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Greenhouse tomato (Solanum lycopersicum) yield in The Netherlands has increased tremendously over the past 50 years. The effects of breeding during this period were investigated. Eight Dutch cultivars and one typical current Japanese cultivar that were released over the past 50 years were compared in a short-term experiment conducted from summer to fall in The Netherlands. Fresh fruit yield of the Dutch cultivars significantly increased '0.9% per year with the year of release from 1950 to 2000. Dry weight fruit yield of the Dutch cultivars also increased with the year of release, whereas the fruit dry matter content was not correlated with the year of release. Total dry matter production of plants increased with the year of release, and the dry matter partitioning to fruit was not correlated with the year of release. An increase in dry matter production was caused not by an increase in fraction of interceptedlight,butbylightuseefficiencybasedoncorrelationsbetweeneachofthemandtheyearofrelease.Thelight extinction coefficient in the plant canopy decreased, whereas leaf photosynthetic rate increased significantly with the year of release. Although fresh fruit yield of the Japanese cultivar was lower than that of the modern Dutch cultivars, fruit dry matter content of the Japanese cultivar was higher than that of the Dutch cultivars. An increase in yield over the past 50 years in Dutch tomato was caused by an increase in light use efficiency resulting from a decrease in light extinction coefficient (a morphological change) and an increase in leaf photosynthetic rate (a physiological change). Greenhouse tomato yield in The Netherlands has doubled from 30 kgm -2 per year in the 1980s to 60 kgm -2 per year in 2005 (Kwantitatieve Informatie voor de Glastuinbouw, 2005). This increase in yield is caused by many factors, e.g., improvements in greenhouse transmissivity and developments in cultivation techniques such as longer cultivation, soilless culture, high wire training systems, CO2 enrichment, and environmental control by computers. Also, breeding for higher yielding cultivars would have played a role in this increase in yield. van der Ploeg et al. (2007) reported that yield of modern tomato cultivars under current conditions was 40% higher than that of old cultivars. The genetic component in yield improvement has been investigated in several crops. Hay (1995) reported in his review that increases in yield of wheat (Triticum aestivum) and barley (Hordeum vulgare) over the past century was caused by in- creased harvest indices (short-stem cultivars). Abbate et al. (1998) also reported that the dry weight of spikes of Argentine wheat cultivars was related to partitioning to spikes, although yield was more closely related with the grain/spike ratio rather than the dry weight of spikes. Saitoh et al. (1993) reported that harvest indices of modern rice (Oryza sativa) cultivars were higher than those of old cultivars. Morrison et al. (1999) reported for soybean (Glycine max) cultivars released from the 1930s to the 1990s in Canada that the yield increased 0.5% per year from the 1930s and that this increase was related to increases in harvest index, photosynthetic rate, and stomatal conductance and a decrease in leaf area index (LAI). These reports show that an increase in yield in these crops by breeding is primarily related to an increase in harvest index rather than total biomass production. However, Hay (1995) reported that an increase in yield of maize (Zea mays) by breeding was not related to harvest index but to total dry matter production. Tollenaar and Aguilera (1992) also reported that solar radiation use efficiency of modern maize cultivars was higher than that of old cultivars, although there were no differences in light extinction co- efficient and light absorption by crops. Ho (1996) reported that the yield of different types of tomatoes such as beef, cherry, and round tomatoes was de- termined by partitioning to the fruit. van der Ploeg et al. (2007) reported that partitioning to fruit of modern tomato cultivars was not higher than that of old cultivars, although there were differences among cultivars. van der Ploeg et al. also reported that an increase in the yield of modern tomato cultivars was caused by an increase in total dry matter production resulting from higher light use efficiency (LUE). In the current work, we look for confirmation of this un- expected result. Our aim of this research is to investigate whether Dutch tomato cultivars that were released between 1950 and 2000 show an increasing trend in yield and altered yield components when grown under the same conditions. Because fresh yield and its components are determined by physiological and morpholog- ical factors such as leaf photosynthetic rate, light extinction coefficient, and LUE, we investigated the relationship between them in the short-term experiment conducted from summer to fall. Total production value of tomato occupies first place among vegetables in Japan (Ministry of Agriculture Forestry and Fisheries of Japan, 2007). Although cultivation techniques

Physiological and Morphological Changes Over the Past 50 Years in Yield Components in Tomato