PPF) on yield. Both grain yield (open circles, r² = 0.986) and vegetative biomass (including roots, closed circles, r² = 0.990) increased with higher PPF, so harvest index remained relatively constant (r² = 0.07) at slightly below 50%. Increase in yield was likely due to higher number of fertile heads at higher PPF (r² = 0.969). No fertile heads were observed at PPF of 200 µmol m^–2 s^–1 or less. Error bars indicate standard error of means (n = 4 for each point).