Background

Maize is the largest food crop in China. At present, it has gradually become an irreversible trend to popularize and apply machinery to harvest maize in China and even in the world. At this time, the breeding and cultivation of maize varieties have brought new challenges to maize production. In the field, harvesting kernels directly by agricultural machinery has become an important direction for the future production and development of maize industry in China. It has become another important goal for maize breeding and cultivation to breed new varieties with fast kernel filling rate at the filling stage, fast kernel dehydration rate at the mature stage and low kernel moisture content at the harvest time (Jiang et al., 2004; Wang et al., 2012).

High moisture content in maize kernels is not conducive to mechanical harvest, resulting in an increase in the harvesting cost. Only when the moisture content is less than 27%, can the maize kernels be suitable for direct mechanical harvesting (Xie et al., 2014). If the moisture content of maize kernels is too high during harvest, it will lead to a series of phenomena, such as serious mechanical blockage, low ear removal rate and high kernel damage rate, resulting in the decline of kernel yield and quality (Liu, 2013). Heilongjiang Province, as the main spring maize producing area, has high latitude, the rapid drop of temperature after autumn, insufficient sunshine and the low accumulated temperature of effective activities, resulting in higher moisture content of maize kernels during harvest, which in turn leads to the decrease of maize kernel yield and commercial quality, the increase of storage and processing costs, and the obvious decrease of maize production benefits (Yang, 2001).

According to the standard of 18% moisture content, when farmers sell maize, the average price per kilogram is about 1 cent lower after each extra moisture is deducted. Assuming 8,000 kilograms per hectare, each kernel contains 5 to 6 more moisture (23%-24%), the income per hectare will be 400-480 RMB less. Therefore, it is imperative to speed up the breeding of maize varieties with low moisture content at physiological maturity and fast dehydration rate at the later stage (Li, 2014). In this study, 36 Chinese major commercial maize single-cross hybrids were used as test materials to determine the natural dehydration rate after kernel physiological maturity, in order to provide experimental reference for breeding new maize varieties with rapid dehydration rate in Northeast China.

1 Results and Analysis

1.1 Establishment of initial identification period based on probe method

In this experiment, the kernel moisture content of the 36 single-cross hybrids (Table 1) was measured once every 4 days from the 10^th day to the 45^th day after pollination by using SK-300 probe-type moisture meter. The moisture content data of all the above-mentioned materials 35 days and 40 days after pollination were respectively selected to make a histogram (Figure 1; Figure 2).

35 days after pollination, the kernel moisture content of single-cross hybrids ranged from 46.71% to 66.12% (Figure 1). During this period, the single-cross hybrids with higher kernel moisture content were Zhengdan 958, WD4436, Denghai 605, Jidan 50 and Hengyu 218. The single-cross hybrids with lower kernel moisture content were Jinongda 212, Jinongda No.2, Jinongda 935, Jidan 27, Yidan 31 and Jidan 565, with the variation range of 46.71%-50.27%.

40 days after pollination, the kernel moisture content of single-cross hybrids ranged from 41.67% to 63.62%, which was lower than that 35 days after pollination, but the minimum moisture content of kernels was still above 40%, indicating that the physiological mature period had not yet reached 40 days after pollination (Figure 2). Therefore, in this study, 40 days after pollination was selected as the initial identification time of kernel moisture content of maize single-cross hybrids.

1.2 Comparison of kernel moisture content at physiological maturity and harvest stage of maize single-cross hybrids for experiment

During physiological maturity and harvest, the average moisture content of maize kernels were 32.06% and 25.28%, the variation range were 23.38%-37.07% and 15.37%-34.34%, and the variation coefficient were 8.90% and 15.96%, respectively. Among them, the inbred line with the lowest kernel moisture content at physiological maturity and harvest stage was Demeiya No.2, and the inbred lines with low moisture content in the two periods were Demeiya No.2, Deyu 919, Demeiya No.1, Xiongyu 581 and Liaodan 565. Heyu 89, Heyu 35, Hengyu 218, Nonghua 101, Damin 899, Zhengdan 958 and Yidan 31 were inbred lines with high moisture content in both periods. For varieties with higher moisture content at physiological maturity, the moisture content at harvest was lower because the kernel moisture content dropped quickly at later stage. The varieties with slower decrease of kernel moisture content at later stage also had lower moisture content at harvest stage because of their lower moisture content at physiological maturity stage (Figure 3).

1.3 Difference analysis of kernel dehydration rate after physiological maturity of maize single-cross hybrids for experiment

The average dehydration rate of 36 maize single-cross hybrids was 0.53%/d, ranging from 0.11% to 0.89%/d, and the intra-group coefficient of variation was 40.02%, which indicated that the kernel dehydration rate in the group varied greatly. The single-cross hybrid with the slowest kernel dehydration rate was Yidan 31, and Demeiya No.1 was the single-cross hybrid with the fastest dehydration rate (Figure 4).

1.4 Establishment of evaluation standard for kernel natural dehydration rate of maize single-cross hybrids

In this study, the average dehydration rate of maize single-cross hybrids were calculated based on the kernel moisture content of 36 single-cross hybrids harvested 10 days after physiological maturity, and the histogram was plotted (Figure 5). The dehydration rate of 36 single-cross hybrids was basically accorded with normal distribution, and the dehydration rate of 50% single-cross hybrids was between 0.427% and 0.777%. Therefore, the evaluation standard of dehydration rate of maize single-cross hybrids was put forward (Table 2).

1.5 Difference analysis of dehydration process and dehydration rate of representative maize single-cross hybrids

According to the evaluation of kernel dehydration rate of maize single-cross hybrids, three representative varieties (Xianyu 335, Demeiya No.3, and Demeiya No.1) with fast dehydration rate (≥ 0.78) and two representative varieties (Jidan 27 and Zhengdan 958) with medium dehydration rate (0.43-0.78) were selected to analyze the dehydration rate of maize kernels. As shown in Figure 6, there were two types of dehydration process for the above varieties: (1) The first type: The kernel physiological dehydration rate was at a high level 39 days after pollination and kept rising rapidly until 42 days after pollination. 42 days after pollination, the rate of physiological dehydration decreased rapidly until physiological maturity. After physiological maturity, the natural dehydration rate kept its original decline until harvest. (2) The second type: The physiological dehydration rate was in the middle-low position 39 days after pollination and kept a slow upward trend until physiological maturity. After physiological maturity, the natural dehydration rate decreased rapidly until harvest.

Demeiya No.3 and Jidan 27 belonged to the first type. 39 days after pollination, the moisture content of Jidan 27 (42.36%) was significantly lower than that of Demeiya No.3 (55.60%), and the physiological dehydration rate of Jidan 27 was higher than that of Demeiya No.3 until 42 days after pollination. 42 days after pollination, Jidan 27 began to decline rapidly from a high physiological dehydration state until physiological maturity. After physiological maturity, the natural dehydration rate of Jidan 27 was at a low level until harvest due to premature senescence at the later stage. However, 42 days after pollination, Demeiya No.3 declined slowly from the medium state and entered the physiological maturity 11 days earlier than Jidan 27, which would provide sufficient protection for its natural dehydration after physiological maturity. Although the moisture content of Demeiya No.3 (36.21%) during physiological maturity was much higher than that of Jidan 27 (28.54%), it maintained a medium natural dehydration rate after physiological maturity until harvest. In a word, the above reasons eventually led to the fast dehydration rate of Demeiya No.3, while Jidan 27 was a variety with medium dehydration rate.

Xianyu 335, Demeiya No.1 and Zhengdan 958 belonged to the second type. 39 days after pollination, kernel moisture content was Zhengdan 958 (59.84%) > Xianyu 335 (56.08%) > Demeiya No.1 (43.34%), and the moisture content of Demeiya No.1 was significantly lower than that of the other two varieties. Although the physiological dehydration rate of all the above three varieties was slowly increasing until physiological maturity, the starting rate was Xianyu 335 > Demeiya No.1 > Zhengdan 958, and the physiological dehydration rate of Demeiya No.1 during physiological maturity was already the same as that of Xianyu 335. At physiological maturity, kernel moisture content was Zhengdan 958 (29.64%) > Xianyu 335 (23.16%) > Demeiya No.1 (16.89%), and the moisture content of Demeiya No.1 was still significantly lower than that of the other two varieties. At the same time, Demeiya No.1 entered physiological maturity 12 days earlier than Xianyu 335 and 18 days earlier than Zhengdan 958, which would provide sufficient guarantee for its natural dehydration after physiological maturity. After physiological maturity, although the natural dehydration rate of all the above three varieties was slowly decreasing, Demeiya No.1 and Xianyu 335 dropped from high to middle, while Zhengdan 958 dropped from middle to low. In short, the above reasons eventually led to the fast dehydration rate of Demeiya No.1 and Xianyu 335, while Zhengdan 958 was a variety with medium dehydration rate.

2 Discussion

2.1 Optimum determination time selection of kernel natural dehydration rate of maize single-cross hybrids

Determination time selection of kernel natural dehydration rate is an important link in breeding new maize varieties with fast dehydration. Kang and Zuber (1989) showed that kernel moisture content at harvest had a very significant negative correlation with dehydration rate (r = -0.62), thus proving that kernel moisture content could be used to evaluate dehydration rate at harvest time. In the study, we measured the kernel moisture content of different single-cross hybrids 10 days after pollination and found that 35 days after pollination, the minimum kernel moisture content of single-cross hybrids was 46.71%, while 40 days after pollination, the minimum kernel moisture content of single-cross hybrids was still above 40%, which indicated that single-cross hybrids still did not enter physiological maturity 40 days after pollination. The results showed that 40 days after pollination was selected as the initial identification time of kernel moisture content for single-cross hybrids. Tan et al. (2008) carried out a correlation analysis on the kernel dehydration rate of some early-maturing maize materials in China, and pointed out that there were significant differences in the average kernel dehydration rate between varieties from 35 days after silking until the physiological maturity, and the dehydration rate could be determined during this period, which was the same as the results of this study.

2.2 Efficient breeding of maize varieties with fast kernel natural dehydration rate

The key to realize mechanical harvesting of maize is to reduce the kernel moisture content to 23%-24% at harvest. Many scholars believe that the kernel moisture content at maize harvest is determined by three factors: dry matter accumulation rate (i.e. filling rate), moisture content during physiological maturity and kernel dehydration rate after physiological maturity (Cross and Kabir, 1989). In order to reach the highest yield, most breeders choose inbred lines with long filling time to prolong the filling time of hybrids, thus the filling time of newly bred varieties is prolonged, but the harvest time remains unchanged, which results in the increase of the moisture content in maize kernels (Gao et al., 1998). Based on the results of previous studies and combined with this study, we analyzed the changes of moisture content and dehydration rate of different maize single-cross hybrids during physiological maturity and harvest, pointed out that it was not advisable to shorten filling time on the premise of sacrificing yield, and it was difficult to reduce maize kernel moisture content by prolonging the days from physiological maturity to harvest. Therefore, selecting maize single-cross hybrids with low moisture content at physiological maturity and fast dehydration rate after physiological maturity could quickly breed single-cross hybrids with low moisture content at harvest time.

3 Materials and Methods

3.1 Materials for testing

In this experiment, 36 Chinese major commercial maize single-cross hybrids were used as test materials, with a growth span of 105-132 days, all of which were well representative (Table 2).

3.2 Experimental design

This experiment was carried out in Gongzhuling Experimental Base of Jilin Academy of Agricultural Sciences in 2016. The randomized block experiment design was adopted with 3 replicates. The planting density was 60,000 plants/ha, with 6 rows. Each row length was 5 m, the row spacing was 0.65 m, and the plot area was 19.5 m².

At 9 am on the 10^th day after pollination, in order to avoid the marginal effect, 2 plants at the head of each plot were removed and 5 fixed plants labeled with tags were selected. Referring to Reid et al. (2010), SK-300 probe-type moisture meter (produced by Harbin Yuda Electronic Technology Co., Ltd.) was used to penetrate the bract leaf in the middle of ear and stab into the kernel. The moisture content was recorded and measured once every 4 days until the moisture content reached 40%.

When the kernel moisture content reached 40%, 2 ears labeled with tags were picked every 4 days, and the moisture content was determined by drying and weight reduction method until harvest. We took 100 homogeneous kernels from the middle part of the ear, weighed their fresh weight, put them into a mesh bag and marked the date. After that, they were placed in an oven to bake at 105°C for 30 minutes, then the oven was adjusted to 80°C and the kernels were dried for 24-36 h to constant weight.

The calculation of kernel moisture content at physiological maturity and harvest period: when dry matter accumulation reached the maximum, the moisture content was taken as the moisture content at physiological maturity. According to the formula: kernel moisture content = [fresh weight of 100 kernels (g) - dry weight of 100 kernels (g)] × 100%/fresh weight of 100 kernels (g), the kernel moisture content at each sampling was calculated after the kernel moisture content reached 40%.

The calculation of kernel natural dehydration rate: natural dehydration rate = [kernel moisture content at physiological maturity (%) - kernel moisture content at harvest (%)]/[days between 2 sampling intervals (d)].

3.3 Data analysis

The data were analyzed by SPSS 22.0 software.

Authors’ contributions

LSF was the executor of experimental design and research in this study, and completed the first draft of the paper. ZCX completed the experimental design and data analysis. LWJ and WH participated in the analysis of the test results. LXH was the designer and responsible person of the project, and guided the data analysis. YDG participated in the writing and revision of the paper. All authors read and approved the final manuscript.

Acknowledgments

This research was jointly supported by Agricultural Science and Technology Innovation Project of Jilin Province (CXGC2017JC001 & CXGC2017TD001) and National Science & Technology Infrastructure (NICGR2018-082).

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