Responses Of Indica Rice Yield And Quality During Pre-And Post-Heading To Extreme Temperature

Rice?Oryza sativa L.?yield and quality are often affected by extreme temperatures?ET?and exposures during reproductive periods usually last for three to seven days.Increasing evidences indicate that ET will be more intense in coming decades and projecting more serious influences on rice.Since,significant efforts have been done to assess the effect of high or low temperatures on rice but the precise sensitive period and underlying mechanism of grain quality changes are still remain unclear.Thus a walk-in climate chamber treated experiment including two indica rice cultivars?V₁=Wanxiangyouhuazhan and V₂=Rongyouhuazhan?was planned:?i?to investigate the impacting trend of ET on yield and quality traits;?ii?to evaluate sensitive periods of yield and quality in ET and?iii?to revel the mechanism of key quality changes in ET.To achieve those objectives a pot experiment led in the farm field of Jiangxi Academy of Agricultural Science,Jiangxi province,China during July to October in 2017.Three temperature treatments–CK:20-28°C,LT:12-20°C and HT:28-36°C as the mean values of night and day temperatures–were implemented for seven days during sensitive growth periods–the last week of pre-heading,and the 1^st,2^nd and 3^rd week of post-heading period–respectively in walk-in climate chambers.The exposure of seven days ET significantly affected heading and harvest dates in rice.HT increased the plant height and decreased growth duration,while LT affected inversely on plant height and growth durations.The grain yield was reduced by 4.9%-13.1%under LT and 12.1%-15.8%under HT.The last week of pre-heading and the 1^st week of post-heading were the most sensitive periods to rice yield under ET.Total biomass and seed setting rate decreased during the last week of pre-heading?S₁?and the 1^st week of post-heading?S₂?,while 1000 grain weight decreased at the 2^nd week of post-heading by ET.ET reduced dry matter translocation,dry matter translocation efficiency and contribution of dry matter assimilation to grain up to 18.2%,19.4%and 11.9%,respectively.Harvest index also reduced under extreme temperatures.Maximum reduction in yield parameters was observed from S₂.However,HT damaged yield and yield components more severely than by LT during the last week of per-heading and the 1^st week of post heading periods.ET decreased chlorophyll content and chlorophyll fluorescence in rice flag leaves mostly at seven days after treatment?DAT?.LT decreased chlorophyll content by 14.8%and chlorophyll fluorescence by 4.1%,while HT reduced by 5.6%and3.0%,respectively.Photosynthetic parameters also significantly reduced under ET,while LT declined41.5%?photosynthetic rate?,34.3%?stomatal conductance?and 51.2%?transpiration rate?in mean across treated periods at 7 DAT.LT was more severely affected compared to HT,though both caused photosynthetic variations and contributed in rice yield and quality reductions.The milling quality of rice was significantly affected by ET,while HT caused the maximum damages in milled rice and head rice percentage during the 2^nd week of post-heading?S₃?.ET also considerably varied the appearance quality especially during S₃ period.The highest decrease in L/W ratio,and increase in chalkiness number,chalkiness in area and transparency were observed under HT,while LT decreased the chalkiness in V₂ cultivar.ET and growth periods showed varied cooking quality mostly during the S₃ period.HT decreased the peak viscosity,trough viscosity,final viscosity,breakdown,and peak time.Gelatinization temperature?GT?was decreased by LT and increased by HT in both tested cultivars.Besides,HT decreased rice amylose content and increased protein content during S₃ period,while LT increased amylose content during the 2^nd and the 3^rdd week of post-heading in V₁ cultivar.The 2^nd week of post-heading was identified as the most sensitive period for grain quality.The 2^nd week of post-heading was most sensitive period for rice amylose and protein contents thus became the focus for further molecular tests.ET changed relative gene expressions?Waxy and OsAAP6gene?responsible for amylose and protein contents in rice grain.HT decreased the relative expression of Waxy gene?75.6%to 82.6%?and increased the relative expression of OsAAP6 gene?14.4%in V₁ and187.2%in V₂?,while LT slightly increase the relative expression of Waxy gene in grain.HT and LT both reduced the activity of granule-bound starch synthase-1?GBSS1?in rice grain,while HT increased glutamate pyruvic aminotransferase?GPT?enzyme activity but LT reduced in both tested cultivars.HT noticeably increased abscisic acid?ABA?activity and reduced the salicylic acid?SA?activity in rice grain during the 2^nd week of post-heading.LT increased both ABA and SA activities,thus neutralized the impact on grain quality.These changes in gene expression,enzyme and hormonal activities during the 2^nd week of post-heading maintained a consistent relation with amylose and protein contents in rice grain.The entire findings from the study suggested that ET damaged photosynthesis and yield components to reduce the yield.The last week of pre-heading and the 1^st week of post heading were identified as the most sensitive for grain yield.HT caused the highest reduction in grain appearance,milling and cooking quality compared to LT.The 2^nd week of post-heading heading was the most sensitive period for rice grain quality.These findings will be useful as references to develop adaptation and neoteric techniques for rice sustainability in future climate changes across the world.