The determination of crop leaf area is widely used in agricultural research and production. The methods mainly include leaf area measuring instrument method, gravimetric method, square paper method, planimeter method, photoelectric method and measurement method. The weight method is simple, but the error is large; the planimeter method and the square paper method are accurate, but time-consuming and labor-consuming; the photoelectric method requires equipment and equipment, and should not be applied at the grass-roots level; the measurement method is simple and easy, and this method uses leaf length. x Leaf width x Leaf area correction coefficient Obtain the leaf area. There are many reports of leaf area correction coefficient (r) values ​​by former researchers, but the r values ​​are different. In order to obtain the accurate value of winter wheat r value, Liu Zihua and Jin Wenlin conducted a preliminary study in Beijing in 1983/84. This paper reports the results of the off-site study on the wheat leaf area correction coefficient and leaf area index (R) in Tangshan City in 1993/94 using four varieties of production in Hebei Province.
1 Materials and Methods The tested varieties were Fengkang 8, Jing 8866, Tangmai 4 and Wanmai 28. The three densities were (1) 7-88,000 seedlings/mu; (2) 15-16 million seedlings/mu; (3) 20-21 million seedlings/mu. The field was designed as a random block with three replicates. Sowed on September 24, 1993. The contents of the survey and measurement methods were to investigate the dynamics of tillering every 7 days after turning greens the following year, and to take 10 samples to investigate the number of leaves; to measure the leaf area per spring for three times, and to measure the area of ​​leaves by using a planimeter every time. , and determine the length and width of each leaf, using the formula r = leaf area / leaf length x leaf width to obtain the r value of each leaf. The leaf area per plant was determined using the number of stems per leaf area per leaf area. The relationship between the leaf area and the number of stems per plant and r value was calculated by regression analysis, and the leaf area index was calculated.
2 Results and Analysis
2.1 Determination of Leaf Area Correction Coefficients The leaf area of ​​each leaf of four cultivars and three densities was measured on April 14, April 29, and May 10, 1994 using a planimeter and the leaves of each leaf were measured. The length and width of the leaf were obtained and the r value of each leaf was obtained. First, compare the different densities and find that there is no difference in r values ​​between different densities. Thus, the r values ​​of the four varieties of three densities were combined separately (Table 1). Table 1 shows that the value of r decreases from large to small with the leaf position. An analysis of variance was performed on the r values ​​of different leaf types of the four varieties. The results showed that there was no difference between the varieties, and there was no interaction between the varieties and the leaves, and the difference between the leaves was extremely significant (Table 2). Further multiple comparisons showed that there was no difference in the 1st and 2nd leaves, but it was significantly different from other leaves; there was no difference between the 3rd, 4th, and 5th leaves, but it was extremely different from the 6th leaves. Six spring leaves were divided into three groups: the first group was the proximal leaves, the phylloscenes were 1, 2; the second group was the stem leaves, the phyllosia was 3, 4, 5; the third group was the flag leaves, the leaf sequence 6. The results of the average r values ​​for the groups were similar to those for the 1983/84 (Table 3). Description There is no difference between years.
2.2 The relationship between the number of stems per plant and leaf area The same crops in the same period, the more the number of stems per plant, the greater the leaf area per plant, there is a positive correlation. Our test results (Table 4) confirm this claim. From Table 4, it can be found that although the number of stems per plant in the previous two surveys was larger, the leaf area of ​​the whole plant was smaller, while the number of single plants was the lowest at the heading stage, and the area of ​​the whole plant leaf was the largest. This is due to the fact that the leaf area of ​​the near root leaves was small and the top two leaves were not fully expanded or extended during the previous two surveys. The leaf area of ​​the four upper green leaves at the time of heading investigation reached the maximum value, and this time was invalid. Mulberry has completely degenerated leading to reduced stems per plant.
The number of stems per plant varies greatly between different cultivars and different densities before heading, and a relatively uniform equation cannot be found to describe the relationship between the number of stems per plant and leaf area. The results of the heading date were collected and the number of stems per plant was found. The following relationship with the leaf area: 
In the formula, Y is the sum of the leaf area of ​​the investigated N plant, Yi is the leaf area of ​​the ith plant, xi is the number of the individual plant of the ith plant, and r is the correction coefficient of the whole plant leaf area at the time (heading period is r=0.76). This theoretical equation was tested with three measured densities of four varieties and the fitness was good (Table 5).
For ease of understanding, (1) can be further simplified as: 
After significant tests, there was no significant difference between the two formulas (t=0.6241, t0.05=2.2280).
2.3 Calculation of population leaf area index Using formula (2), the total leaf area and total leaf area within a certain area of ​​the field can be measured. The land area is the population leaf area index within the survey area. For each wheat field, take P representative samples, each sample area is S, there are n strains in S area, the number of stems of i-th plant is xi, then the average leaf area index (R) of p sample points is: 
3 Discussion The predecessors have conducted many studies on the grouping of winter wheat leaves, and the results of this study are the same as those of them. The predecessors grouped the leaves mainly to study the function of each group of leaves. In this study, the six spring leaves were divided into three groups from the perspective of leaf area correction coefficient to determine leaf area and leaf area index for scientific research and production. However, from this experiment and the 10-year off-site test, the r value has no difference in density and variety. However, the difference in the similar results between the leaves shows that the leaf area correction coefficient is closely related to the genetic factors, and the r value of each leaf. Affected by heredity, not affected by the environment, the r value of each leaf is different, indicating different leaves: the value is determined by different genes. According to the fact that the r value can be divided into three groups, it is inferred that the r value of six spring wheat leaves is determined by three groups of genes. The first group of genes controlled the extension of the near root leaves, the second group of genes controlled jointing to the 3-5 leaves protruding from the booting, and the third group of genes controlled the flag leaves. The expression of r values ​​in these three groups of genes is gradually realized. However, the results of special genetic studies are still required to confirm.
The determination of the value of r for the winter wheat leaf area was determined by the use of a quantitative method. Yan Qisong (1963) proposed that 1/1.2 = 0.833, and most people always use this value. Du Rongsai (1980) proposed using 5/6. The results of this study suggest that the average r-value of the leaves of the whole plant is 0.78. However, it is inaccurate to adopt a value completely when the leaves are different. Using different r values ​​at different growth stages can accurately reflect the leaf area and leaf area index of wheat at that time. According to the results of the two-year off-site experiments, we believe that the r value of the lower 1 and 2 leaves should be taken as 0.835. Close to 1/1.2 or 5/6, the middle 3-5 leaves should take 0.775, and the flag leaf should take 0.73. The joints from the jointing stage to the booting stage involve the leaves of the middle and lower groups. The average value of r should be 0.80. The heading period involves 4 leaves in the middle and upper part. The r value should be 0.7'. It is more reliable to use a r value than the temples.
Leaf area index (R) value can be used formula 
Calculations. In this way, as long as P samples are sampled within each S sample area, calculate the number of stems per plant and calculate the R value using the formula. Using this formula to calculate the R value does not require measuring the leaf area of ​​each leaf, which is very simple. It should be particularly pointed out that this formula is only applicable to heading periods. The simple method of determining the Y and R values ​​of each previous fertility stage is still a subject that needs further study.
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