Rice (Oryza sativa L.) is one of the most important cereal crops in the world. More than half of the world’s population depends on rice as their staple food, and in Africa, the overall demand for rice has outstripped local production (Zenna et al. 2017). Moreover, ongoing global warming exposes rice production to serious risks, such as extreme heavy rain, flooding, and disastrous droughts. To address the demand for food for over nine billion people by 2050, and further, to respond to the challenges posed by global warming, the genetic improvement of yield potential in rice breeding programs is one of the most important strategies.
Since the release in 1966 of IR 8, the first semi-dwarf variety with high yield potential, the International Rice Research Institute (IRRI) has aimed to further improve the yield potential of IR-series varieties and has proposed the new-plant-type varieties (NPTs) based on the “plant type (ideotype) concept” (Donald 1968; Yoshida 1972; Peng et al. 1994, 2008; Khush 1995). NPTs are varieties harboring low tiller number, few unproductive tillers, and a large number of grains per panicle, a phenotype expected to result in high yield potential (Peng et al. 1994). NPTs developed from the crosses between tropical Japonica Group varieties and improved Indica Group varieties have been released since 1993; however, grain yields have been disappointing because of low biomass production and poor grain filling (Peng et al. 2008). To achieve high grain filling and increase yield potential, particularly in terms of improvement of the sink–source balance, it is necessary to have simultaneous improvement of both a high sink capacity (a large number of spikelets per panicle, heavy grains, and stable grain filling) and also a superior source ability (carbohydrate supply) to accommodate the large sink size under high temperatures.
An informative indicator of the sink–source balance is harvest index (HI). HI is defined as the ratio of harvestable parts to biomass and is taken as a measure of biological success in partitioning photosynthates to the harvestable product (Donald and Hamblin 1976; Sinclair 1998). Recent breeding programs have achieved great increases in yield by improving HI. Hay (1995) concluded that yield increase in rice was a consequence of the incorporation of dwarfing genes that increased harvest index by around 0.3 to 0.5. Therefore, HI is an useful indicator to consider in the breeding of new high yielding varieties.
Molecular genetic research has detected several QTLs for HI in rice (Hittalmani et al. 2003; Zhang et al. 2004, 2017; Marri et al. 2005; Sabouri et al. 2009; Li et al. 2012). HI is a quantitative trait controlled by multiple genes (QTLs) for numbers of panicles, grain weight, grain numbers, plant biomass, and photosynthetic carbohydrate accumulation and translocation, and the expressions of these QTLs for HI are affected by the environment (Hittalmani et al. 2003; Li et al. 2012; Zhang et al. 2017). However, there are few studies on the beneficial genetic basis of HI independent of environmental factors.
A total of 334 introgression lines with a genetic background of IR 64 were developed with NPTs under the IRRI–Japan Collaborative Research Project (Fujita et al. 2009). Among them, YTH183 exhibited higher yield and HI than those of IR 64 under various environmental conditions, such as water-saving (aerobic), tropical wet and dry, irrigated lowland and upland, and temperate climate conditions (Kato et al. 2011; Uddin et al. 2016; Ishimaru et al. 2017; Takai et al. 2019). It has been demonstrated that the higher yield potential and greater dehydration avoidance in YTH183 might be conferred, at least in part, by introgressed segments on chromosomes 5 and 6, respectively (Kato et al. 2011). Fujita et al. (2010) and Ishimaru et al. (2017) reported that one QTL for grain weight was mapped on chromosome 5. Thus, genetic analysis using YTH183 is expected to reveal another genetic factor for the sink–source balance or the source ability in addition to grain weight (i.e. sink capacity).
In this study, we attempted to identify QTLs for HI by using recombinant inbred lines (RILs) derived from crosses between IR 64 and YTH183. To confirm the regional effects of detected QTLs on yield-related traits and physiological attributes, a cross-locational investigation was conducted in locations with subtropical and temperate climates in Japan. The present study provides novel information to improve the HI of Indica Group varieties through better sink–source balance or source ability.