Heterosis in plants
In 1876, Charles Darwin noted that cross-pollinated (hybrid) plants have “greater height, weight, and fertility” than their self-pollinated counterparts. The observation that cross-pollinated hybrids are more vigorous than their parents is nowadays commonly referred to as heterosis. Virtually all annual crops exhibit heterosis to a certain extent. The degree of heterosis is calculated as the difference in the phenotypic performance of a trait between a hybrid and the average of its two distinct parents. Naturally cross-pollinating species such as maize or rye typically display a much higher degree of heterosis than self-pollinating plants like wheat and barley. However, hybrid varieties have also been established in selfpollinating species because hybrids often display greater yield stability than line varieties. Heterosis can be monitored for a large number of traits throughout development. In maize, signifi cant size differences between hybrids and their parents can already be observed during embryo development. After germination, seedling root traits including lateral root density, primary root length or seminal root number display differences as early as a few days after germination.
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