Cytoplasmic male sterility offers great potential for facilitating co-existence in maize, and therefore merits close study. Researchers look into many aspects of CMS maize, including a review of past experience with CMS maize lines, checks to see how reliably CMS reduces pollen release, and investigate into the potential use of the plus-hybrid system with modern hybrids.
To gather as much existing information as possible with CMS maize, researchers searched printed publications and the internet to explore the challenges, gaps, and opportunities linked with CMS crops. Special focus was placed on:
The stability of various forms of CMS in maize
Suboptimal seed set due to insufficient pollination
Xenia: prospect of enhancing grain yield and quality
Practical farm experience with CMS maize
To check the degree and stability of male sterility maize, field trials were started with different maize CMS hybrids under a variety of field conditions at three locations in Switzerland, France, Bulgaria, and Germany. The hybrids were checked for the presence of pollen and for the fertility of the produced pollen.
A selected set of hybrids were tested at research sites in France and Switzlerland in 2005 to pinpoint the best combinations of pollen donors and male sterile cultivars to maximise the benefits of the plus-hybrid system. Researchers are taking grain collected from each pairing and measuring total yield and average kernel weight. Flowering synchrony is also being taken into consideration. 2006 a ring test with selected hybrids will be carried out in Switzerland, Germany, Bulgaria and France.
History: The review explains that in 1970, 85% of US hybrid maize seed was produced with CMS. The variant of CMS used at the time, however, made maize susceptible to a disease called Southern Corn Leaf Blight, which suddenly ravaged the country. Seed producers then abandoned CMS for hybrid seed production. Today, some seed producers have re-recruited CMS maize lines due to the high cost of manually removing male flowers.
CMS effect: It has been known since the 19th century that male sterile maize lines yield more than male fertile lines. This was thought to be caused by sparing the plant the exhaustive process of pollen production. It is now postulated that male sterile plants are able to concentrate nutrients to the female reproductive sites when kernels are first being formed, resulting in increased kernel survivability. This phenomenon varies depending on the cultivar and on environmental conditions.
CMS Stability: There is virtually no published information on the stability of CMS in maize. For other crops, CMS stability depends on factors such as temperature, day-length, and water availability. It is generally thought that CMS is most unstable when temperatures are warm and days are long. An intermediate state of male sterility is known as fluctuant. Fluctuant tassels have anthers on the off-shoots from the tassel’s central stalk.
Plus-hybrid system: Not much is known about the xenia effect in maize. Studies in the 1920s and in the 1990s suggest that more distantly related pollen donors favour higher yield. Experiments in Europe have shown that strategic pairings of CMS hybrids and pollen donors can increase yield by 20 percent when male fertile plants make up 15 to 20 percent of the planting.
Farm experience: Experience with CMS maize and pollen donor blends is already available. Specialty high-oil maize was grown in the US on over 400,000 hectares in 1999. With 92 percent of the seeds male sterile and 8 percent acting as pollen donors, yield is slightly lower than a pure field of male fertile maize. It is not clarified if this could be due to incomplete pollination.
Outlook: Planting mixtures of CMS and conventional seed will mean added uncertainty and effort for farmers and extra work for seed producers. Therefore, farmers are not expected to receive CMS maize with open arms unless reliable yield and quality increases are involved.
Twenty maize hybrids were tested to see how well the CMS trait performed under a variety of environmental conditions. Of the twenty hybrids tested, only six had completely male sterile tassels at all of the test sites. Three other hybrids produced anthers (pollen-bearing sacs) at some locations, but never released pollen. Four of the hybrids produced pollen at all of the test sites, although tests are still underway to see if the pollen was viable (capable of fertilising).
The remaining seven hybrids showed different degrees of sterility depending on environmental conditions, suggesting that environmental conditions play an important role in determining how well male sterile maize can prevent unwanted out-crossing.
The Plus-Hybrid system would be a useful tool to achieve an agricultural bio-containment system. For this system, a high level of male sterility must be maintained, as shown by this study. Furthermore, appropriate combinations of CMS hybrids and fertile pollinators can lead to a significant gain in yield that would definitely boost the acceptance of a bio-containment system with cytoplasmic male sterility.
Bio-containment of maize by cytoplasmic male sterility and xenia
|NAME / ORGANISATION||CONTACT INFORMATION|
Swiss Federal Institute of Technology in Zurich (ETHZ), Switzerland
ARVALIS - Institute du végétal, France
AgroBioInstitute (ABI), Bulgaria
|Joachim Schiemann / Alexandra Huesken|
Federal Biological Research Centre for Agriculture and Forestry, Germany