Exploring the limits of crop productivity: Beyond the limits of tipburn in lettuce

The productivity of lettuce in a combination of high light, high temperature, and elevated CO₂ has not been commonly studied because rapid growth usually causes a calcium deficiency in meristems called tipburn, which greatly reduces quality and marketability. We eliminated tipburn by blowing air directly onto the meristem, which allowed us to increase the photosynthetic photon flux (PPF) to 1000 μmol·m ^-2·s^-1 (57.6 mol·m ^-2·d^-1); two to three times higher than normally used for lettuce. Eliminating tipburn doubled edible yield at the highest PPF level. In addition to high PPF, CO₂ was elevated to 1200 μmol·m^-2·mol^-1, which increased the temperature optimum from 25 to 30°C. The higher temperature increased leaf expansion rate, which improved radiation capture and more than doubled yield. Photosynthetic efficiency, measured as canopy quantum yield in a whole-plant gas exchange system, steadily increased up to the highest temperature of 32°C in high CO₂. The highest productivity was 19 g·d^-1 of dry biomass (380 g·d^-1 fresh mass) averaged over the 23 days the plants received light. Without the limitation of tipburn, the combination of high PPF, high temperature, and elevated CO₂ resulted in a 4-fold increase in growth rate over productivity in conventional environments.