The importance of energy metabolism in plant performance and plant productivity is conceptually well recognized. In the eighties, several independent studies in Lolium perenne (ryegrass), Zea mays (maize), and Festuca arundinacea (tall fescue) correlated low respiration rates with high yields. Similar reports in the nineties largely confirmed this correlation in Solanum lycopersicum (tomato) and Cucumis sativus (cucumber). However, selection for reduced respiration does not always result in high-yielding cultivars. Indeed, the ratio between energy content and respiration, defined here as energy efficiency, rather than respiration on its own, has a major impact on the yield potential of a crop. Besides energy efficiency, energy homeostasis, representing the balance between energy production and consumption in a changing environment, also contributes to an enhanced plant performance and this happens mainly through an increased stress tolerance. Although a few single gene approaches look promising, probably whole interacting networks have to be modulated, as is done by classical breeding, to improve the energy status of plants. Recent developments show that both energy efficiency and energy homeostasis have an epigenetic component that can be directed and stabilized by artificial selection (i.e. selective breeding). This novel approach offers new opportunities to improve yield potential and stress tolerance in a wide variety of crops.

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