Field studies with segregating populations under short days showed a significant positive correlation between tuber yield and the light-saturated net photosynthetic rate (Fn) in the 4th leaf from the top, at the tuber formation stage, in two seasons. The leaf area per plant {A) at this stage and also at the tuber bulking stage had a significant positive correlation with tuber yield. The product of PN and A also showed a significant positive correlation with tuber yield at both of these stages, a stronger one than that between tuber yield and A at the tuber formation stage.