Development and evaluation of pyramiding lines carrying early or late heading QTLs in the rice cultivar 'IR64'.

The heading date is an important trait for determining regional and climatic adaptability in rice. To expand the adaptability of the rice cultivar 'IR64', we pyramided multiple early or late heading quantitative trait locus (QTLs) in the 'IR64' genetic background by crossing previously developed near-isogenic lines (NILs) with a single QTL for early or late heading. The effects of pyramiding QTLs were observed in three different climatic zones of the Philippines, Madagascar, and Japan. The early heading pyramiding lines (PYLs) headed 6.2 to 12.8 days earlier than 'IR64' while the late heading PYLs headed 18.8 to 27.1 days later than 'IR64'. The PYLs tended to produce low grain yield compared to 'IR64'. The low yield was not improved by combining , which is a QTL that increases the number of spikelets per panicle. Conversely, 'IR64-PYL(7+10)' carrying and headed earlier, produced more tillers, and more panicles per m than 'IR64', and mitigated the yield decrease in early heading. These results suggest that the effects of pyramided QTLs on heading date were consistent across various environments and PYLs could be used to enhance the adaptation of 'IR64' in other rice growing environments.