What is less clear is the contribution made by plant breeding over this same period. In the short-term experiment described below, eight tomato cultivars - released at different times over the past 50 years or so - were grown at the same time, in the same system of cultivation and in the same aerial and root environment.
The cultivars were Moneymaker (released in 1950), Premier (1960), Extase (1960), Sonatine (1975), Calypso (1982), Liberto (1988), Gourmet (1991) and Encore (2002). Most aspects of their growth were measured but few seemed to correlate with the year the tomatoes had been released.
However, light-use efficiency and leaf photosynthetic rate were both positively correlated with release year, indicating that modern cultivars fix more carbon dioxide and produce more dry matter for each unit of light that falls on them. Leaf photosynthetic rate appeared to have increased by about 15 per cent between 1950 and 2002, thus confirming that plant breeding has also contributed to the observed increase in yields.
Physiological and Morphological Changes Over the Past 50 Years in Yield Components in Tomato by Higashide and Heuvelink (2009). Journal of the American Society for Horticultural Science 134 (4): 460-465. ISHS members can view Journal of the American Society for Horticultural Science from the website www.ishs.org.