Utilizing Dominant Early Maturity Genes of Sterile Line UP-3 s in Hybrid Rice Breeding to Avoid High Temperature Season

A new sterile line UP-3s, which carries the Dominant Early Maturity Gene (DEMG), was bred on the farm of University of Arkansas at Pine Bluff (UAPB). UP-3s and two check sterile lines, Jin23-A and Xie-A which do not carry the Dominant Early Maturity Gene, were crossed with a group of different maturity restorer lines, PB-1R, PB-5R,PB11, PB-13R, PB-20, PB-21, PB-22R, and PB-23R. Eighteen new hybrid rice combinations of these crosses were then tested at UAPB in 2012 and 2013. The results showed that panicle differentiation (PD) of hybrids from female parent UP-3s (DEMG) crossed with the 8 male parents, were earlier than the hybrids from female parent Jin23-A or Xie-A crossed with the 8 male parents. The PD of these earlier hybrids was before Jun 25 and heading was before July 20. Early PD and heading avoided the high temperature (over 34 ̊C) period which usually occurs after July 20 in Arkansas. The yields of these earlier maturity hybrids with female parent UP-3s were higher than those of the late maturity hybrids thatwereF1 progeny of sterile lines Jin23-A or Xie-A (these two female parent checks with non-DEMG). These results showed that the DEMG sterile line UP-3s can be adopted in making crosses with later maturity restorer lines to obtain earlier maturity hybrids to avoid the high temperature period in Arkansas.


Introduction
The rice heterosis (hybrid vigor) has been known for a long time in rice research and production.Utilization of hybrid rice is an important technology to meet the increasing rice demand in the world.Hybrid rice is more profitable and sustainable for yield in production.Hybrid rice delivers a per capita yield advantage of about 20% over inbred rice with lesser inputs to land, such as water and pesticides [1] (Rice-Tech, 2015).Also hybrid rice provides many other attributes in social and environmental aspects [1] (Rice-Tech, 2015).China is the first country to develop hybrid rice commercially.Hybrid rice research was initiated in 1964 [2] (Yuan, 1966).The essential genetic tools for breeding hybrid rice varieties, such as the male sterile line (A line), the maintainer line (B line) and restorer line (R line), were developed in 1973 [3] (Yuan and Virmani, 1988).The first batch of rice hybrid varieties was released commercially in 1976.Hybrid rice accounts for 50 percent of the total rice growing area, and 57 percent of the total rice output were from hybrid in China recent years.The research of hybrid rice in USA began in 1980s, and the first commercial hybrid rice was released in year 2000 by the Rice Tec company in USA [4] (David Bennett, 2010).Hybrid rice has been widely grown in the US, and now account for about 40% of acreage in Arkansas rice production in 2012-2014 [5] [6] (Jarrod T. Hardke, 2014).UAPB initiated its in hybrid rice research in 2008 [7] (Huang, B. 2012).Many breeding materials have be developed, and used in the hybrid rice research activities.
A period of very hot weather usually occurs in the summer in Pine Bluff in Arkansas.There were about 50 days with daily highest temperature over 34˚C from June 25-August 15 in 2012-2013 (Table 1).Rice panicle differentiation (PD) is very sensitive to high temperature.Rice heading usually occurs about 25 days after PD.The germplasm of earlier PD and heading may be away through avoidance to reduce heat-induced sterility at temperature above 34˚C.The best PD growing stage seems to be before June 25 and the heading stage will be before July 20 to avoid the high temperature in Pine Bluff, Arkansas.Therefore developing earlier maturity varieties are very important to the rice production in areas with very high summer temperature.
Breeding for early maturity hybrid rice is normally carried out by using the early maturity female sterile line crossed with an early maturity restore line.But this approach takes time and provides less diversified genetic materials.Another way is to use the sterile line with dominate early maturity gene (DEMG) as female crossed with different late maturity restorer line as male which is quicker, more effective and result in diversified genetics background from restorer materials.
A new sterile line UP-3s, which carries the Dominant Early Maturity Gene, was bred in University of Arkansas at Pine Bluff (UAPB).UP-3s came from Gobo (PI369806, a native rice variety of Surinam in South America)/Zhenshan 97//Xiangzaoxian No. 1, F 6 generation.UP-3s is a sterile line (two line system) with DEMG.Its plant height is about 90 cm and heading days 85 days.The maturity date of its progeny hybrids, crossing with different restorer line, is depending on the maturity of female parent sterile line UP-3s.It can get the earlier hybrids by using this sterile line.Some early maturity hybrid rice combinations have been developed by usin-gUP-3s crossed with male parents of different late maturity restorer lines in UAPB rice research program 2011 and 2012.[7] [8] (Huang et al. 2012(Huang et al. , 2015)).
Xie-A is a sterile line with non-DEMG.Xie-A came from Ignape Catelo (PI 373138, a native rice variety of Senegal in Africa)/Xieqingzao//Xieqingzao, B 6 F 1 generation.Its plant height is 85 cm, heading days is 75 days, and maturity dates of its progeny hybrids, crossing with different restorer line, are depending on the maturity of male parent (restorer lines).Late maturity hybrid rice combinations have been developed by using Xie-A crossed with male parents of different late maturity restorer lines in UAPB rice research program 2011.
Jin-23A also is a sterile line with non-DEMG.Jin-23A came from Ignape Catelo (PI 373138, a native rice variety of Senegal in Africa)/Jin-23//Jin-23, B 6 F 1 generation.Its plant height is about 80 cm, the heading days is about 60 days, and maturity date of its progeny hybrids, crossing with different restorer line, are depending on the maturity of male parent (restorer lines).Late maturity hybrid rice combinations have been developed by using Jin23-A crossed with male parents of different late maturity restorer lines in UAPB rice research program 2012.
The two kinds of hybrids above were tested and compared for their agronomic traits and yield in UAPB farm in 2012 and 2013 separately under the weather condition described in Table 1.

Materials and Methods
Ten hybrids were developed from sterile lines UP-3s (DEMG) and Xie-A (Check, non-DEMG) by separately crossed with 5 restorer lines PB-1R, PB-11R, PB-13R, PB-20R, PB-21R in 2011.These 10 hybrids and their 7 parents were tested for agronomic traits and yield in the farm of UAPB in 2012.Soil texture is silt loam with PH value of 5.3.The test entries were sowed at April 18 in the greenhouse and transplanted to field 20 days after sowing with 10 feet long, 1 foot space row, and 3 replications for each entry.Weeds were controlled with 9.3 L ha-1 of propanil (3', 4'-dichloropropionanilide) mixed with 0.4 kg•ha −1 of quinclorac (3, 7-dichloroquinoline-8-carboxylic acid; Facet, BASF) when the rice were about four-leaf stage.The nitrogen fertilizer was applied pre-flood at 134 kg•N•ha −1 at about the five-leaf stage.The flood (underground water from a well) was maintained throughout the growing season.Heading dates were recorded when 50% of the plants were headed.Pa-nicles were harvested 35 days after heading.Plant heights were measured at harvest.Weights of 100 grains and seed set rate were measured after harvest (seed set rate is the seeds percentage in total of spike lets per panicle), milled rice and head rice and yield were also measured.Another 8 hybrids were developed from sterile lines UP-3s (DEMG) and Jin23-A (Check, non-DEMG) by separately crossed with 4 restorer lines PB-5R, PB-13R, PB-22R, PB-23R in 2012.These 8 hybrids and their 6 parents were tested for agronomic traits and yields in the farm of UAPB in 2013 in a same soil type and management as in 2012.The test entries were sowed in 3 different dates (April 11, 21 and May 1, respectively) in the greenhouse and transplanted to the field 20 days after each sowing date with 5 feet long, 1 foot space row, and 3 replications for each entry.Heading dates were recorded when 50% of the plants were headed.Panicles were harvested 35 days after heading.Plant heights were measured at harvest.Weights of 100 grains and seed set rate were measured after harvest.
Daily temperatures were recorded by Arkansas SCAN (Soil Climate Analysis Network) Site which is about 50 meters away from the field (Table 1).Yields had been analyzed by analysis of variance (ANOVA) among entries (lines) with Duncan multiple comparisons, and descriptive of means and standard errors for each entries (SAS program, v 12.0).
The heading days and yields were similar for the hybrids UP-3s/PB-1R and Xie-A/PB-1R that were developed from sterile lines UP-3s and check Xie-A by crossed with the restorer line PB-1R due to PB-1R having early maturity gene.They headed in July 11-12 with 84 -85 heading days, and yield 10,860 and 10,778 kg/hectare, respectively (Table 2).
The cross UP-3/PB-13R matured 11 days earlier and yielded 16.8% higher than the cross of Xie-A/PB-13R; The cross of UP-3s/PB11R matured 11 days earlier and yielded 27.9% higher than the cross of Xie-A/ PB-11R; The cross of UP-3s/PB-13R matured 14 days earlier and yielded 15.3% higher than that cross of Xie-A/PB-21R; The cross ofUP-3s/PB-20R matured 14 days earlier and yielded 26.4% higher than the cross of Xie-A/PB-20R.All these results are due to the UP-3s has DEMG, and Xie-A has no DEMG (Table 2).
The heading days and yield of the crosses UP-3s/PB1R and Xie-A/PB1R were similar because PB-1R is an earlier season restores line (Table 2).
The results suggest that earlier planting results in higher yields.The yields of hybrid UP-3s/PB-13R in the first stage were 5.8% and 7.1% higher than that in the second and the third stage, respectively; The yields of hybrid UP-3s/PB-5R planted in the first stage were 7.2% and 9.7% higher than that in the second and third stage, respectively; The yields of hybrid UP-3s/PB-22R planted in the first stage were 7.3% and 13.1% higher than that in the second and third stage, respectively; The yields of hybrid UP-3s/PB-23R in the first stage were 5.0% and 8.7% higher than that in the second and third stage, respectively; The same trend also holds for the hybrids from Jin-23A, which does not have DEMG (Table 3).
These results showed that the early maturity hybrids with DEMG sterile female parent had higher yields than late maturity hybrids with non-DEMG sterile female parent, and the yield of earlier planting was higher than that late planting stage for the same hybrid.These results indicate that their PD (the panicle differentiation stage) had avoided the high temperature period.To achieve higher yield under the similar weather condition, the PD should be before June 25 and heading stage should be before July 20.The growing stage of a variety from sowing to heading should be earlier than 90 days to achieve higher yield in rice growing areas with high temperatures.
These results suggest that the better, quicker and more effective way to get the earlier maturity hybrids with the genetic diversity is to cross the DEMG sterile line with the late maturity restorer lines.The Up-3s is an excellent DEMG sterile line to make new early maturity hybrids with late maturity restorer line.This line and can be directly used in the hybrid rice seed production.
These results also suggest that the UP-3s is an ideal line to be used in new sterile line breeding through making crosses with other maintain lines and for screening new DEMG sterile lines in the breeding program.
These results suggest that the early PD and heading maybe one way to reduce heat-induce sterility at temperature above 34˚C.Late maturity restorer lines are normally used to make the late maturity hybrids with normal sterile lines, but they can also be used to breed early maturity hybrids through crossing with DEMG sterile line such as UP-3s which is quicker, more effective and can result in diversified genetic background from restorer materials.This technology could greatly contribute to hybrid rice breeding and seed production in the future.

Table 1 .
The daily highest temperature at Pine Bluff from 2011-2013.