Explained, in part.

It has long been known that hybrid plants such as hybrid corn are more vigorous than their parents. They are larger and have more biomass and bigger seeds. The same is true for plants that are polyploid, meaning that they have two or more sets of chromosomes. Over 70 percent of all flowering plants, including many important agricultural crops such as wheat, cotton, canola, sugarcane and banana, are naturally polyploid.

Until now, the molecular mechanisms for hybrid and polyploid vigor have largely been unknown. . .

The key, Chen and his colleagues studying Arabidopsis plants found, is the increased expression of genes involved in photosynthesis and starch metabolism in hybrids and polyploids. These genes were expressed at high levels during the day, several-fold increases over their parents.

The hybrids and polyploids exhibited increased photosynthesis, higher amounts of chlorophyll and greater starch accumulation than their parents, all of which led to their growing larger.

Also, growth vigor was higher in allotetraploid plants (polyploids formed by combining two different Arabidopsis species) than standard hybrids (formed through combining the same species).

The research team discovered a direct connection between circadian clock regulators and growth vigor in both hybrids and polyploids. Circadian clocks control growth, metabolism and fitness in plants and animals.

They found that some of these regulators, known as transcriptional repressors, were more repressed during the day in the hybrids and polyploids, leading to increases in their photosynthesis and starch accumulation. . .

The hybrid vigor or "heterosis" phenomenon was first observed by Charles Darwin in 1876, and was extensively studied in corn in the early 1900s. All corn in the U.S. is hybrid.

Or said another way, they work longer and harder. They aren't clock watchers.

I've often boggled at the pale green opposition to hybrids. They do just what we need - they produce more phytomass as well as edible seed - and so conserve land, water and labor. They require more fertile soil to produce well, as we should expect since they produce more. They need food to work so hard. But lesser plants in lesser soil over a larger space needing more water and labor is a poor alternative.

Their argumnents usually degenerate into dogma. If the word Monsanto occurs then you know that they are leaning on the stupid button. Same for seed saving. They cling to their childish political agenda while people starve and land is ruined.

But, it may be possible to improve on existing hybrids. Knowing more precisely what makes hybrids vigorous may allow us to create vigorous cultivars that breed true, that are not a cross of heterogeneous parents. Then the seed savers can continue their antiquated hobby farming system and still have productive crops. They'll still need fertility - the plants can't work hard all day on a starvation diet - but one step at a time. Maybe we'll also figure out how to produce crop plants that have nutrient fixing symbionts. That would reduce their productivity, but also reduce inputs and labor.

Agriculture has real problems. We really need to move beyond romantic myths and fuzzy thinking.