Flowering Characteristics of Japonica Rice Male Sterile Lines Adaptive to High Temperature and Summer Drought

Renpeng XIAO,Jingyong LI,Xiong YAO,Xianwei ZHANG,Ming WEN,Yongqun TANG

Chongqing Academy of Agricultural Sciences/Chongqing Ratooning Rice Research Center,Chongqing 402160,China

In the upperreaches ofthe Yangtze Riverrepresented by Chongqing,high temperature and summer drought are major stresses restricting the improvement of rice quality[1].Mechanization for rice production is urgent now due to the massive transfer of rural labors[2].Compared with indica rice,japonica rice cultivars have some advantages in mechanized production and quality improvement[3].However,the differences between indica and japonica rice cultivars in agronomic traits especially in flowering characteristics have restricted the development of japonica rice.Improving the flowering characteristics of japonica rice male sterile lines is a prerequisite to breed new japonica rice hybrids[4].However,elite japonica rice cultivars tolerant to high temperature and the effects of temperature on rice flowering characteristics have rarely been reported.Jinghui 35,the restorer line of a three-line japonica hybrid,and a male sterile line Rejing 1A,both of which were bred by Chongqing Academy of Agricultural Sciences,and approved by Chongqing Crop Variety Approval Committee,successfully improved the yield and quality of japonica rice[5-6].The study of Yanet al.[7]re vealed that the flowering of rice was very sensitive to climatic factors,and thus was unstable under high temperature and drought in summer.Shiet al.[8]believed that hot and humid climate always resulted in good flower-ing characteristics,like early flowering period,high stigma exertion rate,large glume-opening angle and high outcrossing rate.On the contrary,low temperature and drought always caused poor flowering characteristics of rice.Most of the previous studies were performed by simulating high temperature and drought conditions in greenhouse,and thus the correlations of flowering characteristics with high temperature and summerdrought were not evaluated.In recent years,some good indica rice cultivars bred by Chongqing Academy of Agricultural Sciences were crossed with conventional japonica rice cultivars bred by the team of Academician Chen,and the hybrids were then crossed with indica rice male sterile lines(female parent).After several generations of backcross,a series of japonica rice male sterile lines were finally obtained.In the present study,the flowering characteristics of the 23 japonica rice male sterile lines which were more adaptive to high temperature and summer drought were compared with II-32A as the control so as to explore the effects of high temperature and summerdroughton the flowering characteristics of japonica rice male sterile lines and provide some reference for the breeding of elite japonica rice cultivars with high yield and tolerance to drought.

Materials and Methods

Materials

II-32A was an indica rice male sterile line usually used as parental materialin breeding and widely planted,and was selected as the control in this study.One hundred and ninety-eight japonica rice male sterile lines bred by Chongqing Academy of Agricultural Sciences and Chongqing Ratooning Rice Research Center were selected as the experimental materials.

Methods

Field experiment in the summer of 2013 in ChongqingThe 198 japonica sterile lines were planted in Yuxi Crop TestStation ofChongqing Academy of Agricultural Sciences in Yongchuan,Chongqing and screened according to their plant shape,growth period,flowering characteristics,disease resistance,etc.As a result,23 of the sterile lines were selected and classified into two groups:the lines with duration of 70 d from sowing to heading in Group 1,and the lines with duration of 85 d from sowing to heading in Group 2(Table 1).

Field experiment in the autumn of 2013 in HainanThe 23 japonica sterile lines selected above were sown on September 1,2013.The control II-32A was seeded on August 25,September 4 and September 14,respectively.Target items were determined by referring to the methods of Linet al.[9-10],with some modifications.

Survey on flowering peak of rice sterile linesAt the full-flowering stage,10 plants that bloomed simultaneously were selected at random from each line,and one panicle that grew one-third long was selected from each plant and tagged after the opening glumes were cut off.On the next day,newly opened glumes at 15 time periods of 9:01-9:30,9:31-10:00,10:01-10:30,10:31-11:00,11:01-11:30,11:31-12:00,12:01-12:30,12:31-13:00,13:01-13:30,13:31-14:00,14:01-14:30,14:31-15:00,15:01-15:30,15:31-16:00 and 16:01-16:30 were counted for three consecutive sunny days.The means of 10 panicles were taken as the flowering rate at each time period.

Survey on single-panicle flowering duration and single-plant flowering durationAt the initial heading stage,10 plants that bloomed simultaneously were selected at random from each line,and their main panicles were tagged.The days from the first glume opening to the last glume opening of each main panicle were recorded.The mean number of days of 10 panicles’was calculated as the single-panicle flowering duration of the rice line.From the next day,the flowering duration,from the first glume opening to the last glume opening of 10 plants was surveyed.The mean number of days of 10 plants’was calculated as the single-plant flowering duration of the rice line.

Survey on daily flowering dynamics of a single panicleAt the initial heading stage,10 plants that bloomed simultaneously were selected at random from each line,and a single panicle of every plant was tagged.From the next day the opening glumes were counted in the afternoon every day according to the arrangement of anthers in glumes before they were cut off,till the day all the glumes of a single panicle opened,to calculate the daily flowering rate of a single panicle.

Table 1 The information of tested japonica sterile lines in this study

Survey on glume-opening angle,distance between glumes and glume opening durationAt the fullflowering stage,10 glumes about to open of each line were randomly selected and marked with ink.The glumes were observed once every 15 min from 10:30 to measure the opening duration of every glume,and the glume-opening angle and the distance between glumes were measured once every 15 min till the glume-opening angle began to decrease.The maximum glume-opening angle was regarded as the opening angle of the glume,and the corresponding distance between glumes was regarded as the distance between glumes of the glume.These items were measured in three consecutive days,and their averages were finally taken.

Measurement of stigma viability

At the full-flowering stage seven plants that bloomed simultaneously were selected at random from each line.Two panicles that grew about 1/3 long were selected from each plant.Allthe opened glumes and the young glumes at the bottom of the panicles were cut off,and 200 newly opened glumes were maintained and bagged to prevent from outcrossing.And then,the sterile lines were pollinated every day for 7 d.Twenty-five days later,the outcrossing rate of the pollinated panicles was calculated to measure stigma viability of every rice line.

Measurement of enclosed panicle length,enclosed glume rate and stigma exertion rateAt the final flowering stage,10 panicles of every line were selected.The panicle enclosure length was measured;the glumes enclosed in sheathes and the glumes outside of sheathes were counted to calculate sheath-enclosed glume rate by the formula: Sheath-enclosed glume rate=Number of sheath-enclosed glumes/Number of all glumes of a panicle.Among the glumes outside of sheathes,those with two exerted stigmas,only one exerted stigma or no stigma exerted were counted respectively to calculate their proportions.

Double stigma exertion rate=(Number of glumes outside of sheathes with two exerted stigmas/Numberofallglumesoutside of sheathes)×100%

Single stigma exertion rate =(Number of glumes outside of sheathes with only one exerted stigmas/Number of all glumes outside of sheathes)×100%

Stigma exertion rate=(Number of glumes outside of sheathes with one or two exerted stigmas/Number of all glumes outside of sheathes)×100%

Data analysis

The data were processed first using Excel 2003 and then DPS 12.50.

Results and Analysis

The field trials in Hainan revealed that the 11 rice lines of Group 1 began to head in early November,and the 12 rice lines in Group 2 in late November.

Daily flowering peak of single panicle of the rice cultivars

As shown in Fig.1,the daily flowering peak of the lines in Group 1 firstly appeared during 10:30-13:00.Compared with the controlⅡ-32A,the daily flowering peak of 35470A,35485A,35489A,35493A and 35506A was significantly earlier.Among them,the flowering peak of 35470A,35485A and 35506A was observed during 10:00-11:00,while that of the controlⅡ-32A appeared during 11:30-12:30,which was simultaneous with 35489A and 35493A.Early flowering peak is helpful to improving rice yield.The flowering peak of 35482A,35490A and 35524A mainly appeared during 12:00-13:00,and that of 35473A and 35486A at 12:30-13:30,while the flowering peak of 35468A was later,at 14:30-15:30.

As shown in Fig.2,the flowering peak of several rice lines in Group 2,including 35474A,35476A,35478A,35487A, 35510A. 35475A and 35479A,appeared simultaneously with the control II-32A’s,mainly at 12:00-13:00,peak flowering period was mainly at 10:00-11:00,35502A was at 10:30-11:30,35488A at 11:00-12:00,significant different from the control’s.35519A and 35522A’s peak flowering period was slightly earlier than the control’s,at11:30-12:30,and35500A’s laterthanthecontrol’s,at12:00-13:30.

The single-panicle flowering duration and single-plant flowering duration of the rice lines

As could be seen from Table 2,the single-panicle flowering duration and single-plant flowering duration of the controlⅡ-32A were 6 and 14.2 d,respectively,both shorter than those of all the tested lines in Group 1.Among all the tested rice lines in Group 1,35468A had the longest single-panicle flowering duration,8.6 d,while 35486A had the longest single-plant flowering duration,19.2 d.The 35478A,35479A,35487A,35488A,35500A,35510A and 35519A in Group 2 had longer single-panicle flowering duration than the controlⅡ-32A.Among them,35519A had the longest single-panicle flowering duration,10.8 d,while 35510A had the longest single-plant flowering duration, followed by 35502A,35475A,35479A,35519A and 35478A.A longer flowering duration is conducive to the pollination of sterile lines,thus increasing seed setting rate.

Table 2 The single-panicle flowering duration and the single-plant flowering duration of the rice lines in Group 1 and Group 2 d

The flowering peak throughout the whole flowering period of the rice lines

As shown in Fig.3,the number of daily opening glumes differed among the rice lines in Group 1,and peaked mainly on the 3rdday.Compared with the control,whose flowering peak appeared on the 3rdday,35482A and 35485A had the flowering peak on the 2nd day,while the flowering peak of 35486A,35490A and 35524A was not significant,which is not conducive to high yielding.As shown in Fig.4,the flowering peak of the controlⅡ-32A appeared on the 4th day,while that of 35475A and 35478A appeared on the 1stday;35488A and 35519A had no significant flowering peak.

The glume-opening angle,the distance between glumes and glume opening duration of the rice lines

As could be seen from Table 3,among the lines in Group 1,35482A,35489A and 35524A had larger angle of opening glumes and larger distance between glumes,which were helpful to seed production,and theirglume opening duration was shorter than the control’s.Among the lines in Group 2,35474A,35476A,35489A,35493A and 35502A had much larger angle of opening glumes and distance between glumes than the control,which was conducive to outcrossing and seed setting.However,the glume opening duration of all the tested lines in Group 2 was shorter than that of the control.

Sheath-enclosed glumerateand stigma exertion rate of the rice lines

Table 3 The glume-opening angle,the distance between glumes and glume opening duration of the rice lines in Group 1 and Group 2

As could be seen from Table 4,among all the lines in Group 1,35489A had the greatest stigma exertion rate,up to 76.23%,which was 5.69%greater than that of the controlⅡ-32A,and their difference was significant;moreover,35489A also had a sheathenclosed glume rate 5.6%less than the control,which was helpful to outcrossing.35473A and 35482A had the second (75.15%)and the third(74.71%)largest stigma exertion rate,respectively,both significantly greater than the control.Among the lines in Group 2,35475A,35487A,35502A and 35510A had higher stigma exertion rate,67.36%,78.64%,60.09%and 62.76,respectively.The sheathenclosed glume rate of 35476A was the largest in Group 2,up to 13.92%,greaterthan thatofthe control.35478A had a lower stigma exertion rate,but its lower sheath-enclosed glume rate was possible to compensate for the lacks caused by the lower stigma exertion rate.

Stigma viability of the rice lines

As shown in Fig.5,the stigma viability of all the rice lines in Group 1 peaked in the first three days.Among them,the stigma viability of 35468A was the strongest on the 3rd day,and that of 35489A and 35490A on the 2nd day,and that of all the other lines reached the highest level on the 1stday of flowering.Their highest seed setting rate varied in a range of 39.66%-94.92%.In addition,the stigma viability of 35470A, 35482A, 35490A,35493A,35506A and 35524A was relatively high,and maintained for a longer time period,which was significantly better than that of the control.The stigma viability of 35489A was similar to that of the control,but maintained for a longer time period,and it still remained at 33.47%on the 7thday,so that its seed setting rate could be improved by extending its pollination time.

As shown in Fig.6,the stigma viability of all the rice lines in Group 2 also peaked in the first three days.The stigma viability of 35475A,35476A,35479A,35500A,35502A,35522A;35487A and 35488A reached the highest level on the 1st day of flowering,and that of 35474A,35478A,35510A,35519A and the control reached the highest level on the 2ndday.Their highest seed setting rate ranged from 43.51%to 94.92%.Among them,the stigma viability of 35476A,35479A,35500A,35502A,35519A and 35522A was relatively high.The stigma viability of 35478A was slightly lower than that of the control,but maintained for a longer time period,which is helpful to high yielding,even the flowering period of their parents are not completely simultaneous.

Table 4 Panicle enclosure rate and stigma exertion rate of the rice lines in two groups %

To sum up,it was identified that,among the 23 tested rice lines,three(35478A, 35489A and 35502A)showed excellent flowering lines,i.e.,one in Group 1 and two in Group 2.Among them,35478A and 35489A had large panicles,and on average,there were 260.0 and 223.7 glumes respectively on their main panicle,significantly more than that of the control(166.3 glumes)(Fig.7).Large-panicle rice lines are potential resources for the breeding of new high-yield rice varieties.

Conclusion and Discussion

High stigma exertion rate,high stigma viability that can remain for a long time period,significant flowering peak,large glume-opening angle and long glume opening duration are good flowering characteristics of japonica rice male sterile lines[11].The high temperature and frequent drought in summer,especially the continuous sunny and hot days during July-September in the upper reaches of the Yangtze River,result in poor rice quality,decrease milled rice rate and increase chalky rice rate[12-13].To produce highyield and quality rice,Xuet al.[14]revealed that the rice cultivars tolerant to high temperature at the heading stage were more suitable for the high-yield and quality cultivation.

In this study,the flowering characteristics of japonica male sterile lines were investigated under the high temperature and summerdroughtin Chongqing.The results revealed that all the rice lines were different in characteristics,which was consistent with the study of Zhanget al.[15].The study of Daiet al.[16]revealed that the flowering of rice was closely related to field temperature and ecological regions.In detail,at lower temperature,the flowers bloomed lately,and the opening flowers decreased;the flowering traits of different cultivars planted in the same ecological zone were similar.In the present study,all the tested lines were screened at high temperature in the same ecological zone,but they exhibited very different flowering characteristics,which might be related to their genetic relationship among these lines.The writers also found that the glume opening duration of all the tested lines was shorter than that of the control,so it can be used as a selective trait in later material selection.

Finally,three japonica male sterile lines with excellent traits were screened out.Among them,35489A had earlier flowering peak,single-panicle flowering duration, single-plant flowering duration,larger glume-opening angle,largerdistance between glumes,lower sheath-enclosed glume rate,higher stigma exertion rate and higher stigma viability that can remain for a longer time period,than the control.Another rice sterile line 35478A also had earlier flowering peak,lower stigma exertion rate,longer singlepanicle flowering duration,longer single-plantflowering duration,larger opening glume angle,larger distance between glumes,lower sheath-enclosed glume rate and slowly de-creased stigma viability,butlower stigma exertion rate,which can be compensated by its longer flowering duration.The flowering peak of rice sterile line 35502A also appeared earlier,in the first five days;and it had larger glume-opening angle,longer glume opening duration,and lower sheath-enclosed glume rate.Their potential application in breeding should be determined in further research.

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