Researchers analyzed how rice cultivars in the Tohoku region have changed in yield and photosynthetic rate over the 20th century. They grew ten different rice varieties in low- and high-nitrogen conditions and found that newer cultivars released after the 1960s had higher yields than older ones. The increase in yield was linked to a higher photosynthetic rate during the ripening stage, leading to more grains being filled. This suggests that rice cultivars have improved in yield over time due to better photosynthesis, especially in newer varieties.