Fig. 3. Tuber production in response to light (shade or control illumination) and CO₂ (350 or 700 µmol mol^–1) treatments at tuber initiation (TI) and tuber bulking (TB). Plants were harvested 2 (TI) or 4 weeks (TB) after transfer to short days, with exposure to treatments for 2 weeks prior to harvest in each case. Bars represent 5 % confidence intervals; means labelled with different letters differ significantly (P < 0·05).