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NEW DELHI, Feb. 18

EFFORTS to achieve the much-needed yield breakthrough in rice have received a boost with the Manila-based International Rice Research Institute (IRRI) making available its `super rice' lines, which embody a 25 per cent genetic potential yield gain over existing high yielding varieties (HYV).

``We have already made available the new plant type of rice evolved by us to the Indian Council of Agricultural Research (ICAR). It is now up to the ICAR system to use the material, either directly or incorporate it as part of its breeding or crossing programmes'', said Dr Ronald Cantrell, Director-General of IRRI.

Unlike in wheat, scientists have not really succeeded in enhancing the genetic potential yield of rice in the last three decades.

The yield potential in a crop is determined by its total dry matter or biomass and the harvest index (HI), which is the percentage of the plant's mass that is grain.

The traditional tall paddy varieties typically had a total biomass of around 12 tonnes per hectare, of which the harvestable grain matter was 30 per cent. Thus, their maximum yield was only around 4 tonnes per hectare of paddy. The biomass of the traditional varieties could not be increased much by application of nitrogenous fertilisers because these plants tended to grow excessively and lodge badly resulting in reduced rather than higher yields.

What the Green Revolution did was to breed short-statured varieties with strong stalks that could hold more grain and respond better to fertiliser application. The new varieties had a higher biomass of 18-20 tonnes per hectare along with an HI of 50 per cent, raising potential paddy yields to 9-10 tonnes per hectare.

Although the strategy of breeding semi-dwarf strains responsive to higher nutrient application and incorporating higher grain-to-straw ratios succeeded in more than doubling potential yield of rice, it has reached a dead end now. The genetic potential of around 10 tonnes for the best performing varieties today is no more than that of the initial HYVs such as IR-8 or Jaya released in the late 1960s.

What IRRI researchers have done to achieve a fresh breakthrough in rice yields is to re-design a `new plant type'. Rice normally grows as a bunch of 25-30 tillers or stems, each containing around 15 panicles (ears) that bear the flowers and grain. More than a third of these tillers do not produce any grain, which means a large part of the solar energy gets diverted and does not go towards grain-filling.

``The new plant type has only 5-6 tillers per plant, all of which are productive and have about 200-250 grains per panicle as against 150 grains in normal HYVs. The plant also has thicker, more upward-angling leaves, which can trap more sunlight, boosting the rate of photosynthesis. All this together raises the total plant biomass to 22 tonnes per hectare, which along with an HI, raises the potential paddy yield to above 12 tonne per hectare'', Dr Cantrell added.

The Directorate of Rice Research (DRR) in Hyderabad is currently testing out IRRI's new plant type material. While this could well be the foundation breeding material for the next Green Revolution in rice, Dr B. Mishra, Project Director, DRR, however, felt that it would take 3-4 years for the new `super rice' to reach the farmers' fields.

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