Cytoplasmic male sterility (cms) is used increasingly in hybrid seed production, because of its superior cost-efficiency. Non-restored cms-hybrids often yield more than their male-fertile counterparts. An additional positive effect is found when these cms-hybrids are pollinated by unrelated hybrids. This combined effect of cms and genetically dissimilar pollen sources (i.e. xenia) is referred to as the "Plus-Hybrid" effect. The Plus-Hybrid system consists of blending a non-restored cms-hybrid with an unrelated male-fertile hybrid as a pollinator. The objectives of this study were to: (i) determine the combined effect of male sterility and xenia on the grain yield of male-sterile hybrids as pure stands in small-plot experiments and (ii) to evaluate applicable Plus- Hybrids as blends in large-strip mixture trials. Small-plot experiments were conducted for three years in Thailand with detasseled hybrids, and for two years in the USA and three years in Switzerland with cms-hybrids. Compared to their isogenically pollinated male-fertile counterparts, the average grain yield increases of Plus-Hybrids were +4.4 % (p<0.10) with Asian, +4.5 % (p<0.10) with American, and +8.2 % (p<0.01) with European germplasm. To confirm those findings of small-plot trials, large-strip mixture trials were conducted for three years in the USA and one year in Switzerland. All Plus-Hybrids in the large-strip mixture trials outyielded their isogenically pollinated (partially) male-fertile control. The Plus-Hybrid system is an important option for obtaining substantial increases in grain yield also in Asia, especially when cytoplasmic male-sterile versions of elite germplasm are available. Cytoplasmic male sterility (CMS) was extensively studied in the 1950s and 1960s. The outstanding advantage of CMS was that mechanical or manual removal of the tassels, and hence considerable costs in the hybrid seed production, could be avoided. Some of the researchers at that time reported increases in grain yield as a result of CMS. After the epidemic of southern corn leaf blight (Helminthosporium maydis, Cochliobolus heterostrophus) in the US Corn Belt in 1970 (Ullstrup, 1972), the use and, consequently, the research into CMS was abruptly terminated (Duvick and Noble, 1978). Within a few years, hybrid maize companies have switched back completely to normal male- fertile cytoplasm, and detasseling was the method of choice for producing hybrid seed. However, with increasing economic pressure on the profit margin, which seed producers receive per unit sold, CMS has again become more important (B. Fabre, Controleur national mai's SOC, France, personal communication). In Europe and the USA, the number of commercial hybrids produced with CMS is increasing steadily. An adequate amount of pollen is necessary to ensure fertilization of all the ovaries of a maize ear. However, what is an adequate amount? About 14 to 50 million pollen grains are shed per fertile plant (Feil and Schmid, 2002). This corresponds to a minimum of 100,000 pollen grains per fertilized kernel. Already at the end of the 19th century, Watson (1893) concluded from his experiments with detasseled plants: "... the experiments indicate that there is more pollen produced by the corn plant than is necessary to produce a maximum crop and that this over production is an exhaustive process". Xenia is another biological factor affecting flowering, i.e., the effect of non-isogenic pollination. As early as at the beginning of last century, the importance of this effect was reported (Carrier, 1919): "Agronomists have been unreasonably slow in accepting that the pollen which fertilizes the silk may influence the size and the weight of the grain produced as well as its color." The question as to whether detasseling or CMS and, consequently, an interruption of the exhaustive process (Watson, 1893) at an early growth stage have positive effects on seed set may be important for environments where resources are limited. In a first study with the open-pollinated cultivar Suwan 2 in Thailand, we demonstrated that grain yield, number of ears per plant, and harvest index were consistently higher for the CMS version than for the normal male-fertile version, especially after preanthesis drought stress (Stamp et al., 2000). These findings and, moreover, the fact that xenia may indicate that maize grain yield is not solely source- but also sink-limited, should lead us to reconsider the status quo of grain maize production in industrialized countries, where it is common practice to plant one variety per field. Bulant and Gallais (1998) showed that xenia increased the grain yields of inbred lines. Similarly, the hypothesis can be put forward that it may be beneficial to pollinate male-sterile hybrids with pollen from an unrelated hybrid. There are no published studies on the combined effects of male sterility and xenia with regard to maize grain yield. We hypothesized that combining male sterility and xenia to "Plus-Hybrids" is one way of increasing maize grain yields, at least in a range of additive impacts resulting from both these biological factors affecting flowering. In order to keep a fast practical application in mind, the prerequisites for the experiments were: (i) only current commercial or pre-commercial hybrids should be investigated, (ii) all the combinations of germplasm (dent x dent, dent x flint, flint x flint) should be tested in their respective environments, (iii) all three cytoplasms types (T, C and S) should be used. Thus, a collaborative research program was developed to investigate the performance of Plus-Hybrids in Thailand, USA, and Switzerland.

English

0501|AGRIS 0501|AL-Maize Program

Juan Carlos Mendieta

CIMMYT Publications Collection