Effect of Extreme Natural High Temperature on Seed Setting Rate of Rice(Oryza sativa L.)at Heading and Flowering Stage

Zhen ZHU,Qingyong ZHAO,Yadong ZHANG,Tao CHEN,Shu YAO,Lihui ZHOU,Xin YU,Cailin WANG

Institute of Food Crops,Jiangsu Academy of Agricultural Sciences/Jiangsu High Quality Rice R&D Center/Nanjing Branch of China National Center for Rice Improvement,Nanjing 210014,China

As climate warms,the frequency and intensity of high temperature are all increased in summer.In parts of the regions in Yangtze Ricer basin,the ridge of subtropical high pressure often shows a standstill from early and middle July to early August,which is prone to sustained high temperatures[1-2].Heat damage has become one of the major natural disasters in rice[3],seriously affecting the safety of rice production.Numerous studies[4-6]have shown that rice is most sensitive to high temperature at heading and flowering stage,and high temperature will lead to decreased seed setting rate in rice,ultimately leading to reduced yield.The main reason for reduced rice yield by high temperature is to hinder the pollen maturity and anther dehiscence,as well as the germination of pollens on stigmas and elongation ofpollen tubes.Eventually,pollination failure and infertility are induced.However,the quantitative analysis on the effect of high temperature on seed setting rate is mostly carried out under the condition of artificial high temperature.There are rare researches on the effect of extreme natural high temperature on seed setting rate of rice.In this study,the rice canopies were shown for several consecutive days in 2013,and during that time,the daily highest temperatures all exceeded 35℃.The effects of extreme natural high temperature on seed setting rates of different rice cultivars(lines)at heading and flowering stage were investigated by making full use of the favorable conditions above,and several heat-resistant materials were preliminarily screened.

Materials and Methods

Materials

The 198 new rice cultivars(lines)and combinations were introduced and bred by the Institute of Food Crops,Jiangsu Academy of Agricultural Sciences,including 33 conventionalindicarestorer lines(A1-A33),66 new conventionaljaponicacultivars(lines)(B1-B66)and 99indicahybrids.

Methods

All the tested materials were cultivated in the experimental fields of the Institute of Food Crops,Jiangsu Academy of Agricultural Sciences in 2013,and their field management was all the same.For each of the tested materials,the firstly appeared 10 panicles were labeled,and after the maturity,their seed setting rates were surveyed.The data were processed and analyzed using Excel.For the correlation analysis,arcsine transformation was carried out for the data about seed setting rates.The daily highest temperature in the rice canopy layer was detected using temperature and humidity monitor(HOBO U23-001 model).

Results and Analysis

Temporal distribution of heading stages of tested rice materials and daily highest temperatures

The daily highest temperatures from July 21 to August 30 were shown in Fig.1.The instant temperatures in the rice canopy layer were recorded very one hour by the temperature and humidity monitor.For the 13 consecutive days from August 6 to August 18,the daily highest temperatures were all higher than 36℃,which was rarely shown in the past 40 years.During that period,the temperature was highest on August 12 (38.73℃),and was lowest on August 16(36.25℃).Fig.2 showed the temporal distribution of heading stages of the 198 tested materials from August 6 to August 18.In the heading and flowering stage,all the tested materials suffered at least one day of heat stress(＞36℃).

Effects of high temperature on seed setting rates of conventional indica restorer lines

The seed setting rates of the 33 conventionalindicarestorer lines under heat stress at heading and flowering stage were shown in Table 1 and Table 2.Their heading stages were during August 9 and August 18 and average seed setting rate was 73.09%,which was significantly lower than those of normal materials.The seed setting rates (ranging from 23.02%to 89.13%)of the 33 conventional indica restorer lines ranged from 23.02%to 89.13%with variation coefficient of 17.56%.There were 5 restorer lines of which the seed setting rates were higher than 85%and close to the normal level,indicating weaker effects of high temperature;there were 4 restorer lines of which the seed setting rates were lower than 60%(the lowest was 23.02%)and significantly lower than the normallevel, indicating greater effects of high temperature;the seed setting rates of most the tested materials ranged from 60%to 85%and were a littler lower than the normal level,indicating certain effects of high temperature.It indicated that the sustained high temperature had certain effects on the 33indicarestorer lines,and there were significant differences among different cultivars(lines).The seed setting rates of Ninghuiguangkangzhan (89.13%),Shuhui 527(88.27%),Chenghui 3203(87.94%)and Xianyin-8 (87.89%)were close to the normal level(90%).The correlation analysis showed that the seed setting rates of the 33 conventionalindicarestorer lines were negatively related to daily highest temperatures(y=-0.651 7x+86.996(n=33),r=0.066).

Table 1 Statistical analysis of seed setting rates of tested rice materials

Table 2 Seed setting rates of 33 conventional indica restorer lines

Effects of high temperature on seed setting rates of new conventional japonica cultivars(lines)

The average seed setting rate of the 66 new conventionaljaponicacultivars (lines)under heat stress was 70.59%with range of 32.78%-89.05%and variation coefficient of 19.91%(Table 1).As shown in Table 3,there were 21 tested materials of which the seed setting rates were lower than 65%,accounting for 31.81%of the total tested rice materials,and they were greatly affected by extreme natural high temperature;the seed setting rates of most(34)of the tested materials ranged from 70%to 85%,accounting for 51.52%of the total tested materials,and the high temperature show certain effects on them;there were 7 materials of which the seed setting rates ranged from 85%to 90%and were close to the normal level,and they were weakly affected by the high temperature.The seed setting rates of Wuyinjinghui(B2),Nanjing 44//W3660/Nanjing 44(B11),Nanjing 44//W3660/Nanjing 44 (B12)and Wuyun 2330/JD6011(B22)all approached the normal level.The correlation analysis showed that the seed setting rates of the 66 new conventional japonica cultivars(lines)were negatively related to daily highest temperatures(y=-1.756 2x+127.58(n=66);r=0.117).

The intermediate cultivars(lines)were classified statistically according to combinations.As shown in Table 3,the average seed setting rate of all the cultivars(lines)of Nanjing 44//W3660/Nanjing 44 was highest(88.13%),and among them,the lowest seed setting rate also exceeded 85%.They were affected most weakly by the extreme natural high temperature.The average seed setting rate of all the cultivars (lines)of Nanjing 44/JD6202 ranked second (82.47%)with range of 79.89%-85.13%.The heat stress showed relatively weak effect on them.There were great differences in seed setting rate among the cultivars(lines)of Wuyun 2330/JD6011,Wuyunjing 7//Kanto 194/Wuyunjing 7,Wuyu 5021/Kanto 194 and Nanjing 44/JD6301,and the effects of extreme natural high temperature also differed significantly among their cultivars(lines).The differences in seed setting rate among the cultivars (lines)of Wujing 15//Wujing 13/Kanto 194 and 2394/Xudao 3 were smaller,but the heat stress showed greateffects on alltheircultivars(lines).It indicated that combination has great influence on the heat resistance of new conventionaljaponicacultivars(lines).

Table 3 Seed setting rates of hybrids derived from different combinations

Table 4 Comparison of seed setting rates among F1hybrids derived from the same sterile lines

Effects of high temperature on seed setting rates of indica hybrids

The average seed setting rate of the 99indicaF1hybrids was 72.84%with range of 20.65%-91.95%and variation coefficient of 16.99%(Table 1).Fig.4 showed that there were 20 tested materials of which the seed setting rates were lower than 65%,accounting for 20.2%of the total tested materials,and they were greatly affected by the high temperature.There were 18 materials of which the seed setting rates ranged from 80%to 85%,accounting for 18.18%,and they were weakly affected by heat stress.There were 13 materials whose seed setting rates were higher than 85%,accounting for 13.13%of the total tested materials.Their seed setting rates were most close to the normal level(90%).There were 48 materials of which the seed setting rates ranged from 65%to 80%,accounting for 48.48%of the total materials.They were affected by the extreme natural high temperature to some extent.

The F1hybrids were classified according to sterile lines (Table 4).The seed setting rates of F1hybrids derived from N25A,19S,Yuexiang A and Yanxian 2S were relatively high with average seed setting rate of about 80%.Their seed setting rates were relatively stable among different F1hybrids with variation coefficients of 5.97%,7.25%,8.08%and 6.99%,respectively.The extreme natural high temperature showed relatively weak effects on them.The average seed setting rate of F1hybrids derived from Yanxian 1S was close to 80%,but the variation range (55.51%-91.95%)of seed setting rates was relatively large.The seed setting rates of F1hybrids derived from N113A,N22A and Wuxiang S were relatively low with average seed setting rate lower than 65%.There were great differences in seed setting rate among different combinations.The sterile line showed certain effect on heat resistance of F1hybrids.

The seed setting rates of F1hybrids derived from different restorer lines were analyzed statistically(Table 5).There were great differences in seed setting rates among F1hybrids derived from different restorer liens.The average seed setting rates of F1hybrids derived from the 24 restorer lines ranged from 53.32%to 89.00%with variation coefficients ranging from 0.9%to 39.29%.The average seed setting rates of F1hybrids derived from Xianyin-8 and Ninghuiguangkangzhan all exceeded 85%.Comparative analysis of seed setting rate was carried out between F1hybrids and their restorer lines.The results showed that the seed setting rates of most F1hybrids derived from restorer lines with good heat resistance were relatively high,but there were also some restorer lines whose F1hybrids were affected greatly by heat stress,such as Zhonghui 8006 and Shuhui 158.The seed setting rates of Zhonghui 8006 and Shuhui 158 were 82.21%and 80.30%,and theywere affected weakly by the extreme natural high temperature.However,the lowest seed setting rate among F1hybrids derived from Zhonghui 8006 was only 44.74%,and derived from Shuhui 158 was only 41.19%;their F1hybrids were all affect greatly by the heat stress.Similarly,there were also some heatsensitive restorer lines whose F1hybrids were weakly affected by the high temperature,such as Neixianghui 1,Chenghui 727 and Zhenhui 084.The seed setting rates of the three restorer lineswere54.22%,67.56% and 67.54%,respectively,but the highest seed setting rates among their F1hybridswere89.52%,89.43% and 87.67%,which were close to the normal level.The correlation analysis(Fig.5)showed that under heat stress,the seed setting rates of F1hybrids were positively related to those of their restorer lines(r=0.470*(n=24)).

Table 5 Comparison of seed setting rates among F1hybrids derived from the same restorer lines

Conclusions and Discussion

Previous studies[7-9]have shown that rice is most sensitive to high temperature before the flowering of spikelets,and heat damage at heading and flowering stage is prone to leading to greatly increased percentage of empty grains and decreased seed setting rate,ultimately leading to reduced yield.There have been some researches on critical temperature for heat damage in rice at flowering stage.It is generally considered that the critical temperature for heat damage in rice at flowering stage is 35℃.The International Rice Research Institute ever treated total 84 rice lines with temperatures of 35 and 38℃,respectively,and the seed setting rates of about 75%of the selected lines were lower than 20%under the temperature of 38℃.Therefore,35℃is considered to be more preferable for heat stress[10].The Shanghai Institute of Plant Physiology treated Erjiuqing with different high temperatures at flowering stage in artificial climate chamber.They found that the 5-consecutive-d treatment of 30℃showed significant damage to the flowering and fruiting of Erjiuqing,and the 5-consecutive-d treatment of 38℃made Erjiuqing harvest failure.Therefore,they believed that the critical temperature for longterm heat damage to indica rice was 30℃at flowering stage,and for shortterm heat damage was 35℃[11].Renet al.[12]found that hybrid rice was most sensitive to high temperature at flowering stage;the critical daily average temperature for heat damage was 30℃,and the critical daily highest temperature was 35℃.Tanet al.treated the daily average temperature of 30℃and daily highest temperature of 35℃as the critical temperatures for heat damage caused by natural high temperature[13].Tanget al.[14]found that the seed setting rates of both hybrid rice and conventionaljaponicarice were all reduced after 1-d treatment of 35℃at the heading stage.In this study,the heading stages of the tested materials were during August 6 to August 18.During the interval,the daily highest temperatures in the rice canopy layer were all higher than 36℃,and the tested materials all suffered at least one day of heat stress (36℃),basically meeting the requirements by heat damage test.

Zeng and Huanget al.found that there were great differences in heat resistance among different rice cultivars[15-16].In this study,the average seed setting rates of the three types of rice were all lower than the normal level,and significant differences were found in heat resistance among different materials,which were basically consistent with previous study results.The correlation analysis showed that the seed setting rates of both conventionalindicarestorer lines and conventionaljaponicacultivars (lines)under heat stress were all negatively related to daily highest temperature(P＞0.05).Leiet al.[17]investigated the effects of natural and artificial high temperatures on seed setting rates of rice combinations and their restorer lines.The results showed that there were great differences in seed setting rate among different combinations,and the heat resistance capacities of rice combinations were linearly related to the heat resistance capacities of their male parents (P＜0.05).This study found that parents had certain effects on the heat resistance capacities of their F1hybrids.The seed setting rates of most the F1hybrids derived from restorer lines with good heat resistance were higher,but there were also some F1hybrids affected greatly by high temperature.Some of the F1hybrids derived from restorer lines with bad heat resistance were affected weakly by heat stress.The correlation analysis showed that under heat stress,the seed setting rates of F1hybrids were positively related to those of their restorer lines.

High temperature is a natural disaster,and is irresistible.The heat damagecanonlybereduced or avoided by active defensive measures.From the perspective of cultivation,reasonable arrangement of sowing period is an effective measure to avoid heat damage.The arrangement of sowing period should follow the principles that heading stage avoids high temperature in hot summer,heading is completed before the safe full heading stage,and the sowing of next-season crop is not affected.Studies have shown that there are greatdifferences in sensitivity to high temperature among different cultivars.In terms of species,the selection of heat-tolerant cultivars can effectively reduce heat damage.Moreover,parents have a significant impact on heat resistance oftheir future offspring.So the screening of heat-tolerant resources plays an important role in breeding of heat-tolerantcultivars.There have been many studies on screening of heat-tolerant resources[18-20].A group of heat-tolerantresources have been screened out,but most of them are local cultivars,of which the agronomic traits are poor and cannot be utilized directly.In this study,total 4 conventionaljaponicacultivars and 4 conventionalindicarestorer lines are preliminarily screened out,and they are all new cultivars (lines)with better agronomic traits after generations of selection.After the heat resistances are identified repeatedly and confirmed,the screened cultivars (lines)can directly used in production or as heat-tolerant resources.

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