Direct-Seeded Rice: Genetic Improvement of Game-Changing Traits for Better Adaption

The sustainability of rice production continues to be a subject of uncertainty and inquiry attributed to shifts in climatic conditions. In light of the impending climate change crisis and the high labor and water costs accompanying it, direct-seeded rice(DSR) is unquestionably one of the most practical solutions. Despite its resource and climate-friendly advantages, early maturing rice faces weed competitiveness and seedling establishment challenges. Resolving these issues is crucial for promoting its wider adoption among farmers, presenting it as a more effective sustainable rice cultivation method globally. Diverse traditional and contemporary breeding methods are employed to mitigate the limitations of the DSR approach, leveraging advanced techniques such as speed breeding and genome editing. Focusing on key traits like mesocotyl length elongation, early seedling vigor, root system architecture, and weed competitiveness holds promise for transformative improvements in DSR adaptation at a broader scale within farming communities. This review aims to summarize how these features contribute to increased crop production in DSR conditions and explore the research efforts focusing on enhancing DSR adaptation through these traits. Emphasizing the pivotal role of these game-changing traits in DSR adaptation, our analysis sheds light on their potential transformative impact and offers valuable insights for advancing DSR practices.

Deep placement of nitrogen fertilizer increases rice yield and nitrogen use efficiency with fewer greenhouse gas emissions in a mechanical direct-seeded cropping system

Deep placement of nitrogen fertilizer is a key strategy for improving nitrogen use efficiency. A two-year field experiment was conducted during the early rice growing seasons(March–July) of 2016 and 2017.The experimental treatments comprised two rice cultivars: Wufengyou 615(WFY 615) and Yuxiangyouzhan(YXYZ), and three N treatments: mechanical deep placement of all fertilizers as basal dose at 10 cm soil depth(one-time deep-placement fertilization, namely OTDP fertilization);manual surface broadcast(the common farmer practice) of 40% N fertilizer at one day before sowing(basal fertilizer)followed by broadcast application of 30% each at tillering and panicle initiation stages;and no fertilizer application at any growth stage as a control. One-time deep-placement fertilization increased grain yield of both rice cultivars by 11.8%–19.6%, total nitrogen accumulation by 10.3%–13.1%, nitrogen grain production efficiency by 29.7%–31.5%, nitrogen harvest index by 27.8%–30.0%, nitrogen agronomic efficiency by 71.3%–77.2%, and nitrogen recovery efficiency by 42.4%–56.7% for both rice cultivars, compared with the multiple-broadcast treatment. One-time deep-placement fertilization reduced CH4-induced global warming potential(GWP) by 20.7%–25.3%, N2O-induced GWP by 7.2%–12.3%, and total GWP by 14.7%–22.9% for both rice cultivars relative to the multiple-broadcast treatment. The activities of glutamine synthetase and nitrate reductase were increased at both panicle-initiation and heading stages in both rice cultivars following one-time deep-placement fertilization treatment. Larger leaf area index at heading stage and more favorable root morphological traits expressed as larger total root length, mean root diameter, and total root volume per hill were also observed. One-time deep-placement fertilization could be an effective strategy for increasing grain yield and nitrogen use efficiency and lowering greenhouse-gas emissions under mechanical direct-seeded cropping systems.

Soil Nitrogen Distribution and Plant Nitrogen Utilization in Direct-Seeded Rice in Response to Deep Placement of Basal Fertilizer-Nitrogen

Deep placement of controlled-release fertilizer increases nitrogen (N) use efficiency in rice planting but is expensive. Few studies on direct-seeded rice have examined the effects of deep placement of conventional fertilizer. With prilled urea serving as N fertilizer, a two-year field experiment with two N rates (120 and 195 kg/hm2) and four basal N application treatments (B50, all fertilizer was broadcast with 50% as basal N;D50, D70 and D100 corresponded to 50%, 70% and 100% of N deeply placed as basal N, respectively) were conducted in direct-seeded rice in 2013 and 2014. Soil N distribution and plant N uptake were analyzed. The results showed that deep placement of basal N significantly increased total N concentrations in soil. Significantly greater soil N concentrations were observed in D100 compared with B50 at 0, 6 and 12 cm (lateral distance) from the fertilizer application point both at mid-tillering and heading stages. D100 presented the highest values of dry matter and N accumulation from seeding to mid-tillering stages, but it presented the lowest values from heading to maturity stages and the lowest grain yield for no sufficient N supply at the reproductive stage. The grain yield of D50 was the highest, however, no significant difference was observed in grain yield, N agronomic efficiency or N recovery efficiency between D70 and D50, or between D70 and B50, while D70 was more labor saving than D50 for only one topdressing was applied in D70 compared with twice in other treatments. The above results indicated that 70% of fertilizer-N deeply placed as a basal fertilizer and 30% of fertilizer-N topdressed as a panicle fertilizer constituted an ideal approach for direct-seeded rice. This recommendation was further verified through on-farm demonstration experiments in 2015, in which D70 produced in similar grain yields as B50 did.

Effect of irrigation regime on grain yield,water productivity,and methane emissions in dry direct-seeded rice grown in raised beds with wheat straw incorporation

Dry direct-seeded rice grown in raised beds is becoming an important practice in the wheat–rice rotation system in China.However,little information has been available on the effect of various irrigation regimes on grain yield,water productivity(WP),nitrogen use efficiency(NUE),and greenhouse gas emission in this practice.This study investigated the question using two rice cultivars in 2015 and 2016 grown in soil with wheat straw incorporated into it.Rice seeds were directly seeded into raised beds,which were maintained under aerobic conditions during the early seedling period.Three irrigation regimes:continuous flooding(CF),alternate wetting and drying(AWD),and furrow irrigation(FI),were applied from 4.5-leaf-stage to maturity.Compared with CF,both AWD and FI significantly increased grain yield,WP,and internal NUE,with greater increases under the FI regime.The two cultivars showed the same tendency in both years.Both AWD and FI markedly increased soil redox potential,root and shoot biomass,root oxidation activity,leaf photosynthetic NUE,and harvest index and markedly decreased global warming potential,owing to substantial reduction in seasonalThe results demonstrate that adoption of either AWD or FI could increase grain yield and resource-use efficiency and reduce environmental risks in dry direct-seeded rice grown on raised beds with wheat straw incorporation in the wheat–rice rotation system.

Increasing the appropriate seedling density for higher yield in dry direct-seeded rice sown by a multifunctional seeder after wheatstraw return

Dry direct-seeded rice(DDR) sown using a multifunctional seeder that performs synchronous rotary tillage and sowing has received increased attention because it is highly efficient,relatively cheap,and environmentally friendly.However,this method of rice production may produce lower yields in a rice–wheat rotation system because of its poor seedling establishment.To address this problem,we performed field experiments to determine the rice yield at five seedling density levels(B1,B2,B3,B4,and B5=100,190,280,370,and 460 seedlings m-2,respectively) and clarify the physiological basis of yield formation.We selected a representative high-quality rice variety and a multifunctional seeder that used in a typical rice–wheat rotation area in 2016 and 2018.The proportion of main stem panicle increased with increasing seedling density.There was a parabolic relationship between yield and seedling density,and the maximum yield(9.34-9.47 t ha-1) was obtained under B3.The maximum yield was associated with a higher total spikelet number m-2 and greater biomass accumulation from heading to maturity.The higher total spikelet number m-2 under B3 was attributed to an increase in panicle number m-2 compared with B1 and B2.Although the panicle numbers also increased under B4 and B5,these increases were insufficient to compensate for the reduced spikelet numbers per panicle.Lower biomass,smaller leaf area,and lower N uptake per plant from the stem elongation stage to the heading stage were partially responsible for the smaller panicle size at higher seedling density levels such as B5.The higher biomass accumulation under B3 was ascribed to the increases in the photosynthetic rate of the top three leaves m-2 of land,crop growth rate,net assimilation rate,and leaf area index.Furthermore,the B3 rice population was marked by a higher grain–leaf ratio,as well as a lower export ratio and transport ratio of biomass per stem-sheath.A quadratic function predicted that 260-290 seedlings m-2 is the optimum seedling density for achieving maximum yield.Together,these results suggested that appropriately increasing the seedling density,and thereby increasing the proportion of panicles formed by the main stem,is an effective approach for obtaining a higher yield in DDR sown using a multifunctional seeder in a rice–wheat rotation system.

Grain yield and nitrogen use efficiency of an ultrashort-duration variety grown under different nitrogen and seeding rates in direct-seeded and double-season rice in Central China

Nitrogen(N) and seeding rates are important factors affecting grain yield and N use efficiency(NUE) in directseeded rice. However, these factors have not been adequately investigated on direct-seeded and double-season rice(DDR) in Central China. The objective of this study was to evaluate the effects of various N and seeding rates on the grain yield and NUE of an ultrashort-duration variety grown under DDR. Field experiments were conducted in 2018 in Wuxue County and 2019 in Qichun County, Hubei Province, China with four N rates and three seeding rates.The results showed that the grain yield of the ultrashort-duration variety ranged from 6.32 to 8.23 t ha–1with a total growth duration of 85 to 97 days across all treatments with N application. Grain yield was increased significantly by N application in most cases, but seeding rate had an inconsistent effect on grain yield. Furthermore, the response of grain yield to the N rates was much higher than the response to seeding rates. The moderate N rates of 100–150 and 70–120 kg N ha–1in the early and late seasons, respectively, could fully express the yield potential of the ultrashort-duration variety grown under DDR. Remarkably higher N responses and agronomic NUE levels were achieved in the early-season rice compared with the late-season rice due to the difference in indigenous soil N supply capacity(INS) between the two seasons. Seasonal differences in INS and N response should be considered when crop management practices are optimized for achieving high grain yield and NUE in ultrashort-duration variety grown under DDR.

Mitigating N2O and NO Emissions from Direct-Seeded Rice with Nitrification Inhibitor and Urea Deep Placement

Soil-emitted nitrous oxide(N2O) and nitric oxide(NO) in crop production are harmful nitrogen(N) emissions that may contribute both directly and indirectly to global warming. Application of nitrification inhibitors, such as dicyandiamide(DCD), and urea deep placement(UDP), are considered effective approaches to reduce these emissions. This study investigated the effects of DCD and UDP, compared to urea and potassium nitrate, on emissions, nitrogen use efficiency and grain yields under direct-seeded rice. High-frequency measurements of N2O and NO emissions were conducted using the automated closed chamber method throughout the crop-growing season and during the ratoon crop. Both UDP and DCD were effective in reducing N2O emissions by 95% and 73%, respectively. The highest emission factor(1.53% of applied N) was observed in urea, while the lowest was in UDP(0.08%). Emission peaks were mainly associated with fertilization events and appeared within one to two weeks of fertilization. Those emission peaks contributed to 65%–98% of the total seasonal emissions. Residual effects of fertilizer treatments on the N2O emissions from the ratoon crop were not significant;however, the urea treatment contributed 2%, whereas UDP contributed to 44% of the total annual emissions. On the other hand, cumulative NO emissions were not significant in either the rice or ratoon crops. UDP and DCD increased grain yields by 16%–19% and N recovery efficiency by 30%–40% over urea. The results suggested that the use of DCD and UDP could mitigate N2O emissions and increase grain yields and nitrogen use efficiency under direct-seeded rice condition.

Accumulation of Heavy Metals in Maga-Pouss Rice Fields (Far-North Region, Cameroon) and Transfer to Rice Grains

Monitoring of heavy metals contamination of agricultural products and their transfer and bioaccumulation in crops like rice has become a hot topic worldwide over the last two decades. The present study was carried out to determine the accumulation of heavy metals in rice fields and their transfer to rice grains. Soil, irrigation water and rice grains samples were gathered in Maga-Pouss, Far-North, Cameroon. Concentrations of six heavy metals (lead, cadmium, zinc, copper, iron and mercury) were evaluated by Atomic Absorption Spectrophotometer (AAS). Mercury was not detected in this study. Average concentrations of metals were in this order (in mg/kg): Fe (188.60 ± 97.06) > Pb (63.63 ± 7.11) > Cd (2.59 ± 0.29) > Zn (1.10 ± 1.05) > Cu (0.80 ± 0.73) in water and Pb (105.50 ± 31.11) > Fe (105.50 ± 31.11) > Cu (45.93 ± 14.39) > Zn (22.52 ± 6.40) > Cd (3.15 ± 0.49) in soil. Water in Maga-Pouss rice fields appears to be more harmful than the soil, notably for lead, cadmium and copper. In rice grains, heavy metals were found in this order (mg/kg): Fe (188.01 ± 82.62) > Cu (27.20 ± 0.00) > Zn (23.61 ± 12.42) > Pb (19.50 ± 19.91) > Cd (2.02 ± 1.05). The mean bioconcentration factor (BCF) of metals from soil to rice grains was in the following order: Fe (2.60) > Zn (1.05) > Cd (0.64) > Cu (0.59) > Pb (0.18). From water to rice grains, the order is: Cu (37.26) > Zn (22.49) > Cd (6.97) > Pb (2.74) > Fe (1.94). Rice field pH and electrical conductivity favored the uptake of lead, copper and cadmium by rice grains. The findings of this study will be good documentation for risk assessment, and decision-making by environmental managers in this region.

Effects of Nitrogen on the Competitiveness of Echinochloa colona and Amaranthus viridis with Direct-seeded Rice

Information on weed competitiveness responses to added nitrogen （N） is required to assist with the development of appropriate fertilizer management strategies where weed competition is anticipated. A greenhouse study was conducted to examine the effects of four N rates on the competitive ability ofEchinochloa colona and Amaranthus viridis grown together with direct-seeded rice. Rice and each weed species were grown in a replacement series design at added N rates of 0, 50, 100, and 150 kg ha1. Replacement series diagrams for relative yield showed that competitive ability ofE. colona increased with added soil N. Values of weed aggressivity index for E. colona also significantly increased with the addition of N in the soil. In the absence of added N, A. viridis was more competitive than rice but this relationship slightly changed as N was added. However, values of weed aggressivity index of were statistically similar at all N rates. The results suggest that the competitiveness ofE. colona increased with added soil N, and A. viridis unchanged by soil N levels. Both weed species were different in their response to higher N levels. Information gained in this study could be used to demonstrate the importance of effective weed and fertilizer management.

Temporal Dynamics of Antioxidant Defence System in Relation to Polyamine Catabolism in Rice under Direct-Seeded and Transplanted Conditions

Six rice cultivars viz. PR120, PR116, Feng Ai Zan, PR115, PAU201 and Punjab Mehak 1 under the direct-seeded and transplanted conditions were used to investigate the involvement of antioxidative defence system in relation to polyamine catabolism in temporal regulation of developing grains. Activities of ascorbate peroxidase （APx）, guaiacol peroxidase （GPx）, catalase （CAT）, superoxide dismutase （SOD）, polyamine oxidases （PAO） and contents of ascorbate, a-tocopherol, proline and polyamines increased gradually until mid-milky stage and then declined towards maturity stage under both planting conditions. The transplanted condition led to higher activities of antioxidative enzymes （APx, GPx and CAT） and contents of ascorbate, a-tocopherol and proline whereas the direct-seeded condition had elevated levels of PAO and SOD activities and contents of polyamines, lipid peroxide and hydrogen peroxide. Cultivars Feng Ai Zan and PR120 exhibited superior tolerance over other cultivars by accumulating higher contents of ascorbate, a-tocopherol and proline with increasing level of PAO and SOD activities under the direct-seeded condition. However, under the transplanted condition PR116 and PAU201 showed higher activities of antioxidative enzymes with decreasing content of lipid peroxide. Therefore, we concluded that under the direct-seeded condition, enhancements of polyamines content and PAO activity enabled rice cultivars more tolerant to oxidative stress, while under the transplanted condition, antioxidative defence with decreasing of lipid peroxide content was closely associated with the protection of grains by maintaining membrane integrity during rice grain filling. The results indicated that temporal dynamics of H2O2 metabolic machinery was strongly up-regulated especially at the mid-milky stage.

The Effect of Three New Fungicides against Rice Sheath Blight in Field Experiment

[Objective] The aim was to study the effect of three new fungicides against rice sheath blight in field experiment. [Methods] The experiment set up 7 treatments with three times of repetition and designed by random grouping. By using 5 sampling points in each plot, and investigating continuous 4 holes of each point, total plants, diseased plants and disease degrees were recorded. Then disease index and control efficiency were calculated, and variance analysis was carried out. [Results] 300 or 450 ml/hm^2 azoxystrobin ＋ difenoconazole 325 g/L SC had better control efficiency to rice sheath blight and had no phytotoxicity effect, we should use it at the initial disease stage and continuously spray 2-3 times. [Conclusion] The experiment provided a theoretical basis for controlling rice sheath blight using fungicides.

Estimate of CH_4 Emissions from Year-Round Flooded Rice Fields During Rice Growing Season in China

A special kind of rice field exists in China that is flooded year-round. These rice fields have substantially large CH4 emissions during the rice-growing season and emit CH4 continuously in the non-rice growing season. CH4 emission factors were used to estimate the CH4 emissions from year-round flooded rice fields during the rice-growing season in China. The CH4 emissions for the year-round flooded rice fields in China for the rice growing season over a total area of 2.66 Mha were estimated to be 2.44 Tg CH…

Nitrogen Losses from Flooded Rice Field

A field microplot experiment was conducted during the tillering stage of paddy rice to investigate nitrogen(N) Iosses from flooded rice fields following fertilizer application. After application of ammonium bicarbonate,most of nitrogen in the floodwater was present as NH4-N and its concentration varied widely with time.Concentrations of both NO3-N and NO2-N in the floodwater were low due to the weakened nitrification.Under flooded anaerobic reducing conditions, soil solution concentrations of NO3-N and NH4-N were nothigh, ranging from 0.6 mg L-1 to 4.8 mg L-1, and decreased with soil depth. However, the groundwater wasstill contaminated with NO3-N and NH4-N. Rainfall simulation tests showed that the N losses via runoff inrice fields were closely related to the time intervals between fertilizer applications and rainfall events. Whena large rain fell for a short period after fertilizer application, the N losses via runoff could be large, whichcould have a considerable effect on surface water quality. Both irrigation and N fertilizer application mustbe controlled and managed with great care to minimize N losses via runoff from agricultural land.

Phosphorus Concentration and Forms in Surface and Subsurface Drainage Water from Wetland Rice Fields in the Shaoxing Plain

Phosphorus (P) is the limiting factor for eutrophication in most freshwater ecosystems. In China, Ptransported from intensively cultivated land has been reported as an important source of P in surface waters.In this study, we investigated P concentration and forms in surface and subsurface drainage from wetland ricefields in the Shaoxing plain, Zhejiang Province, China. From selected rice fields, surface drainage sampleswere collected at rice-growing, non-growing and fertilization periods, and subsurface drainage samples atdrought and rewetting (irrigation or precipitation after 5～10 d drought period in the surface soils) and wet(drainage under long-term wet soil condition) periods. Water samples were characterized for their totalreactive P (TRP), dissolved reactive P (DRP) and particulate reactive P (PRP). Concentrations of the TRPand DRP in the surface drainage ranged from 0.08 to 1.50 and 0.06 to 1.27 mg L-1, respectively. The TRPand DRP were dependent on field operation activities, and decreased in the order of fertilization period ＞rice-growing period ＞ non-growing period. Phosphorus concentration of runoff receiving P fertilizer can bean environmental concern. The PRP concentration in the surface drainage, ranging from 0.01 to 0.57 mgL-1, accounted for 8%～78% of the TRP. Concentration of the TRP in the subsurface drainage was from0.026 to 0.090 mg L-1, consisting of 29%～90 % of the DRP and 10%～71% of the PRP. In the droughtand rewetting period, the PRP accounted for, on average, 63% of the TRP, much higher than in the wetperiod (23%), suggesting that there was transport of P in preferential flow during drainage events after ashort-term drought period in the surface soils. Therefore, P losses in particulate form may be importantin the subsurface drainage from rice fields when surface soils form cracks and favor rapid flow downwardthrough the soil profiles, suggesting the important role of water-dispersible colloid particles in mediating andco-transporting P in the subsurface drainage of rice fields.

Field effect of Cnaphalocrocis medinalis granulovirus (CnmeGV) on the pest of rice leaffolder

Rice leaffolder,Cnaphalocrocis medinalis(Guenée),has become a major pest throughout the rice cultivating areas of China and caused severe damage to rice production.Cnaphalocrocis medinalis granulovirus(CnmeGV),a naturally occurring baculovirus,is revealed as a potential microbial agent for the pest control.Field applications of CnmeGV were conducted against rice leaffolder larvae in rice paddies.CnmeGV infected the larvae not only in the current generation but also in the successive generation,resulting in a sustained infection in the larva population for at least 48 days.Under diferent concentrations of CnmeGV(7.5×1011 and 1.125×1012 occlusion body(OB)ha-1)at 30 days after spraying,larval population reduced up to 76.32%and rice leaf rolled rate kept in 15.42%.Simultaneously,CnmeGV had no impact on arthropod predators of C.medinalis,with abundances ranging from 2.39 to 3.79 per ten hills.These results revealed that CnmeGV is suitable as a bio-pesticide for rice leaffolder management in rice paddies.

N_2O Emission from Paddy Field under Different Rice Planting Modes

Measurements of N2O emissions from conventional rice cultivation （CRC）, CRC with straw mulching, system of rice intensification （SRI） and SRI with plastic film mulching were conducted through static chamber/gas-chromatography techniques. The results show that daily fluctuation of N2O emissions in jointing stage are much higher than in others. A type peak of N2O seasonal emission presented between jointing and bearing stages companying with high daily average temperature and low precipitation. Biomass and leaf stomatal conductance were observed. Total quantities of N2O emission were budgeted. The results showed that after jointing stage the average N2O emission flux of SRI with plastic film mulching increased significantly than CRC with straw mulching and SRI, the leaf stomatal conductance of those showed the same trend （p〈 0.05）. Yield and total quantity of N20 emission in CRC with straw mulching enhanced 13. 7% and 10.7% compared with those of CRC, respectively. The total quantity of N20 emissions reduced 3. 6% in SRI with plastic film mulching compared with CRC, however, the yield increase of that was not significant.

Model for Methane Emission from Rice Fields and Its Application in Southern China

A process model has been developed. The model has been used to calculate the methane emission from rice fields. The influence of climate conditions, field water management, organic fertilizers and soil types on methane emission from rice fields are considered. There are three major segments which are highly interactive in nature in the model:rice growth, decomposition of soil organic matter and methane production, transport efficiency and methane emission rate. Explicit equations for modeling each segment mentioned above are given. The main results of the model are: 1. The seasonal variation of methane emission of the model output agrees with that of field experiments. The deviation of seasonal average methane emission rate between modeled value and experimental data is about 10%. 2. In the whole rice growing period, model output is similar to experimental data in the seasonal variation of transport ability of rice plant. 3. Soil organic matter content and soil physics and chemistry are major factors that determine the total season average emission rate, while soil temperature controls the temporal variation of methane emission from rice fields.

Control Efficiency and Crop Safety of 20% Cyhalofop-butyl WP on Grass Weeds in Direct Seeded Rice Fields

The purpose was to define the control efficiency and safety of 20% cy- halofop-butyl WP on grass weeds in direct seeded rice fields, in order to provide the basis for chemical weeding. This study measured Leptochloa chinensis（L.） Nees, Echinoch/oa crusgalli （L.） Beauv and other gramineous weed control efficiency with four concentrations of 20% cyhalofop-dutyl WP and 100 g/L cyhalofop-dutyl EC in direct seeded rice fields, and analyzed the yield-increasing effect and safety of rice. The results showed that 20% cyhalofop-butyl WP had a good control efficiency on grass weeds such as Leptochloa chinensis（L.） Nees, Echinoch/oa crusgalli（L.） Beauv and other grasses. The effective dosage of 90-150 g/hm2 was over 90.7% on Lep- foch/oa chinensis（L.） Nees and the comprehensive control effect of the grass weeds was above 86.7%, which was basically consistent with 100 g/L cyhalofop-dutyl EC. Furthermore, 20% cyhalofop-dutyl WP was high security for direct seeded rice fields. The yield of rice was increased by 10.18%-11.22% after spraying herbicide. There- fore, 20% of cyhalofop-dutyl WP can be used as a special herbicide for controlling Leptochloa chinensis（L.） Beauv in direct seeded rice fields, and has a good applica- tion prospect.

Greenhouse Gas Emissions from Northeast China Rice Fields in Fallow Season

CH4, N2O and CO2 emissions from northeast Chinese rice fields were measured in the fallow season （November to March） to investigate the effects of freezing-thawing on the emissions. Both CH4 emission from and atmospheric CH4 oxidation by the soil occurred, but the flux was small. During the fallow season, rice fields acted as a minor source of atmospheric CH4, which accounted for about 1% of the CH4 emission during the rice growing period. The field was also a substantial source of atmospheric N20, which ranged between 40 to 77 mg m-2 and eu=counted for 40%-50% of the annual N20 emission. The largest N20 flux was observed in the thawing period during the fallow season. Laboratory incubation tests showed that the largest N20 flux came from the release of N20 trapped in frozen soil. Tillage and rice straw application （either mulched on the soil surface or incorporated in the soil） stimulated the CH4 and CO2 emissions during the fallow season, but only straw application stimulated N2O emission substantially.

Field identification of morphological and physiological traits in two special mutants with strong tolerance and high sensitivity to drought stress in upland rice(Oryza sativa L.)

The two mutants idr1-1 and 297-28, which were obtained from the radiation mutation of HD297 and IAPAR9, were used as experimental materials in this study for a 2-year(2012 and 2013) experiment about field drought resistance identification in Beijing, China. Key agronomic traits and water-related physiological indexes were observed and measured, including the leaf anti-dead level(LADL), days to heading, plant height, setting percentage, aboveground biomass, leaf water potential(LWP), net photosynthetic rate(Pn) and transpiration rate. The results showed that the mutant idr1-1 that was under drought stress(DS) conditions for 2 years had the highest LADL grades(1.3 and 2.0) among all the materials, and they were 2–3 grades stronger than the wild-type IAPAR9 with an average that was 21.4% higher for the setting percentage than the wild type. Compared with the IAPAR9 for the 2-year average delay in the days to heading and the reduction rates in the plant height, setting percentage, and aboveground biomass under DS compared with the well-watered(WW) treatment, idr1-1 showed 3.2% less delay and 19.1, 16.4, and 6.1% less reduction, respectively. The idr1-1 in the LWP always exhibited the highest performance among all the materials. The Pn of idr1-1 under severe and mild DS comparing with that under WW was slightly decreased and even slightly increased, respectively, leading to an average reduction rate of only 0.92%, which was 26.93% less than that of IAPAR9. Under the severe DS, idr1-1 still showed the highest value of 16.88 μmol CO2 m–2 s–1 among all the materials and was significantly higher than that of IAPAR9(11.66 μmol CO2 m–2 s–1). Furthermore, only idr1-1 had the increased and the highest transpiration rate values(7.6 and 6.04 mmol H2 O m–2 s–1) under both mild and severe DS compared with the values under WW, when the transpiration rate of all the other materials significantly decreased. By contrast, the 297-28 in terms of the LADL grade under DS was the lowest(7.0), and it was four grades weaker than its wildtype HD297 and even one grade weaker than the drought-sensitive paddy rice SN265. For the 2-year average reduction rates in aboveground biomass and plant heights under DS compared with those under the WW, 297-28 was 31.6 and 31.8% higher than HD297, respectively. Meanwhile, 297-28 showed the worst performance for the LWP, Pn, and transpiration rate. These results suggest that idr1-1 might be a superior drought tolerant mutant of upland rice found in China. It has a strong ability to maintain and even enhance leaf transpiration while maintaining a high plant water potential under DS, thus supporting a high Pn and alleviating the delay in agronomic trait development and yield loss effectively. 297-28 is a much more highly drought-sensitive mutant that is even more sensitive than paddy rice varieties. The two mutants could be used as drought tolerance controls for rice germplasm identification and the drought resistant mechanism studies in the future. idr1-1 is also suitable for breeding drought-tolerant and lodging-resistant high-yield rice varieties.