Investigating The Climate Adaptability Mechanisms Of Different Japonica Rice(Oryza Sativa L.)Cultivars In Northeast China

Assessment of response mechanisms of Japonica rice under different climatic conditions has an important practical and theoretical significance.Sustainable cultivation of the Japonica rice in mid-high-latitude regions requires the investigation of adaptability mechanisms at critical growth stages.Due to global warming,the Japonica-type rice has been expanded to high latitudes,while the growth phases and grain yield of single and early rice is greatly influenced from the high and low temperature stress due to greater climatic variation,how to sustain the rice planting in high-latitude areas is a big challenge.The objectives of the current research were to investigate the response mechanisms of different Japonica rice cultivars under varying climatic environments in Northeast China(NEC).Genotypic variation in duration of anthesis is considered as an escape mechanism from the temperature stress for better grain-filling,but still partially understood regarding its dependency on environmental conditions at high-latitude regions.Generally,it is assumed that panicle temperature during anthesis determines the spikelet fertility.So,assessment of the sensitivity of time of day of anthesis to changes in air temperature and other environmental variables can be an effective eco-physiological adaptive tool with the prospects for improved approaches in the assessment of thermal sterility in Japonica-type rice.Therefore,rationale of this research was to evaluate that how climate variability impacts the Japonica-type rice in term of growth,grain yield and quality to target further research and render essential insights into how a Japonica-type rice is likely to adapt in mid-high-latitude regions.Undergoing this rationale and objectives,four cultivars(late-maturing:Longdao-18 and Longdao-21,early-maturing:Longjing-21 and Suijing-18)were chosen for field experiments with Randomized Complete Block Design(RCBD)at two sites of Harbin and Qiqihar in Heilongjiang Province during 2017,2018,and 2019.Difference in climatic features of Harbin and Qiqihar showed significant phenological variations in growth durations.The overall growth duration at Qiqihar was a bit longer than Harbin.Specific leaf area(SLA)increased significantly under high night temperatures and directed,together with the high leaf dry matter under high night humidity,to high growth rates.The effect of relative humidity on leaf area and crop growth rate(CGR)was bigger than temperature effect,particularly low humidity had a significant negative impact.Maximum SLA at Harbin and Qiqihar was observed for Longdao-18 in 2018 with values of 32.88m² kg^–1 and 30.95 m² kg^–1,respectively.Physiological differences showed that mean onset of anthesis was earliest for Longdao-21 at Harbin whereas latest for Longdao-18 at Qiqihar.Maximum time to end anthesis was taken by Longdao-18 which was 9.0 hasr in 2019.Highest mean duration of anthesis was noticed in Longdao-18 which was 4.2h in 2018.The results demonstrated that high temperature at the onset of anthesis at the start of the day enhanced the escape from high temperature later during the day.Average temperature during early weeks of grain-filling was in optimum range(20–27°C)at Harbin than Qiqihar.Time to reach the maximum grain-filling for superior and inferior spikelets showed obvious variations,indicating that the difference of environmental conditions between the two places had varying degrees of influence on each cultivar.For better apprehension of the response mechanisms during grain-filling phase,the filling period was divided into 3 sub-phases,the early,middle and late.Due to optimized environmental conditions,most of the grain-filling occurred during middle period at Harbin which shared nearly 60%of the whole 45 days filling phase.Whereas,at Qiqihar,most of the grain-filling was seen at late phase of filling period as the temperature during early and middle phases was not in optimum ranges,therefore filling period prolonged at Qiqihar towards the late sub-phase.Correlative data between grain-filling and yield showed yield differences for Suijing-18,Longjing-21,Longdao-21,and Longdao-18 were 8.02%,12.78%,17.19%and20.53%higher at Qiqihar than Harbin,respectively during 2017 growing season,and the change trend in2018 and 2019 was the same as in 2017.Values for yield components including fertile tillers,1000-grain weight,and branches per panicle were higher at Qiqihar.The grain-filling parameters correlation with yield components showed that filling duration in Qiqihar was conducive to increase the yield there but main limiting factor in grain yield in Harbin was the low dry weights of inferior grains.Maximum grain yield was noticed for Longdao-18 at both Harbin and Qiqihar study sites with values of 9,500 kg ha^–1 and 13,250 kg ha^–1,respectively in 2018.Differences in quality features showed that average values of protein contents were higher at Harbin in 2017 and 2018(8.54%and 9.13%,respectively)than Qiqihar(8.34%and 9.14%),respectively.Amylose contents were also higher at Harbin in 2017 and 2018(20.03%and 22.27%,respectively)than Qiqihar(14.44%and 14.67%,respectively).Chalkiness percentage was higher at Qiqihar than Harbin which demonstrated that Harbin produced good quality rice.Different large-and small-panicles holding Japonica-type rice cultivars more often failed to accomplish their yield potential due to poor grain-filling and changes(late or early)in anthesis of superior and inferior spikelets.This study strived to evaluate the importance of investigating the morphological,physiological,and biochemical adaptability mechanisms of Japonica-type rice at mid-high-latitudes during anthesis,and grain-filling of inferior and superior spikelets.Moreover,this study is to understand that it is implemental whether alterations in adjustments of agronomic management measures could sustain the grain yield and quality for Japonica-rice at mid-high-latitude regions.Aiming to sustain the future Japonica rice production,awareness of climate-smart agriculture and optimized use of inputs is necessary.Strengthening the adjustments of technological programs to offset the negative impacts of climate variabilities is indispensable.Use of more advanced statistical techniques for deep studies integrated with mechanized approaches should be explored impacts assessment of climatic variables at mid-high-latitudes.Additionally,it is suggested that temperature-based indices over all climatic variables such as(growing degree days)GDD,and meteorological standard indices should be applied in future climate-rice studies in NEC for relationship analysis.Cold periods during critical growth phases caused chilling injuries and yield decline which suggested to necessarily have chilling injury indices in future research.Therefore,the results of this study suggested the need of have modern GDD methods in evaluation of temperature impacts on critical growth phases and to have interannual shifts in Japonica rice yield in NEC because these methods consider different threshold levels of the environmental variables such as temperature.The findings emphasize the importance of considering relative humidity and radiation hours during focusing on response mechanisms to temperature,and strongly propose that due to irregular shifts in climatic conditions especially daily temperature changes,biomass partitioning rather than biomass itself will be impacted.This study necessarily suggests to utilize the improved modeling measures in the prediction of thermal sterility for Japonica-type rice.