Effects Of Simulated Drought At Booting Stage On Chalkiness Formation Of Japonica Rice

With the increasing frequency and intensity of combined high temperature and water-deficit stress under climate change,physiology of drought stress has become a major research topic in crop science.High temperature and drought occur frequently in Yangtze River Basin,one of the main rice areas in China,causing serious reduction in rice yield and quality.However,current studies are focused on rice yield,with little information concerning the response of rice quality to drought and its physiological mechanisms.Chalkiness is the white opaque part of rice endosperm,being the most important appearance quality indicator and a crucial factor contributing to the market price of rice.This study is conducted to investigate the response of grain chalkiness to drought,as simulated by the application of KI solution,and to explore its physiology foundation from perspective of carbon and nitrogen metabolism.The main results are shown as follows.(1)Drought stress at booting stage had significant effect on rice yield and grain quality.Grains are grouped into six kinds of samples,i.e.grains of top primary rachis,top secondary rachis,middle primary rachis,middle secondary rachis,bottom primary rachis,and bottom secondary rachis.The results showed that grain number of primary branch and two branches increased significantly,and the seed setting rate decreased under slight drought.Drought at booting stage of rice mainly affected seed setting rate and grain weight.In addition,the grain filling rate was affected by drought stress,the dry matter accumulation rate was faster in the early stage and the later stage was slow.Severe drought stress at booting stage of rice led to a significant increase in the chalky rice rate of the superior and inferior grains.(2)Drought at booting stage affected the content of chlorophyll in leaves.The chlorophyll a and b content increased first and then decreased with the increase of filling days.The contents of chlorophyll a and b increased,and then decreased gradually 5-10 days after drought treatment.In moderate drought treatment,the chlorophyll a and b content decreased in the DAF5-10,indicating that drought stress accelerated the decomposition of chlorophyll a and b.The content of Rubisco in flag leaves decreased with the increase of the number of days.At the DAF-5 and DAF-20 and the mature period,the content of Rubisco was significantly lower than that of the control in the same period,suggestive of the accelerated hydrolysis of leaf Rubisco under drought.(3)Chalkiness occurrence under drought is partially due to the low accumulation of starch and protein in grains.Grain amylose content during drought treatment after maturity was significantly lower than that of CK,but there were no significant differences between the content of superior grain amylose and amylopectin.The total starch content of drought treatment was significantly lower than that of control treatment at 15 DAF and mature period.Drought affected significantly storage proteins of albumin,globulin and glutenin,and mainly in the early and middle filling stage.Proteins play a critical role in the adhesion between starch granules,and the low accumulation of protein is suggested as the cause of chalkiness occurence under drought.(4)The increase in chalkiness rate under drought stress may be related to the up-regulation of a-amylase-related genes in grains.SEM photos showed that a large number of micropores occurred on the surface of the starch granules in the chalky part of grains produced by severe drought.We detected the expression of seven a-amylase genes,and found that the expression of Amy 1C,Amy2A,Amy 3B were upregulated in response to drought stress,indicating the hydrolysis of starch during grain filling.In summary,we investigated the effects of drought stress on yield and quality at the booting stage of rice by simulated drought conditions.It was found that booting stage significantly reduced the number of grains per spike,seed setting rate and grain weight,causing reduction in grain yield.Chalkiness grain rate increased significantly under drought stress,which is not only related to the low accumulation of starch and protein but also the hydrolysis of starch granules by ?-amylase.