Jun-Ichi Sakagami

Differential Salinity Tolerance among Oryza glaberrima, Oryza sativa and Their Interspecies Including NERICA

Abstract Salinity tolerance has been extensively studied in Oryza sativa, but little is known about the salt tolerance levels in Oryza glaberrima and the interspecific progenies including New Rice for Africa (NERICA). In this study, the salinity tolerance of the three cultivated rice species, O. glaberrima (54 genotypes), the interspecific progenies (21) including NERICA (7) and O. sativa (41) mainly grown in West Africa were examined comparatively. At 10 days after sowing (DAS) 80 mM NaCl was added to the culture solution, and the plants were grown for 10 more d. The ratio of shoot biomass in the 80 mM NaCl solution to that in the control was significantly higher in the interspecific progenies than in the other two species, and the relative root biomass was significantly lower in O. glaberrima than in the others. The vegetative growth of six genotypes including the salt tolerant Pokkali, and NERICA4 and its parents were evaluated further in pot experiments irrigated with 80 mM NaCl solution from 22 to 52 d after sowing. At 30 d of the salt stress, CG14 and Mala noir IV (O. glaberrima) were killed by salt, while WAB56-104 and NERICA4 survived; Pokkali maintained the highest relative shoot biomass growth at all sampling times of 10 d intervals. These results indicate that O. glaberrima is relatively weaker to NaCl salinity, while the interspecific progenies are fairly tolerant during the seedling stage, and that the relatively high salt stress tolerance of NERICA4 is derived from the O. sativa parent, WAB56-104.

Role of canopy coverage in water use efficiency of lowland rice in early growth period in semi-arid region.

Abstract An appropriate combination of rice cultivar and cropping system that maximizes water use efficiency (WUE) may improve yield of rainfed lowland rice. In the paddy field, a large amount of water is consumed by evaporation during the early growth period, and it can be reduced by canopy coverage especially in semi-arid regions. Therefore, we evaluated the role of canopy coverage in WUE of rice in the early growth period in semi-arid region. A pot experiment was conducted in Namibia to investigate the genotypic and species difference in WUE, and another pot and a field experiment were conducted to investigate the effects of planting density on WUE. Although no significant difference was observed among species, the mean WUE was in the decreasing order of Oryza sativa, and Oryza glaberrima followed by the interspecific progenies including NERICA. In contrast, there was a significant difference in WUE at the genotypic level. Highly tillering genotypes such as WAB1159-2-12-11-5-1 and WITA 2 showed a high WUE. Furthermore, WUE was significantly correlated with the number of tillers (R2 = 0.453), and higher planting density resulted in a higher WUE. In contrast, stomatal conductance had no significant correlation with WUE (R2 = 0.081). Thus, the physical conditions affected by number of tillers and planting density had greater impacts on WUE than physiological characteristics such as stomatal conductance. The suppression of surface water evaporation by coverage was significant, probably contributing to WUE improvement. To increase WUE in semi-arid regions, we recommend the increase of canopy coverage and higher planting density.

Identification and linkage analysis of a new rice bacterial blight resistance gene from XM14, a mutant line from IR24

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.

Relationship between shoot elongation and dry matter weight during submergence in Oryza sativa L. and O. glaberrima Steud. rice cultivars.

Abstract Rice plants are damaged by flash floods with a rapid increase in water level caused by a heavy rain. However, rice plants cope with the flash floods either by an “escape strategy” involving rapid shoot elongation or by a “quiescence strategy” involving surviving under water with minimal activity. As we found in previous experiments, Saligbeli cultivar adapted well to flash floods through rapid shoot elongation. To understand the vigorous growth process during submergence, we studied the relationship between shoot elongation and changes in dry matter weight (DMW) during submergence. O. glaberrima Steud. cv. Saligbeli and O. sativa L. cv. Ballawé and IR 49830-7-1-2-2 were used. Saligbeli and Ballawé exhibit shoot elongation, and IR 49830-7-1-2-2 exhibits flash-flood tolerance due to the presence of the Sub-1 gene. Twelve-day-old seedlings were submerged for 7 days and the plant length and DMW were measured. The plant length ratio of submerged to control plants in Saligbeli was higher than that of other cultivars during 2-6 days of submergence but IR 49830-7-1-2-2 shoot elongation was inhibited by submergence. In all three cultivars, the elongation of the developing leaf sheath conferred shoot elongation during submergence. The plant length of all submerged plants showed a strong positive correlation with DMW of the leaves developed during submergence. Submerged Saligbeli and Ballawé showed strong negative correlations between DMW of the leaves developed before and during submergence (r = –0.786 for Saligbeli and –0.772 for Ballawé , P < 0.05), suggesting that the enhancement of shoot elongation during submergence is accomplished by using dry matter of the leaves developed before submergence. However, the correlation was not observed in the submerged IR 49830-7-1-2-2. Further details from studies using isotopes are also needed to understand the plant growth during submergence.

Seed Germination and Coleoptile Growth of New Rice Lines Adapted to Hypoxic Conditions

We investigated the morpho-physiological traits of rice (Oryza sativa L.) during the germination and post-germination phases to explore avoidance of hypoxic conditions. We compared four lines selected for anaerobic germination (AG lines) with the variety IR42. The germination capacity of AG lines was higher than that of IR42. The germination percentages and coleoptile elongation differed among the four AG lines; IR06F459 showed the fastest germination and rapid coleoptile elongation. The coleoptiles of IR06F459 were significantly longer than those of IR42. The α-amylase activity in germinating seeds was significantly higher in IR06F459 than in IR42. At 2 days after sowing, the sucrose and glucose concentrations in germinating seeds were higher in IR06F459 than in IR42. These results show that IR06F459, an AG line with a long coleoptile, has high α-amylase activity and high sucrose and glucose concentrations in germinating seeds. These attributes partly explain its vigorous germination and coleoptile growth under hypoxic conditions.

High-resolution mapping and characterization of xa42, a resistance gene against multiple Xanthomonas oryzae pv. oryzae races in rice (Oryza sativa L.)

Improvement of resistance against rice bacterial blight (BB) disease is an important breeding strategy in breeding programs across the world, especially in Africa and southern Asia where BB is more prevalent. This report describes a high-resolution map and characterization of xa42 at XA42 locus, a rice BB resistance gene in XM14, a mutant line originating from IR24. The candidate gene region was narrowed down from 582 kb, which had been obtained in our previous study, to 57 kb. XM14 shows brown spots in its leaves like lesion mimic mutants. This line also shows a shorter stature than the original cultivar IR24. In XA42 gene segregating populations, homozygotes of xa42 allele were consistently resistant to the six Japanese Xanthomonas oryzae pv. oryzae races used for this study. They also showed brown spots and markedly short stature compared with the other genotypes, suggesting that xa42 gene exhibits pleiotropic effects.

Saving Water in Arid and Semi-Arid Countries as a Result of Optimising Crop Evapotranspiration

Although water covers about 71 percent of our planet surface, 98 percent of it has too high salt content to be used for drinking water, for irrigation, or even for most industrial purposes. Fresh water represents one percent of all the water on the earth and is distributed unevenly on the earth surface. As a result of the dramatic increase in population, in economic activities, and a subsequent increase in water usage, the world fresh water resources became scarce during the past decades (Postel et al., 1996; Hoekstra & Chapagain, 2007). Shortage of water currently plagues almost every country in North Africa and the Middle East (MENA) to the extent that hampers economic growth and threats social stability. Most importantly, the scenarios for global environmental change suggest a future increase in aridity and in the frequency of extreme events in many areas of the earth. Economists believe that water problem cannot be solved until water is considered as economic good. There is an urgent need to develop appropriate concepts and tools to do so. Evapotranspiration (ET) is defined simply as the sum of the amount of water returned to the atmosphere through the processes of evaporation (moisture loss from the soil) and transpiration (biological use and release of water by vegetation). Crops are different in their response to water stress at a given growth stage. Therefore, estimating ET is an important tool to calculate the actual crop water requirements in given conditions. As a result, optimizing ET may contribute in solving water shortage problems at two levels, the farm level and the national/global level. Firstly, at the farm level, the process of water irrigation losses has two main components: one due to evapotranspiration losses, and the other including the losses resulting from the percolation of water beneath the root zone in excess of any required leaching for salinity management. Therefore, saving water on the farm level can be achieved by using deficit