| Because of the intensification of anthropogenic disturbance, global temperature hasincreased 0.6℃averagely in the past one hundred years. Furthermore, it is predicted thatthe earth will be warmer 1.4～5.8℃by the end of this century. At the same time, thefrequency of extreme climate events (such as extremely high temperature) will be higher,and their duration will be longer. Thus, the crop system would be challenged by morefrequent high temperature. Rice is one of the most important grain crops in China, which iseasily confronted with summer high temperature in its sensitive growth period. In recentyears, the heat injury of rice has been intensified in southern China, especiallyYangtze-Huaihe region. For instance, Anhui and Wuhan provinces endured above 35℃high temperature continuously in flowering stage of medium rice, causing rice heat injuryabout 2.8 million Mu, resulting in a decrement of 20×108 kg in rice grain yield in 2003.Therefore, it is very important in China to understand the occurrence law of rice heat injuryand explore its mechanisms of rice heat avoidance and resistance.The objectives were to study: (1)genotypic differences in effects of high airtemperature in field on rice yield components and grain quality during heading stage;(2)responses of pollen activity and seed setting of rice spikelet under different developmentstages to high temperature of heading period; (3)effects of fertilization amount on spiketemperature at spiking stage of rice; (4)impact of irrigated water depth on rice spiketemperature at spiking stage. The results were as follows:(1)High temperature significantly influenced seed setting rate (p＜0.05) with anaverage decrease of 25% forsix tested varieties, but no significant impact was found on thegrain number per panicle and 1000-grain weight. In addition, extremely high temperaturealso reduced the head rice rate and increased the chalkiness rate and degree (p＜0.05), and there was no significant impact on brown rice rate. Although there were significantgenotypic differences in high temperature effects among six varieties, the heat injury wereserious on rice yield and grain quality of each tested variety. These evidences suggest thatnew variety breeding with resistance to high temperature be one of the key technologies toreduce heat injury. In the near future, more attention should be paid to cropping systemadjustment and rice cultivation technique improvement to let rice plant avoid hightemperature and to improve rice physiological resistance to heat injury during the headingstage.(2)Rice pollen activity decreased along with flowering sequence under normaltemperature, and that extremely high temperature significantly reduced rice pollen activity(p＜0.05). The difference of pollen activity between high and normal temperaturesdecreased along with the flowering sequence, resulting in the highest decrement occurringin the first day or second day of flowering. Pollen germination rate on stigmas, pollengermination rate on culture medium and stainable pollen grain rate of the four varieties onaverage under high temperature at the 1st or the 2nd were 16.00, 25.85 and 11.74percentage lower than those of under normal temperature, and at the 5th day they were only8.49, 6.63 and 6.02 percentage lower than those under normal temperature, respectively.Similar trends occurring in seed setting characteristics, the seed setting rate of four cultivarson average under high temperature at the 1st were 14.04 percentage lower than that undernormal temperature, and at the 5th day it was only 5.95 percentage lower than that undernormal temperature, but empty grain and unfilled grain rates of four cultivars on averageunder high temperature at the 1st were 10.06 and 3.98 percentage higher than those undernormal temperature, and at the 5th day they were only 3.98 and 1.97 percentage higher thanthose under normal temperature, respectively. In addition, significant difference oftemperature effects on pollen activity and seed setting existed among the four cultivars withthe least impact occurring in Shanyou 559. According to the regression analysis, the rates ofseed setting, empty grain and unfilled grain showed a significantly linear correlation withthe rates of pollen germination on stigmas and on culture medium (p＜0.05), respectively.(3)Same trends were found between the atmospheric and spike temperature withhighest temperature occurring during 10:30～15:30. The average spike temperature ofthree treatments of YangDao 6 and YangJing 9538 were 3.58℃and 3.04℃lower than theatmospheric temperature, respectively. In addition, the discrepancy between air and spiketemperature of the treatments of the high and middle fertilization amount was 1.54℃and 0.90℃higher than that of the low fertilization amount. The discrepancy between air andspike temperature of YangDao 6 was also 0.85℃higher than that of YangJing 9538 at thesame fertilization amount. The discrepancy between air and spike temperature of the threetreatments showed a negative relationship with the penetrated light ratio of spike, butpositive relationship with total LAI; LAI above the spike and number of stem andtillers(coefficient of correlation: -0.8796**, 0.9436**, 0.9552**, 0.943**, 0.9110**,respectively). These results indicate that a good structure of rice canopy and populationfrom optimized field management, for example reasonable fertilization amount candecrease the spike temperature, and reduce heat injury.(4)Irrigated water depth in the paddy field significantly affected the temperatures ofthe field surface, the middle of rice plant and rice spike, resulting in lower temperaturesoccurring along with the increase of field water depth. The temperature differencesbetween air and rice spike, the middle of rice plant and field surface were 1.38℃,2.97℃and 4.11℃higher in average of three cultivars in the plot with deep water than that of theplot without water, and 0.66℃, 1.57℃and 2.14℃higher than that of the plot with shallowwater, respectively. In addition, the temperature differences were 0.72℃, 1.40℃and1.97℃higher in the plot with shallow depth than that of the plot without water. Theanalysis of the temperature transfer between field surface, the middle of plant and ricespike indicated that the temperature transfer patterns under all water management regimeswere up to the energy transfer rule. The above results suggest that keeping water depth inthe field contribute great to decrease rice spike temperature and alleviate rice heat injury.
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