Effects of Tiller-inhibitor on Growth and Yield Formation of Super Early Hybrid Rice Jinyou458

[Objective]The paper was to study the effect of tiller-inhibitor on the growth and yield formation of super early rice Jinyou 458.[Method] Two treatments including spraying tiller-inhibitor（TI） and spraying water（CK） at the SN-n stage were set in the test,the effect of tiller-inhibitor on yield and its components,leaf and plant morphology,field microclimate at booting stage and quality characteristics of rice population were studied.[Result] Spraying tiller-inhibitor could effectively reduce the occurrence of invalid and inefficient tillers,increase the proportion of high effective tillers（tiller with 4 or more leaves） in tiller composition at the maximal tiller stage.The panicle length,spikelets per panicle,spikelets density,number of secondary branches and the secondary spikelets,seed setting rate of rice plant sprayed with tiller-inhibitor were significantly higher than CK.Meanwhile,spraying tiller-inhibitor could increase plant height,biomass weight and leaf area index of rice population during middle and late stages of rice growing,improve the leaf temperature of top three leaves and the light transmittance of rice population at the booting stage,increase the leaf SPAD value,thus enhance photosynthetic capacity of rice plants.It also showed the characteristics of elongating the internode of rice plant and decreasing the stem thickness,etc.[Conclusion] The panicle-bearing rate of stems and tillers,effective panicles,spikelets per panicle,seed setting rate and 1 000-grain weight had coordinately increased after spraying tiller-inhibitor,thus increased the yield.

Relationship between Leaf Area Index and Yield of Hybrid Rice

In the research, 20 indica hybrid rice varieties were investigated under high-dense and low-amount, mid-dense and mid-amount and low-dense and high-amount fertilization conditions in order to explore correlation between LAI in ful heading stage with yield and to analyze yield differences, yield components, material accumulation and transferring, as wel as correlation between key cultivation factors and yield. Final y, approaches and key technology for yield breakthrough were dis-cussed, which provides theoretical and technological references for rice high yield.

Combining Ability Analysis on Yield Traits of Recombinant Inbred Lines in Hybrid Rice

A population of 140 recombinant inbred lines at F8 generation were obtained after seven successive generations of self-pollination using single seed descent（SSD） method from the F2 hybrids of three-line restorers Luhui 8258 with high combining ability and Yanghui 34. Then, the 140 inbred lines obtained above and their parents Luhui 8258 and Yanghui 34 were crossed with three different types of cyto-plasmic male sterile（CMS） lines（Gang 46 A, Ⅱ-32 A and Lu 98A） according to NCⅡ design. The resulting 426 combinations were planted at Deyang and Suining bases to test the combining ability and inheritance of five yield traits： yield per plant, panicle number per plant, filled grain number per panicle, seed setting rate and 1 000-grain weight. The results showed that the variances of both general and specific combining abilities of the five traits all reached a significant or extremely significant level at the two sites. The broad and narrow heritability of the yield traits（except 1 000-grain weight whose broad and narrow heritability were both over70%） were all below 50% at the two experimental bases, suggesting that the four traits were easily subjected to environment influence. Very significant positive correlation of general combining ability was found between yield per plant and other traits except 1 000-grain weight. The general combining ability variance showed a normal distribution among the recombinant inbred lines at two sites, right in line with inheritance of quantitative traits. So, the genes controlling rice general combining ability can be targeted by QTL mapping.

Effects of Specific Gravity-based Seed Grading on Seed Germination, Seedling Emergence and Grain Yield of Hybrid Rice

Using the commercial seeds of two hybrid rice varieties including Lu- liangyou 996 and Liangyoupeijiu as the materials, four specific gravity-based seed grading treatments, Le., the specific gravity of 〈1.0 （T1）, 1.0-1.09 （T2）, 1.1-1.19（T3） and ≥1.2 （T4）, by selection with different saline solutions, and the control without seed grading （CK） were designed to study the effects of seed grading on seed germination, seedling emergence, seedling quality and grain yield. The results showed that the treatments of T2, T3 and T4 had higher or significantly higher seed germination rate, germination index and vigor index, seedling emergence rate and adult seedling rate than the CK, while T1 had significantly lower values of these traits than the CK. Compared with the CK, the number of spikelets per pani- cle was found to be the main reason for the yield increase of these treatments with high seed viability.

Effects of Nitrogen Rate on Blast Resistance and Grain Yield of Super Early Hybrid Rice Luliangyou 996

Super early hybrid rice Luliangyou 996 was the materials for this experiment. This paper focused on the influences of different nitrogen rates on blast resistance and grain yield. Studies suggested that with the increase of nitrogen rate,there were no distinct changes in the sick grain rate and sickness index, but the fifth grade of panicle blast rate rose gradually. When the nitrogen was too much（270 kg/hm^2）, the sick grain rate and sickness index were the highest. The yield rose along with the addition of nitrogen if the amount of applied nitrogen was within the range of 0 to 180 kg/hm^2, and the yield would decrease with the addition of nitrogen if the amount of applied nitrogen was over 180 kg/hm^2. Considering the sickness, output and economic benefit of rice, it is better to applied 90 to 135 kg/hm^2 of nitrogen for Luliangyou 996.

Comparative Analysis of Main Characters of New Hybrid Rice Combinations with Different Yield Levels

The main characters and yield of 22 new indica hybrid rice combinations were analyzed in order to sort out the common characteristics in the performance of main characters of high-yielding varieties （combinations）.The results showed at the confidence level of 95%, the whole growth pedod ranged from 131.3 to 137.3 d, the effective ear number ranged from 2 495 000 to 3 179 000 ears/hm2, the plant height was in the range of 119.5-131.7 cm, the average ear length ranged from 24.6 to 27.2 cm, the spikelet number per panicle was in the range of 164.2-225.4, the seed setting percentage ranged from 69.4% to 82.3%, and the 1 000-grain weight was in the range of 26.9-31.2 g. The correlation analysis between single traits and yield of rice varieties （combinations） showed that in order to achieve high yield, the medium indica hybrid rice variety with moderate growth period, higher plant height or biological yield, larger panicle and bigger kernel should be selected. In addition, the maximum tiller number should be controlled and adjust- ed, and the spike rate should be improved to ensure sufficient effective ears in the cultivation process.

Effect of Different Fertilization Methods on Grain Yield and Nitrogen Agronomy Efficiency of TwoLine Super Hybrid Rice

ln order to explore the effect of broadcast application of urea （BR-U） and surface concentrated-fertilization on grain yield and nitrogen agronomy efficiency of two-line super hybrid rice, this experiment was conducted with super hybrid rice Y-Liangyou 1 and Liangyou 0293 to determine til ering dynamics, SPAD, LAl and dry matter accumulation. lts total N application was 180 kg/hm2 and different application rate and topdressing methods were set respectively. The results showed that grain yield and nitrogen agronomy efficiency were significantly improved with the T3 and T4 nitrogenous fertilizer application model. The improving of grain yield and nitrogen agronomy efficiency was benefited from rational postponing N application. However, neither single application of base fertilizer （T2） nor single application of topdressing fertilizer （T5 and T6） can coordinate the relationship between source and sink. Both grain yield and nitrogen agronomy efficiency were insignificantly under the broadcast application of urea and surface concentrated fertilization.

The Principal Component Analysis on Yielding and Agronomic Traits of Hybrid Rice of Liangyou 2111

In order to define the relationship between yield and important agronomic traits of two lines hybrid Uangyou 2111, the principal component analysis method was used to analyze the expadmental data of six test points in Yunnan Province. The results showed that the main factors influencing the production of Liangyou 2111 were grain number, grains seed number, panicle length, growth padod and panicle rate; then were 1 O00-grain weight, seed setting rate, effective panicle and highest stem tillers number; again was plant height. Therefore, when hybrid rice of Uangyou 2111 will be planted widely in yunnan province, we should focus on en- sudng the panicle traits, especially increase grain number and grain seed number, and coordinately develop other traits to achieve high yield.

Relationship Between Grain Yield and Yield Components in Super Hybrid Rice

Chinese super hybrid rice breeding project has developed many new varieties with great yield potential. It is controversial which yield component should be emphasized in super hybrid rice production. The present study was conducted to compare super hybrid rice with common hybrid and super inbred rice and analyze contributions of yield components to grain yield of super hybrid rice under experimental conditions, and evaluate relationships between grain yield and yield components of super hybrid rice in farmer’s paddy fields. Field experiments were done in Changsha, Guidong, and Nanxian, Hunan Province, China, from 2007 to 2009. Eight super hybrid varieties, one common hybrid variety, and one super inbred variety were grown in each location and year. Rice production investigation was undertaken in high-yielding （Guidong）, moderate-yielding （Nanxian）, and low-yielding （Ningxiang） regions of Hunan Province, China, in 2009. Grain yield and yield components were measured in both the field experiments and rice production investigation. Super hybrid rice varieties outyielded common hybrid and super inbred varieties across three locations and years. Yield potential has been increased by 11.4% in super hybrid rice varieties compared with common and super inbred varieties. The higher yield of super hybrid varieties was attributed to improvement in panicle size. Panicles per m2 had the highest positive contribution to grain yield with the exception under yield level of 10.0 to 12.0 t ha-1, and was positively related to grain yield in farmer’s field at all of the high-, moderate-, and low-yielding regions. Our study suggests that panicle per m2 ought to be emphasized in super hybrid rice production.

Yield potential and stability in super hybrid rice and its production strategies

China＇s Super Hybrid Rice Breeding Program has made significant progress over the past two decades. In this paper, we reviewed our studies on the yield potential and stability in super hybrid rice and discussed the strategies for super hybrid rice production. The results of our studies show that rice yield potential has been increased by 12% in super hybrid cultivars as compared with ordinary hybrid and inbred cultivars. The higher grain yields in super hybrid rice cultivars are attributed to larger panicle size coupled with higher biomass production or higher harvest index. However, grain yields in super hybrid rice cultivars vary widely among locations depending on soil and climatic factors. Therefore, it is important to tailor target yield to local conditions in super hybrid rice production. The target yield for super hybrid rice production can be determined by the average yield method or the regression model method. Improving soil quality is critical to achieving the target yield in super hybrid rice production. Favorable crop rotations such as rice-oilseed rape and novel soil management practices, such as biochar addition, are effective approaches to improve soil quality. It is needed to develop simplified cultivation tech- nologies for super hybrid rice to meet the changes in socioeconomic environments during the period of transition. There are such technologies as no-tillage direct seeding and mechanized transplanting at high hill density with single seedling per hill.

Suitable growing zone and yield potential for late-maturity type of Yongyou japonica/indica hybrid rice in the lower reaches of Yangtze River, China

Late-maturity type of Yongyou japonica/indica hybrids series （LMYS） have shown great yield potential, and are being widely planted in the lower reaches of Yangtze River, China. Knowledge about suitable growing zone and evaluation of yield advantage is of practicall importance for LMYS in this region. Fifteen LMYS, two high-yielding inbred japonica check varieties （CK-J） and two high-yielding hybrid indica check varieties （CK-I） were grown at Xinghua （119.57°E, 33.05°N） of Lixiahe region, Yangzhou （119.25°E, 32.30°N）of Yanjiang region, Changshu （120.46°E, 31.41°N）of Taihu Lake region, and Ningbo （121.31°E, 29.45°N） of Ningshao Plain in 2013 and 2014. The results showed that maturity dates of the 15 were later than the secure maturity date at Xinghua and 6, 14 and 15 LMYS were mature before the secure maturity date at Yangzhou, Changshu and Ningbo, respectively. One variety was identified as high-yielding variety among LMYS （HYYS） at Yangzhou, 8 HYYS in 201：3 and 9 HYYS in 2014 at Changshu, 9 HYYS at Ningbo. HYYS here referred to the variety among LMYS that was mature before the secure maturity date and had at least 8% higher grain yield than both CK-J and CK-I at each experimental site. Grain yield of HYYS at each experimental site was about 12.0 t ha-1 or higher, and was significantly higher than CK varieties. High yield of HYYS was mainly attributed to larger sink size due to more spikelets per panicle. Plant height of HYYS was about 140 cm, and was significantly higher than check varieties. Significant positive correlations were recorded between duration from heading to maturity stage and grain yield, and also between whole growth periods and grain yield. HYYS had obvious advantage over check varieties in biomass accumulation and leaf area duration from heading to maturity stage. Comprehensive consideration about safe matudty and yield performance of LMYS at each experimental site, Taihu Lake region （representative site Changshu） and Ningshao Plain （representative site Ningbo） were thought suitable growing zones for LMYS in the lower reaches of Yangtze River. The main factors underlying high yield ofHYYS were larger sink size, higher plant height, longer duration from heading to maturity stage and whole growth periods, and higher biomass accumulation and leaf area duration during grain filling stage.

Effects of Plant Density and Nitrogen Application Rate on Grain Yield and Nitrogen Uptake of Super Hybrid Rice

The nitrogen uptake, yield and its components for two super-high-yielding hybrid rice combinations, Guodao 6 and Eryou 7954 were investigated under different plant densities （15, 18, and 21 plants/m^2） and different nitrogen application rates （120, 150, 180, and 210 kg/hm^2）. The experiment was conducted on loam soil during 2004-2006 at the experimental farm of the China National Rice Research Institute in Hangzhou, China. In these years, the two hybrid rice cleady showed higher yield at a plant density of 15 plants/m^2 with a nitrogen application rate of 180 kg/hm^2. Guodao 6 produced an average grain yield of 10 215.6 kg/hm^2 across the three years, while the yield of Eryou 7954 was 9 633.0 kg/hm^2. With fewer plants per unit-area and larger plants in the plots, the two hybrid rice produced more panicles per plant in three years. The highest nitrogen uptake of the two hybrid rice was at a plant density of 15 plants/m^2 with a nitrogen application rate of 180 kg/hm^2. Further increasing nitrogen application rate was not advantageous for nitrogen uptake in super-high-yielding rice under the same plant density.

Why high grain yield can be achieved in single seedling machine-transplanted hybrid rice under dense planting conditions?

This study was conducted to identify the factors associated with high grain yield in single seedling machine-transplanted hybrid rice under dense planting conditions. Field experiments were done in Yong＇an Town, Hunan Province, China in 2015 and 2016. Two hybrid rice cultivars were grown under single seedling machine transplanting （SMT） and conventional machine transplanting （CMT） at a high planting density in each year. Grain yield and yield attributes were compared between SMT and CMT. Averaged across cultivars and years, grain yield was 12％ higher under SMT than under CMT. Plant height, basal stem width, and shoot and root dry weights were higher in seedlings for SMT than those for CMT. SMT had less maximum tiller number per m2 and consequently less panicle number per m2 than did CMT. Branch number per panicle, especially the secondary branch number per panicle, and spikelet number per cm of panicle length were more under SMT than under CMT, which resulted in more spikelet number per panicle under SMT than under CMT. SMT had higher or equal spikelet filling percentage than did CMT. The difference in grain weight between SMT and CMT was relatively small and inconsistent cross years. SMT had higher or equal total biomass and harvest index than did CMT. Dry weight per stem under SMT was heavier than that under CMT. Larger leaf area per stem was partly responsible for the heavier dry weight per stem under SMT than under CMT. Our study suggests that improvement in seedling quality, panicle size, and dry weight per stem are critical to the high grain yield in single seedling machine-transplanted hybrid rice under dense planting conditions.

Effect of Nitrogen Regimes on Grain Yield,Nitrogen Utilization,Radiation Use Efficiency,and Sheath Blight Disease Intensity in Super Hybrid Rice

Poor nitrogen use efficiency in rice production is a critical issue in China. Site-specific N managements （SSNM） such as real-time N management （RTNM） and fixed-time adjustable-dose N management （FTNM） improve fertilizer-N use efficiency of irrigated rice. This study was aimed to compare the different nitrogen （N） rates and application methods （FFP, SSNM, and RTNM methods） under with- and without-fungicide application conditions on grain yield, yield components, solar radiation use efficiency （RUE）, agronomic-nitrogen use efficiency （AEN）, and sheath blight disease intensity. Field experiments were carried out at Liuyang County, Hunan Province, China, during 2006 and 2007. A super hybrid rice Liangyou 293 （LY293） was used as experimental material. The results showed that RTNM and SSNM have great potential for improving agronomic-nitrogen use efficiency without sacrificing the grain yield. There were significant differences in light interception rate, sheath blight disease incidence （DI） and the disease index （ShBI）, and total dry matter among the different nitrogen management methods. The radiation use efficiency was increased in a certain level of applied N. But, the harvest index （HI） decreased with the increase in applied N. There is a quadratic curve relationship between grain yield and applied N rates. With the same N fertilizer rate, different fertilizer-N application methods affected the RUE and grain yield. The fungicide application not only improved the canopy light interception rate, RUE, grain filling, and harvest index, but also reduced the degree of sheath blight disease. The treatment of RTNM under the SPAD threshold value 40 obtained the highest yield. While the treatment of SSNM led to the highest nitrogen agronomic efficiency and higher rice yield, and decreased the infestation of sheath blight disease dramatically as well. Nitrogen application regimes and diseases control in rice caused obvious effects on light interception rate, RUE, and HI. Optimal N rate is helpful to get higher light interception rate, RUE, and HI. Disease control with fungicide application decreased and delayed the negative effects of the high N on rice yield formation. SSNM and RTNM under the proper SPAD threshold value obtained high-yield with high efficiency and could alleviate environmental pollution in rice production.

Grain yield,nitrogen use efficiency and physiological performance of indica/japonica hybrid rice in response to various nitrogen rates

Utilizing the heterosis of indica/japonica hybrid rice(IJHR)is an effective way to further increase rice grain yield.Rational application of nitrogen(N)fertilizer plays a very important role in using the heterosis of IJHR to achieve its great yield potential.However,the responses of the grain yield and N utilization of IJHR to N application rates and the underlying physiological mechanism remain elusive.The purpose of this study was to clarify these issues.Three rice cultivars currently used in rice production,an IJHR cultivar Yongyou 2640(YY2640),a japonica cultivar Lianjing 7(LJ-7)and an indica cultivar Yangdao 6(YD-6),were grown in the field with six N rates(0,100,200,300,400,and 500 kg ha^(-1))in 2018 and 2019.The results showed that with the increase in N application rates,the grain yield of each test cultivar increased at first and then decreased,and the highest grain yield was at the N rate of 400 kg ha^(-1)for YY2640,with a grain yield of 13.4 t ha^(-1),and at 300 kg ha^(-1)for LJ-7 and YD-6,with grain yields of 9.4–10.6 t ha^(-1).The grain yield and N use efficiency(NUE)of YY2640 were higher than those of LJ-7 or YD-6 at the same N rate,especially at the higher N rates.When compared with LJ-7 or YD-6,YY2640 exhibited better physiological traits,including greater root oxidation activity and leaf photosynthetic rate,higher cytokinin content in the roots and leaves,and more remobilization of assimilates from the stem to the grain during grain filling.The results suggest that IJHR could attain both higher grain yield and higher NUE than inbred rice at either low or high N application rates.Improved shoot and root traits of the IJHR contribute to its higher grain yield and NUE,and a higher content of cytokinins in the IJHR plants plays a vital role in their responses to N application rates and also benefits other physiological processes.

Relationship of Parental Indica-Japonica Indexes with Yield and Grain Quality Traits of Japonica Hybrid Rice in Northern China

Taking the main parents （10 male sterile lines and 10 restorer lines） and their 100 combinations of japonica hybrid rice in northern China as materials, the relationships of parental indica-japonica indexes determined by the methods of the Cheng’s index as well as simple sequence repeat （SSR） markers with yield and grain quality traits of hybrid rice were studied. For the parents, the Cheng’s index （Chi） ranged from 13.5 to 19.3 and the indica index in SSR markers （ADi） were from 0.12 to 0.38. The classification of parents by Chi was not completely consistent with that by ADi. The Chi of male parent was more closely related to hybrid traits than that of female parent, as contrasted to ADi. At the same time, the difference between parents （PD） in Chi was more closely related to hybrid traits than that in ADi. The indica-japonica indexes of parents and their difference between parents appeared quadratic relationship to hybrid traits with the critical extremum. The directions of the correlation of indica-japonica indexes of parents and their differences with hybrid yield traits were on the opposition to those with hybrid grain quality traits. Therefore, the female parent should match the male parent moderately in indica-japonica index to obtain the optimum of hybrid traits, high yield as well as good quality.

Geographic variation in the yield formation of single-season high-yielding hybrid rice in southern China

Environmental conditions greatly affect the growth of rice. To investigate the geographic differences in yield formation of single-season high-yielding hybrid rice in southern China, experiments were conducted in 2017 and 2018 in the upper and middle–lower reaches of the Yangtze River with 10–30 main locally planted high-yielding hybrid cultivars used as materials. Compared with rice planted in the middle–lower reaches of the Yangtze River, rice planted in the upper reaches has a longer tillering duration, higher accumulated temperature(≥10℃) during tillering period, but lower accumulated temperature and solar radiation from initial booting to maturity. Yield traits comparison between the upper and the middle–lower reaches of Yangtze River showed that the former had 48.1% more panicles per unit area while the latter had 46.4% more grains per panicle;the rice yield in the former was positively correlated with the seed setting rate and the dry matter accumulation before heading, while the latter was positively correlated with grains per panicle and dry matter accumulation from booting to maturity. Comparison of the same variety Tianyouhuazhan planted in different regions showed there was a significant positive correlation between panicle number and the duration of and accumulated temperature during the tillering period(r=0.982^(**), r=0.993^(**), respectively), and between grains per panicle and accumulated solar radiation during booting period(r=0.952~*). In the upper reaches of the Yangtze River, more than 90% of cultivars with an yield of greater than 11 t ha^(–1) had an effective panicle number of 250–340 m^(–2), and there was a significant negative correlation between seed setting rate and grains per panicle;therefore, the high-yielding rice production in these regions with a long effective tillering period(>40 d) should choose varieties with moderate grains per panicle, adopt crop managements such as good fertilizer and water measures during vegetative growth period to ensure a certain number of effective panicles, and to increase the dry matter accumulation before heading. While in regions with a short effective tillering period(<20 d) but good sunshine conditions during the reproductive growth period, such as the middle–lower reaches of the Yangtze River, high-yielding rice production should choose cultivars with large panicles, adopt good water and fertilizer managements during the reproductive growth period to ensure the formation of large panicles and the increase of dry matter accumulation after heading.

Combining Ability Analysis for Yield and Yield Components of Yinshui-type Japonica Hybrid Rice

[Objective] This study aimed to analyze the yield potential of newly developed Yinshui-type japonica hybrid rice parents and provide the theoretical basis for breeding of Yinshui-type japonica hybrid rice. [Method] An incomplete diallel cross of four cytoplasmic male sterile （CMS） lines and four restorer lines was designed to investigate the combining ability for yield and yield components of Yinshui-type japonica hybrid rice. [Result] The variances of general combining ability （GCA） for yield traits were extremely significantly different. GCA variances of male parents were higher than that of female parents except for seed-setting rate. The variances of specific combining ability （SCA） for grain yield per plant, number of effective spikes and seed-setting rate were significantly or extremely significantly different. 139A and Wanlun 422 exhibited higher GCA effects. Most yield traits of these two parents be- longed to type II. Combinations 139AxWanlun 422 and Liaojingl0AxWanlun 422 had relatively high grain yield per plant and SCA effects. GCA effects of restorer lines and SCA effects of combinations greatly affected yield traits, while GCA ef- fects of CMS lines slightly affected yield traits. [Conclusion] In this study, restorer lines exhibited greater genetic differences and impact on yield traits than CMS lines; 139A and Wanlun 422 possessed higher application potential.

Irrigating with cooler water does not reverse high temperature impact on grain yield and quality in hybrid rice

Rice grain yield and quality are negatively impacted by high temperature stress.Irrigation water temperature significantly affects rice growth and development,thus influencing yield and quality.The role of cooler irrigation water in counteracting high temperature induced damages in rice grain yield and quality are not explored.Hence,in the present study two rice hybrids,Liangyoupeijiu(LYPJ)and IIyou 602(IIY602)were exposed to heat stress and irrigated with water having different temperatures in a splitsplit plot experimental design.The stress was imposed starting from heading until maturity under field-based heat tents,over two consecutive years.The maximum day temperature inside the heat tents was set at 38℃.For the irrigation treatments,two different water sources were used including belowground water with cooler water temperature and pond water with relatively higher water temperature.Daytime mean temperatures in the heat tents were increased by 1.2–2.0℃ across two years,while nighttime temperature remained similar at both within and outside the heat tents.Cooler belowground water irrigation did have little effect on air temperature at the canopy level but decreased soil temperature(0.2–1.4℃)especially under control.Heat stress significantly reduced grain yield(33%to 43%),panicles m^(-2)(9%to 10%),spikelets m^(-2)(15%to 22%),grain-filling percentage(13%to 26%)and 1000-grain weight(3%to 5%).Heat stress significantly increased chalkiness and protein content and decreased grain length and amylose content.Grain yield was negatively related to air temperature at the canopy level and soil temperature.Whereas grain quality parameters like chalkiness recorded a significantly positive association with both air and soil temperatures.Irrigating with cooler belowground water reduced the negative effect of heat stress on grain yield by 8.8%in LYPJ,while the same effect was not seen in IIY602,indicating cultivar differences in their response to irrigation water temperature.Our findings reveal that irrigating with cooler belowground water would not significantly mitigate yield loss or improve grain quality under realistic field condition.The outcome of this study adds to the scientific knowledge in understanding the interaction between heat stress and irrigation as a mitigation tool.Irrigation water temperature regulation at the rhizosphere was unable to counteract heat stress damages in rice and hence a more integrated management and genetic options at canopy levels should be explored in the future.

Grain Yield and Nitrogen Use Efficiency of Hybrid Rice in Response to High Plant Density and Nitrogen Rate

Increased plant density with low N rate was a recommended strategy to increase grain yield and N use efficiency(NUE);however,grain yield,NUE and the total N uptake(TNU)responses of hybrid rice to this strategy at different yield levels(medium yielding site(MYS)Luzhou City and high yielding site(HYS)Deyang City had not been described.Field experiments with hybrid rice Rongyou1015 were conducted to study the effects of two plant densities.High plant density(HD),low plant density(LD)and four N rates(without N,N_(0);a recommended N rate of 195 kg•hm^(-2),N_(CK);a 23%reduction in N rate,N_(-23%);a 46%reduction in N rate,N_(-46%)on yield attributes,grain yield,TNU and NUE of hybrid rice were studied under different yield levels in 2016-2017.The results showed that the grain yield and NUE of hybrid rice in response to plant density and N rate varied with yield levels.For MYS,reducing N rate by 46%result in significantly lower grain yield at LD treatment;whereas at HD treatment the grain yield of hybrid rice under N_(-46%) and N_(CK) were equal.For HYS,reducing N rate by 46% result in significantly lower grain yield regardless of low plant density and high plant density;however,a reduction in N rate by 23%increased grain yield,AE_(N) by 36%,PFP_(N) by 31% and RE_(N) by 11% over N_(CK) at HD treatment.Higher grain yield of hybrid rice under the combination of HD with low N rate was attributable to improvement in spikelets per panicle and harvest index.The results suggested that high plant density with low N rate might be an effective approach to improve grain yield and NUE in rice production,but reduction in N application rate was determined,according to yield levels.