Hybrid Rice Research and Development in Pakistan

MATERIALS AND METHODS

The establishment of testcross nursery to identify restorers and maintainers is the first step in three-line heterosis breeding. For this purpose, a source nursery comprising of uniform rice genotypes was transplanted on three different dates i.e., June 20th, July 5th and July 20th each year. Standard agronomic and plant protection measures were adopted during the years. From kharif 2000 to 2004, 608 test crosses were made among different elite lines and cms lines to categorize the existing germplasm as a restorer or maintainer line. Data on the agronomic characters of the potential restorers and maintainers were collected from 5 selected plants. Pollen studies were carried out for their fertility/sterility of testcross F₁ plants. For the purpose, 15-20 spikelets from the just emerged panicles of 3 randomly selected plants were collected in a vial containing 70% ethanol. All the anthers from 6 spikelets were taken out with the help of a forceps and placed on a glass slide with a drop of 1% Iodine Potassium Iodide (IKI) stain. the pollen grains were gently crushed with a needle to release the anthers. After removing the debris, a cover glass was placed and the slide was observed under the research microscope. The criteria for classifying the parental lines as maintainers and restorers were used as proposed by Virmani et al. (1997).

From 1993 to 2006 different IRRI hybrids received in the form of International Hybrid Rice Observational Nursery (IRHON) were evaluated for yield and other economic characteristics. Augmented in Randomized Complete Block Design has been adopted. Hybrids were transplanted with the spacing of 20x20 cm with one seedling per hill. The recommended plot size was 5x1.4 m for each entry. Standard agronomic and plant protection measures were adopted. Fertilizers at the rate of 170-100-0 NPK kg ha^-1 were applied. Chemical protection against diseases and insect pests was need-based level. The data on heading days, plant height, tillers per plant, phenotypic acceptability, grain yield and cooking quality were collected in each year. The same procedure was adopted for testing of local hybrids.

For hybrid seed production, the restorer (Basmati 385) was seeded 3 times with intervals of three days between seeding. CMS (A) line i.e., IR 58025A was seeded 21 days after second seeding of restorer. Accordingly 30 days old nursery of these parents were transplanted in the field on their scheduled/repeated dates. Six row ratio treatments considered were as follows.

Row length per treatment was 15 m. Plant to plant distance was 15 cm in all the three seeding of restorers. However, cms line seeding distance was spaced at 30x15 CMS. A space isolation of 50 m was provided for seed production plot to avoid the foreign pollen interception. To ensure purity of hybrid seed, roughing was done during different growth stages. At booting stage with the help of a sharp sickle ½ or 2/3 of the A line’s flag leaf was removed from just above the flag leaf joint with the tiller. Two applications of GA₃ spray were made at the rate of 3 g/1000 m², the first when 15-20% of the tillers started heading and the second 2 days after first spray. A ULV/Knapsack sprayer was used for this purpose. A rope was used at the time of flowering for supplementary pollination. This operation was done 3-4 times daily at peak anthesis for ten days (Virmani et al., 1997). Restorer parent was harvested first. A line seed was harvested and threshed separately.

For seed multiplication of cms line (A), IR58025A and IR73328A were seeded on 20.05.2006. The seeding of respective maintainer (B) lines, were done on 22.05.2006 and 25.05.2006 to prolong the pollen availability period. The CMS lines along with their maintainer lines were transplanted in the field accordingly with plant spacing of 20x20 cm. At flowering, supplementary pollination by shaking maintainer lines with the help of a rope was carried out to ensure the proper pollination for having good seed setting of CMS line.

For evaluation of cms lines, twenty two Cytoplasmic Male Sterile (CMS) lines and their respective maintainer lines were transplanted in the field on 15.07.2005 in rows of 30 plants spaced at 22.5 cm from each side At flowering, all CMS lines were crossed with their respective maintainers to produce the nucleus seed of CMS lines as well. CMS lines were evaluated for pollen sterility (%), Out Crossing Rate (OCR) and agronomic traits i.e., Plant height, No. of tillers/plant, spikelets per panicle and days to 50% flowering.

The general reference for data collection was the Standard Evaluation System for Rice (Anonymous, 1996).

RESULTS AND DISCUSSION

Evaluation of genetic material: Out of 608 test crosses, 38 lines were identified as restorers. Among these restorer lines, 16 belong to Basmati group while the rest fall in coarse rice (Table 1). Similarly 52 maintainers were found from the tested test crosses. Out of these maintainer lines, 26 lines were Basmati and the rest were coarse lines (Table 2). The frequency of restorers and maintainers were 6 and 9% respectively amongst the tested genotypes. A similar results were also found by Ali and Khan (1996) Sabar and Akhter (2003) and Virmani and Kumar (2004) but are in contradict with the findings of McWilliam et al. (1995). Important characteristics like plant height, productive tillers per plant, maturity days and number of grains per panicle of potential restorers are given in Table 3.

Table 1:	Elite genotypes identified as restorers

Table 2:	Elite genotypes identified as maintainers

Table 3:	Characteristics of potential restorers

Table 4:	Performance of exotic rice hybrids

Evaluation of exotic rice hybrids: From 1993 to 2006, 800 hybrids were evaluated for yield, adaptability and standard heterosis. Heading days, height, tillers, phenotypic acceptability and cooking of hybrids were studied from 1996 to 2006 (Table 4). Standard heterosis of hybrids varied from 8 to 142% over KS 282 (local check variety). Maximum yield of 11.76 t ha^-1 was given by the hybrid IR76715H, while the lowest yield of 5.23 t ha^-1 was given by the IR83212H. The present data depicted that there were rice hybrids those have higher yield than approved check variety i.e., KS 282. Ali (1998), Ali and Khan (1995) and (Virmani and Kumar (2004) also found similar results.

Table 5:	Performance of promising local rice hybrids
+ increase over check

Table 6:	CMS Lines and their characteristics

Cooking quality of the out yielding hybrid was recorded as poor compared to the check variety. These results are in accordance with the findings of Singh et al. (2000) and Khush et al. (1988).

Heading days of the tested hybrids ranged from 64 to 120 days compared to check variety which has about 72 days (Table 4). Early maturing hybrid was IR83212H while the late maturing hybrid was IR76715H. The tested hybrid IR 80785H obtained the shortest height (84 cm), while the tallest hybrid was IR 83212 H with the plant height of 127 cm. The check variety obtained the height of 105 cm. Regarding tillers per m², most of the hybrids had more number of tillers than the check variety (KS 282).

IR 73410H exhibited excellent crop stand and grain appearance having rank No. 1 for phenotypic acceptability, while two hybrids had rank No. 7 for the same trait. Cooking quality of the out yielding hybrid was recorded as poor compared to the check variety except IR 83212H. These results are in accordance with the findings of Singh et al. (2000) and Khush et al. (1988). Grain quality characteristics are very important parameters for the determining consumer favorable reception for any hybrid. It has been found that the grain quality of hybrids in not comparable to that of premier quality varieties such as basmati 385 and KS 282. Since most of the out yielding hybrids have inferior cooking quality and poor phenotypic acceptability, therefore, there is a dire need to develop local parental lines for the development of rice hybrids having high yield, good adaptability and acceptable cooking and eating quality.

Evaluation of new rice hybrids: In Basmati group, LH-1 was the only local test hybrid. LH-1 gave 20% standard Heterosis for paddy yield over the check variety Basmati 385 (Table 5). The maturity days of LH-1 were at par with Basmati 385. In coarse group, 2 rice test hybrids LH-10 and LH-15 performed better than the check variety KS282. Standard Heterosis for paddy of LH-15 and LH-10 was 21 and 14%, respectively. Plant height and maturity days were at par with KS282.

Evaluation of CMS lines: Morphological characteristics of these 22 CMS lines were recorded. Complete pollen sterility was observed in all lines. All CMS lines were also evaluated for agronomic traits (Table 6). Semi dwarf plant height was observed in all lines except IR75606A which had intermediate height. Better tillering ability was observed in all CMS lines. The range of spikelets per panicle was 68-170. IR 69616A was remained on the top with 170 spikelets/panicle whereas IR 79128A was remained at the bottom. Six CMS line were late in maturity i.e.,>100 days for 50% flowering. IR 68897A gave maximum out crossing rate i.e., 46% amongst all the cms lines followed by IR 70369A having 40% out crossing rate.

On the basis of the traits evaluated, IR58025A, IR79156A, IR68897A, IR68886A, IR73328A, IR75596A, IR70369A and IR79128A can be recommended for hybrid rice seed production on the basis of their better floral and agronomic characteristics. For the development of new local cms lines forty six maintainers are in CMS conversion programme. These are 3, 3, 6 and 34 lines, in BC₅, BC₃, BC₂ and BC₁, respectively.

Hybrid rice seed production: Highest seed yield of >1.24 t ha^-1 was recorded in row ratio 2:10. However, the seed yield from 0.80-1.10 t ha^-1 was produced by row ratios 2:12, 3:10 and 3:12. Lowest hybrid seed yield was produced in row ratio 3:8. These findings were also observed by Ali et al. (2005). Better synchronization and more number of tillers of the restorers and CMS line can produce > 3.0 t ha^-1.

CONCLUSION

The study was a good start to categorize the local germplasm for hybrid rice research studies. Basmati 385, 99722, 40265, PK3699-43 and LG22 were identified as potential restorers. IR58025A, IR79156A, IR68897A, IR68886A, IR73328A, IR75596A, IR70369A and IR79128A were selected for the development of hybrid rice on the basis of their better floral and agronomic characteristics. For hybrid seed production, the row ratio 2:10 (Restorer: CMS) gave the maximum seed yield i.e., >1.5 t ha^-1. So far, ten rice hybrids have been developed and evaluated for yield and grain quality characteristics. Rice hybrids having higher magnitude of heterosis, better grain quality and resistance to major pests and diseases are needed to be developed. Top priority has to be given to maintain the purity of parental lines and to produce high quality hybrid seed. Involvement of seed agencies from private sector will be crucial to meet the increased demand for hybrid seed.

ACKNOWLEDGMENT

This study was supported by the research grant from the Government of the Punjab and from Agriculture Linkages Programme, Pakistan Agricultural Research Council, Islamabad. The authors would like to thank Dr. Fangming Xie and Dr. S.S.Virmani, Hybrid rice scientists, IRRI, Philippines for their technical assistance.