Diurnal Variations Of The Temperature Difference Between Superior And Inferior Spikelet Of Japonica Rice

Heat stress has been one of the major problems facing rice production,causing both quantitative and qualitative losses.Therefore,it is imperative to elucidate the eco-physiological mechanism underlying the detrimental effect of heat on rice yield and quality.The temperature of rice plant is governed by environmental factors like solar irradiation,air temperature,humidity,and wind,and is also associated with physiological activities including transpiration and respiration,therefore being considered as an important eco-physiological index for heat tolerance.Current study of plant temperature mainly focuses on canopy temperature at population level and leaf,stem and panicle temperature at plant level.However,the study of temperature variation,influencing factors and physiological significance at grain level has not been carried out.In this study,the typical japonica rice varieties with canopy structure were selected as materials,field experiments were combined with intelligent greenhouse,and non-destructive analysis equipment such as infrared imaging and infrared thermometer were used to explore the diurnal variation law of temperature difference between superior and inferior spikelet.Meanwhile,the physiological and ecological basis of diurnal variation of grain temperature was comprehensively explored by biochemical analysis method.It is hoped that the results will provide theoretical reference for stress-resistant cultivation and genetic improvement of rice.The main conclusions are as follows:1.The diurnal variation of temperature difference between superior and inferior spikelet of rice was revealed.Using Ningjing8,Wuyunjing7 and Wuyujing3 as materials,the temperature of superior and inferior spikelet of rice was measured by infrared thermometer and infrared thermal imager at sunrise,9 o’clock,12 o’clock,15 o’clock,sunset and 24 o’clock respectively during the early,middle and late stage of grain filling,and the diurnal variation law of temperature difference between superior and inferior spikelet was analyzed.At the same time,using Wuyujing3 and its mutant DY1102 as materials,high temperature treatment(day/night:35℃/28℃)and moderate temperature control(day/night:31℃/24℃)were set up to study the change characteristics of temperature of superior and inferior spikelet under high temperature.The results showed that the temperature of superior spikelet was higher than that of inferior spikelet in daytime,especially in the early stage of grain filling.There was no significant difference in the temperature of superior and inferior spikelet at night,and the temperature of inferior spikelet might be higher than that of superior spikelet in the late stage of grain filling.The temperature difference between superior and inferior spikelet is larger in daytime,smaller at night,larger in the middle and early stages of grain filling and smaller in the late stage of grain filling.High temperature treatment did not change the diurnal variation of the temperature of superior and inferior spikelet.The diurnal variation of the temperature of superior spikelet was higher than that of inferior spikelet,and there was no significant difference at night or the temperature of inferior spikelet was higher than that of superior spikelet.However,the effect of high temperature on different varieties was different,which increased the temperature difference between superior and inferior spikelet of DY1102,but had no significant effect on Wuyujing3.2.The effects of population environment on temperature difference between superior and inferior spikelet of rice were evaluated.Taking Ningjing8,Wuyunjing7 and Wuyujing3 as materials,reference plants with physiological inactivation were created three days before the temperature of superior and inferior spikelet was measured during grain filling period.The reference plants were placed side by side with normal plants to determine the temperature of grain location,evaluate and eliminate the effect of grain location,and quantify the physiological effects accurately.The results showed that the position temperature of superior spikelet was significantly higher than that of inferior spikelet in daytime,and then there was no significant difference between them,even the opposite trend.At night,the position temperature of inferior spikelet was higher than that of superior spikelet.At sunrise and sunset,the position temperature of inferior spikelet of Wuyunjing7 was higher than that of superior spikelet,while there was no significant difference between Ningjing8 and Wuyujing3.Quantitative studies on physiological metabolic effects of superior and inferior spikelet showed that the temperature of superior spikelet was significantly higher than that of inferior spikelet in the daytime,and there was no significant difference in the temperature of superior and inferior spikelet at night,sunrise and sunset,some of which were consistent with the trend in the daytime.3.The physiological effects of temperature difference between superior and inferior spikelet in rice were preliminarily revealed.Using Ningjing8 as material,bracts morphology and color changes were observed by ultra-depth-of-field stereoscope.At the same time,the samples were sampled and fixed.After dehydration,replacement and drying,the stomatal density and size of bracts of superior and inferior spikelet were observed by scanning electron microscopy.After dehydration,transparency,embedding,sectioning and staining,the shape and area of vascular bundles of bracts of superior and inferior spikelet were observed by optical microscopy.The transpiration rate were measured by portable photosynthetic instrument.Half-panicle sampling was used to analyze the diurnal variation of assimilation products such as sucrose and starch in superior and inferior spikelet.The results showed that the bracts of superior and inferior spikelet were green from 0 to 10 days after anthesis;the bracts of superior spikelet were yellowing after 20 days after anthesis,and the yellowing time of inferior spikelet was later than that of superior spikelet.There was no significant difference in stomatal density and width between bracts of superior and inferior spikelet,but stomatal length of bracts of superior spikelet was significantly longer than that of inferior spikelet.There was no significant difference in the number and area of vascular bundles between bracts of superior and inferior spikelet.The transpiration rate of superior spikelet was significantly lower than that of inferior spikelet in the early stage of grain filling,but there was no significant difference in the later stage.There was no significant difference in transpiration rate at night.The sucrose content of bracts and grains of superior spikelet was lower than that of inferior spikelet,but the starch content was the opposite.The content of starch and sucrose in bracts oscillated,i.e.sunrise was lower than sunset.The starch content of bracts of superior and inferior spikelet at sunrise was 79.97%and 83.10%at sunset,respectively,while the sucrose content at sunrise was 66.72%and 73.50%at sunset.However,there was no diurnal variation in grains.Based on the above results,this study revealed the diurnal variation of temperature difference between superior and inferior spikelet and the physiological and ecological basis.The temperature difference between superior and inferior spikelet in rice has obvious diurnal variation law,which shows that the temperature difference between superior and inferior spikelet in the daytime is larger than that at night.Besides the influence of grain location on the temperature difference between superior and inferior spikelet,the physiological metabolic activities such as transpiration(daytime)and respiration(night)of grains themselves also have significant effects on grain temperature.