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May, 2007
Source:
Consultative Group on
International Agricultural Research (CGIAR)
CGIAR
Story of the Month
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 (photo).
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.
Other news
from
ICRISAT, from
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