Heterosis and combining ability of hybrid rice and its relation to Japonica-indica index of parents 

RJ. zhang1, S. ming1, C.W. Xu1, L.S. yang1, Y.S. bai1, C.Q. sun2 and X.K. wanc2

1) Anhui Academy of Agricultural Sciences, Hefei, 230031 China

2) China Agricultural University, Beijing, 100094 China

 Utilization of heterosis of F1 hybrid between Japonica and Indica has been one of the main directions of hybrid rice breeding in the recent years. But several problems of F1 hybrid between Japonica and Indica have not been solved; for example, tall plant height. long growth period, low seed set and low degree of grain filling, etc. Thus, direct utilization of F1 hybrid between Japonica and Indica has been restricted. Under such situation, use of Japonica-like rice (J?) which is not typical but Japonica-inclined type is expected to be one of the effective ways to improve grain yield of hybrid rice.

Twelve parents (Table 1) were crossed according to NC II design and thirty-two F hybrids were obtained. The F1 s, their parents, and CK Shanyou 63 were grown in Hefei in midseason in 1995, in three replications of a randomized block design. Twelve parents were classified by Cheng's index (classification index of Indica and Japonica rice. Cheng 1993). Eight characters, i.e., days to heading, plant height, panicle length, number of effective tillers, number of grains per panicle, seed set, 1000-grain weight and grain yield per plant were observed in F1 s and parents.

The effects of female, male and their interaction on hybrids were estimated in terms of % variance. As shown in Table 2, the male effect was greater than female and interaction effects in all characters except for 1000-grain weight in which female effect was greater than the sum of male and interaction effect. Therefore, characters carried by R-line (male parent) was extremely important in breeding of Japonica Two-line hybrid rice.

Relative heterosis (RH) and standard heterosis (SH) were also estimated (Table 3). Positive crosses outnumbered negative crosses in all characters for RH, while only in three characters for SH. It was found that degree of heterosis remarkably diferred among crosses. In number of grains per panicle, heterosis ranged from -1.1% to 107.9% with a mean 33.5% in RH, and from -17.1% to 82.7% with a mean 23.1% in SH. In grain yield, SH

Research Notes
Table 1. Cheng's index and specific property of parents
Parents Cheng's index1) Specific property2)
4008s 22(J) WCV PGMS
8087s 21(J) PGMS
7001s 21(J) PGMS
80-4B 22(J) B-line
Cbao 19(J) R-line
HP121 16 (J') WCV R-line
HP214 15 (J?) WCV R-line
PA64 4(I) WCV R-line
LH422 16 (J?) WCV R-line
YHIOI 14 (J?) WCV
02428 20 (J) WCV
Neken 1 22(J) WGV
1) J: Japonica, J': Japonica-like, I: Indica.

2) WCV: wide compatible variety, PGMS: photoperiod sensitive genie male sterile, B-line: maintainer, R-line: restorer.
Table 2.Contributions of female, male and their interactions to hybrid variance (%)
Items Days to

heading

 Plant

Height(cm)

 Panicle

Length (cm)

 Effective tillers Grains per

panicle

 Seed

set(%)

 1000-gram

weight(g)

 Grain yield

per plant(g)
Female 4.3 3.2 14.1 8.2 17.2 15.6 57.3 10.6
Male 87.5 82.7 71.5 59.4 57.7 45.0 23.0 55.7
Female X male 8.2 14.1  14.4 32.4 25.1 39.3 19.7 33.7

 
 
Table 3. Comparison of degree of heterosis among different cross groups
Characters Relative heterosis1) Average of RH Standard heterosis2) Average of SH
P3) N4) A5) J X J J X J? J X I P N A J X J J X J? J X I
Days to heading 30 2 8.8 5.1 7.3 22.6 32 0 10.2 7.4 9.4 22.3
Plant height 32 0 30.1 21.3 33.9 41.8 8 24 -6.0 -7.9 -5.7 -1.1
Panicle length 32 0 15.1 11.7 14.0 30.3 2 30 -11.9 -16.1 -10.8 -4.1
Effective tillers 22 10 9.1 1.8 9.7 28.4 17 15 4.0 -0.3 0.1 32.7
Grains per panicle 31 1 33.5 22.7 33.6 65.2 28 4 23.1 6.0 36.3 21.8
Seed set 3 29 -7.4 -5.4 -8.4 -9.9
1000-grain weight 0 32 -16.0 -15.7 -15.8 -17.5
Grain yield 12 20 -2.4 -17.0 3.5 18.0
  1. Relative heterosis (RH): (F1 -MP) x 100 / MP, MP =(P1 +P2 ) / 2. 2) Standard heterosis (SH): (F1 -CK) x 100 / CK.
3) P: Number of positive crosses. 4) N: Number of negative crosses. 

5) A: Average of all crosses.

36 Rice Genetics Newsletter Vol. 14

Fig. 1. Relationship between relative heterosis (RH) and the difference in Cheng's index between

the two parents in number of effective tillers and number of grains per panicle. showed a range of -36.8% to 33.6% with a mean -2.4%. SH of seed setting rate ranged from -23.5% to 3.4% with a mean -7.4%.

Next, heterosis was compared among three cross-combination groups; J x J, J x F, and J x 1. In general, heterosis showed an order of J x J < J x J' <J x I, indicating that remotely related parents showed high degree of heterosis as expected. The differences in Cheng's index between the two parents were significantly correlated with respective RH (r = 0.52-0.72). The cases with number of effective tillers and number of grains per panicle are shown in Fig. 1. In SH, however, number of grains per panicle showed J x J < J x J' > J x I, suggesting that a large number of grains per panicle which is one of the important yielding components is expected in J x J ' crosses. But seed set and 1000-grain weight showed a trend of J x J > J x J ' > J x 1. The above results suggest that J x J ' cross may contribute to increase number of spikelets per panicle but all spikelets are not always fully filled.

Japonica PGMS line 4008s, Indica R-line PA64 and Japonica-like WCV YH101 had higher G.C.A. (general combining ability) value of grain yield. Hybrids with high S.C.A. (special combining ability) value of grain yield were 80-4B/YH101, 80-4B/02428 and 4008s/HP214. The difference in Cheng's index between the two parents had very little effect on S.C.A. values of seven characters, correlation coefficients ranging from -0.02 to 0.10 except for effective tillers (-0.36).

Reference 

Cheng Kan-Sheng, 1993. Discrimination of Indica and Japonica subspecies in Asian cultivated rice. Yunnan

Science and Technology Press. Kunming, China, pp.45.