| Pre-harvest sprouting in hybrid rice seed production fields has become a natural disaster for decades, and is an urgent problem that needs to be solved at all times. In Sichuan, a major province of hybrid rice seed production, Gang You Series, D You Series and II You Series are often threatened and challenged by pre-harvest sprouting. When it goes severe, amount of seeds produced and transferred to other localities would be affected. In Jiangsu, another major province of seed production and export sales, pre-harvest sprouting is also common in ShanYou, XieYou, Gang You andⅡYou series hybrid rice. In 2007, from the end of September till the beginning of October, typhoon "Krosa" struck the north of Jiangsu Province, went with the long spell of rainy weather, thousands hectares of hybrid rice lodged in different degrees, and pre-harvest sprouting occurred badly. Pre-harvest sprouting rates of hybrid rice seeds were over 40% in a lot of fields, especially in Yan Du,Jian Hun,Fu Ning,Jin Hu Counties. After screening and winnowing, qualified seeds that produced by some farmers were only 750 kg/hm2, economic loss was serious. According to a rough estimation, all over the country, there were about 133,000 hectares of hybrid rice seed production fields, of which rice pre-harvest sprouting rate was about 10%. Thus, the output loss of seeds was about 35 million kilograms, which cost 700 million Yuan (RMB)and even more when disasters hit badly.Rice is a self pollinated crop, like cross pollinated crop when CMS lines were used to hybrid seed production. The phenomenon of glume-opened seeds of hybrid rice was serious, because the glumes opening angle of CMS lines was wide, and some glumes could not closed completely. Spraying Gibberellic acid at heading stage, which had the effect on promoting germination, was often used as a conventionality, for solving the panicle enclosure of CMS lines in hybrid rice seed production. In the main areas of hybrid rice seed production, rainy weather and typhoon disasters often occurred at pre-harvest stage, which cause serious pre-harvest sprouting of hybrid rice seed production. Fundamentally, rice seed dormancy was the determinants of pre-harvest sprouting.Researches showed that seed dormancy of CMS lines and maintainer lines of indica rice used commonly was quite weak or absented in seed dormancy, and seed dormancy of the most of the restorer lines were low, of the small part of the restorer lines had certain seed dormancy (Zhao Ming, et. al.,2004). There was not hybrid rice combination that could resist to pre-harvest sprouting in hybrid rice seed production now yet. So that, improving seed dormancy of the parents of hybrid rice and breeding hybrid rice combination with high seed dormancy were urgent tasks, for preventing pre-harvest sprouting economically and effectively.Research contents of this paper mainly include 5 parts:1. Forming and releasing time of seed dormancy of different rice cultivars; 2. Influence on seed dormancy of hulls and seed coat of different rice cultivars; 3. Genetic characteristics of seed dormancy of IR112-12 and its derived lineⅡ112B; 4. Correlation between parents and F1 and combining ability of parents in seed dormancy for indica hybrid rice; 5. Studies on pollen fertility, flowering habits and agronomic traits ofⅡ112A and the combining ability for 14 agronomic traits.1. The forming and changing trend of seed dormancy were studied in 35 indica rice cultivars (Oryza sativa L.), which were cultivated recently or used commonly in hybrid rice seed production, by detecting the seed dormancy index of every cultivar at 15-40 days after heading. Seed dormancy disappearing time were studied by using 6 cultivars selected from those 35 cultivars after harvesting, and seed dormancy indexes were detected in different days in three conditions as stored at in-door natural conditions, stored at ice box (0℃) and treated in dry oven (50℃) for different days. The results showed as follows:(1) Rice seed dormancy formed in the middle stage of seed development (15d-20 d after heading), which means the seeds were protected by seed dormancy as soon as the seeds have potential germination ability. According to the seed dormancy decreasing, speed, 35 rice cultivars could be classified into three groups. The first groups were strong seed dormancy cultivars, seed dormancy of which decreased slowly, keeping strong seed dormancy even at the mature period (35 d after heading). The second groups were no seed dormancy or weak seed dormancy cultivars, seed dormancy of which decreased so fast that the seed dormancy would be closed to zero at the mature period (35 d after heading). The third groups were moderate seed dormancy cultivars, seed dormancy of which didn't decrease slow or fast, keeping middle level at the mature period (35 d after heading). (2) Seed dormancy decreasing speed was determined by both internal and external factors. Viewing from internal factors, seed dormancy of strong seed dormancy cultivars decreased slowly, while seed dormancy of weak seed dormancy cultivars decreased fast. When strong seed dormancy cultivars were stored at natural in-door conditions, seed dormancy went on decreasing, and could keep about 50 days until the seed dormancy was relieved finally. Strong or weak of seed dormancy of different rice cultivars was determined by decreasing speed of seed dormancy, while the resistance to pre-harvest sprouting of different rice cultivars was determined by seed dormancy at the late maturing phase. Viewing from external factors, seed dormancy of cultivars decreased slowly when stored at low temperature conditions, and decreased fast when stored at high temperature conditions. Seed dormancy of strong seed dormancy cultivars could be relieved by treating in dry oven for 2 days (50℃). In the process of seed reproduction in Hainan, hybrid rice seed production and storage, to meet the requirements of the practical production, seed dormancy should be broken or maintained by some measures.2. The effect of hull of rice on seed dormancy was studied by detecting the seed dormancy indexes of hull intact and de-hulled seeds of 5 rice cultivars, which had different seed dormancy indexes. The effect of seed capsule (include pericarp) was studied by detecting the seed dormancy indexes of de-hulled seeds and seed capsule destroyed seeds of 5 rice cultivars. Inhibitor on seed germination in hull of rice were studied by detecting the seed germination rate, sprout length and root length of rape seed (no seed dormancy) andⅡ-32B de-hulled seeds (no seed dormancy), which were treated by soaked-solution of hull of different rice cultivars. The results showed that:(1) Seed dormancy indexes of weak dormancy cultivars were zero at hull intact and de-hulled conditions. It showed that hull, seed capsule and pericarp had no control effect on seed dormancy in this condition. Seed dormancy indexes of cultivars with strong seed dormancy would appeare two cases:One case was that seed dormancy index of N22, a strong dormancy cultivar, was 100% at hull intact conditions, and 99% at de-hulled conditions. It showed that non-hull factors, including seed capsule, pericarp, embryo and endosperm had control effect on seed dormancy. Another case was that seed dormancy indexes ofⅡ112B, IR112-12 and IR64, which with strong dormancy, were 95.3%,99.7%, 98.5% at hull intact conditions, while seed dormancy indexes were 15.7%,81.3%,80% at de-hulled conditions respectively. The results showed that hull, seed capsule, pericarp, embryo and endosperm had strong control actions on seed dormancy, but the degrees of control action were different among different cultivars.Seed dormancy disappeared when destroyed the seed capsule, pericarp and partial of endosperm of de-hulled seed of IR112-12,Ⅱ112B, N22 and IR64. It showed that seed capsule and pericarp had some control actions on seed dormancy, but the influence mechanism of the seed capsule and pericarp remain unknown.(2) Rape seeds with no seed dormancy were soaked by soaked-solution of rice hull of IRl 12-12,Ⅱ112B, N22, IR64 andⅡ-32B. Risuts indicated that the inhibition rate to seed germination rate were 16.5%,16%,14%,12% and 0, respectively. The inhibition rate to sprout length were 97.4%,92.21%,79.22%,29.87%,12.99%, respectively. The inhibition rate to root length were 89.12%,78.29%,61.68%,41.97%,9.49%, respectively. De-hulled seeds ofⅡ-32B were soaked by soaked-solution of rice hull of IRl 12-12,Ⅱ112B, N22, IR64 andⅡ-32B. Risuts showed that the inhibition rate to seed germination rate were 18%,15%,15%,11%and 0, respectively. The inhibition rate to sprout length were 9.94%,9.42%,7.45%,6.21%,3.11%, respectively. The inhibition rate to root length were 42.98%,40.35%,38.6%,21.05%,8.77%, respectively. These results showed that, soaked-solution of rice hull of strong seed dormancy rice cultivars had control effects on rape seeds orⅡ-32B de-hulled seeds, but the control degree were different. It suggested that rice seed hull would hed inhibitor on germination, and the quantity of inhibitor of different rice cultivars were different. The hull of rice seeds with strong dormancy should be had more inhibitor than that of seeds with less strong dormancy. The inference of soaked-solution of rice hull of no dormancy rice cultivars on the germination rate of rape seeds orⅡ-32B de-hulled seeds were not found. It indicated that the hull had no inhibitor or the amount was very little.(3) Soaked-solution of rice hull of IRl 12-12,Ⅱ112B,N22 and IR64 would had no control effect on seed germination after treating in dry oven at 50℃conditions for 2 days. It suggested that inhibitor on seed germination in the hull would be sensitive to temperature. Inhibitor would be disappeared after treating in dry oven at 50℃conditions for 2 days. 3. Genetic characteristics of seed dormancy were studied by using P1, P2, F1, F2, B1, B2 generations ofⅡ-32B/IR112-12 andⅡ-32B/Ⅱ112B, and by using P1, P2, F1, F2 generations of IR64/LongtepuB and Kang85/IR64. Among them,Ⅱ112B was a maintainer line, and IR112-12 was a restorer line. Both of them had strong seed dormancy.Ⅱ-32B and Longtepu B were maintainer lines with low seed dormancy index, while IR64 and Kang85 were restorer lines. Results showed that as follows: (1) Rice seed dormancy was a quantitative characteristic, inherit patterns were different. Seed dormancy of some rice cultivars had major gene effect, while seed dormancy of some rice cultivars had polygene effect. Some had dominant gene effect, and some had recessive gene effect.(2) Seed dormancy of IR112-12 were controlled by polygene, meanwhile had dominant major gene effect. Seed dormancy ofⅡ112B, compared with IR112-12, dominant degree of seed dormancy of F1 generation had declined. It showed that the genes controlled seed dormancy was abandoned at the process of crossing and backcrossing. Seed dormancy of IR64 was controlled by polygene, and had recessive gene mainly. Genetic characteristic of seed dormancy of Kang85 was same as that of IR112-12, were controlled by polygene, and had dominant major gene effect. But Kang85 compared with IR112-12, total effect of genes controlled seed dormancy and dominant degree of seed dormancy of F1 generation were lower than that of IR112-12.(3) Seed dormancy of every cultivar at Nanjing was the same as that at Hainan, and stable for many years. It showed that the heritability of seed dormancy was very high. Broad-sense heritability of the dormancy was 96%-98%, while narrow-sense heritability was 62%-65%, calculated by using variance of every generation.(4) Furthermore, correlation relationship between seed dormancy and agronomic characters were analyzed by using every plant of F2 generations, results noticed that there were linear positive correlation between seed dormancy index and DSH, determine coefficient was 0.5765, the regressive equation was Y=0.0278X-2.2384. The reason would be that genes controlled seed dormancy and DSH were in linkage. Correlation relationship between seed dormancy and agronomic characters except DSH were not distinct.4. Dormancy indexes of hulled and dehulled seeds were investigated by using 109 F1 hybrids and their 19 sytoplasmic male sterile (CMS) lines and 9 restorer lines in indica hybrid rice. The seeds of each F1 and the parents were harvested at 35 days after heading. Meanwhile, combining ability was analyzed in 25 combinations made by 5 CMS lines and 5 restorer lines (NCⅡgenetic design). The results showed that as follows:(1) The seed dormancy index of F1 was positively and highly significantly correlated with those of their parents and mid-parent value. Out of the 109 combinations,82 combinations showed mid-parent heterosis, and 43 combinations heterobeltiosis.(2) Seed dormancy index of F1 and their parents declined dramatically in dehulled seeds compared with hulled seed, indicating that the hull played an important role in seed dormancy. However, the trend was similar in hulled and dehulled seeds in terms of relationship between F1 and their parents in seed dormancy.(3) The influence of hull on seed dormancy depended mainly on F1 genotype, not on hull itself of female parent.(4) Test results of 5×5 incomplete diallel cross (NCⅡ) design indicated that the general combing ability (GCA) of all the characters were significant at 1% level. The variance of general combining ability (GCA) in female and male parents occupied 59.2% and 31.1% of total variance, respectively. The variance of specific combining ability (SCA) in combinations occupied 9.7% of total variance, indicating that gene additive effects were principal. Among the 5 CMS lines,Ⅱ112A had the highest GCA effect for seed dormancy, flowed by D62A. Among the 5 restorer lines, IR112 had the highest GCA effect for seed dormancy followed by 2786. They were excellent materials as parents for breeding F1 hybrid rice with stronger seed dormancy.5. Pollen fertility, flowering habits and agronomic traits ofⅡ112A were compared with that ofⅡ-32A, and the combining ability of 14 agronomic traits were studied with 5 CMS lines and 5 restorer lines by the way of NCⅡdesign. The results showed as follows:(1) Seed dormancy indexes ofⅡ112A were more than 90% and stable for many years, the sterile plant rate and sterile degree ofⅡ112A, a CMS line, reached 100% and 99.9%, respectively. Irregularly shaped pollen abortion rate and round-shaped pollen abortion rate ofⅡ112A were high, and flowering habits was excellent. The sterility and out crossing habits coming fromⅡ-32A were maintained.(2) Test results of NCⅡdesign indicated that the general combining ability (GCA) of all the characters were significant at 1% level. Effect of the general combining ability (GCA) of grain weight per plant ofⅡ112A was strong, because effect of the general combining ability (GCA) for number of panicles per plant and seed setting rate were high. Maturity stage of descendant ofⅡ112A was earlier than that ofⅡ-32A, and could decrease the plant height, but could not increase the length of grains.Ⅱ112A/05-7 andⅡ112A/Kang85 were two excellent combinations, with high grain weight per plant and good comprehensive characters.(3) Correlation coefficient between grain weight per plant and seed dormancy index in 25 combinations was-0.0161, not significant at 5% level. It indicated that improving seed dormancy would not influence upon the grain weight per plant. |
PDF Full Text Request