Re-designing rice plant for high paddy yield
The country attained self-sufficiency in rice production due to adaptation of modern varieties and technologies developed by BRRI. Revolution in rice production mainly comes from widespread of Boro varieties: BRRI dhan28 and BRRI dhan29, as well spread of Aman varieties: BR11, BRRI dhan49 and BRRI dhan52. Future yield increase is much difficult in the face of limited land, population increase and natural disasters.
Yield potential needs to be increased to get more yields from unit land. Yield is a complex trait governed by many genes. According to ideotype breeding concept, knowledge and relationships between yield and yield contributing physiological traits is important. Increased photosynthetic efficiency, decreased respiration, more dry matter, increased starch and greater sink size are possible approaches to increase yield.
Selection of semi-dwarf short type rice varieties in late 60s is successful history of green revolution. IRRI bred semi-dwarf rice with recessive sd-1 allele have reduced Gebberilic Acid3 (GA3) synthesis and shorter plant height - could give higher yield. Later, IRRI proposed new plant type with sturdy culms, low tillering (5-6), and dense panicle, and yield 13-15 t/ha- little success was obtained for increasing yield.
In China, Professor Yuan Long ping (father of hybrid rice) designed super high yielding varieties emphasizing on more light, water and CO2 absorption to increase photosynthetic efficiency and to enhance carbohydrate storage. According to Long ping most important morphological features are- lodging tolerance and more dry matter with long, erect, v-shaped and thick upper 3 leaves. Other traits are: tallness, vigorous roots, late senescent, higher photosynthetic area, huge assimilates and high translocation.
Professor Yuan Long ping realized the need of increasing dry matter through tallness. If 300 effective panicles per square meter of land could be maintained then 15 tons/ha grain yield could be obtained at harvest index (HI) of 0.50 with 15 tons/ha biomass yield. Long ping's super high yielding variety gave 15 t/ha yield following 50:50 yield and biomass ratio. Thus, increased dry matter production is important.
However, it is difficult to enhance harvest index (HI of 0.50 is fixed) and lodging tolerance of tall plant. But, tallness, robustness and lodging tolerance with more dry matter production is essential. As HI could not be increased, only option is to increase biomass as a 1st objective.
The ultimate goal is to develop super high yielding varieties (13 t/ha) for breaking present yield ceiling of modern varieties. We designed special plant architectures to improve morpho-physiological traits of 2 plant types (intermediate tall and tall). The 2nd objective is to increase photosynthesis efficiency i.e. increased storage of starch in stems. A lodging tolerance erect plant with late senescence and very deep green leaves and stems could absorb more sun light for increasing photosynthesis.
Vigorous roots have more fertilizer and water take up efficiency. High tillering ability, robust and tall plants contribute to more photosynthetic area with more CO2 absorbing capability. All these favorable traits increased photosynthesis efficiency i.e. increased storage of starch in stems. The success of our research lies on storage of more carbohydrate/assimilates that means enhanced source capacity of rice plant.
Tallness is desirable for flood adaptation and for more biomass. Tallness and yield have negative correlation; it is difficult to develop a high yielding tall genotype. Without strong culms, a tall plant results in lodging, decreased HI and yield. Scientists have realized that if lodging of tall rice can be protected- a tall plant with more biomass would contribute to increased yield.
Lodging tolerance is key traits of intermediate and tall plant types for enhancing translocation of assimilates to grain. If there is lodging- translocation of assimilates reduce sharply. We transferred a number of genes/traits for enhancing mechanical strength of culms: culm stiffness, higher stem diameter, higher culm wall thickness, shorter lower internodes length, high biomass, as well higher cellulose/lignin in culm to obtain lodging tolerance. Innovation of new genotype with strong base as like bamboo - makes intermediate and tall plants more tolerant to lodging.
With development of a intermediate tall genotypes (BRH13-2-4-6-4B & BRH13-2-4-7-2B) with strong base - higher yield (10.1 t/ha) under irrigated Boro areas was achieved with higher number (450- 500) of filled grains/ panicle, higher number effective of tillers/plant (18-22). 10.1 t/ha yield with HI=0.50 was up to the expected level- this is a great success. But research showed that storage of assimilates has some limitation; there are some un-filled grains in panicles that could be minimized using integrated fertilizer management practices.
Research showed that in tall BRRI dhan91(190 cm tall)- storage of assimilates is 3 times more and biomass is 1.6 times more than local deep water varieties. A tall deep water genotype with strong base - gave satisfactory higher yield (4.5 t/ha) in semi deep water areas. Seed reserve and seed protein (9.6%) content is much better with HI=0.35. This is also a great success for a tall variety.
For tall deep water BRRI dhan91- genetic gains were achieved in physiological traits: strong stems; more stem carbohydrate; chlorophyll content, vigorous growth, late senescence and more biomass. As lodging problem has been protected- a tall plant - contributed to better partitioning to yield. However, challenge is that even by increasing tallness, lodging tolerance, and carbohydrate reserve- harvest index is (0.35) could not be increased up to mark (0.50). The challenge for tall genotype is to increase HI.
A noble recombination of major beneficial genes for physiological traits in BRH13-2-4-7-2B enhanced higher number (450- 500) of filled grains/panicle and higher yield (10.1 t/ha) that is much more than popular modern Boro varieties. The challenge for intermediate tall Boro genotype with higher number (450- 500) of filled grains/panicle is to increase assimilates storage up to expected level. Strategic research could be conducted for further improvement of yield of irrigated Boro rice.
The government could give priority for dissemination of BRRI dhan91 in 0.7 m. ha of fallow of shallow deep areas - give priority to release and disseminate BRH13-2-4-7-2B during Boro season for greater food security in our country.
The writer is a chief scientific officer, Bangladesh Rice Research Institute