Preliminary Study on the Waterlogging Tolerance of 116 Corn Materials

Shilong ZHANG　Haitao JIA　Yinshan GU　Zhenghua HE　Yiqin HUANG

Abstract Maize is one of the most important food crops in the world. With the global warming， waterlogging stress has become an important abiotic stress factor that affects crop growth， including maize. Waterlogging seriously affects 10% of the arable land and can lead to a 15%-80% reduction in crop yield[1]. In this study， 115 inbred line materials commonly used in spring maize planting areas in the Jianghan Plain， Hubei， and maize inbred line B73 with complete genome information， were collected and stressed by waterlogging for two weeks in the seven-leaf and one-heart stage， and the survival rate was statistically compared and analyzed， aiming to screen germplasms with strong waterlogging tolerance for the genetic improvement of waterlogging tolerance of Hubei maize lines.

Key words Maize; Waterlogging tolerance; Survival rate; Waterlogging stress; Genetic improvement

Received： September 3， 2020  Accepted： November 4， 2020

Supported by "Seven Major Crop Breeding" Special Project （2018YFD01-00102）.

Shilong ZHANG （1973-）， male， P. R. China， associate professor， PhD， devoted to research about maize genetic breeding.

*Corresponding author. E-mail： hyqhzau@163.com.

Periodic or long-term waterlogging is a common environmental stress factor suffered by plants. Since the beginning of this century， with the global warming and frequent floods， waterlogging has become one of the most important abiotic stress factors after drought in the world[2-3]. Waterlogging stress makes plants in a state of hypoxia， restricts aerobic respiration， and poses a serious threat to plant growth and even survival[4]. Maize （Zea mays L.） is very sensitive to waterlogging stress. During its life cycle， due to poor drainage or excessive rainfall， it often suffers from waterlogging stress[5]. Especially in Southeast Asia， 25% of the maize planting area is affected by waterlogging caused by seasonal rainfall， which causes an average annual reduction of maize production by about 15%[6]. In the normal cultivation season of southern China， the maize belt is prone to low-temperature spring rain during the seedling stage， and long-lasting rainy season during the flowering period. In the soil environment with poor drainage and irrigation systems and high groundwater levels， the maize root system is in a state of low oxygen for a long time， and waterlogging has severely restricted the stable and high yield of southern maize and the expansion of planting area. Cultivating new varieties with water tolerance is the most economical and effective way to reduce maize loss， increase yield per unit area， and expand maize planting area[7-8]. More than 100 QTLs for waterlogging-related traits have been reported in the maize genome. Due to the complex genetic composition of maize genome information and waterlogging stress， so far， no QTL has been cloned from forward genetics， and there is little information about the selected sites in the process of genetic improvement of maize waterlogging tolerance. Therefore， discovering and identifying new germplasms tolerant to waterlogging and cultivating new varieties tolerant to waterlogging are still major issues facing geneticists and breeders.

Materials and Methods

Experimental materials

116 spring maize inbred lines in Jianghan Plain， Hubei were selected. The specific code， blood relationship and survival rate after two weeks of waterlogging are shown in Table 1.

Experimental methods

The seedling transplanting method was adopted. Single maize seeds were sown on the aperture disks. When the seedlings grew to the two-leaf and one-heart period， seedlings with the same growth potential were selected from each inbred line material for transplantation. The planting plot had single row with a row length of 2.0 m， row spacing of 0.5 m， plant spacing of 0.25 m. The experiment was done in three replicates， each including 8-11 plants.

Waterlogging survival rate survey and data processing

At the stage of seven leaves and one heart， the seedlings were flooded to the growth point， and the survival rate was calculated after two weeks. Data was processed software SPSS19. The survival rate of each inbred line was taken as the average of three replicates. The survival rate of waterlogging≥70% was defined as stronger tolerance to waterlogging; the survival rate of waterlogging between 60% and 70% was defined as strong waterlogging tolerance; the survival rate of waterlogging between 50% and 60% was defined as general waterlogging tolerance; and less than 50% was defined as intolerant to waterlogging.

Results and Analysis

The survival rates of the 116 inbred lines after waterlogging for two weeks averaged 41.3%， with the highest survival rate of 100% and the lowest survival rate of 0， and the kurtosis and skewness were -1.31 and 0.045， respectively. The survival rate distribution is shown in Fig. 1. 23 inbred line materials showed a survival rate greater than 70%， and were defined as having stronger stain resistance， such as Xianyu 335s parent 6WC， and its derivative and improved line HA83008[（4CV/F085 ） F2-3-1-1-1]. There were 17 inbred line materials with a survival rate of 60%-70%， which were defined as having strong waterlogging resistance， such as Zheng 58s improved line HA83015 [（Zheng 58/337） F2-7-2-2-1-1] and Weike 702s parent material. There were 11 inbred materials with a survival rate between 50% and 60%， which were defined as having general waterlogging resistance. There were 66 inbred lines with a survival rate of less than 50%， which were defined as being intolerant to waterlogging. Among them， there were 27 inbred lines with a survival rate of less than 10%， and 6 inbred lines having a survival rate of 0， which meant that they were extremely sensitive to waterlogging （Table 1）.

Conclusions and Discussion

Waterlogging is a serious environmental pressure. Higher plants have an absolute demand for oxygen to maintain metabolism and growth. However， the diffusion rate of atmospheric oxygen in waterlogged soil is about 320 000 times lower than that in non-waterlogged soil， which hinders the transfer of oxygen between the soil and the atmosphere and affects the physiological and metabolic processes of plant roots. During the period of waterlogging stress， the reduction of oxygen content will also lead to the production of reactive oxygen species， carbohydrate fermentation and sulfate reduction to sulfides， and changes in all these physiological indicators or metabolites are potentially toxic to the normal growth of plants[9]. Under waterlogging conditions， the photosynthetic rate is also reduced by 30%-40%. These changes seriously affect plant growth， dry matter accumulation and final yield[10]. Only by fully understanding the many characteristics of waterlogging-tolerant and flood-tolerant crop varieties can it be possible to develop waterlogging-tolerant and flood-tolerant crop varieties. For example， SUB1A is determined to be the determinant of the waterlogging tolerance of rice. Based on this site， a waterlogging-tolerant rice variety， usually called "scuba rice"， is cultivated. Researchers have introduced the waterlogging resistance gene SUB1 into maize through modern biotechnology and obtained New varieties of waterlogged corn[11-12]. For example， SUB1A is determined to be the determinant of the waterlogging tolerance of rice. Based on this site， waterlogging-tolerant rice varieties， usually called "scuba rice"， have been cultivated. Researchers have introduced the waterlogging resistance gene SUB1 into maize through modern biotechnology and obtained new maize varieties resistant to waterlogging[11-12].

The selection of strains with stronger waterlogging tolerance is the key to genetic improvement of waterlogging tolerance. Improving the productivity of plants in waterlogged soil and cultivating new varieties that can better adapt to waterlogging stress are economic and environmentally friendly ways to improve crop tolerance to waterlogging[13]. However， due to the narrow germplasm resources used in breeding and the high complexity of waterlogging stress traits， so far we know little about the genetic basis of maize waterlogging tolerance and information of sites selected in the process of improving maize waterlogging resistance. The waterlogging tolerance at the seedling stage of maize is an extremely complex comprehensive trait， which is greatly affected by the environment， genotype， and genotype-environmental interactions， and the evaluation indexes of maize waterlogging tolerance are not uniform， mainly in adventitious root formation， waterlogging tolerance coefficient， leaf yellow index and identification of physiological indicators related to stress. These indicators are labor-intensive and time-consuming for the primary positioning population， and are greatly affected by genetic background. Waterlogging survival rate is the most intuitive manifestation of the tolerance of maize to waterlogging stress. In this study， the waterlogging survival rate was used as the evaluation index to grade the waterlogging tolerance of 116 inbred lines. A batch of waterlogging-tolerant materials with a two-week survival rate still above 90%， were identified， such as HA83050， HA83008 and H83123. Meanwhile， in this study， we also identified some extremely sensitive materials for waterlogging that almost all died of waterlogging for two weeks， including HA83102， HA83137， HA83139， HA83141， HA83149 and HA83185. On the basis of theoretical research， we can use the maize materials with strong waterlogging tolerance and intolerant materials to construct a basic group for linkage positioning， and identify and dig out the genes that are resistant to waterlogging. In production and application， we can improve the materials that are being used in production but do not have waterlogging tolerance using the donor parents of inbred line crops with strong waterlogging tolerance based on backcrossing and breeding. This study is of great significance for improving the waterlogging tolerance of Hubei maize varieties.

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Editor： Yingzhi GUANG  Proofreader： Xinxiu ZHU