Herbicide-Tolerant Rice Varieties

• 28 Sep 2021

Why in News

Recently, the Indian Agricultural Research Institute (IARI) has developed the country’s first-ever non-GM (genetically modified) herbicide-tolerant rice varieties (Pusa Basmati 1979 and Pusa Basmati 1985).

• These varieties can be directly seeded and significantly save water and labour compared to conventional transplanting.
• ICAR-IARI is a deemed university.

Key Points

• About the New Varieties of Rice:
  
  • The new varieties contain a mutated AcetoLactate Synthase (ALS) gene making it possible for farmers to spray Imazethapyr, a broad-spectrum herbicide, to control weeds.
    
    • The ALS gene in rice codes for an enzyme (protein) that synthesises amino acids for crop growth and development.
    • The herbicide sprayed on normal rice plants binds itself to the ALS enzymes, inhibiting their production of amino acids.
  • Imazethapyr, effective against a range of broadleaf, grassy and sedge weeds, can’t be used on normal paddy, as the chemical does not distinguish between the crop and the invasive plants.
  • However, the new basmati varieties contain a mutated ALS gene whose DNA sequence has been altered using ethyl methanesulfonate, a chemical mutant.
    
    • As a result, the ALS enzymes no longer have binding sites for Imazethapyr and amino acid synthesis isn’t inhibited.
  • The plants can now “tolerate” application of the herbicide, and hence it kills only the weeds.
  • It is important to note that, as there is no foreign gene involved in the process, the herbicide-tolerance is through mutation breeding. Thus, it is not a Genetically modified organism.
• Advantages of These Varieties:
  
  • Direct Seeding of Rice Activity: The new varieties simply replace water with Imazethapyr and there’s no need for nursery, puddling, transplanting and flooding of fields.
    
    • Water is a natural herbicide that takes care of weeds in the paddy crop’s early-growth period.
    • The new varieties will help in Direct Seeding of Rice (DSR) which has several advantages over paddy transplantation.
  • Cheaper Option: DSR cultivation is currently based on two herbicides, Pendimethalin and Bispyribac-sodium.
    
    • However, Imazethapyr is cheaper than these two options.
  • Safer Option: Imazethapyr, moreover, has a wider weed-control range and is safer, as the ALS gene isn’t present in humans and mammals.

Paddy Transplantation vs Direct Seeding of Rice

• Paddy Transplantation:
  
  • The field where the seedlings are transplanted has to be “puddled” or tilled in standing water.
  • For the first three weeks or so after transplanting, the plants are irrigated almost daily to maintain a water depth of 4-5 cm.
  • Farmers continue giving water every two-three days even for the next four-five weeks when the crop is in tillering (stem development) stage.
  • Paddy transplantation is both labour- and water-intensive.
• Direct Seeding of Rice (DSR):
  
  • In DSR, the pre-germinated seeds are directly drilled into the field by a tractor-powered machine.
  • There is no nursery preparation or transplantation involved in this method.
  • Farmers have to only level their land and give one pre-sowing irrigation.
• Advantages with Direct Seeding of Rice:
  
  • Water savings.
  • Less numbers of labourers required.
  • Saves labour cost.
  • Reduce methane emissions due to a shorter flooding period and decreased soil disturbance compared to transplanting rice seedlings.
• Drawbacks of Direct Seeding of Rice:
  
  • The seed requirement for DSR is also high, 8-10 kg/acre, compared to 4-5 kg/acre in transplanting.
  • Further, laser land levelling is compulsory in DSR. This is not so in transplanting.
  • The sowing needs to be done timely so that the plants have come out properly before the monsoon rains arrive.

Source: IE