Hybrid rice has established its superiority over high yielding varieties in respect of growth, yield and tolerance to stresses. The first commercially usable Cytoplasmic Male Sterile (CMS) line was developed in China during 1973 from a spontaneous male sterile plant isolated in a population of the wild rice Oryza sativa f. Spontanea in Hainan Island (Cheng, 2012). The discovery of this source, designated as Wild Abortive (WA) type is considered a landmark in the history of hybrid rice. With the availability of effective seed production through CMS-fertility restorer system, commercial utilization of heterosis has become practically feasible in rice. Following the success of hybrid rice in China, the International Rice Research Institute (IRRI), Philippines in 1979 augmented research to evolve hybrid ideally suited to tropical environment. IRRI Developed stable male sterile lines including, IR 58025 A, IR 62829 A, IR 68888 A, IR 68897 A and many others in the background of indica rice varieties suited for tropical conditions and has facilitated intensive hybrid breeding research in many countries. Recognizing the potential of this technology, the Indian Council of Agricultural Research (ICAR) launched a mission mode project on hybrid rice during 1989. As a result of concerted efforts for more than 2 decades, a total of 59 rice hybridshave been released up to 2012 for commercial cultivation in India (Viraktamath et al., 2012). However, for development of region and situation specific hybrids to enhance the area under hybrid rice cultivation, it is important to estimate the magnitude of heterosis in number of hybrid combinations. Identification of superior hybrid combinations is prerequisite for development of new hybrids. As rice is being cultivated in varying land and agro-climatic conditions, baskets of choice of hybrids are required to fulfill the need of the stakeholders. Variable types of restorer lines may be tested with different CMS lines for studying the magnitude of yield heterosis along-with heterosis for other yield attributing traits for searching suitable hybrids to meet the region specific requirements. Keeping these points in view the present investigation was formulated to study the heterosis in rice.

Results and Discussion

The analysis of variance (Table 1) revealed that highly significant differences among lines (female) for several traits under study except for 50 % flowering, days to maturity, plant height (cm), number of effective tillers plant^-1, number of spikelet panicle^-1, number of grains panicle^-1, 1000 grains weight (g), while variance among males (testers) were highly significant for almost all the traits. The variance among crosses due to males and females (lines×testers) interaction component were highly significant for all the traits. The present findings confirming the earlier findings of Saravanan et al (2008) and Tiwari et al (2011).

Table 1 ANOVA for L×T analysis for eighteen characters in rice

Heterosis was estimated as percent increase or decrease of F₁ values over better parent (BP) and standard hybrid (SH); Pro Agro-6444. The nature and magnitude of heterosis are presented in Table 2.

Table 2 Estimates of heterosis based on better parent (BP) and standard hybrid check, Arize 6444 (SV) for different characters in 48 rice hybrids

Negative heterosis is desirable for days to 50% flowering because this will help the hybrid to mature earlier (Table 3). The nature and magnitude of heterosis revealed that none of the hybrids showed negative and significant heterosis over BP, whereas 14 hybrids showed significant negative heterosis over standard hybrid (Pro Agro 6444) among them top three exhibiting earliness were IR68897A´Krishna Hansa, IR58025A´CR2340-1 and IR68897A´CR2340-1. Heterosis in positive as well as negative direction for days to 50% flowering has also been reported by Lingaraju et al (1999) and Patil et al (2003).

Table 3 Ranking of five desirable hybrids on the basis of per se performance, heterobeltiosis and economic Heterosis for different characters in rice

The short duration varieties are of breeder’s interest to fit them into rainfed cultivation, therefore, heterosis in negative direction is desirable for days to maturity. All the hybrids showed negative and significant heterosis over SH (Pro Agro 6444), among them top three hybrids exhibiting short duration were IR68897A´Krishna Hansa, IR58025A´CR2340-1, and IR 68897A×CR 2340-1. These finding are in agreement with those of Bhave et al (2002) and Sarial et al (2006).

Out of 48 hybrids, 28 exhibited significantly shorter plant stature over SH (Pro Agro 6444), but none over BP. Heterotic effects were observed with highest magnitude over the check (Pro Agro 6444) in crosses Pusa 6A×MTU-7029, IR68897A×MTU-7029 and IR58025A×MTU-7029. Dwarf stature is preferable as it is less prone to lodging (Janardhanam et al., 2001).

Higher number of effective tillers plant^-1 generally contributes to higher grain yield. In this study, 6 hybrids possessed positive and significant heterosis over BP and SH (Pro Agro 6444), the hybrids showing highest magnitude of heterosis over Pro Agro 6444 were;Pusa6A×Krishna Hansa, Pusa6A×Badshahbhog, IR68897A×Badshahbhog and Pusa 6 A×Type-3. Such range of heterosis for this trait was also reported Singh (2000) and Patil et al (2003).

Generally, larger panicle is associated with high number of grains panicle^-1 resulting into higher productivity, therefore, positive heterosis for panicle length are desirable (Tiwari et al., 2011). Out of 48 hybrids, 16 showed positive heterosis with significant values over better parent and none of the hybrids manifested heterosis over SH (Pro Agro 6444). The similar results were reported by Patil et al (2003), Verma et al (2005) and Eradasappa et al (2007).

Positiveheterosis for number of spikelets panicle^-1 is desirable. Sixteen of the hybrids studied expressed high heterotic estimates with positive and significant values over respective better parent. The result of present study indicated that a total of 12 crosses exhibited desirable heterosis over SH (Pro Agro 6444), The heterotic hybrids, IR68897A´Malviya-36, Pusa6A´BPT5204, Pusa6A´Pusa Sugandha-4 and Pusa6A´Malviya-36 are the best hybrids for this trait. These findings are in close conformity with the result of Tiwari et al (2011).

Number of grains panicle^-1 is the major yield attributing character, hence significant positive heterobeltiosis and standard heterosis is desirable. Fourteen crosses showed significant positive heterosis over BP and four hybrids over SH (Pro Agro 6444), respectively. The crosses IR68897A×Malviya-36, Pusa6A×Pusa Sugandha-4, IR68897A×BPT-5204 and IR 58025A×Sarju-52 are the top heterotic hybrids for number of grainspanicle^-1. These findings are in close conformity with the results of Singh and Haque (2000) and, Saravanan et al (2008).

1000-grainweight is important yield attributing trait. Out of 48 hybrids studied none of the hybrid showed positive and significant heterosis over BP, while 12 hybrids exhibited positive and significant heterosis over the SH (Pro Agro 6444). Top three crosses viz., IR58025A×Pusa Sugandha-4, IR68897A´Narendra-359 and IR 58025A×HUR 3022 exhibited significant positive heterosis over SH. Sarial et al (2006) reported similar observations.

Heterobeltiosis (%)=  

Economic heterosis (%) = 

Where, F_{1 is} the average performance of crosses, BP is the average performance of better parent, and SV is the average performance of standard variety (standard hybrid in this study, Arize 6444).

Wynne J.C., Emmer D.A., and Rice P.W., 1970, Combining ability estimates in Arachis hypogaea L. II. Field perfomramne of F1 hybrids, Crop Sci., 10: 713-715
http://dx.doi.org/10.2135/cropsci1970.0011183X001000060036x