Recent progress in genomics has promoted understanding of genetic mechanisms controlling complex crop production traits.In rice, yield is one of the most complex traits and generally determined by relationship between sink and source ability. While several quantitative trait locus (QTL) genes have been identified for sink-size traits, there has been little progress in genetic analysis for source ability. One of the reasons is that the source ability such as photosynthesis is a dynamic trait, hampering efforts to make precise measurements due to the technical difficulty involved in handling so many plants simultaneously. To facilitate genetic studies for source ability, we developed rapid and precise methods to evaluate leaf photosynthesis, one of the important factors for source ability in rice. QTL analysis using such methods successfully detected several promising QTLs and verified the potential utility of the evaluation methods. Our work reflects the importance of steady observation of rice plants in paddy fields and includes the application of the latest technology used in various areas of science to develop improved criteria for each trait toward advanced genetic studies.