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Wild Plants to the Rescue

Efforts to domesticate new, high-yield, perennial grain crops require patience and persistence—but such plants could transform agriculture

David Van Tassel, Lee DeHaan

Amber Waves of Sunflowers

Early-stage plant domestication is truly a process of trial and error. We make observations on many candidate species and carry a number of them through several generations of selection. At some point, entire species may have to be culled so we can invest intensely in more promising candidates. Another species that at first seemed less promising may respond to a few cycles of selection in a dramatic way. We have learned that a wild species’ average traits, particularly if measured on plants growing in a natural ecosystem, tells us little about what potential it will have when growing under agricultural conditions—and even less about the potential of rare individuals.

Maximilian sunflower (H. maximiliani) has been our top perennial oilseed candidate for domestication since 2000. Fast growing and widely adapted, this species produces many tiny seeds. But recently a close relative and another subject of our research, Silphium integrifolium, has emerged as another promising plant. In its wild state, Silphium appears to be a poor crop candidate. Its traits are almost the opposite of maximilian sunflower: It establishes slowly, flowering only in its second year. Its seeds are very large for a wild plant, but it produces only 15 to 20 per head in the wild. The flower heads have hundreds of florets, but the vast majority are staminate, producing only pollen rather than developing into seeds. In this genus, the seed-bearing florets are found on the rim of the head, where they produce one very long petal and several tiny ones. Collectively, these inconspicuous florets create the large ring of showy sunflower petals that attracts the attention of bees and painters—and plant breeders. It was those large seeds that kept us from completely disregarding this species.

Was there genetic variation for the number of pistillate florets in Silphium, we wondered? In 2004 we grew thousands of plants and spent days walking through the field counting the number of long petals. That number indicates the maximum number of seeds the head can produce. We found and flagged a few dozen plants with heads bearing more than 25 petals, and we covered the next unopened head on each plant with a cloth bag to keep out pollen carried by butterflies and other insects. We checked these bags every day or two. If a bagged head was shedding pollen, we brushed it off into a small jar. Then we went back through the bagged plants, checking for heads with receptive stigmas and pollinating them with this mixed pollen. (Silphium is self-incompatible, so a plant’s own pollen will not fertilize its ovules.) We collected the resulting seeds and planted them the next year. A year later we repeated the process, this time raising the bar: Only plants with 35 long petals were flagged and bagged. Again and again we repeated this cycle of selection and intermating. In 2012 several plants had heads with more than 100 long petals, and one plant had more than 150.

This is the kind of low-tech, protracted genetic work that could never have been funded through competitive grants. It was a back-burner project even at the Land Institute, and we did not keep careful records of average petal number or calculate heritabilities. We simply tried to find the most extreme plants in the population and intermate them. Recently, we began to feel confident that this trait responded well enough to selection that the number of seeds per head was not likely to impose serious limitations.

This year we estimated yields more carefully. In our unfertilized breeding nursery, the average seed yield in 2012 was 278 pounds per acre, although some plant families did much better. That’s not very impressive compared with the typical yields of 1,500 to 2,000 pounds per acre obtained from commercial annual hybrid cultivars in a nearby experiment at Kansas State University’s South Central Kansas Experiment Field. However, this was not a typical couple of years. Our 2012 Silphium yield compares quite favorably with that of the commercial sunflowers in 2011, 263 pounds per acre, and 2012—0 pounds per acre. During these drought years, Silphium grew normally while native grasses, annual sunflower cultivars and even some perennial grain species showed severe stress and stunting.

Prairie ecologist John Weaver noted in 1935 that S. integrifolium flowered normally in the great drought of 1934, when many other prairie plants turned brown and dormant. He attributed this success to Silphium’s deep roots. Perhaps it will be the next slightly domesticated perennial grain to move up through the crop-development pipeline. Years of somewhat informal breeding seem to have greatly increased the species’ yield potential, although a few cycles of selection for actual yield, not just petal number, are needed to increase the average yields. We have also seen great variation in the amount of seed shattering, although we have not done much selection for that. The oil content of the grains is similar to cultivated sunflower, and Silphium has more protein than its domesticated cousin. Suddenly we find that this candidate, with its large seeds and upright growth form, is more promising than the interspecific perennial–annual hybrids and the maximilian sunflower on which we have spent much more effort.

Once we have a population with reduced shattering and have demonstrated that Silphium can be harvested mechanically at the 5- to 10-acre scale, this candidate should be much more attractive to potential university collaborators around the world. We still need to learn much more about its soil fertility requirements, pathology, ecology and oil chemistry. But its surprise potential illustrates the need for careful research over a broad range of species—even those that may not initially seem that promising.

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