Vertical Distribution Characteristics of the Phenotypic Traits of High-Yielding Wheat Varieties Under Limited Irrigation and Nitrogen Supply

To clarify the vertical distribution characteristics of the phenotypic traits of high-yielding wheat varieties, the canopy was used as the system, with the leaf position and node position as the level, strati�ed from top to bottom. The morphological structure characteristics of the leaf layer, node layer and light transmittance of the high-yielding wheat varieties under limited irrigation and nitrogen supply were studied and discussed. The results showed that two-year test yield and nitrogen fertilizer production e�ciency (NPF) were the highest in the spring irrigation at jointing and anthesis stages (W2) and 192 kg/hm2 (N1), which achieved high yield and e�cient nitrogen utilization. With the decrease of the leaf layer from top to bottom, the leaf area of the two wheat varieties displayed a trend of being higher in the upper layer and lower in the lower layer, while the light transmittance showed a decreasing trend. As the node layer decreased from top to bottom, the node length of the two wheat varieties decreased sequentially. Comprehensive research suggested that appropriate irrigation and nitrogen input improved the vertical distribution characteristics of wheat phenotypic traits, and can obtain higher grain yield and nitrogen fertilizer production e�ciency.


Introduction
Wheat is an important food crop in North China, and an increase in its output is important for the country's food security (Yang et al. 2021; Zhao et al. 2017; Zang et al. 2012).However, in actual production, farmers commonly apply excessive amounts of irrigation and nitrogen, leading to environmental pollution and reduced irrigation and nitrogen use e ciencies as well as product quality (Jing et al. 2007; Drinkwater et al. 2007; Wang et al. 2006; Zhang et al. 2007).Therefore, appropriate irrigation and nitrogen supply promote the effective distribution of plant components substances, and ensured production is an important way to achieve high yields and e cient utilization of irrigation and nitrogen by wheat.
Phenotypic traits are the combination form and overall expression of comprehensive biological traits formed by crop yield.Clari cation of the phenotypic trait distribution characteristics of high-yielding wheat varieties can provide references for breeding water-saving and high-yield wheat.The ideal plant type selection was the basis for water-saving cultivation and high-yield breeding of dryland wheat, leaf type was mainly featured by erect leaf, the ag leaf length and width ratio was moderate, and the length of the rst two leaves was approximately 20 cm (Li et al. 2011).Studies have shown that nitrogen application signi cantly increases the leaf length, single leaf area and SPAD value of different wheat varieties (Li et al. 2011;Vos et al. 2005), and excessive nitrogen application leads to higher plant growth and longer internodes (Laghari et al. 2010).Nitrogen reduction can increase the light transmittance ratio of the lower canopy after anthesis and can improve the light transmittance through of the lower canopy (Ma et al. 2019).As the amount of nitrogen increased, the plant height, leaf area index and photosynthetically active radiation intercepted by the canopy increased (Zhang et al. 2018).Lower nitrogen supply promotes the transfer of dry matter from the third leaves, the fourth leaves, the third nodes, the fourth nodes and remaining nodes of wheat plants to the grain, which was bene cial to increase the grain yield (Jiang et al. 2019).Through hydroponic experiments to study the effects of low nitrogen supply on the photosynthesis and nitrogen status of different wheat leaves, which indicated that the area of the rst leaf of two wheat varieties was signi cantly reduced, but that of the second and third leaves did not vary signi cantly (Li et al. 2013).At a nitrogen application rate of 225 kg/hm 2 , the thicknesses of the second and third internodes at the base of two wheat cultivars were greater than those of other nitrogen treatments (Zhou et al. 2019).Previous studies mainly focused on the leaf shape, and there are few studies on the combination of the leaf position and node position.The vertical distributions of the leaf position and the node position of different wheat varieties are less reported under water-limited irrigation, and knowledge of these topics is unclear, especially under the limited irrigation resources in North China.Therefore, this study selected two main wheat varieties (high-yield, watere cient and drought-resistant varieties) in the experimental area to systematically examine the vertical distribution changes of canopy morphology and physiological characteristics and to explore the individual plant types of wheat under different irrigation and nitrogen regimes.This study is expected to provide a scienti c basis and technical guidance for the shaping of a highly e cient canopy structure for water-e cient wheat cultivation.

Test site and meteorological conditions
Field trials were carried out at the Wuqiao experimental station of China Agricultural University in Wuqiao County, Hebei Province, from 2009 to 2011.Wuqiao is located in the middle of the Heilonggang Basin of the Haihe Plain, which has a warm temperate monsoon climate.The test site was located at 116°37′23″ east longitude and 37°41′02″ north latitude, with an average annual sunshine duration of 2767.1 hours and an average annual temperature of 12.5 ℃.The average annual rainfall is 676.9 mm, and the rainfall in the wheat season from 2009 to 2010 was 116.3 mm, which was a normal year.The rainfall in the wheat season from 2010 to 2011 was 62.4 mm (Table 1).The rst crop of the test eld was silage summer maize, and the soil was loamy bottom clay aquic soil.See Table 2 for the basic soil fertility of the 0-20 cm soil layer before planting.

Experimental design
Test one (2009~2010) The tested wheat varieties were Jimai 22 and Shimai 15.Each variety had two water-e cient irrigation modes (W1, W2) and two nitrogen levels (N1, N2).On the basis of suitable moisture during sowing, two water-e cient irrigation modes were established: W1 (spring irrigation at jointing stage) and W2 (spring irrigation at jointing and anthesis stages); and two nitrogen fertilizer treatments: N1 (base fertilizer: 123 kg/hm 2 + topdressing 69 kg/hm 2 at the jointing stage) and N2 (base fertilizer: 123 kg/hm 2 + topdressing 147 kg/hm 2 at the jointing stage).The combinations were W1N1, W1N2, W2N1 and W2N2.The area of each plot was 54 m 2 , with three replicates, and the distance between plots was 0.6 m.The seedlings were sown on October 16, 2009, with a basic seedling density of 510×10 4 plants/hm 2 and a row spacing of 16 cm.
Before the wheat was sown, the corn stalks were crushed and returned to the eld, after which organic fertilizer (50% chicken manure + 50% soil manure), 15 m 3 /hm 2 ; urea, 150 kg/hm 2 ; diammonium phosphate, 300 kg/hm 2 ; and potassium sulfate, 225 kg/hm 2 were applied.A random block arrangement was adopted and repeated three times.In the jointing period, the abovementioned treatments were combined with irrigation and nitrogen fertilizer topdressing.The irrigation water measurements and other eld management measures refer to water-e cient and high-yield cultivation techniques for wheat.The wheat was harvested on June 15, 2010.See Table 3 for the speci c treatments.The tested wheat variety was Shimai 15.A water-nitrogen model test was established, and the speci c codes for each regime are shown in Table 4.The plot area was 54 m 2 , the plots were arranged in random blocks, repeated 3 times, and the plots were separated by 0.6 m.The seedlings were sown on October 17, 2010, with a basic seedling density of 530×10 4 plants/hm 2 and a row spacing of 15 cm.Water plantar moisture before sowing wheat, and top dressing was carried out in conjunction with irrigation during the jointing stage.In each treatment, the corn stalks were crushed and returned to the eld before sowing, and organic fertilizer (50% chicken manure + 50% soil manure), 15 m 3 /hm 2 ; urea, 150 kg/hm 2 ; diammonium phosphate, 300 kg/hm 2 ; potassium sulfate, 225 kg/hm 2 ; and zinc sulfate, 15 kg/hm 2 were applied.A random block arrangement was adopted and repeated three times.In the jointing period, the abovementioned treatments were combined with irrigation and nitrogen fertilizer topdressing.Irrigation water measurements and other eld management measures refer to water-e cient and high-yield cultivation techniques for wheat.The wheat was harvested on June 20, 2011.See Table 4 for the speci c treatments.Test items and methods

Leaf shape determination
Sampling was conducted during wheat anthesis, and representative samples of 2×50 cm were collected from each test plot and transferred indoors.Twenty plants with uniform growth were selected, and the length and width of the ag leaf, the top two leaves, the top three leaves and the top four leaves were measured with a ruler.The leaf length was measured as the distance from the base of the leaf to the tip (cm), the leaf width was measured as the width from the base of the leaf to the widest point (cm), and the green leaf area of the population was measured at the same time.From 2009 to 2010, the length and width coe cient method was used to determine the leaf area at different leaf positions.From 2010 to 2011, the single leaf area (cm 2 ) was measured using an LI-3000 leaf area meter produced by the American LI-COR company.

Stem node morphology determination
To determine the leaf morphology, the plant height, ear length and the length of each node were measured with a ruler.The stem thickness was measured with a Vernier caliper.Both tested varieties had ve internodes, with the inferior internode as the rst internode, followed by the second internode, the third internode, the fourth internode, and the belowground internode as the inverted fth internode.The wheat plant height composition index was calculated as follows: I L = (L 1 + L 2 )/L, and the index between two adjacent nodes was calculated as follows: , where L is the plant height, n is the node position from top to bottom, L 1 is the internode length beneath the panicle, and L 5 is the basal internode length.

Determination of canopy transmittance
At the booting stage, owering stage, and 10 days and 20 days after owering, the photosynthetically active radiation was measured by an LI-191SA linear light quantum sensor produced in the United States on a sunny and windless day from 11:00 to 13:00.The wheat canopy was divided into the upper layer, middle layer and lower layer.The natural light intensity above the wheat corresponded to the upper layer, the middle layer was 2/3 of the plant height based on the distance from the ground, and the lower layer was 1/3 of the plant height from the ground.The formula for calculating the light transmittance of each layer was as follows: T=I r /I 0 ×100%, I r is the light intensity of a speci c leaf layer, and I 0 is the natural light intensity at the top of the wheat canopy.

Grain yield
At maturity, 4 m 2 sample point were selected from each plot, and were harvested separately by hand.After threshing, they were air-dried to calculate yield.The moisture content of the grain was 13%.
Nitrogen production e ciency

Results And Analysis
Wheat grain yield and nitrogen fertilizer production e ciency Table 5 shows that under the W1 level, the grain yield and nitrogen fertilizer production e ciency of Jimai 22 were not signi cantly different between the N1 and N2 treatments, while the grain yield and nitrogen production e ciency of Shimai 15 in the N1 regime were signi cantly higher than those in the N2 treatment.Under the W2 level, the grain yield and nitrogen production e ciency of the two wheat varieties in the N1 regime were signi cantly higher than those in the N2 treatment.Under the same level of nitrogen application, the grain yield and nitrogen production e ciency of the two wheat varieties in the W2 regime were higher than those in the W1 regime from 2009 to 2010, while in 2010~2011, the grain yield and nitrogen production e ciency of the W2 regime were higher than those in the W1 treatment.This showed that under the two waterlimited irrigation regimes in this experiment, the W2 regime was more conducive to improving the grain yield and nitrogen fertilizer production e ciency than the W1 regime.Under the W2 level, the N1 regime resulted in high wheat grain yield and nitrogen production e ciency.Vertical change in the wheat canopy shape

Leaf layer morphological characteristics
Leaves are the most important photosynthetic organs of crops, and the size and spatial distribution of leaves are determined by the genetic characteristics of the variety and are also affected by the cultivation measures.Leaf characteristics are usually based on measurements of the leaf length, width and area.Table 6 and Table 7 show that with the decrease of the leaf layer from top to bottom, the leaf area of the two wheat varieties displayed a trend of being higher in the upper layer and lower in the lower layer.Under the same limited irrigation conditions, the area of each leaf layer in the N2 regime was higher than that in the N1 regime for both varieties.Table 6 shows that compared with the N1 treatment, the leaf area of a single stem of Jimai 22 in the N2 regime increased by 5.7%, and the leaf area of a single stem of Shimai 15 in the N2 regime increased by 11.1%.The comparison between varieties showed that the leaf area of Jimai 22 in each layer was signi cantly higher than that of Shimai 15, which indicated that Jimai 22 had a relatively strong stabilization ability of the leaf area.

Changes in the leaf area index
The leaf area index is an important indicator of the characteristics of light radiation in wheat populations.Table 8 and Table 9 show that as the leaf layer decreased from top to bottom, the leaf area index of the two wheat varieties decreased.Under the same limited irrigation conditions, the leaf area index in each layer in the N2 regime was higher than that in the N1 regime for both varieties.The leaf area index of Jimai 22 was higher than that of Shimai 15 in each layer.This vertical distribution pattern with a higher leaf area index in the middle and upper parts and a lower leaf area index at the bottom was not only conducive to the transmission of light from the upper to the lower layer but was also bene cial for the interception of light energy in the middle and lower layers.

Morphological characteristics of nodal layers
Differences in plant height depend mainly on differences in node length, and the plant height composition index re ects the increasing trend of the length of each node from bottom to top.Table 10 and Table 11 show that as the node layer decreased from top to bottom, the length of each internode of both wheat varieties showed a decreasing trend, while the plant height composition index of two adjacent internodes showed an overall upward trend of increasing, followed by a decrease and then an increase.Under the same limited irrigation conditions, the length of the upper three nodes at the owering stage accounted for approximately 70% of the plant height.The plant height and ear length of both varieties in the N2 regime were greater than those in the N1 treatment.The two wheat varieties had a similar plant height, but the length of the rst ve internodes of Jimai 22 was shorter than those of Shimai 15, which indicated that Jimai 22 had stronger lodging resistance.The N1 regime shortened the length of the basal internodes and enhanced the lodging resistance of both wheat varieties.This not only conserves nitrogen but is also an effective lodging-resistant cultivation method.Suitable application of nitrogen at the jointing stage can effectively prevent wheat from lodging, which can increase yield.

Variation characteristics of stem thickness
At the late stage of wheat growth and development, due to the large amount of stalk storage material transported to the grain, the size of the medullary cavity between the stalk nodes changed.
Table 12 and Table 13 show that as the node layer decreased from top to bottom, the stem thickness of the two wheat varieties became the thickest in the rst three nodes.Under the same limited irrigation conditions, the thickness of the internodes of the N2 regime was higher than that of the N1 regime in both varieties, except for ve stems thick.Comparing the varieties, the stem thickness of each node layer of Jimai 22 was higher than that of Shimai 15, which indicated that Jimai 22 had a higher lodging resistance than Shimai 15.

Dynamic characteristics of the temporal and spatial distribution wheat canopy transmittance
Table 14 shows that with increasing canopy height, the light transmittance of the two wheat varieties showed an increasing trend, and the light transmittance of the middle part of the canopy was higher than that of the lower part.During the growth and development process, the light transmittance of the middle and lower parts increased and was highest at 20 days after owering and lowest at the booting stage.Under the same limited irrigation conditions, the light transmittance of the N1 regime was higher than that of the N2 regime for both varieties.Under the same nitrogen level, the light transmittance of the W1 regime was higher than that of the W2 regime for both varieties.Under different water and nitrogen regimes, the light transmittance of Jimai 22 was generally higher than that of Shimai 15. Figure 1 shows that with the decrease in the canopy level, the light transmittance decreased, reaching the maximum value at the ag leaf.Under the same limited irrigation conditions, the light transmittance decreased with increasing nitrogen application rate.Therefore, appropriate irrigation and nitrogen supply could improve the light distribution of the canopy and increase the light transmittance of the middle and lower layers at the jointing stage.

Morphological characteristics of leaves
Leaf morphological characteristics are mainly based on the leaf length, leaf width and leaf area, which are among the important factors that affect the light distribution and interception in the canopy.A reasonable canopy structure is the basis for the formation of high crop yields, and nitrogen supply is an important factor that regulates the leaf area index (Zhang et al. 2011;Wang et al. 2011).Within a certain range, the leaf area index increases with increasing nitrogen application rate, which optimizes the light receiving posture for wheat plants (Huang et al. 2019).Studies have pointed out that under optimal fertilization treatment, the upper three leaves of wheat shorten, and the leaf area and leaf area index of the upper three leaves decrease, which increases the light transmittance and the light interception in the middle and lower layers of the wheat canopy (Li et al. 2007).Some scholars believe that the ag leaf should not be too long or too wide, the length of the rst two leaves should be approximately 20 cm, and the leaves should be tower-like, this kind of plant structure allows good light transmittance.The results of this study showed that as the leaf layer decreased from top to bottom, the leaf area and leaf area index of both varieties decreased.Under the same limited irrigation conditions, the leaf length, leaf width and leaf area of both varieties were higher in the N2 regime than in the N1 treatment.With the increase in the nitrogen application rate, the leaf area index increased, which is consistent with previous research results (Han et al. 2020) and Liu et al. (Liu et al. 2021).In each canopy layer, the leaf length, leaf width and leaf area of Jimai 22 were higher than those of Shimai 15.Therefore, the nitrogen fertilizer signi cantly increased the leaf length, leaf width and leaf area index of the upper three leaves of both varieties.the ratio of the two basal nodes to plant height had a speci c threshold that would not reduce the lodging resistance of the stem (Huang et al.

2021)
. With an increase in the nitrogen application rate and topdressing ratio, the basal internodes of wheat became increasingly thinner, and the stalk strength decreased (Zhang et al. 2014).This study showed that as the node layer decreased from top to bottom, the internode length of the two wheat varieties showed a decreasing trend, and the stem thickness reached the highest value at the third internode.Under the same limited irrigation conditions, the length of the upper three nodes at anthesis accounted for approximately 70% of the plant height, except for the ve internode length and the ve stem diameter, the length of each internode and the stem thickness of both varieties in the N2 regimewere higher than those in the N1 treatment.With an increase in the nitrogen application rate, the plant height, internode length and stem thickness increased.
Comparing between varieties, the plant heights and the stem thickness of each node layer of Jimai 22 was higher than that of Shimai 15.
proper irrigation and nitrogen supply has a certain lodging resistance, which is conducive to obtaining high yields.

Characteristics of light transmittance
Light transmittance is an important parameter that re ects the relationship between light distribution and canopy structure in a plant population.
The leaves play the most important role in the canopy structure, while the light distribution is strongly affected by the leaf position.Studies have shown that with an increase in wheat planting density, the light transmittance in the middle and low canopy levels is relatively low (Ding et al. 2020).With an increase in nitrogen fertilizer, the light transmittance in the middle and lower parts of a winter wheat canopy showed a trend of rst decreasing and then increasing (Lei et al. 2017).The results of this study showed that under the same limited irrigation conditions, the light transmittance in the N1 regime was higher than that in the N2 regime for both varieties.Under the same nitrogen level, the light transmittance in the W1 regime was higher than that in the W2 regime for both varieties, and under different irrigation and nitrogen regimes, the light transmittance of Jimai 22 was generally higher than that of Shimai 15.With increasing canopy height, the light transmittance of the population showed an increasing trend, and the light transmittance of the middle part was higher than that of the lower part.During the growth and development process, the light transmittance of both varieties showed an overall increasing trend.With an increase in the nitrogen application rate, the light transmittance in each leaf layer showed a downward trend.
Based on the above conclusions, nitrogen application can alter the characteristics of the leaf layer and node layer in wheat, shorten the length of the basal internode, and increase the light transmittance in the middle and lower parts of the population.Under the condition of limited irrigation, when the nitrogen application rate was 192 kg/hm 2 , the yield of the two wheat varieties reached 9954.61 kg/hm 2 , and the nitrogen fertilizer production e ciency was 23.49 kg/kg, which can be used as a reference for actual production.
Canopy distribution of the light transmission rate at anthesis under limited irrigation and nitrogen supply (2010~2011)

Table 1
Meteorological conditions during the winter wheat growing period from 2009∼2011

Table 4
-way analysis of variance (ANOVA) was performed in SPSS 19.0 (IBM Corp., Armonk, NY, USA).GraphPad Prism 5.0 (GraphPad Software, Inc., California, USA) was used to draw the gures.Regime means were compared using the least signi cant difference test considering P < 0.05 as signi cant.
NPE was calculated according to the methods of Dordas and Sioulas (2009): NPE (kg/kg)=grain yield / N fertilizer rate Data analyses One

Table 5
Grain yield and nitrogen fertilizer production e ciency of winter wheat under limited irrigation and nitrogen supply From 2009 to 2010, the grain yields of the Jimai 22 and Shimai 15 in the W2N1 regime were 9349.04 kg/hm 2 and 9557.48 kg/hm 2 , and the nitrogen fertilizer production e ciencies of the Jimai 22 and Shimai 15 were 23.44 kg/kg and 22.74 kg/kg.There was a large difference between the varieties: under different water and nitrogen regimes, the grain yield and nitrogen fertilizer production e ciency of Jimai 22 were higher than those of Shimai 15.The W2N1 regime resulted in the highest yield and nitrogen fertilizer production e ciency of both wheat varieties.Similarly, from 2010 to 2011, the two wheat varieties had the highest grain yield and nitrogen fertilizer production e ciency in the W2N1 treatment.

Table 6
Leaf con guration of individual plants at anthesis under limited irrigation and nitrogen supply (2009~2010)

Table 8
Leaf area index of individual plants at anthesis under limited irrigation and nitrogen supply

Table 10
Internode length and plant height component index at anthesis under limited irrigation and nitrogen supply (2009~2010)