Ever since the introduction of short statured varieties that enabled tropical Asia witness a major yield breakthrough in indica rice, there has been serious efforts to replicate the strategy in the japonica growing temperate and semi temperate countries like Korea, Japan, USA, etc. The dwarfing gene (sd1) in the donor variety Dee geo wu gen could not however, be exploited for development of similar plant type in japonica rice, because of the constraint of intersubspecific hybrid sterility. Breeding strategy to overcome the indica/japonica hybrid sterility barrier, if to use the DGWG source or finding compatible alternate dwarfing gene source are the two options for developing semidwarf statured high yielding japonica varieties. South Korea was the first to successfully recombine the DGWG dwarfing gene in japonica background by crossing the japonica variety Yukara with T(N)1. To overcome F1 spikelet sterility and combine all other desirable characteristics, the F1 was backcrossed with the semidwarf indica variety IR8. Of many derivatives obtained therefrom, the line Suweon213 (IR667) was named ‘Tongil’ in 1971 and released in 1972. Despite its impressive plant type and yield potential, its grain quality was less acceptable to Korean consumers, who are used to the quality of traditional japonica varieties. Also, its level of resistance to rice blast, bacterial blight brown planthopper and low temperature conditions was far less than desired. Also, it was not suitable for late season planting. Nevertheless, Tongil varieties proved good ‘bridge parents’ for progressively improving the productivity quality and adaptation to the stresses (Fig 9). Among the bridge parents Iri317 (Jinhaung/IR2622) proved to be the best combiner. Intensive breeding using such parents with the objective of correcting the deficiencies resulted in 18 Tongil type varieties between 1974 and 1979 and almost as many were released between 1979 and 1989. Some of the best varieties among them included Milyang30, Suweon284, Milyang 42, Iri338, etc. The breeders’ success in recombining the desired quality and cold tolerance is attributed to their strategy of backcrossing or top-crossing with indica parent as against the earlier Japanese practice of backcrossing with japonica parents. Korean breeders have been preferring always to use genotypes of Tongil plant type as one of the parents to improve rice varieties in view of their excellent morpho-physiological frame.
Fig Bridge improvement from japonica-indica crosses
As for the adoption of Tongil varieties, despite the improvement made, less than acceptable cooking quality, declining yield advantage over the traditional japonica varieities, relatively low price offered in the market, and no respite from pests have together contributed to the decline of area under Tongil varieties. Further improvements being effected on yield and quality, diversification of the varieties and government support are expected Tongils to continue to contribute immensely to the country’s future production goals. In evolving higher yielding semidwarf varieties, Japan and the USA also faced the same problem of indica-japonica hybrid sterility when attempted to take advantage of DGWG gene. Limited progress in recombining the DGWG gene with popular tall/semi tall japonica varieties necessitated search for dwarfness donors in japonica genepool itself. In Japan, identification of three different dwarfing gene sources of spontaneous and induced origin enabled enhancement of productivity of temperate japonica varieties. By crossing of Jikokku, a native dwarf with Zensho26 several high yielding varieties like Hoyoku, Kokumasari, Shiranui, Reihou, Nishihomare, Minaminishiki, etc were developed for southern China. Many more high yielding varieties have been developed subsequently through hybridization of them with other varieties (Fig 10). The variety Reimei widely cultivated in north China in the 70s is a gamma radiation induced dwarf mutant isolated from the variety Fujiminori. Using Reimei as the dwarding gene source with Toyanizhiki, the popular variety Akihikari was developed. It ranked fourth in area coverage, by occupying over 1,20,000 ha in 1979. Another high yielding dwarf variety Kochihibiki released in Central Japan is a product of the cross involving Shiro Senbon, a local dwarf of spontaneous origin (Kichuchi, 1986).
High yielding varieties developed using spontaneous and induced dwarf mutants in temperate Japonica resistance to lodging, responsiveness to fertilizer and grain quality to US market standards. Varietal improvement, in keeping with the market preference, has been for development and sustenance of short and medium grain type temperate japonicas and indica – like long grain type tropical japonicas (earlier believed to be of indica-japonica origin) respectively of low and intermediate amylose content. The breeding trend continued until 60s, when semidwarf indica varieties marked a major yield breakthrough in tropical Asia. Rice improvement strategy since then progressed from complete reliance on japonica germplasm to backcrossing of single genes from indicas into japonica and increasingly of improved cultivars from indica by japonica crossing (Rutger, 2004). To evolve semidwarf varieties of high yield potential without compromising on japonica grain quality, a breeding strategy using two different dwarfing gene sources has been adopted. Beginning in the late 60s, breeders started introgressing semidwarfness successfully from the widely used DGWG of the indica source through backcrossing to local quality germplasm. Two significant achievements of this breeding strategy have been M9, the popular semidwarf cultivar developed from the cross of IR8 with California quality germplasm (Carnahan et al., 1978) and Lemont developed of crosses involving Blubelle/T(N)1 derived semidwarf varieties as dwarfing source and traditional long grain varieties of southern US (Bollich et al., 1985). Whereas M9 served as the parental source for the development of many semidwarf cultivars (Mackill and McKensic, 2003), Lemont became the ultimate semidwarf donor for several semidwarf quality varieties in the southern U.S. Under the ‘Base-broadening’ programme, many indica varieties that performed far superior to local varieties have been increasingly involved in US rice breeding programme (Rutger, 2004). The other strategy was to use dwarf mutants induced in locally popular varieties as dwarfing gene source. Use of such sources help to overcome the quality problem, while improving the plant type. An example of how such improvement achieved was the induction of the semidwarfing gene sd1 in the cultivar Calrose, which was released as Calrose76, the first semidwarf cultivar in California (Rutger et al., 1977). The mutant cultivar, though not widely grown, served as the potential dwarfing gene source for nine additional California released cultivars (Rutger, 1992). The induced mutation approach was subsequently used to isolate 11 semi dwarf mutants in Arkansas cultivars all non allelic to sd1. Egypt is yet another traditionally japonica rice growing country. Its experiment with long grain indica rice varieties like IR28 from IRRI failed to catch up with farmers as well as consumers despite their higher yield and resistance to blast disease. Keeping the consumer quality central to its breeding efforts, Egypt placing emphasis on earliness for crop intensification, high per day yield and resistance to blast, has come up over the last 20 years, with varieties that raised the productivity to 10t/ha. Significantly, the ruling varieties have in some way involved indica-japonica derivatives and induced dwarf mutant variety of the US as sources of dwarfness. For instance, popular varieties of today such as Giza 178 and Sakha101 and Giza175 have involved respectively Tongil type germplasm like Milyang49, Milyang79 and Giza 175, which involved indica line 1394-10-1 in their development and Giza176 involved Calrose76, an induced dwarf mutant variety of US. Increasingly the country’s rice germplasm is rich enough with sources for resistance to lodging, blast disease and salinity as well as for yield enhancement-all in the background of japonica grain quality.
FOUR DECADES OF RICE BREEDING – ACHIEVEMENTS, IMPACT, FUTURE DEMANDS AND STRATEGIES -Dr.E.A.Siddiq