Studies And Application Of Thermo-photo-sensitive Two-line System Hybrid Wheat

Heterosis utilization is one of the effective ways in raising crop yield further, whichhas been confirmed in many crops such as maize and rice. Since 1992, two-line systemhybrid wheat (Triticum aestivum L.) based on thermo-photo-sensitive genic male sterile(TPSGMS) has become one of the main ways of heterosis utilization in wheat in China, forits broad restoring sources in common wheat and simplification of F₁ seed productioncompared with traditional three-line system. According to the progress in past years, andaimed at stablizing seed purity, raising yield and simplifying techniques of F₁ seedproduction, improvements in fertility stability, adaptability and out-crossing ability forC49S-87, a TPSGMS line introduced from Crops Research Institute of ChongQing, China,were conducted combining with the natural advantages of ecological diversity in Yunnan,and researches in techniques of fertility purification and multiplication for TPSGMS lines,F₁ seed production with high purity and high yield, cultivation of hybrid wheat with lowcost, etc. were also done. The results were listed as follows:By crossing an advanced line 96B-138, a material accidentally discovered in testcrosswhere the hybrid F₁ of C49S-87/96B-138 performed highly sterile, with C49S-87, severalimproved TPSGMS lines such as K78S and K1564S were developed in 2001. Results fromfertility and adaptability experiments during 2001-2004 in 5 different locations withaltitudes varying from 1,530m to 2,400m in Yunnan showed that, in natural conditions ofKunming, the critical temperature and daylength for fertility alteration of K78S were 12.8℃and 12.41h, 2.1℃and 0.93h higher or longer than that of C49S-87, respectively, andthe sterile heading duration of K78S with bagged sterile rate＞98% were 24 to 43 daysfrom altitudes of 1,530m to 2,400m, at least 18 days longer than that of C49S-87, whichprovide K78S better fertility stability and adaptability. In out-crossing ability, K78S had20%～60% of stigma exsertion rate and 8～10cm of ear-neck length, while C49S-87 was 0and 1～3cm, separately, which raised the out-crossing rate from 40%～50% of C49S-87 to70%～80% of K78S. The significant improvements of K78S in fertility and out-crossingcharacteristics not only laid a good base for minimizing the risk in F₁ seed production,guaranteeing the seed purity and significantly raising the yield of F₁ seed, but also provideda better platform for breeding new kings of sterile lines suitable for different usage.The breeding of K78S indicated that, in transfer of TPSGMS line, screening andutilizing materials having no or low restoring ability to sterile line as parent is a feasibleway of raising the critical values for fertility alteration, which provides a new way forgenetically improving the sterility characteristic when available TPSGMS gene resources islimited. Two hybrid varieties Yunza5 (K78S/01Y1-1069, making test-cross in 2001 and released in2004) and Yunza6 (K78S/01Y1-608, released in 2006) were developed with yield advantagesof 17.8%～43.9% compared with conventional CK in regional tests. And the successfulbreeding of Yunza5 indicated that, the translocation line 6VS/6AL of Triticumaestivum-Haynaldia villosa, developed by Cytogenetics Institute of Nanjing AgriculturalUniversity, is one of the best diseases resistants both for sterile and restoring lines improvementin powdery mildew and yellow rust resistance. Combining with hybridization breeding indisease resistance, two-line system hybrid breeding could further raise the effectiveness ofbreeding materials utilization and accelerate the development of fine combinations.Both C49S-87 and K78S could guarantee the high sterility (bagged sterile rate＞95%)of anterior 5 tiller ears at least for each plant when sowed in optimum date and area, whilethe practical average numbers of tiller ear per plant for all wheat varieties was less than 3according to recent 15 years' data of regional tests in Yunnan. Therefore, in normal fieldseedling emergence, tiller ears of sterile line did not affect the purity of F₁ seedsignificantly, the main factor was the fertility degeneration of sterile line population causedby fertility variation of individuals and its accumulation during seasons of multiplication,which could be effectively controlled by measures of fertility purification and selectingsuitable sites for sterile line multiplication where the sterility should be restored as high aspossible.In F₁ seed production, the optimum ratio of male: female for Yunza5 and Yunza6 is2:6, plus the better out-crossing and fertility characteristics of K78S, the purity could bekept to above 98%, and in the same time, the average yield of F₁ seed could be up to3750kg/hm², 70% higher than that of Yunza3 developed with C49S-87, which significantlydecreased the seed cost and raised the benefit of seed production. It is the application ofK78S that the techniques of F₁ seed production were largely simplified as: separatelysowing and harvesting the male and female + artificially-aided pollination + normaltechniques for seed multiplication of conventional variety, which is easy to operate forfarmers, and basically reached the goal of establishing simple, high effectiveness and lowcost techniques for F₁ seed production.The general strategy for hybrid wheat cultivation was fully utilizing the tillering advantageof hybrid to construct coordinated population, and exert the yield advantage in the end, whichhas been verified by repeated studies and practices of application. In irrigated area in Yunnan,the optimum basic seedling for hybrid wheat is 90～105×10⁴/hm², and 120～135×10⁴/hm² inupland, accounting for 60-75kg/hm² and 40%～50% of conventional varieties in seed rate,which decreased the seed cost to being equal to that of conventional varieties or lower, andpromoted the application of hybrid wheat in production in Yunnan. By continuous improvement in sterile and restoring lines, fertility purification of sterilelines, combination screening, and technical researches and practices in F₁ seed production andcultivation, a stable and practical two-line system for heterosis utilization in wheat based onTPSGMS was established in Yunnan, which was further verified by more than 20000hm² ofapplication of Yunza3, Yunza5 and Yunza6, with average yield increase at 15%～20%.In addition, principles of combination screening, especially for the breeding and screeningof combinations with stable fertility in abnormal lower temperature, ways of raising theheterosis further and measures of guaranteeing the purity of F₁ seed when exceptional "hightemperature" occurs during fertility sensitive period were also discussed in the paper.