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Virk, PS. Breeding to enhance.pdf

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5 IRRN 29.1 MINI REVIEW R ice is the staple food for the largest number of people on Earth. According to one estimate, global rice production must reach 800 million t from the present 585 million t in 2003 to meet the demand in 2025. Because irrigated rice contributes more than 75% of the total rice production, enhancing its yield potential would be key to meeting the global rice requirement for an additional 215 million t. Therefore, the average yield of irrigated rice varieti
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       R ice is the staple food for the largest number of people on Earth. According to oneestimate, global rice production must reach 800 million t from the present 585million t in 2003 to meet the demand in 2025. Because irrigated rice contributesmore than 75% of the total rice production, enhancing its yield potential would be key tomeeting the global rice requirement for an additional 215 million t. Therefore, the averageyield of irrigated rice varieties must increase in tropical rice lands from 5 to 8.5 t ha –1 (Peng et al 1999). To achieve the target yield level, rice varieties with a yield advantage of about 20% over widely grown varieties under tropical conditions must be developed.Alternatively, we must increase the area under rice, but there is little scope for expandingirrigated rice area. In fact, the cultivated rice area is decreasing as a result of urbanizationand industrialization.Yield potential is defined as the yield of a variety when grown in environments towhich it is most adapted, with nutrients and water nonlimiting and pests and diseasesand other stresses effectively controlled (Evans 1993). During the 1960s, the yield potentialof the irrigated rice crop increased from 6 to 10 t ha –1  in the tropics. This was accomplishedat IRRI primarily by reducing plant height through the incorporation of a recessive gene, sd1 , for short stature from Chinese variety Dee-geo-woo-gen. In 1962, IRRI plant breedersmade a cross that resulted in the development and release of IR8 in 1966, the first semidwarf variety in the tropics. IR8 represented a new plant architecture—short stature, high                       tillering, sturdy stems, and dark green and erectleaves (Jennings 1964). The plant type represented by IR8 and succeeding varieties was extremelyeffective in increasing the productivity of irrigatedrice lands (Khush 1995). The yield potential of IR8was about 9.5 t ha –1  during the dry season in thetropics when it was released in 1966. However, itnow yields about 7.5–8.0 t ha –1  under the bestmanagement (Peng et al 1999). Several subsequentIR varieties have outyielded IR8 by 15–20%.Moreover, several of them have shorter growthduration and their per day productivity is evenhigher.Plant breeders and physiologists at IRRI in thelate 1980s postulated that the plant type of thesevarieties may limit further improvement in yieldpotential. They produce a large number of unproductive tillers and have excessive leaf areathat may cause mutual shading and a reduction incanopy photosynthesis and sink size, especiallywhen grown under direct-seeded conditions(Dingkuhn et al 1991). Most of these varieties havehigh tillering capacity and small panicles. The largenumber of unproductive tillers, which limit sink sizeand contribute to lodging susceptibility, wasidentified as the major constraint to yieldimprovement in these varieties. Furthermore,simulation models predicted that a 25% increase inyield potential was possible by modifying thefollowing traits of the semidwarf plant type(Dingkuhn et al 1991): (1) enhanced leaf growth incombination with reduced tillering during earlyvegetative growth, (2) reduced leaf growth alongwith sustained high foliar N concentration duringthe reproductive stage and late vegetative growth,(3) a steeper slope of vertical N concentrationgradient in the leaf canopy with more N present inthe top, (4) expanded storage capacity of the stems,and (5) improved reproductive sink capacity alongwith an extended grain-filling period.These factors prompted IRRI scientists topropose modifications to the high-yielding indicaplant type in the late 1980s in order to increase theyield potential .  The proposed new plant type (NPT)has low tillering capacity (3–4 tillers when directseeded, 8–10 tillers when transplanted); fewunproductive tillers; 200–250 grains per panicle; aplant height of 90–100 cm; thick and sturdy stems;leaves that are thick, dark green, and erect; avigorous root system; 100–130-d growth duration;and increased harvest index (Peng et al 1994, Khush1995). This ideotype became the “new plant type”highlighted in IRRI’s strategic plan (IRRI 1989). Thegoal was to develop an NPT with yield potential20–25% higher than that of existing semidwarf varieties of rice in the tropical environment duringthe dry season.However, at the time, the ideotype approachto plant breeding was not considered novel. In fact,it was proposed by Donald (1968), who defined“crop ideotype” as an idealized plant type with aspecific combination of characteristics favorable forphotosynthesis, growth, and grain production based on knowledge of plant and crop physiologyand morphology. He anticipated that it would bemore efficient to define a plant type that wastheoretically efficient and then breed for thisideotype (Hamblin 1993).Meanwhile, IRRI has also been looking athybrid rice breeding to increase the yield potentialof rice in the tropics. (However, this topic is beyondthe scope of this paper and will be reviewed inanother paper.)          Yield is a function of total dry matter and harvestindex (grain to straw ratio). Therefore, yield can beincreased by enhancing the total dry matter orharvest index or both. Modern, high-yielding,semidwarf varieties produce about 18–19 t of  biomass ha –1  and their harvest index is around 0.45–0.50. Cultivars producing 22 t of biomass and witha harvest index of 0.55 should produce about 12 t of grain per hectare. The harvest index can beincreased by increasing the sink size. For example,we can raise the number of grains per panicle. Onthe other hand, we need to develop plants withsturdier stems so that nutrients can be applied athigher rates to enhance total biomass.The choice of traits to breed for an ideal planttype for the irrigated lowland came from severaldifferent perspectives. Further details on reducedtillering, large panicles, grain density, grain-fillingpercentage, leaf characteristics, growth duration,root system, and disease and insect resistance have been discussed elsewhere (Peng et al 1994, Khush1995, Khush and Peng 1996).      Breeding work for the development of NPT startedas early as 1989 (Khush 1995). Bulu varieties or javanicas from Indonesia have low tillering, largepanicles, and sturdy stems. This germplasm is nowreferred to as the tropical japonicas (Khush 1995).(For clarity, we shall refer to this NPT as NPT-TJ.)     Around 2,000 bulu varieties were assessed in thefield during 1989 and donors for developing NPT-TJ were identified. A few examples of donors usedin the preliminary breeding were Gendjah Wangkal(for the low-tillering trait), Ketan Gubat (for largepanicles), Senhkeu (for thick stems), and Shen Nung89-366 (for short stature) (Peng et al 1994). Inaddition to bulus from Indonesia, many tropical japonica donors were identified in germplasm fromMalaysia, Thailand, Myanmar, Laos, Vietnam, andthe Philippines (Virk and Khush 2003).      Hybridization was undertaken in 1990. Since mostof the bulu varieties were tall, these were crossedwith a semidwarf breeding line, Sheng Nung 89-366, obtained from Shenyang AgriculturalUniversity, China. The selected donors were crossedand breeding lines with the proposed ideotype wereselected. Since then, more than 2,000 crosses have been made, 100,000 pedigree lines produced, breeding lines with the desired morphologicalideotype traits selected, and more than 500 NPT-TJlines evaluated in observational yield trials. TheNPT-TJ lines based on tropical japonica germplasmwere developed in less than 5 years. They weregrown in a replicated observational trial for the firsttime in late 1993. As intended, the first batch of NPT-TJ lines had large panicles, few unproductive tillers,thick stems, and large and dark green flag leaves.The grain yield of these lines, however, was notencouraging. We attributed this to low biomassproduction and poor grain filling. Reduced tilleringcapacity contributed to low biomass production because the crop growth rate during the vegetativestage of NPT-TJ lines was lower than that of indicavarieties (Khush and Peng 1996). Less biomassproduction was also associated with poor grainfilling, but the cause-and-effect relationship has not been established. The poor grain filling of NPT-TJlines was probably due to the lack of apicaldominance within a panicle (Yamagishi et al 1996),the compact arrangement of spikelets on the panicle(Khush and Peng 1996), and a limited number of large vascular bundles for assimilate transport (S.Akita, pers. commun.). The NPT-TJ lines were alsosusceptible to diseases and insects and had poorgrain quality. Fortunately, we traced the unfilledgrain problem to certain donor parents such asSongkeu, Djawa Pelet, and Ribon and were able toovercome this problem by including only parentsthat produce a high percentage of filled grains insubsequent hybridization programs (Virk andKhush 2003). As a result, the yield performance of later NPT-TJ lines was higher than that of previousNPT-TJs and the indica check variety. For example,the best NPT-TJ line outyielded IR72 by 9.5% in anobservational field trial conducted at IRRI duringthe 1998 dry season.Several NPT-TJ lines were shared with theYunnan Academy of Agricultural Sciences. From2000 to 2003, after evaluations under localconditions, Chinese rice breeders released threeNPT-TJ varieties, Dianchao 1, Dianchao 3, andDianchao 2, developed from IR64446-7-10-5 andIR69097-AC2-1, two IRRI NPT-TJ lines. The NPT-TJ lines perform very well in temperate areas wheredisease pressure is low and consumers prefer stickyand bold grain types.NPT-TJ lines also serve as an importantreservoir of hitherto underused genetic diversity,especially in the tropics, and breeders are startingto use them in their crossing programs. For example,in Indonesia, breeders using IRRI’s NPT-TJ lineIR66154-521-2-2 as one of the parents releasedvariety Ciapus during 2003. In China and Vietnam,some promising lines srcinating from crosses withNPT-TJ lines are being evaluated in farmers’ fields.        Further fine-tuning was necessary as the NPTshowed traits that could still be improved. Anincrease in tillering capacity of the NPT-TJ lines wasenvisaged to increase biomass production.Moreover, most of the NPT-TJ lines lackedresistance to tropical diseases and insects as theparents used for developing these lines weresusceptible. For example, there were no donors forresistance to brown planthopper and tungro virusin tropical japonica germplasm. Farmers andconsumers in tropical rice-growing countries prefervarieties with long and slender grains andintermediate amylose content.As the fine-tuning process continued, in 1995,modern high-yielding indica varieties/elite lineswere included in the hybridization program. Withthis, the development of improved NPT lines began.Since these lines are derivatives from crosses between indica and japonica germplasm, we shallcall them NPT-IJs. This was necessary, as explainedabove, to increase biomass, incorporate genes forresistance to tropical diseases and insects, andchange grain appearance and quality, and therebyensure wider acceptability among farmers andconsumers in Asia.     More than 400 NPT-IJ lines have beenevaluated in observational yield trials. During 2001,several NPT-IJ lines outyielded the best indica checkvariety by up to 30% in breeders’ replicated yieldtrials (Khush et al 2001). Encouraged by the superiorperformance of the NPT-IJs, we subjected them tomore rigorous yield testing during 2002 and 2003.In the 2002 dry and wet seasons, several NPT-IJ linessignificantly outyielded check variety IR72 (seetable). The NPT-IJ lines approached the 10 t ha –1 yield barrier. However, IR72158-16-3-3-1 andIR72967-12-2-3 might not have expressed their yieldpotential fully since their harvest index was below50% and grain filling was not greater than 80%(Peng et al 2004). In the 2003 dry season, NPT-IJ lineIR72967-12-2-3 was the top yielder. It produced10.16 t ha –1 , which was significantly higher thanindica check variety yield.It is obvious that yield improvement wasachieved with the NPT-IJ lines. The yield increasewas attributed to increased panicle number per m 2 ,improved grain-filling percentage, larger panicleswith a large number of spikelets, more biomass, andhigher harvest index. These lines with crop growthduration of 115–125 d belong to the early tomedium-maturing group and will have a minimumeffect on the intensification of cropping systems inthe tropics.To achieve a 10% increase in yield potential of irrigated lowland rice in the tropics, the followingare the target traits: 330 panicles per m 2 , 150 spikeletsper panicle, >80% grain filling, 25 mg grain weight(oven-dry), 22 t ha –1  aboveground total biomass (at14% moisture content), and 50% harvest index (Pengand Khush 2003). Among these traits, the key is apanicle size of 150 spikelets per panicle. During2004, we have initiated a breeding program toexamine these issues. A crop management strategyhas to be developed to fully express the yieldpotential of NPT-IJ lines with large panicles. As breeding efforts continue, it is expected that moreelite NPT-IJ lines with improved yield potential,disease and insect resistance, and grain quality will be developed.It is important to note, however, that the NPTproject has succeeded quite dramatically fromanother perspective. Before this project, the japonicagenepool was essentially excluded from irrigated breeding programs in the tropics. In fact, crosses between the two subspecies had very limitedsuccess. Notable exceptions are Tongil rice in Koreaand Mahsuri in the rainfed lowland tropics. Atpresent, we have NPT-IJ lines derived from singleor three-way crosses between the japonica NPT-TJsand the elite indica HYV genepool. It appears thatthis new “mixed” genepool can generateimprovements in secondary traits such as lodgingresistance and nutritional quality.Related to this is the potential to exploit thisnew genetic variation in heterosis of F 1  hybrids. Thecurrent hybrids grown in the tropics are nearly allderived from the indica-indica hybrids developedat IRRI. However, recent results in China haveshown immense potential for intersubspecificheterosis. The ability of the NPT-TJ and NPT-IJ linesto produce such hybrids for the tropics is being                                                                                                                                                                            

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Jul 23, 2017
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