Rice is the main food crop in China. In the past ten years, with the development of the economy, the area of ​​cultivated land has decreased, and the stability and increase in total rice production have only depended on the increase in yield. The improvement of rice plant type has an important effect on increasing the yield. From the two breakthroughs in rice yield in China, the main reason for the increase in yield of dwarf varieties compared with high-stalk varieties and the increase in yield of hybrid rice varieties compared to dwarf varieties is the continuous improvement of plant type improvement. The first stage of plant type improvement is dwarf breeding, and the second stage is ideal plant type breeding. Its development direction is the combination of form and function, and the ideal plant type is combined with advantage utilization. Therefore, the research on ideal plant type has received increasing attention and concern from breeders.
1 Development history of ideal plant type The research on plant type of crops actually started before the concept of “plant type†appeared. As early as 1932, Heath et al. pointed out that the attitude and quantity of the leaves are important factors that determine the differences in the production of the materials. Therefore, the plant type of the crops is proposed. In the 1950s, Moji and Saeko introduced the concept of canopy density and extinction coefficient into the analysis of crop canopy structure, thus laying a solid foundation for the study of plant type theory. In the late 1950s, dwarf breeding in China was the initial stage of plant type improvement. In 1960, Kakudanji Saburo first put forward the relationship between fertility and plant type and the ideal type of stem and leaf assimilation system under different fertilizer and water conditions based on his experimental research on rice, soybean and sweet potato. However, until 1968, when Australian Donald systematically proposed the ideal plant type concept and minimum competition theory of crops, the ideal plant type of crops really attracted the attention of breeders in various countries.
In the 1970s, Songdao Province studied the ideal plant type from the perspective of high-yield cultivation of rice, and put forward the theory and method of “ideal riceâ€, which led to the development of ideal rice plant breeding. In the 1980s, Yang Shou-ren et al. proposed a comprehensive view of resistance to fertilizer, large amount of growth, and appropriate economic coefficient from the requirements of actual production and high yield of rice production. Later, many scholars studied the problem of rice plant type from high yield breeding and cultivation. In the late 1980s and early 1990s, Yang Shouren et al proposed and further improved the super-high-yielding rice breeding theory combining optimal plant type and superior utilization, optimized trait mating, and selection standards and methods for hybrid offspring. In the same period, scientists from the International Rice Research Institute proposed and implemented a new plant breeding program. They believe that plant type breakthrough is the key to achieving breakthrough in production. The future research direction is to use heterosis to shape the ideal plant type and further increase rice yield. Since then, Sichuan Agricultural University, IRRI, Shenyang Agricultural University, Guangdong Academy of Agricultural Sciences, and Hubei Academy of Agricultural Sciences have successively designed ideal super-high-yield plant models suitable for this region.
2 Research progress on morphological, physiological and ecological characters of rice ideal plant type breeding
2.1 Study on stalk traits Lodging is one of the important limiting factors for high yield in rice, and plant lodging resistance is also determined mainly by plant height. In the 50s to 60s of the 20th century, the dwarf breeding stage in China was the first stage of development combined with practice and plant type theory. The research center at this stage was the plant height of crops, and the plant's fertility and resistance were reduced by reducing plant height. Inverted and dense planting significantly increased, and then increase the leaf area index and biological yield, thereby increasing the yield of the crop population, and selected a series of dwarf Nantes and a series of dwarf high product species. With the deepening of the research on plant type theory and the development of production practices, there has been a new understanding of plant height. Dwarfism has mainly improved the economic coefficient, and there has been no significant change in biological production. It is believed that there must be a large breakthrough in production must be achieved in the There has been a significant increase in production. Appropriately increasing the plant height can reduce the leaf area density, favor the CO2 diffusion and the light reception of the middle and lower leaves, and is obviously beneficial to the growth and late grain filling. At the same time, studies have also shown that there is a significant positive correlation between plant height and biomass yield, especially in high-yield conditions, and an increase in biomass production is the material basis for the increase in grains per spike and 1000-grain weight. Xu Zhengjin et al. clearly pointed out that one of the distinctive features of the high-yield bred crops in the northern japonica rice region in recent years is the high biomass yield. To further increase the yield, the increase in biological output is the main route, and it is unlikely that the ratio of grain to straw will continue to increase. Yang Shouren et al. proposed that the “dwarf†of the northern rice in the first season is set at 90 to 100 cm, the standard height determined by the IRRI is 95 to 100 cm, and Chen Wenfu and Others believe that the plant height of the new plant rice should reach 95-105 cm.
Zhang Zhongxu et al. considered that the lodging resistance was significantly negatively correlated with the length of the first elongate node, and was highly significantly positively correlated with the thickness and thickness of the stem. Okawa et al. also found that there are significant differences in the bending stress, section modulus, and leaf sheath strength in stem physical properties, which are directly related to lodging resistance, and indicate the direction for improving the lodging resistance of tall straw varieties. Ling Qihong's research suggests that by controlling the extension of the internodes of the base, strong stalks that are resistant to fall are formed, and at the same time, they control the growth of ineffective leaf areas and inefficient leaf areas. The promotion of internode elongation under the ear, but also can reduce the degradation of spikelets, increase the conceivable flowers and increase the area of ​​flag leaf, so that the density of the leaf layer is appropriate, is conducive to the use of light energy, and improve the population of grain-leaf ratio. Ling Qihong believes that strong stalks are the organizational structure and material basis of big spikes. Plants with large stems and stems and sheaths have a large amount of substances stored in the stems and sheaths, which can play a role in stabilizing the seed setting rate and grain weight and improve the lodging resistance. Based on the material basis, with the increase of the weight of single stem sheath, the specific leaf weight of leaves increases, the leaves stand upright, and the decay rate of the leaf area during fruiting period decreases, thus increasing the photosynthetic productivity during the grain filling period. The thickness of rice stems not only affects the lodging resistance of the population, but also determines the size of the ear. When Zhao Zhengwu studied hybrid rice, the increase in stem diameter was 1 mm, the total number of grains per ear increased by 30.15, and the number of grains increased by 22.23 grains. In the past, the study on the thickness of rice stem stalks was mostly an empirical description. It is believed that thick stalks are an important indicator of plant type breeding, and the physiological and ecological role of stalk weight in the population remains to be further studied.
2.2 Study of leaf traits
2.2.1 Leaf morphology research shows that the photosynthetic efficiency of upright leaf populations is higher than that of flat or bent leaves, the upright leaves, the small angle of the leaves is conducive to the light on both sides of the leaf, the appropriate leaf area index is improved, and the reflectance to sunlight is small In order to increase canopy photosynthetic rate, increase the amount of material production, increase the amount of light at the base of the canopy, increase the vitality of the root system, and increase the lodging resistance. Songdao Province has proposed six criteria for “ideal rice,†one of which is that the stem leaves should be gradually shortened, especially the upper three leaves should be short, thick, and erect. He believes that the photosynthetic efficiency of the erect leaf group is 11% to 17% more than that of the bent leaf group. The yield of rice variety Miyang 23 with no significant difference in photosynthetic rate was higher than that of Nipponbare. The upper leaves, especially the flag leaf, were long and erect. This was considered to be one of the main reasons. Gao Liang et al. simulated the photosynthetic capacity of four typical rice plant types, and concluded that the photosynthetic capacity of the group was the highest in the early stage of rice growth, the highest in the middle stage, and the highest in the later stage; The results of analysis on the relationship between the plant type characteristics and the yield of effective rice tiller leaf stage (Nn) showed that the high-yield group had larger leaf opening angles in Nn phase, moderate and long leaf lengths, and greater leaf looseness; thicker leaves , High chlorophyll content; Leaf area growth is slow, leaf area index is small; Sheath sheath is accumulating fast, Leaf area carried by unit leaf sheath weight is small.
Kakuda has shown that thick leaves generally have more mesophyll cells and RUBP carboxylase compared to thin leaves. Under conditions of sufficient CO2, photosynthesis is beneficial. The photosynthetic intensity of thick leaf cultivars was significantly higher than that of thin leaf cultivars. In the case of overgrowth, the longer the total length of the upper three pieces was, the lower the seed setting rate was. Ling Qihong et al. found that the upper three leaves should be longer than the leaves of the base, and the differentiation and development of the ear of rice should be synchronized with the upper three leaves, and the upper three leaves should be high-efficiency functional leaves in the grain-filling stage of rice to increase its leaf area. The ratio must inevitably be accompanied by high rate of spike formation and large spike formation, high total florets, and increased grain-leaf ratio. Moreover, since the top three leaves are young leaves with good light conditions and late senescence, they should be properly increased in the population. The ratio can reduce the aging rate of the leaf area after heading, increase the photosynthetic potential after heading, and the net assimilation rate, which is favorable for achieving super high yield. Yuan Longping's breeding model for super high-yielding hybrid rice requires the length of the flag leaf to be more than 50 cm, and the cross-section is curled, which is conducive to maintaining the upright plant type design of the leaves, and supports the view of Ling Qihong. Under the condition of similar plant height, the light transmittance of the rice varieties with leaf rolling traits was significantly higher than that of non-butterfly cultivars. Therefore, the appropriate leaf curling traits can be expected to improve the photosynthetic production and ventilation of the cultivars and combinations. situation.
2.2.2 Physiological Characters of Leaves Stomata are the main channel for water and gas exchange between the leaf and the external environment. Stomatal characteristics are closely related to photosynthetic, respiratory, and transpiration of the leaves. People have conducted extensive research from the perspective of cultivation physiology, and believe that the stomatal density has a very significant positive correlation with stomatal diffusion conductance and net photosynthetic rate, and is significantly negatively correlated with specific leaf weight. There was a significant difference in the stomatal density of different types of rice, ç±¼ type> ç±¼/ç²³ partial ç±¼ type> ç±¼/ç²³ partial type ç²³ type ç²³ type. The high photosynthetic rate of japonica rice leaves has a direct relationship with its large stomatal density and small gas diffusion resistance. There was a significant positive correlation between stomatal aperture and stomatal diffusion conductance and net photosynthetic rate. Therefore, it is an effective way to improve the utilization of light energy by adjusting the leaf photosynthetic performance through the improvement of leaf stomatal properties. The studies of Liu Zhenqi, Chen Wenfu, and Zhang Longbu also showed that the specific leaf weight was positively correlated with the chlorophyll content, but the chlorophyll content in rice leaves was also positively correlated with the net photosynthetic rate, but the chlorophyll a/b was negatively correlated with the net photosynthetic rate. . Peng Yingcai believes that the specific leaf weight is also significantly positively correlated with the net assimilation rate. Higher specific leaf weights and larger stomatal densities are beneficial to improve photosynthetic efficiency. How to break the stomatal density and the negative correlation with the leaf weight, the integration of the two into one is the key to the problem. Due to the high heritability of stomatal traits, it may be an effective way to improve the quality of the leaves by indica rice with thicker leaves, lower stomatal densities and indica hybrids with thin leaves and large stomatal densities and multiple backcross or multiple crosses. one.
2.3 Research on Spike Characters
2.3.1 Research on spike shape Yin Hongzhang et al. pointed out that in the middle and late stages of grain filling, the gradual bending of the ear affects the photosynthesis of the population. Xu Zhengjin has further confirmed that the erect panicle type is better than the semi-vertical or curved panicle type, it is beneficial to improve the population structure and the light state, the population growth rate is high, and the biological yield is significantly higher than the semi-vertical and curved panicle type. The intensity of light intensity in the middle of the erect panicle group was higher than that of the semi-vertical or curving panicle type in one day, which was an important factor influencing the photosynthesis of the population. Regardless of the natural wind speed, the diffusion efficiency of the population C02 is also higher in the erect panicle than in the semi-vertical and curved panicles. This difference in panicle type indicates that the erect panicle type is more conducive to the accumulation of photosynthetic products. From the point of view of population light energy utilization, erect panicle type may be another important morphological evolution to meet the requirement of super-high yield after dwarfing and ideal plant type. The erect panicle type generally has a higher yield potential, and its yield structure is characterized by a high panicle number and panicle number, while the 1000-grain weight and seed setting rate are low, but this is not directly related to the erection of the panicle. The percentage of open grains in the branches of the lower branch of the erect ear was high, and the grain weight was also significantly lower than that of the one branch. Therefore, the ratio of the seeds of the first branch was high, and the seeds of the second branch were distributed in the upper part of the cob. Panicle varieties will help improve the firmness and quality. Imaiznmi et al. found that rice ear not only has the photosynthetic characteristics of C3 plants, but also has high C4 photosynthetic enzyme activity, and has the ability to fix CO2 equivalent to 30% of flag leaves. On the other hand, the Tsunods believe that the photosynthetic capacity of the panicle itself is much lower than that of wheat and can be regarded as a non-assimilating organ.
2.3.2 Panicle number research The rice yield yield factor has the closest correlation between total grain number per unit area and yield, and has the largest contribution to increase yield. The number of spikes is the main factor that restricts the total number of grains per unit area. In the ideal high-yield plant design, it should be considered that rice varieties have better ability to deliver. The importance of birth ability has been mentioned in some rice plant designs in China. Xu Zhengjin believes that the ultra-high-yielding species is based on a significant reduction in the number of panicles in exchange for a greater increase in the number of grains per spike and a significant increase in grain weight, and high biomass production is the main reason for its high yield. Li Yizhen analyzed the high-yielding structure of hybrid rice and found that the total grain number was most closely related to yield, and the contribution rate to yield increase was the highest, while the spike number was the main factor restricting total grain number. Chen Wenfu research pointed out that common features of new plant types are increased plant height, reduced number of tillers, increased spike success rate, large spikes, and both biological yield and economic coefficient. Xiao Yinghui analyzed the yield components of large panicle type rice and concluded that the lack of effective panicles per plant is the main factor affecting the yield of big panicle type rice. The effective panicle composition of one plant also affects the yield formation of big panicle type rice.
Appropriate fertility can increase the population's biological output, and the delivery is also the basis for yield formation. A large number of experiments have shown that the excessively weak delivery force often leads to poor automatic mediation, and it is not easy to form high-yield groups in the early stage, which leads to low biological output and limits the increase of production. The excessive force of delivery can easily cause the stalks to be weak, the lodging resistance is reduced, and it is also unfavorable to production. This requires a modest delivery force. The International Rice Research Institute, on the basis of studying the tillers and panicles of panicle type and panicle heavy cultivars and their contribution to yield, believes that 5-6 panicles per plant are reasonable. It is proposed that the rate of succession of panicles should be increased to achieve the unit of large panicles. The area has enough spikelets to achieve high yields. Kim et al. also proposed that the reasonable number of panicles per plant in tropical varieties was 6. These panicle panicles had the highest ear panicle weight, and were significantly different from other stems and panicles. Zhu Defeng believes that 9 to 10 single plants per plant are reasonable. These planters have a high rate of spike formation, a large panicle type and a high seed setting rate, and the plant height, heading date, and maturity period are relatively consistent. Most of the other plant type models set the intermediate fertility as the breeding goal. The above conclusions may be due to differences in ecological environment and growth period.
2.4 Study of root traits The root volume of 70% to 80% of rice was distributed in the soil layer of 0 to 10 cm, and the root depth was as large as 50 to 60 cm, and the lateral stretch was 40 cm. Chuantian believes that the upper roots are distributed in the upper layer of the soil and stretch horizontally or diagonally; the lower roots are different in the internodes, and the stretching direction and distribution area are different; the root diameter is small, and most of the lateral expansion occurs; the roots occurring in the tillering stage are thick and oblique. Stretching in the lower direction, distributed in the middle and lower layers; the roots that grow before and after the spikes form are the thickest and mostly extend downward. Yoshiba and Singh's research showed that there are great differences in rice root types and varieties, and the distribution of roots is closely related to soil moisture. Wu Zhiqiang's research shows that the roots of flooded fields are distributed in the upper layers of the soil; the root systems of wet irrigation and dry farming are mainly distributed in the middle and lower layers. The root volume and rooting power of hybrid rice were significantly better than that of conventional varieties and had a deeper distribution and stronger drought resistance. Wu Yuexuan analyzed the effects of temperature on the growth and development of hybrid rice roots, and pointed out that high temperatures are conducive to the occurrence and growth of roots, but later in the process will accelerate root senescence. According to Zhao Shilong's analysis of rice roots, the root growth of thin-cultivated or large-spike varieties was significantly better than that of common cultivation or spikelet-type varieties. The distribution of roots is related to the posture of the leaves. The root distribution is shallow, the root volume is small, the leaf angle is large, and the leaves are scattered; while the roots are deep and deep, the root volume is large, the root activity is high, the angle of the leaves is small and compact, and the leaves are easy. Stay upright. The higher the photosynthetic rate is.
Yang Shouren pointed out long ago that the roots are fundamental and that there are many roots and many spikes. When the number of spikes is the same, the number of roots is larger and the panicles are larger, and the amount of roots is higher. Related studies have also demonstrated that the number of effective panicles per hole, the number of grains, and the number of grains per panicle are extremely significantly positively correlated with the root volume. Each additional root adds approximately 10 grains. Shi Qinghua's results of genetic studies on the root morphological characteristics and aboveground traits of 12 rice varieties with different panicle types showed that the number of roots, root weight, and root diameter were significantly related to the above-ground traits, respectively, and could be used as root selection indicators for rice breeding. . Yoshida investigated the relationship between plant height and tillering and root growth of 1081 rice varieties. It is believed that the varieties with deeper root distribution have higher plant height, fewer tillers, and longer and more main stem and early tillering nodes. The spike-type cultivars have fine roots and are mostly distributed on the surface of the soil. Panicle-heavy cultivars have thicker roots and a higher ratio of direct roots to lower roots. The number of roots at the heading stage is larger and their activity is stronger. The multiple-panicle cultivars are more important than the large panicle cultivars. Dwarf and multi-spike cultivars had fewer roots and shorter cultivars, while tall culms and large ear cultivars had more roots and longer roots, and the oxidative activity of roots was positively related to leaf angle. Ling Qihong studies that the distribution of rice roots is geometrically related to the leaf angles. When the roots are deeper and more vertical, the leaf angles are smaller. When the roots are more distributed and less vertical, the larger leaf angles tend to drape. Wang Yulong studied the relationship between spikelet root activity and seed stalk fertility. It was found that the rooting activity of spikelet during the grain-filling period was significantly and positively related to the grain grouting intensity, seed setting rate, and heavier weight, and it was considered that the increase in spikelet root activity was observed. It is an effective way to increase the seed setting rate and grain weight of rice.
3 Prospects and prospects of ideal plant type research The characteristics of rice plant type mainly depend on the genetic basis, while the population characteristics are mainly controlled by environmental factors. There exists a relationship of opposites between the two. To achieve high yield and high quality, it is necessary to coordinate the two. The contradictions will bring about the greatest unity of plant type and environmental advantages. In the future, research on the ideal plant type of rice should be done in the following six areas:
(1) The ideal plant type must also be adapted to the requirements of local production practices and ecological conditions. Ideal strains should be determined from the interrelationships among groups and the environment, between groups and individuals, between individuals and individuals, and within individual parts. type.
(2) Study the evaluation principle of the ecological suitability of different ideal plant types for a large range of rice, and achieve a reasonable division of its planting area.
(3) To study the expression mechanism of the superior plant type traits of different plant type rice varieties and the ecological performance under the group conditions, in order to realize the artificial regulation of rice gene expression of different ideal plant types. (4) Establish a mathematic simulation system for ecological indicators of the ideal plant type breeding.
(5) Create a super high-yield cultivation management system that is suitable for the ideal plant type in each region.
(6) While emphasizing the cultivation of ideal plant types, attention should also be paid to the combination of high quality and stress resistance. Such plant type breeding has practical significance.
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