Halopriming in the submergence-tolerant rice variety improved the resilience to salinity and combined salinity-submergence at the seedling stage.

The role of halopriming in alleviating the detrimental effects of salinity and combined salinity-submergence was evaluated using two rice genotypes, "IR06F148" (anaerobic germination + submergence tolerant [Sub1]) and "Salt-star" (salt tolerant) with contrasting levels of tolerance. Nonprimed seeds and those primed with 1% calcium chloride (CaCl2) were germinated, and the seedlings were exposed to salinity (50 or 100 mM sodium chloride [NaCl]) and submergence (nonsaline or saline water). Salinity substantially inhibited plant height, shoot/root dry mass, and leaf area. Priming improved the resilience to 50 mM NaCl by increasing the chlorophyll content and lowering hydrogen peroxide (H2O2) production; and to 100 mM NaCl by increasing the total soluble sugars. However, apparent differences in the responses of primed "Salt-star", such as an increase in the Na+, K+, and Ca2+ levels, indicated that halopriming differentially affected the response to salt based on the salinity tolerance of the variety. Submergence reduced the shoot biomass, chlorophyll, and photosynthetic efficiency to a greater extent in "Salt-star" than in "IR06F148". Priming, especially in "Salt-star", caused a lesser reduction in the chlorophyll (Chl) and maximum quantum yield of photosystem II (Fv/Fm) but increased the total soluble sugars post-submergence, indicating a boost in the photosynthetic efficiency. The responses of the two varieties to submergence depended on their tolerance, and halopriming affected each variety differently. The metabolic and molecular changes induced by halopriming in submergence-tolerant rice may be explored further to understand the underlying mechanisms of improved resilience.

Spatial O2 Profile in Coix lacryma-jobi and Sorghum bicolor along the Gas Diffusion Pathway under Waterlogging Conditions.

While internal aeration in plants is critical for adaptation to waterlogging, there is a gap in understanding the differences in oxygen diffusion gradients from shoots to roots between hypoxia-tolerant and -sensitive species. This study aims to elucidate the differences in tissue oxygen concentration at various locations on the shoot and root between a hypoxia-tolerant species and a -sensitive species using a microneedle sensor that allows for spatial oxygen profiling. Job's tears, a hypoxia-tolerant species, and sorghum, a hypoxia-susceptible species, were tested. Plants aged 10 days were acclimated to a hypoxic agar solution for 12 days. Oxygen was profiled near the root tip, root base, root shoot junction, stem, and leaf. An anatomical analysis was also performed on the roots used for the O2 profile. The oxygen partial pressure (pO2) values at the root base and tip of sorghum were significantly lower than that of the root of Job's tears. At the base of the root of Job's tears, pO2 rapidly decreased from the root cortex to the surface, indicating a function to inhibit oxygen leakage. No significant differences in pO2 between the species were identified in the shoot part. The root cortex to stele ratio was significantly higher from the root tip to the base in Job's tears compared to sorghum. The pO2 gradient began to differ greatly at the root shoot junction and root base longitudinally, and between the cortex and stele radially, between Job's tears and sorghum. Differences in the root oxygen retention capacity and the cortex to stele ratio are considered to be related to differences in pO2.

Different survival strategies involve carbon translocation rather than de novo C assimilation under complete submergence in rice plant.

This study investigated the effect of transient submergence on the recovery of photosynthetic activity and translocation of photosynthate in IR67520 (Sub1A genotype) and IR72442 (non-Sub1A genotype) using 13C-labeled tracer, coupled with some photosynthetic physiological assessments. Plant growth, photosynthetic capacity, and photosynthetic recovery were studied by treating the two rice genotypes without or completely submerged for 7 days in transparent acrylic tanks filled with water to a depth of 80 cm, followed by 7 days of reaeration. Results revealed that the IR67520 was able to obtain new carbon source for assimilation during at 7 days of recovery periods. The IR72442 genotype partitioned 13C to the newly developed upper leaves more than the IR67520 genotype did. This was due to its inability to obtain CO2 from other source during post submergence. Recovery of chlorophyll content, ability to retain higher biomass, and ability to grow faster at 7 days of recovery periods also indicated the ability of Sub1A genotype to reactivate its photosynthetic capacity.

Combinational Variation Temperature and Soil Water Response of Stomata and Biomass Production in Maize, Millet, Sorghum and Rice.

Environmental responses of stomatal conductance (gs) as basic information for a photosynthesis-transpiration-coupled model have been increasing under global warming. This study identified the impact of gs behavior under different soil water statuses and temperatures in rice, maize, millet, and sorghum. The experiments consisted of various soil moisture statuses from flooding to drying and combination of soil moisture status and temperature. There was a reduction in shoot biomass of maize and sorghum caused by decreasing of gs, photosynthesis (A), and transpiration (E) in early imposed waterlogging without dependent temperature, whereas millet and rice were dependent on temperature variation. The effect of gradual soil drying, gs, A, and E of maize, millet, and sorghum were caused by low temperature, except rice. The impact of the combination of various soil water statuses and temperatures on gs is important for the trade-off between A and E, and consequently shoot biomass. However, we discovered that an ability to sustain gs is essential for photo assimilation and maintaining leaf temperature through evapotranspiration for biomass production, a mechanism of crop avoidance in variable soil water status and temperature.

Genetic Diversity of Hydro Priming Effects on Rice Seed Emergence and Subsequent Growth under Different Moisture Conditions.

Seed priming refers to seed enhancement methods that stimulate seed metabolism. This study evaluated the genetic diversity of hydro priming efficacy in 27 different genotypes of rice under dry to wet soil moisture conditions. The genotypes included 21 genotypes of Oryza sativa, five genotypes of Oryza glaberrima, and one genotype of NERICA (New Rice for Africa). The treated rice seeds were sown in plastic boxes under four soil moisture conditions (5%, 10%, 15%, and 20% (w/w)). The genotypes were categorized into six groups based on growth parameters using hierarchical cluster analysis. Furthermore, emergence properties were investigated by using principal component analysis based on the mean emergence time of control and primed seeds. Seed priming enhanced growth performance under the moderate dry conditions of 10% and 15% soil moisture. Meanwhile, priming efficacy was low in water stress conditions of 5% and 20% soil moisture. There were wide-ranging genotypic differences of priming efficacy under 20% soil moisture condition. Our findings indicate that the anaerobic-tolerant genotypes tend to exhibit priming efficacy under high soil moisture conditions. Furthermore, one group included all upland genotypes of O. sativa. This group originally adapted to 10% and 15% of dry conditions, and seed priming improved their features greatly.

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.

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.

Contrasting physiological responses by cultivars of Oryza sativa and O. glaberrima to prolonged submergence.

BACKGROUND AND AIMS: Oryza glaberrima is widely grown in flood-prone areas of African river basins and is subject to prolonged periods of annual submergence. The effects of submergence on shoot elongation, shoot biomass, leaf area and CO(2) uptake were studied and compared with those of O. sativa. METHODS: A wide selection of lines of O. sativa and O. glaberrima, including some classified as submergence tolerant, were compared in field and pot experiments. Plants were submerged completely for 31 d in a field experiment, and partially or completely for 37 d in a pot experiment in a growth chamber. KEY RESULTS: Leaf elongation and growth in shoot biomass during complete submergence in the field were significantly greater in O. glaberrima than in O. sativa. So-called submergence-tolerant cultivars of O. sativa were unable to survive prolonged complete submergence for 31-37 d. This indicates that the mechanism of suppressed leaf elongation that confers increased survival of short-term submergence is inadequate for surviving long periods underwater. The O. sativa deepwater cultivar 'Nylon' and the 'Yélé1A' cultivar of O. glaberrima succeeded in emerging above the floodwater. This resulted in greatly increased shoot length, shoot biomass and leaf area, in association with an increased net assimilation rate compared with the lowland-adapted O. sativa 'Banjoulou'. CONCLUSIONS: The superior tolerance of deepwater O. sativa and O. glaberrima genotypes to prolonged complete submergence appears to be due to their greater photosynthetic capacity developed by leaves newly emerged above the floodwater. Vigorous upward leaf elongation during prolonged submergence is therefore critical for ensuring shoot emergence from water, leaf area extension above the water surface and a subsequent strong increase in shoot biomass.

Morphological and physiological responses of rice seedlings to complete submergence (flash flooding).

BACKGROUND AND AIMS: Reducing damage to rice seedlings caused by flash flooding will improve the productivity of rainfed lowland rice in West Africa. Accordingly, the morphological and physiological responses of different forms of rice to complete submergence were examined in field and pot experiments to identify primary causes of damage. METHODS: To characterize the physiological responses, seedlings from a wide genetic base including Oryza sativa, O. glaberrima and interspecific hybrids were compared using principle component analysis. KEY RESULTS: Important factors linked to flash-flood tolerance included minimal shoot elongation underwater, increase in dry matter weight during submergence and post-submergence resistance to lodging. In particular, fast shoot elongation during submergence negatively affected plant growth after de-submergence. Also shoot-elongating cultivars showed a strong negative correlation between dry matter weight of the leaves that developed before submergence and leaves developing during submergence. CONCLUSIONS: 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.