The fresh fruit yield of tomatoes (Solanum lycopersicum L.) is determined by yield components and related traits. In a low-truss cultivation in Japan, there was no significant difference in light use efficiency between different planting densities and stages. Accordingly, dry matter production is described by a function of intercepted light: Total dry-matter production = light use efficiency × Intercepted light. Light interception is determined by leaf area index (LAI) and the light extinction coefficient in the plant canopy. The efficiency of dry matter production per unit of intercepted light, i.e., light use efficiency, is determined by not only the leaf photosynthetic rate but also light extinction coefficient. The higher yield of modern tomato cultivars in The Netherlands is due to the increase in total dry matter production of plants, not by an increase in dry matter partitioning to fruits. An increase in the photosynthetic rate and a decrease in the light extinction coefficient may result in an increase in light use efficiency among Dutch cultivars. The yield of greenhouse tomatoes in Japan has increased little since the 1980s. Recently, Japanese researchers have attempted to improve the yield using current Japanese cultivars. Elevation of CO2 and fogging in a greenhouse improved the fresh fruit yield and total dry matter production in Japanese cultivars. However, the fraction of dry matter distribution to fruit in ‘Momotaro York’ was significantly decreased. The fruit yield was also improved by grafting the Japanese scion ‘Momotaro York’ onto Dutch rootstock ‘Maxifort’ (S. lycopersicum × S. habrochaites). The increases in yield and total dry matter production were mainly determined by the increase in light use efficiency.
