Hybrid Pigeonpea: Breaking a Yield Barrier

Scientists working to improve the grain legume pigeonpea - Cajanus cajan (L.) Millsp. - announced recently that the new hybrid ICPH 2671 produces nearly 50 percent more grain than the popular Indian cultivar Maruti, definitively breaking the yield barrier that has had them and farmers frustrated for many years.

Development of the world's first commercially viable system for producing hybrid pigeonpea seed was completed 2 years ago by the International Crops Research Institute for the Semi-arid Tropics (ICRISAT), working in close collaboration with the Indian Council of Agricultural Research (ICAR). A public good resulting from more than two decades of sometimes frustrating research, the technology represents "a major breakthrough," says ICRISAT Director General William Dar, permitting pigeonpea yields of 3 to 4 tons per hectare.

Eminent agricultural scientist M.S. Swaminathan predicts that the new pigeonpea hybrids, with their "quantum leap in yield," could open the way for a revolution in the production of this important pulse, similar to the transformation of wheat and rice production made possible several decades ago by novel semi-dwarf varieties.

According to K.B. Saxena, the ICRISAT scientist who led development of the new pigeonpea technology, 100 to 150 tons of hybrid seed, enough to plant about 25,000 hectares, should be available to farmers in 2008. It is being produced with the help of 16 public and private seed companies. Meanwhile, Swaminathan is overseeing a project that will seek to make the hybrids accessible even to the poorest growers.

Early Gains

ICRISAT and ICAR scientists embarked on a collaborative program of pigeonpea improvement in the mid-1970s and quickly registered important gains in their efforts to raise productivity.

By the 1980s, they had developed early maturing varieties, which can be harvested in just 3 to 4 months, compared to the standard growing period of 6 to 9 months. These varieties are now being grown in rotation with wheat in northern India, resulting in a more diverse and sustainable cropping system. Scientists also succeeded in developing resistance to two major pigeonpea diseases, fusarium wilt and the sterility mosaic virus. But despite the release of dozens of improved varieties over the years, all of them conventional inbred lines, research was unable to make a dent in average yields, which remained near 700 kilograms per hectare.

One proven approach to boosting the productivity of crops involves exploiting the phenomenon of hybrid vigor, or heterosis, in which crossbred plants exhibit marked superiority to their parents. By 1991, pigeonpea breeders had developed hybrids based on genetic male sterility hybrids with a yield advantage of 25 to 40 percent, but these did not produce seed efficiently enough for commercial production.

Cytoplasmic Solution

With a partially self-pollinating plant like pigeonpea, large-scale production of hybrids requires that the female parent produce no viable pollen. Only then can it be crossed with the chosen male parent to produce progeny showing hybrid vigor.

One way to accomplish this is by physically removing the anthers, or male reproductive organs, from the female parent. That approach is manageable for crops like vegetables or fruits, in which a single pollination results in large number of seeds. But it is quite laborious and expensive for crops that are predominantly self-pollinating like pigeonpea, in which the male and female reproductive organs occur within the same flower and a single pollination yields only three or four seeds

The far more efficient approach with these crops is to identify a male-sterile line that is unable to self-pollinate but rather depends entirely on the separate male line for seed formation. Male-sterile lines were used in the first pigeonpea hybrids, but their male sterility was governed by nuclear genes, which segregate during meiosis. As a result, pollen was absent in only half of the female parent plants, posing a serious obstacle to large-scale hybrid seed production.

The solution was to identify plants with cytoplasmic nuclear male sterility, that is, plants in which this trait is conferred by genes in the cytoplasm, or cell fluid around the nucleus. Because these genes are not involved in meiosis, all the offspring of cytoplasmic nuclear male-sterile plants are male sterile. Thus, when they are used as female parents in cross-breeding, all of their seed is hybrid.

Finding this solution was highly complex, requiring years of painstaking effort and skilled use of a pigeonpea wild relative (Cajanus cajanifolius), collected from the forests of India's Madhya Pradesh State. The hard-won result consisted of cytoplasmic nuclear male-sterile parents with high enough seed yields to permit viable commercial production of pigeonpea hybrids. In the 2 years since this system was devised, ICRISAT has tested more than 300 experimental hybrids, finding ICPH 2671 to be the most outstanding.

Equitable Access

While the pigeonpea hybrids offer a sizable benefit, there is also a cost. Rather than produce their own pigeonpea seed year after year, farmers will need to obtain new supplies of certified hybrid seed each year, just as farmers do with hybrid maize and rice. The reason for this is that hybrid vigor is expressed only in the first generation of progeny that result from crossbreeding. If farmers grow seed harvested from hybrid pigeonpea plants, they will see a sharp decline in crop yields.

ICRISAT is working closely with a consortium of private- and public-sector seed companies in India to commercialize pigeonpea hybrids and ensure that ample supplies can be made widely available within the next couple of years. Moreover, as reported in the April issue of the prestigious magazine Science, the M.S. Swaminathan Foundation has launched a project that will enable women farmers to produce pigeonpea hybrid seed themselves for their own use and for sale to neighbors. The idea is to create low-cost sources of seed for small farmers, while also fostering small-scale enterprises that generate employment and income for women.

Researchers in the Philippines, Myanmar and China are also using the new cytoplasmic nuclear male-sterile system to develop pigeonpea hybrids for their countries.

Precious Pigeonpea

India , where pigeonpea is referred to as "red gram," accounts for nearly 85 percent of world production. But the crop is also grown in many other developing countries across Africa, Latin America and Southeast Asia. It is especially well suited to poor soils in drylands.

In terms of area and production, pigeonpea trails other major grain legumes, such as common bean, cowpea and chickpea. Arguably, though, it has more diverse uses in bolstering food security, generating cash income and making agricultural production more sustainable.

The grain provides poor consumers with a vital source of cheap protein, while the pods are consumed as a green vegetable. Pigeonpea plants serve as animal feed as well (for both livestock and fish), and the crop is commonly grown to prevent soil erosion, raise soil fertility and rehabilitate degraded lands. In China, where pigeonpea has undergone a major revival in recent years, food technologists have even developed a variety of processed foods and drinks from pigeonpea seeds.

Area planted to the crop worldwide has steadily expanded - from 2.2 million hectares in the early 1950s to 3.8 million by the late 1990s. But average yields have declined during this period - from 780 to 753 kilograms per hectare. As a result of depressed productivity, the price of pigeonpea has soared, causing particular concern in India, where recent shortages of the pulse have prompted the government to ban exports.

ICRISAT and its national partners are hopeful that new pigeonpea hybrids will soon make those concerns a thing of the past.

For more information, contact K.B. Saxena (k.saxena@cgiar.org) and consult the ICRISAT Web site .