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Directorate of Wheat Research, Karnal 132 001, Haryana, India

COMMERCIAL HETEROSIS IN WHEAT – AN OVERVIEW

 Authors: Vinay Mahajan, S Nagarajan, Manoj Srivastav ,Vineet Kumar and N V P R Ganga Rao

 Corresponding Author: Dr Vinay Mahajan, Email: vimahan@hotmail.com

 Address: Directorate of Wheat Research, P O Box 158, Karnal, Haryana, India

 Contents

Table 1: Heterotic advantage for grain yield in bigger wheat plots under normal planting 

Introduction
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Exploitation of heterosis through hybrid wheat is more attractive than the conventional plant breeding methods due to lower yield gain of 1% per year in North Western Plains Zone – the bread bowl of India. The traditional breeding methods had so far exploited inter-genomic heterosis making it higher yielding than other diploid crops however, the advantage of intra-genomic heterosis in the three genomes can be harvested through the use of hybrid wheat. In India, efforts on hybrid wheat were initiated in sixties following cytoplasmic male sterility (CMS) approach however no significant results were obtained. In 1995, Directorate of Wheat Research, Karnal had decided to re-address hybrid wheat with an emphasis on chemical hybridizing agent (CHA) approach. The hybrid wheat was addressed primarily at two fronts in CHA approach (i) to evaluate and operationalize chemical hybridizing agent in producing hybrids and (ii) to identify parental lines that exhibit commercial heterosis.

 The advantage of commercial heterosis in hybrids over the best check is exploited in many cross-pollinated crops as well as self-pollinated crops such as corn, pearl millet, sorghum, sunflower, cotton, pigeon pea, rice and a number of vegetable crops for commercially important characters.

Heterosis in Wheat

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Since mid sixties another issue before the wheat group is identification/ development of superior hybrids and parents with exceptional combining ability. Four year results of Livers and Heyne (1968) revealed a yield increase of about 30% in hybrid over the best cultivar. By the end of 1970’s when parents with high specific combining ability were used, it resulted in hybrids with 10-15% yield superiority over the best variety. In pre-semi-dwarf period in wheat commercially viable wheat had several limitations such as:

a. In early 1970’s when tall parents were used, the hybrids had excessive biomass production and were too tall.

b. The hybrids produced by hand pollination on emasculated ears were grown in high input managed fields with few replications hence the real yield potential of commercial hybrids was still questioned.

c. Due to limited number of tests and paucity of information on genotype x environment interaction, commercial hybrids were not always the best.

 These limitations can be overcome by developing simple and perfect methodology for producing large number of out cross seed for large number of cross combinations and evaluating them for their per se performance over a range of environmental conditions. The revolutionary chemical formulations like CH 9701, CH 9708, CH 9832 etc.. may simplify hybrid seed production in wheat. More than 35 chemicals with diverse group of structural families have been patented as potential CHAs (Lucken and Johnson 1988).

Wheat is one of the most systematically evolved crops at national and international level, wherein there is extensive introgression of one gene pool into another gene pool in different wheat based agro-climatic zones, still maintaining their own identity. At present the success of hybrids not only depends upon the exceptional specific combining ability but also to the suitability to CHAs based methodology for hybrid seed production.

The issues in identification of heterosis can be addressed through components like

1.   Matching the yield components to achieve yield maximization in hybrids

2.   Diversity in parental lines

3.   Hybrid x environment interaction

4.   Use of other characters like disease resistance, quality as a pre-requisite to develop superior hybrids

5.   Exploring the advantage of allopolyploidy in wheat

 Extent of Heterosis

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Enough reports are available in literature indicating high heterosis over mid-parent or the best parent however the real commercial feasibility of hybrid wheat depends upon the heterotic advantage over the best ruling variety of that agro-climatic zone. Wheat group dealing with various aspects of hybrid wheat found that the standard heterosis for grain yield on bigger plot basis (Table 1) ranged from 6% (Borghi et al 1988) to as high as 41% (Zehr et al 1997). Fabrizino and co-workers (1998) are of the view that the expression of heterosis was due in part to genetic diversity but was unpredictable and also depended on factors not elucidated by their study.

Genetics of Yield

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For majority of characters, gca (general combining ability) was of greater importance than sca (specific combining ability). Among few papers based on large scale trials (Borghi et al 1989; Bears and Bingham 1989; Morgan et al 1989; Perenzin et al 1992; Borghi and Perenzin 1994) also confirm greater importance of gca. Due to reciprocal compensation among traits the large sca effects of some yield components are difficult to exploit. Non-additive genetic variance or sca is best expressed in space planting (Patrico & Patterson 1973; Windner and Labsock 1973; Mani and Rao 1977; Cregan and Busch 1978; Rehman 1978; Quick 1978; Mihaljer 1980; Virmani and Edwards 1983; Lucken 1986).

In high yielding hybrids although several yield components appear to be important in determining grain yield (Liver and Heyne, 1968), the role played by the number of spikelets per spike appeared important (Borghi et al 1988). The kernel weight tented to be higher in hybrids but this does not contribute significantly to increased yields (Borghi et al 1988). A positive association was observed between grain yield and harvest index for the best combinations (Sinha and Khanna 1975). It was suggested that hybrids perform better because of their superior capacity in producing and partitioning biomass. The high grain yield of hybrids was associated with an increase of plant height while the harvest index was slightly higher than found for varieties (Edward et al 1980).

Hybrid x Environment interaction

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Wheat hybrids are found to be stable for their performance in different environments and seasons as observed by Wienhues (1968) and Stroike (1987), while Boland and Walcott (1985) and Borghi and Perenzin (1990) reported that the yield stability of the hybrids was intermediate to that of the parents.

Disease Resistance in Hybrids

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Resistance genes could be accumulated in the hybrids (Stroike 1987). Resistance to disease is dominant and is expressed in the heterozygote, it could be quickly incorporated in the hybrids (Johnson and Schmidt 1968). Hybrid seed production system could be more feasible to match with the evolution of the parasite population (Driscoll 1981).

Wheat quality of Hybrids

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Among various quality parameters, protein quality is key element in determining bread-baking performance. Even though, the protein quality is a genetic character but is significantly affected by environmental conditions like high nitrogen application.

Hybrids are generally intermediate to the parents in flour yield, dough properties and baking quality (Johnson and Schmidt 1968; Bequette and Fischer 1980). The wheat hybrids producing large amount of dry matter compared to their parents exhibited a positive correlation between protein content and total biomass (Corbellini and Borghi 1985; Borghi et al 1986) which suggest that high protein content in hybrids may be partly due to the enhanced source. Edwards (1987) who analysed several hybrids from hard x soft red wheat crosses, emphasized the necessity to use parents with very strong mixing properties to offset the soft wheat effects and concluded that complementation for quality characters does appear to be the main advantage offered by the hybrid genotypes. Perenzin et al (1992) observed that some hybrids derived from crosses between low quality-high yielding cvs. and high quality-low yielding cvs. revealed a yield level approaching highest yield cvs. coupled with a bread-making quality corresponding to the first class of the Italian market (W>250, P/L<1). Borghi and Perenzin (1994) reported that hybrid Maestra x Golia was not only statistically at par for yield and agronomically superior to the best check Eridano due to reduced plant height, but also had superior grain quality which represents a 30% higher selling price. Grain and bread making quality characters (protein and SDS sedimentation) were generally not adversely affected in the hybrids and depends on the parental material (Cukadar et al 1997). The present literature revealed that a satisfactory bread making properties combined with high yields can be obtained with at least first generation of hybrids. Parental lines having superior quality parameters when combined with genetically diverse high yielding superior ideotype may result in hybrid combinations which may be superior in economic yield or quality over the best check. No information is traceable on the effect of allopolyploidy on heterosis in wheat.

Future Prospects

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Heterotic advantage up to 41 per cent on large plot basis has been reported so far. The desired expression of economic heterosis in wheat can be achieved by matching yield components, quality and disease resistance from genetically diverse parents. Though genetic diversity may be one of the important factors in search of commercial heterosis, but there could be certain unexplored factors that may limit the understanding and use of heterosis in wheat. Some less explored genetic diversity of China and Australia may be useful material for future genetic stocks in hybrid program to develop trait specific gene pools. At present the hybrid wheat in India had to pass through the barrier of free flow of information among public and private organizations and will emerge as a winner to make hybrid wheat an attractive reality.

LITERATURE CITED
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