Bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) is a chief factor limiting rice productivity worldwide. XM14, a rice mutant line resistant to Xoo, has been obtained by treating IR24, which is susceptible to six Philippine Xoo races and six Japanese Xoo races, with N-methyl-N-nitrosourea. XM14 showed resistance to six Japanese Xoo races. The F2 population from XM14 × IR24 clearly showed 1 resistant : 3 susceptible segregation, suggesting control of resistance by a recessive gene. The approximate chromosomal location of the resistance gene was determined using 10 plants with shortest lesion length in the F2 population from XM14 × Koshihikari, which is susceptible to Japanese Xoo races. DNA marker-assisted analysis revealed that the gene was located on chromosome 3. IAS16 line carries IR24 genetic background with a Japonica cultivar Asominori segment of chromosome 3, on which the resistance gene locus was thought to be located. The F2 population from IAS16 × XM14 showed a discrete distribution. Linkage analysis indicated that the gene is located around the centromeric region. The resistance gene in XM14 was a new gene, named XA42. This gene is expected to be useful for resistance breeding programs and for genetic analysis of Xoo resistance.

Seed priming is a physiological seed enhancement method. Hydro-or osmotic priming can advance germination under adequate moisture conditions. During direct seeding on well-drained paddy and upland fields, rice seeds occasionally encounter low soil moisture conditions. Under these conditions, rice seeds need to undergo rapid germination and secure emergence through improved water absorption capacity and seed bioactive. This study aims to clarify the effects of seed hydropriming on germination and seedling vigor in rice under different soil moisture conditions. The study employed three hydration conditions such as priming, soaking, and control. The seeds to be primed and soaked were submerged in tap water at 30˚C for 12 h. For priming, the seeds were subsequently dried to attain their initial seed weight, but the seeds for soaking were not dried before sowing, and the control seeds were untreated. In addition, different soil moisture conditions, such as 3%, 6%, 8%, 11%, 15%, and 20%, were set. Therefore, emergence time shortened with seed priming at 3% -11% soil moisture contents. In particular, at 8% soil moisture content, priming and soaking decreased emergence time by 26.8 h and 21.7 h, respectively compared with that of the control. At 8% -15% soil moisture contents, shoot elongation rate obtained with the priming seeds increased >1.2 times compared with that obtained with the control and soaking seeds. At >8% soil moisture content, shoot dry weight obtained with the priming seeds increased >1.3 times compared with that obtained with the soaking seeds. Furthermore, at declining soil moisture conditions, the increase in root dry weight is promoted by seed priming compared with that in the control seeds. These results suggest that increased root growth affects plant water absorption under low soil moisture conditions due to priming. This study demonstrates that seed priming facilitates rapid emergence and seedling vigor, unless extremely dry or flooded soil moisture conditions are present at seeding.

Enhancement of shoot elongation during submergence in water that is too deep to permit re-emergence by small seedlings represents a futile escape strategy that takes place at the expense of existing dry matter in circumstances where underwater photosynthetic carbon fixation is negligible. Consequently, it compromises survival or recovery growth once flood water levels recede and plants are re-exposed to the aerial environment. Tolerance is greater in cultivars where acceleration of elongation caused by submergence is minimal.

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