Deep root system architecture from a combination of <i>DRO1</i> and <i>qRL6.1</i> maintains high photosynthesis during rice grain-filling in paddy fields

The source capacity affects grain yields. Many efforts have been made to enhance grain yields by introducing quantitative trait loci (QTLs) responsible for shoot-related traits associated with source capacity. Root-related traits could improve the source capacity by improving the uptake of water and nutrients; however, studies have been limited. In this study, to elucidate the root traits related to source capacity in paddy fields, we conducted a physio-morphological analyses and field trials for yield-related traits for three years. We evaluated high-yielding lowland rice, IR64, along with four near-isogenic lines (NILs) and pyramiding lines (PYLs) that introduced upland rice alleles for four root QTLs (root growth angle, thickness, length, and volume). DRO1-NIL with the root growth angle QTL, DEEPER ROOTING 1 (DRO1), exhibited a larger rooting depth index (RDI) but a shorter total root length (TRL) than IR64. qRL6.1-NIL with a root length QTL, qRL6.1, displayed a similar RDI but a longer TRL than IR64. In contrast, qRL6.1+DRO1-PYL with both DRO1 and qRL6.1 not only had a larger RDI than IR64 but also a TRL comparable to that of IR64. qRL6.1+DRO1-PYL significantly increased grain weight per plant compared to IR64; however, neither of the NILs improved grain weight per plant. Furthermore, qRL6.1+DRO1-PYL exhibited a higher bleeding rate and nitrogen content, along with an integrated net CO2 assimilation rate (Aall) superior to that of IR64, which may have contributed to its enhanced source capacity. Our study highlights that pyramiding root QTLs has the potential to enhance source capacity, including photosynthetic ability.