Effect of the dwarfing gene d18-k on cool tolerance at booting stage under the genetic background of an extremely cool-tolerant rice line norin-PL11, and its rolation with water depth

Norin-PL11 (PL11) is a highly cool-tolerant line which contains genes for cool tolerance introduced from 'Padi Labou Alumbis', a landrace of Borneo. We investigated whether the dwarfing gene dl8-k (kotaketamanishiki dwarf) exerts its pleiotropic effect of enhancing the cool tolerance at the booting stage under the genetic background of PL11, using the dl8-k isogenic line of the recurrent parent PL11 (D11). During the booting stage, two cool-air treatments at 12 deg C for five days with and without deep water of 15cm depth (DW and CA treatments, respectively) were conducted in growth chambers illuminated at about 600 micromol PAR m**-2 s**-1. Besides spikelet fertility, the ratio (%) of the fertilized-spikelet number of each treated panicle to the varietal mean of fertilized-spikelet number per panicle in the control, viz. FS-T/C was adopted in order to estimate the extent of cool damage including the factor of spikelet degeneration. The CA treatment induced notable spikelet degeneration in PL11, but not in D11. The parameter L sub(5), which is the average of the lowest five FS-T/C values (or spikelet fertilities) on five consecutive days in each line, was used for evaluating the cool tolerances of the lines. In the CA treatment, D11 had a higher L sub(5) in FS-T/C than PL11, indicating that D11 is more cool-tolerant than PL11. D11 showed a significantly higher L sub(5) of FS-T/C in the DW treatment than in the CA treatment, but PL11 showed no significant difference of L sub(5) between the CA and DW treatments. Similar results were obtained for spikelet fertility. In D11, almost the whole panicle at the most susceptible stage was under the surface of the deep water. On the other hand, the whole panicle was above the water surface at that stage in PL11. Consequently, D11 almost eluded the damage from the severe cool-air treatment when combined with the deep water. D11 exceeded PL11 not only in ripened pollen grain number per anther in the control but also in an indicator of pollen fertility after the CA treatment owing to the effects of d18-k. It is concluded that dl8-k may be useful for developing super-highly cool-tolerant cultivars for cool-weather areas.