Rye (Secale cereale L.)

Abstract

Rye (Secale cereale L.) is mainly a European cereal with about 75% of the global production growing in Russia, Belarus, Poland, Germany, and Ukraine. It has the best overwintering ability, and the highest tolerance to drought, salt, or aluminium stress from all small-grain cereals. Harvest is used for bread making, feed, and in growing demands for ethanol and biomethane production as a renewable energy source. Hybrid rye is competitive to triticale and wheat also on better soils and grown in Germany on about 70% of the total rye acreage. Rye developed in the Middle East as a secondary crop, cultivated rye has its greatest diversity in landraces and populations from Central and East Europe. Their utility for breeding has considerably increased by progress in marker-based introgression of donor chromosome segments. Resistance breeding is presently focused on leaf and stem rust (Puccinia recondita, P. graminis f.sp. secalis), ergot (Claviceps purpurea), and Fusarium diseases. Leaf blotch (Rhynchosporium secalis) and soilborne viruses might gain more attention in the future. Main breeding goals are grain yield, straw shortness, lodging resistance, high kernel weight, tolerances to pre-harvest sprouting and abiotic stresses. Population varieties comprise open-pollinated and synthetic varieties. Both are derived from self-incompatible breeding populations which are steadily improved by recurrent half- or full-sib selection. Open pollinated varieties (OPVs) constitute selected fractions of those populations whereas synthetic varieties are composed of specifically selected parents from which they can identically be reconstituted. Most modern population varieties contain germplasm from two or more genetically distant gene pools. Hybrid breeding is based on self-fertile gene pools and cytoplasmic genic male sterility (CMS) is used as hybridizing mechanism. Long-lasting breeding cycles are needed for the development of seed parent lines since testcrossing is only possible after the inbred lines have been converted to CMS analogues by repeated backcrossing. Options to speed up this process are discussed. Development of restorer lines is straightforward once effective restorer genes have been introduced to the respective breeding populations. Recurrent improvement of fertility restoration is most efficiently accomplished by recombining selected inbred lines after the first or second testcrossing stage. Commercial hybrid seed production requires well-skilled farmers, careful seed processing, and deliberate logistics since rye produces huge amounts of pollen which may be transported over long distances. Even the slightest genetic contamination of the CMS pre-basis and basis seed production may render the respective seed lots worthless for subsequent multiplication. To reduce the cost of the final step of seed production, the CMS seed parent and the pollinator parent are grown as a mixture in a 95:5 ratio. Thus, only about 95% of the certified seed consists of true hybrid seed. Whereas the remainder 5% are randomly intermated plants of the pollinator. However, the latter generally are poor competitors and therefore do not impair the yielding performance of the ‘hybrid’ stand. In the last decades, population and hybrid breeding led to substantial progress in grain yield and other traits.