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Article

Improvement of the Photosynthetic Characteristics and Yield of Wheat by Regulating the Proportion of Nitrogen Fertilizer Base and Topdressing

by
Yaoyuan Zhang
1,
Haiqi Wang
2,
Rongrong Wang
1,
Fangfang He
1,
Guiying Jiang
1,* and
Jianwei Xu
1,*
1
College of Agronomy, Shihezi University, Shihezi 832003, China
2
Agricultural Science Research Institute of 12th Division, Urumqi University, Urumqi 830046, China
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(4), 899; https://doi.org/10.3390/agronomy15040899
Submission received: 17 March 2025 / Revised: 31 March 2025 / Accepted: 2 April 2025 / Published: 3 April 2025
(This article belongs to the Section Soil and Plant Nutrition)
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Abstract

:
This study developed a nitrogen management framework that simultaneously addresses photosynthetic limitations and water scarcity challenges, providing a scalable solution for sustainable wheat production in arid farming systems. Focusing on Xinjiang’s arid region, we investigated how different ratios of basal to topdressed nitrogen fertilization affect photosynthetic mechanisms in drip-irrigated spring wheat. We implemented a split-plot design during the 2020–2021 growing seasons, using two wheat cultivars as main plots: strong-gluten Xinchun 37 (XC37) and medium-gluten Xinchun 6 (XC6). The subplots consisted of five N application treatments: N00: (no nitrogen application, control), N28 (base fertilizer 20%, top dressing 80%, and so on), N37 (3:7), N46 (4:6), and N55 (5:5). The vast majority of indicators performed best under N37 treatment. And LAI, RuBPC (ribulose-1,5-diphosphate ribulose carboxylase) activity, net photosynthetic rate (Pn), yield, and its composition were higher than the rest of the treatments by 0.21~31.75%, 6.94~25.21%, 7.42~40.78%, 0.86~25.44%, and 0.44~12.02%. And intercellular CO2, concentration (Ci) was lower than other treatments by 7.63~50.60%. Yield showed q highly significant positive correlation with Pn, Gs, Tr, ΦPSⅡ, and chlorophyll fluorescence parameters, but a negative correlation with Ci. Stepwise regression analysis showed that LAI, Pn, Ci, and RuBPC activity had a significant impact on yield and its compositions. In addition, all index performances of XC37 were better than XC6. Under drip irrigation in arid zones, allocating 30% basal + 70% topdressed N optimally enhances photosynthetic capacity and yield formation in spring wheat, offering a practical pathway for sustainable intensification in water-limited agroecosystems.

1. Introduction

The formation of crop yield is primarily achieved through the photosynthesis of normal growing green leaves [1,2], and the key to breaking through the limitation of crop yield is to enhance and tap the photosynthetic performance potential of crops and improve the light energy utilization rate [3,4,5]. This challenge is particularly relevant in Xinjiang, an arid-to-semi-arid region where agricultural productivity is frequently constrained by water scarcity [6]. Wheat is the most important grain crop in Xinjiang [7], and as a globally important staple crop, wheat faces growing production demands, with global requirements projected to increase by approximately 50% by 2050 to meet the needs of an expanding population [8]. To ensure China’s food security, it is crucial to consistently raise the yield of wheat produced per unit of land area. Drip irrigation is the main cultivating technology of wheat production in the arid zone of Xinjiang in recent years [6,7]. Xinjiang wheat relies on drip irrigation technology precision controlling fertilizer [9,10] and the implementation of the action plan for zero growth in fertilizer application by 2020 [11]. These lead to a positive development in the use of fertilizers. Under the condition of maintaining high yield, N application declines from 300 kg·hm−2 to 255 kg·hm−2 for wheat during the reproductive period, and the utilization rate of fertilizer increases [12,13]. Although widespread adoption of soil testing and formula fertilization technologies has substantially improved fertilization practices in Xinjiang, challenges persist, including suboptimal fertilization structures, imbalanced nutrient ratios, and concerns regarding soil fertility depletion and environmental degradation [14,15,16]. Furthermore, nitrogen applied during early growth stages is subject to substantial losses through dissolution, leaching, and volatilization [17,18], resulting in typical nitrogen use efficiencies of only 30–40% in wheat systems [19]. When there is a nitrogen deficiency, topdressing can usually maintain high yield by providing additional nutrients for the following growth stages. Therefore, this experiment is based on different nitrogen fertilizer base–topdressing ratios to synergistically optimize photosynthetic performance and yield formation under water-limited drip irrigation. And it seeks to break through the traditional fertilizer application mode and improves the nitrogen fertilizer utilization rate, which is of great practical significance to the sustainable development of the wheat industry in arid regions of Xinjiang.
The level and mode of nitrogen supply affect photosynthetic parameters to a certain extent [20]. Appropriate nitrogen application can promote the uptake of luminous energy and increase the activity of PSII in wheat, which can lead to the efficient conversion of luminous energy into chemical energy, offer enough energy for carbon assimilation, and increase photosynthetic rate, ultimately showing better spike characteristics and higher yields [21,22,23,24,25]. RuBPC is a key enzyme in photosynthetic carbon assimilation, which is significantly correlated with crop photosynthetic rate [26]. Wheat as a C3 plant and PEPC enzyme of wheat has a high affinity for substrates [27,28], and enables effective refixation of respiratory CO2 [29]. Nitrogen fertilization can promote the enhancement of RuBPC and PEPC in wheat flag leaves, thereby promoting the increase in the rate of photosynthesis and the accumulation of assimilates [30]. In general, with standard N application increasing, the photosynthetic gas exchange parameters firstly increase then decrease [31]. When the fertilizer application rate increased from 150 kg·hm−2 to 225 kg·hm−2 and 300 kg·hm−2 at the flowering stage, the photosynthetic rate of winter wheat increased, thereby boosting wheat yield [32,33]. In addition, research showed that nitrogen application in the range of 60 kg·hm−2~120 kg·hm−2, Fv/Fm, and ΦPSⅡ increased with increasing nitrogen application [34]. However, excessive application of nitrogen fertilizer actually led to a decrease in photosynthesis in spring wheat [35]. An appropriate nitrogen fertilizer basal and topdressing mode could increase the crop’s leaf chlorophyll content, improve photosynthetic performance, extend green leaves’ functional period, and enhance the accrual of photosynthetic outgrowth, thereby increasing wheat yield. For different crops, the optimal nitrogen fertilizer base to topdressing ratio are also varies for achieving high yields. It was shown that when N application was 150 kg·hm−2 and post-seedling topdressing was 70%, it could reduce the light compensation point, increase ΦPSⅡ, and remarkably increase sugar yield by 5~29% during the expansion period of sugar beet tubers [36]. The pattern of a nitrogen fertilizer basal and topdressing ratio of 4:6 can regulate the differences between cotton monocultures and clusters, improve nitrogen accumulation and transport in all plant organs, maintain a higher number of bolls per plant, and obtain high yields [37]. In the study of wheat, it was found that, at the same N level, the Pn, Gs and Tr of wheat were highest and more favorable to increase canopy LAI, SPAD, and the photosynthesis rate when the nitrogen fertilizer basal and topdressing mode were 6:4 and 3:7 [20]. The maximum prices of Pn, Gs, Ci and Tr were at 300 kg·N·hm−2 followed by 225 kg·N·hm−2, and these traits were highest at anthesis in the ratio of 6:4 in winter wheat [13]. These varietal and environmental differences highlight the importance of context-specific N management. Proper N allocation regulates key photosynthetic enzymes, optimizes gas exchange parameters, and enhances chlorophyll fluorescence characteristics, collectively determining photosynthetic efficiency and yield potential in drip-irrigated spring wheat systems.
Rational nitrogen management represents a critical lever for yield improvement in wheat cultivation, given nitrogen’s fundamental role as a primary macronutrient [38,39]. Staged fertilization strategies have demonstrated particular efficacy in delaying leaf senescence, improving nitrogen use efficiency, and ultimately boosting grain yields [40]. While previous research has extensively examined the impacts of N application rates and methods on wheat growth dynamics, physiological responses, and yield components [13,20]. However, under root layer nitrogen reduction, there is a lack of systematic research on the regulation of photosynthetic physiology and yield formation in drip-irrigated spring wheat under different basal and top dressing modes, which limits the potential for high yield of drip irrigation wheat in arid areas. Therefore, we assume that drip irrigation of wheat under different nitrogen fertilizer base to topdressing ratios can improve photosynthesis, thereby establishing a physiological foundation for achieving both high and stable yields in water-limited environments. Our findings will provide essential agronomic insights for developing sustainable, high-efficiency production systems for drip-irrigated spring wheat in arid regions.

2. Materials and Methods

2.1. Overview of the Experimental Site

We carried out experiment from April to July 2020−2021 at the Xinjiang Shihezi University (85°59′ E, 44°18′ N). It has a typical continental climate, far from the sea, with high evapotranspiration, high radiation, and low rainfall. The total solar radiation in the region was 1722 kWh·m−2 (2020) and 1740 kWh·m−2 (2021), and the changes in meteorological indexes during the wheat reproduction period are shown in Figure 1. The test soil type was classified as a calcisol, the soil depth was 0~40 cm, and its basic information is listed in Table 1.

2.2. Experimental Design

This experiment adopted a split plot design. Spring wheat varieties were set as the main zone, and nitrogen fertilizer basal and topdressing modes were set as the secondary zone. The test varieties were Xinchun37 (XC37, protein content 16.30%, strong adaptability and high stability) and Xinchun6 (XC6, protein content 13.50%, strong resilience). Five nitrogen fertilizer basal and topdressing treatments were set up as shown in Table 2, where N28 was the conventional topdressing mode of nitrogen fertilizer in Xinjiang. All treatments were triplicate.
The test plot area was 12 m2. We used a wide narrow row and a one tube, four rows planting method, as shown in Figure 2 (drip irrigation tape: diameter 16 mm, drip head spacing 30 cm, flow rate 2.6 L·h−1). Plot was tilled into the soil with 120 kg·hm−2 of P2O5 (calcium superphosphate) as basal fertilizer, and 250 kg·hm−2 pure nitrogen was applied during the growth period (urea, N = 46%). Firstly, sowing, plowing, raking, and sunning the ground were carried out. Then, we applied different amounts of base fertilizer using different treatments. The topdressing was applied along with water through drip irrigation belts. After the preliminary work of sowing was completed, we sowed seeds and installed the drip irrigation system. During the growth period of wheat, we needed to observe the growth status of wheat and take timely measures. The total irrigation volume was 6000 m3·hm−2 over the entire life cycle. And the amount of irrigation water in each period was precisely controlled by a water meter. We planted on 1 April 2020 and 4 April 2021 with a sowing amount of 345 kg·hm−2, and harvested on 4 July 2020 and 7 July 2021.

2.3. Measurement Items and Methods

For each treatment of each variety, single stems with healthy growth and essentially similar length were chosen and tagged for material. Twenty plants per plot were randomly selected to measure the leaf region index, relative chlorophyll content, blade gas exchange parameters, and leaf chlorophyll fluorescence parameters at the tillering, jointing, booting, flowering, milky maturity, and dough stages. All treatments were repeated three times. And the sampling and measurement time was from 11:00−13:00 at noon. A total of 30 pieces of flag leaves of each treatment were taken at the flowering, milky maturity, and dough stages, and then immediately laid in liquid nitrogen for 30 min. Finally, these were preserved in a refrigerator at −80 °C to determine the activity of key photosynthetic enzymes.

2.3.1. Leaf Region Index (LAI) and Relative Chlorophyll Content (SPAD)

A leaf region meter (Li-3100C, Li-Cor, Lincoln, NE, USA) was used to measure the leaf area and the measurements were repeated three times in the period described above. The leaf area meters measured the leaf area. Leaf area index (LAI) = leaf area/unit land area. A chlorophyll meter (SPAD-502, Konica Minolta, Tokyo, Japan) was used to measure SPAD values in 20 flag leaves (the penultimate spreading leaf was measured before the booting stage) on the main stem of wheat in the above period.

2.3.2. Photosynthetic Enzyme Activity

RuBPC activity and PEPC activity were measured by spectrophotometric means with reference to Racker [41].

2.3.3. Gas Exchange

Pn (μmol CO2 m−2·s−1), Gs (mol H2O m−2·s−1), Ci (μmol CO2 m−2·s−1), and Tr (mmol H2O m−2·s−1) were determined with a photosynthesis analyzer (LI−6400, Li−Cor, Lincoln, NE, USA) using a rotametry method in the uppermost spreading leaves of the tillering and jointing stages of the plant, and the flagging leaves of the earliest booting stage, the flowering, the milky maturity, and the dough stages. Data calibration should be performed before determining the parameters. (1) FLOW Meter Zero: the flow rate signal should be within the range of −0.5–+0.5; (2) IRGA Zero (zero calibration of CO2 and H2O): the zero point of CO2 is generally calibrated first, followed by H2O. CO2R and CO2S are close to ‘0’, and the range of fluctuation is within the range of ±0.1 μmol·mol−1, and the zero adjustment is completed; H2OR or H2OS are near ‘0’, the fluctuation range is within ±0.01 mmol·mol−1, the zero adjustment is completed. For the determination of photosynthetic parameters, the temperature was set at 25 °C, the CO2 concentration was set at 400 μmol·mol−1, and the light source was set at 1300 μmol·m−2·s−1 with an artificial light source. Five leaves of uniform length were measured per treatment from 11:00 to 13:00 noon.

2.3.4. Leaf Chlorophyll Fluorescence

The chlorophyll fluorescence parameters of the uppermost spreading leaves of plants at tillering and jointing stages, and the flag leaves at booting, flowering, and milky maturity stages were determined using the FMS-2 (Hansatech, Norfolk, UK) portable fluorometer round robin method, and the number of leaves was 5. ΦPSⅡ was measured from 11:00–13:00 p.m., and Fv/Fm was determined after dark reaction lasting 30 min.

2.3.5. Grain Yield

At maturity, wheat plants were selected from 1 m2 region within each treatment, harvested, and threshed manually. The grain was naturally air-dried until the moisture content of the grain was about 12.5% and the grain yield was calculated. For each treatment plot, 20 g was taken randomly three times and weighed as W1. Seven days later, it was weighed as W2; if moisture content was higher than 12.5%, drying continued until the moisture content was 12.5%. Seed moisture content = (W1 − W2)/W1 × 100%. Twenty wheat plants were randomly selected from 1 m2 (samples were taken diagonally within the sample plot until twenty plants were selected) for determining grain per spike and the 1000-grain weight.

2.4. Data Analysis

Analysis of variance (ANOVA) and Duncan’s multiple range test were performed using SPSS 20 software (SPSS, Chicago, IL, USA). The Pearson correlation coefficient was used to analyze the degree of correlation between the indicators. To evaluate the degree of determination of different indicators on yield and yield composition, we used stepwise regression to analyze the measured LAI (X1), SPAD (X2), Pn (X3), Gs (X4), Tr (X5), Ci (X6), Fv/Fm (X7), ΦPSⅡ (X8), RuBPC (X9). We used Origin 2021 software for drawing.

3. Results

3.1. Leaf Area Index (LAI) and Relative Chlorophyll Content (SPAD)

LAI showed a trend of “Λ” with wheat fertility period changed (Figure 3). LAI reaches its maximum at the booting stage. In addition, the decrease in LAI from milky maturity to dough stage (69.79~71.32%) was significantly higher than the decrease from flowering to milky maturity stage (32.76~35.12%) in 2020–2021. Under the nitrogen fertilizer basal and topdressing modes, the LAI values showed N37 ≥ N46 > N5 > N28 > N00. During the two-year period 2020–2021, N37 was higher than N00, N28, N46, and N55 by 4.94~46.52%, 1.10~27.93%, 0.21~6.91%, and 0.45~13.82%. And XC37 was 4.50~4.51% higher than XC6. It indicated that a proper backward shifting (3:7) of the nitrogen fertilizer basal and topdressing mode was beneficial for ensuring a larger photosynthetic leaf region during wheat fertility.
SPAD showed a trend of “Λ” with changing wheat fertility periods (Figure 4). SPAD reached its maximum at the flowering stage. The decrease in SPAD from the milky maturity to dough stages (20.98~38.01%) was slightly greater than the decrease from the flowering to milky maturity stages (15.09~24.53%) in 2020–2021. Under the different nitrogen fertilizer basal and topdressing modes, the overall changes in SPAD values of wheat leaves showed that N37 > N46 > N55 > N28 > N00 (Figure 4). During the two-year period 2020–2021, N37 was higher than the rest of the treatments by 7.59~11.77%, 10.70~12.65%, 11.87~14.39%, and 24.67~29.08% at the flowering stage. Comparisons between varieties showed that XC37 was on average higher than XC6 by 1.06~16.04% in 2020–2021.

3.2. Activities of Key Photosynthetic Enzymes in Flag Leaves

As displayed in Figure 5, RuBPC and PEPC activity showed a decreasing trend as the wheat fertility period developed. Comparing the RuBPC and PEPC activities of the two varieties, we found that XC37 was higher than XC6 by 0.44~9.72% and 1.26~12.81% in 2020–2021. The response of the activity of the two enzymes to different nitrogen fertilizer base-to-topdressing ratios was consistent. With the proportion of topdressing decreases, the activities of the two photosynthetic enzymes revealed increasing firstly then a decrease, which was manifested as N37 > N46 > N55 > N28 > N00. During the period 2020–2021, the RuBPC activity in N37 was significantly increased by 17.23~34.63%, 10.47~16.23%, 7.46~11.23% and 8.33~12.92%, and the PEPC activity in N37 was significantly increased by 18.76~66.94%, 9.98~38.46%, 7.01~16.88%, and 8.47~28.17% compared to N00, N28, N46, and N55 treatments.

3.3. Gas Exchange Parameters

As shown in Figure 6A, as the reproductive period progressed, Pn varied, increasing and then decreasing in 2020–2021. The nitrogen fertilizer basal and topdressing mode had a remarkable influence on leaves’ gas exchange parameters. The effects of nitrogen fertilizer basal and topdressing mode on two varieties is consistent. Under different nitrogen fertilizer basal-to-topdressing ratios treatments, N37 was remarkably higher than other treatments (p < 0.05). Pn reached its peak at the flowering stage, when N37 was higher than the remaining treatments by 27.29~33.28%, 9.21~15.70%, 7.75~13.38%, and 8.74~14.80%, respectively. Comparing the two varieties, it was found that XC37 was higher than XC6 by 0.74~11.46% on average (except for the dough stage).
As the growth period progressed, Gs showed a trend of first increasing and then decreasing, reaching its maximum value during the flowering period (Figure 6B). Both Pn and Gs declined faster in the later stages of fertility. The response of Gs to nitrogen fertilizer in the basal and topdressing modes had the same trend with Pn. During 2020–2021, the Gs of N37 treatments were higher than N00, N28, N46, and N55 by 21.58~80.27%, 13.98~16.77%, 4.17~9.41%, and 5.37~14.25%. Comparing two varieties, it is concluded that XC37 was on average higher than XC by 60.76~20.87% (except for the jointing stage).
Under the same basal and topdressing mode, Ci increased first and then decreased from the tillering to milky maturity stages (Figure 6C). The decrease in Ci from the milky maturity to dough stages (5.36~13.85%) was smaller than the decrease from the flowering stage to the milky maturity stage (12.59~17.77%), which was opposite to the decreasing trends in Pn and Gs. The changing trends of the two varieties under different nitrogen fertilizer from the base to topdressing modes are consistent, but XC37 was higher than XC6 by 0.11~10.40%. With the increase in the basal and topdressing modes, it first decreased and then increased during the same reproductive period. And N37 was significantly lower than the rest of the treatments. The Ci values of N37 was 295~309 μmol CO2·mol−1 at the flowering stage in both years, which was lower than other treatments by 18.48~20.98%, 12.95~13.91%, 5.21~9.27% and 9.97~11.28%.
The variation mode of Tr was basically consistent with the trends of Pn and Gs, with the highest expression observed during the flowering stage (Figure 6D), and N37 showed the highest performance. During the flowering stage of two years, the Tr of N37 treatment were 8.56~9.14 mmol H2O m−2·s−1, which were 21.18~42.15%, 14.54~16.82%, 5.91~9.84%, and 6.45~16.29% higher than N00, N28, N46, and N55 during the same period, respectively. In addition, XC37 was higher than XC6 by 0.44~17.77%.

3.4. Chlorophyll Fluorescence Parameters

In the nitrogen fertilizer basal and topdressing mode experiment, Fv/Fm increased first and then decreased with the fertility period, and the overall change was relatively gentle (Figure 7). Under the different basal and topdressing mode, it manifested as N37 > N46 > N55 > N28 > N00. During the flowering stage, N37 was significantly higher than other treatments by 13.80~26.14%, 7.53~12.43%, 4.94~8.70%, and 6.28~9.84% Under the same basal and topdressing mode, XC37 was higher than XC6 with an increase of approximately 0.39~5.19% (except for the jointing stage). With the increasing nitrogen fertilizer basal and topdressing ratio, ΦPSⅡ increased then decreased during the same period, and it remained between 0.78~0.85 in the N37 treatment at the flowering stage, which was higher than N00, N28, N46, and N55 by 20.99~44.76%, 13.88~22.60%, 8.66~18.53%, and 11.61~21.76%, respectively. XC37 was 1.50~17.63% higher than XC6 during the whole growth period.

3.5. The Relationship Between Photosynthetic Physiological Parameters and Grain Yield

3.5.1. Grain Yield

The effects of nitrogen fertilizer basal and topdressing modes and varieties interaction on yield and compositions were consistent over two years (Table 3), and both had significant effects. The highest yield was recorded in the N37 mode for both varieties and a further increase resulted in an insignificant increase in yield and the compositions of basal and topdressing ratio (4:6 and 5:5). Compared with N28 treatment, N37 treatment increased the 1000-grain weight, the spike number, the grain per spike, and yield by 5.86~12.70%, 11.53~11.86%, 11.22~13.65%, and 28.90~33.52%.

3.5.2. Correlation and Pathway Analysis of Photosynthetic Physiological Parameters with Yield Formation

Yield (Y1) and the spike number (Y2), grain per spike (Y3), and 1000-grain weight (Y4) had remarkable and tally positive relationships with SPAD, Pn, Gs, Tr, Fv/Fm, ΦPSⅡ, RuBPC, and PEPC (except for Y3), and all achieved the best results in N37 treatment. These indicators show a trend of first increasing and then decreasing with the increase in topdressing ratio. Yield and its composition remarkably and tally negative relationships with Ci, and achieved the best results in N00 treatment. With an increase in topdressing, Ci shows a “V” shape and remarkable and tally negative relationships with Ci. LAI and SPAD showed remarkable tally positive correlations with Pn, Tr, Gs, Fv/Fm, ΦPSⅡ, RuBPC, and PEPC. Pn, Gs, Tr, Fv/Fm, and ΦPSⅡ showed highly and remarkably positive relationships with RuBPC and PEPC activities (Figure 8), which indicate that different nitrogen fertilizer-based topdressing modes can affect photosynthesis by affecting the activity of key photosynthetic enzymes, ultimately achieving the goal of high and stable yield.
Parameters of photosynthetic physiological parameters, yield, and its components were subjected to principal component analysis under the nitrogen fertilizer basal and topdressing mode (Figure 9). From Figure 9, it can be seen that the contribution rates of Principal Component 1 (PC1) and 2 (PC2) were 90.3% and 3.7%, with a cumulative contribution rate of 93.99%. Therefore, two principal components can explain the information on 14 indicators, which were significantly representative. PC1 represented Y1, Y2, Y3, Y4, LAI, SPAD, RuBPC, PEPC, Pn, Gs, Tr, Fv/Fm, and ΦPSⅡ. Compared with other indicators, the angle between Y1 and Ci was the largest, which indicated the highest negative correlation between Ci and Y1 under the nitrogen fertilizer basal and topdressing mode. The small angle between Y1 and Y3, Y4, LAI, SPAD, and Gs indicated a prominently positive correlation between these indicators and grain yield. In addition, the angles between photosynthesis enzymes and Fv/Fm, ΦPSⅡ, Pn were all less than 30°, which indicated that there was a closer connection between Fv/Fm, ΦPSⅡ, Pn and photosynthesis enzymes. In summary, Y3, Y4, LAI, SPAD, and Gs had greater effects on seed yield and can characterize seed yield to some extent.
Stepwise regression analysis and pathway analysis were performed with LAI (X1), SPAD (X2), Pn (X3), Gs (X4), Tr (X5), Ci (X6), Fv/Fm (X7), ΦPSⅡ (X8), RuBPC (X9), and PEPC (X10) as the independent variables, and yield (Y1), spike number (Y2), grain per spike (Y3), and 1000-grain weight (Y4) as dependent variables (Table 4). It was found that the immediate influences of various photosynthetic indicators on yield were Fv/Fm (|0.7650|) > Ci (|−0.5580|); on the spike number were LAI (|1.3990|) > RuBPC (|1.0470|) > Pn (|1.0150|) > Ci (|−0.4820|); and on the grain per spike were LAI (|1.6190|) > RuBPC (|1.3270|) > Pn (|0.8950|) > Ci (|−0.3850|). The direct impacts on 1000-grain weight were LAI (|1.3360|) > RuBPC (|1.1830|) > Pn (|0.8050|) > Ci (|−0.4890|). This indicated that increasing Fv/Fm and decreasing Ci can increase yield; increasing LAI, RuBPC, Pn and decreasing Ci can increase the spike number, the grain per spike, and the 1000-grain weight.

4. Discussion

4.1. Effects of Nitrogen Fertilizer Basal and Topdressing Mode on Photosynthetic Physiology of Wheat Leaves

LAI and SPAD values are important indicators for measuring leaf photosynthetic function, and maintaining a high green leaf region and SPAD value can effectively improve wheat biomass and yield [42,43,44]. Nitrogen (N) nutrition plays a pivotal role in regulating the synthesis of photosynthetic pigments, such as chlorophyll, and in sustaining photosynthetic efficiency throughout the growth cycle. A nitrogen fertilizer basal to topdressing ratio of 5:5 (topdressing: jointing stage) dominantly increased SPAD value of wheat plants in the plains wheat region of northern China [24,45]. Similarly, in the Huang Huai Hai wheat-growing region, a N application rate of 240 kg·hm−2 was found to effectively slow chlorophyll degradation and delay leaf senescence compared to lower (180 kg·hm−2) or higher (300 kg·hm−2) application rates. Under this optimal N rate, a basal-to-topdressing ratio of 3:7 further enhanced LAI and SPAD values, underscoring the synergistic effects of N rate and application timing [46]. Our experiments have led to consistent conclusions. In our experiment, when the nitrogen fertilizer base application ratio was 3:7, the LAI and SPAD values of drip-irrigated wheat increased compared to other treatments. This indicated that appropriate nitrogen application rates and nitrogen fertilizer base and topdressing application ratios can increase the chlorophyll content and green leaf area of wheat leaves to a certain extent and maintain them until the later stage of growth.
The enzymatic activities of RuBPC and PEPC are central to carbon metabolism and photosynthetic efficiency in wheat. RuBPC, the primary enzyme in the Calvin cycle, directly influences carbon fixation, while PEPC plays a complementary role in carbon assimilation, particularly under stress conditions [26,29]. Rational N management has been shown to enhance the activities of these enzymes in flag leaves, thereby promoting photosynthetic rates and assimilate accumulation [30]. Research has found that a basal to topdressing ratio of 3:7 (topdressing applied together with irrigation water. Topdressing: jointing stage, booting stage, anthesis stage, filling stage) can significantly increase the chlorophyll content of flag leaves, delay flag leaf senescence, and improve the net photosynthetic rate of flag leaves [47]. Our experimental conclusions were the same. In our study, we found that when the basal and topdressing mode was 3:7, the activity of RuBPC and PEPC effectively increased in the wheat region. Further analyzing our results showed that Pn, Tr, and Gs displayed highly prominent positive relationships with RuBPC and PEPC activities and a tally of prominent negative relationships with Ci, respectively. Therefore, an appropriate nitrogen fertilizer basal and topdressing mode can improve and maintain the activity of key photosynthetic enzymes and regulate gas exchange parameters, thereby optimizing wheat photosynthesis.
The appropriate nitrogen application method affects the transfer rate of photosynthetic electrons towards the photochemical direction in wheat leaves, and regulates the increase in photosynthetic rate [24,48,49]. The response of different regional environments and wheat varieties to the nitrogen fertilizer base in topdressing modes also varies. Research has shown that using a N amount of 225 kg·hm−2 and a basal and topdressing mode of 6:4 (topdressing: jointing stage, flowering stage and filling stage) can effectively improve the photosynthetic efficiency of wheat [50]. The basal and topdressing mode of 3:7 and 4:6 (topdressing: tillering stage, jointing stage, booting stage, flowering stage) can increase the Pn, Gs of wheat and alleviate the decline of the photosynthetic rate in the northwest wheat region of China [51,52]. In our research, we found that Pn, Tr, and Gs showed a difference of increasing and subsequently decreasing with decreases in amount of topdressing, and the highest Pn, Tr, and Gs were observed in the N37 treatment, while Ci showed the opposite change. In addition, insufficient topdressing fertilization (such as N55 and N00), and low leaf nitrogen content resulted in much lower RuBPC activity being directly involved in photosynthesis and low photosynthetic efficiency, which led to a greater reduction in Pn. This may be due to the fact that inadequate N supply reduces Pn and Gs, limiting CO2 availability and reducing CO2 fixation, thereby reducing photosynthetic efficiency. This indicates that the appropriate basal and topdressing mode was more favorable, improving the photosynthetic parameters of wheat and enhancing the photosynthetic performance of leaves.
Chlorophyll fluorescence parameters can provide real-time insights into the photochemical efficiency of photosynthesis in wheat. Chlorophyll fluorescence parameters exhibit high sensitivity to nitrogen (N) supply due to the critical role of N in chlorophyll synthesis [53]. Previous studies have demonstrated that optimizing N application strategies, including the basal-to-topdressing ratio and timing, can significantly enhance chlorophyll fluorescence characteristics, thereby improving the light-trapping ability and photochemical efficiency of PSII. Topdressing N at critical growth stages (e.g., tillering, jointing, booting, and flowering) ensures a steady supply of N during periods of high photosynthetic demand [12,34]. For example, at a N application rate of 225 kg·hm−2, basal-to-topdressing ratios of 3:7 and 4:6 (with topdressing applied at the tillering, jointing, booting, and flowering stages) were shown to increase Fv/Fm and ΦPSII, leading to higher net photosynthetic rates and improved yield potential [52]. Our study found that the trends of change in ΦPSⅡ and Fv/Fm were consistent, with N37 treatment being the highest. It indicated that an appropriate nitrogen fertilizer basal and topdressing mode can significantly enhance the chlorophyll fluorescence characteristics of wheat leaves, thereby improving the maximum luminous energy translation efficiency and the potential activity of photosystem II. Compared with the XC6, XC37 exhibited higher photosynthetic enzyme activity, gas exchange parameters, and fluorescence parameters. This suggests that genotypic differences in N utilization efficiency and photosynthetic capacity play a significant role in determining the sustainability of photosynthesis. Strong gluten wheat varieties like XC37 may possess superior mechanisms for maintaining high RuBPC activity and stable PSII function under varying N conditions, thereby enhancing photosynthetic efficiency and assimilate accumulation. Based on previous research, further studies are needed on how different nitrogen fertilizer basal-to-topdressing ratios regulate the process of photosynthesis. Despite these advances, key knowledge gaps remain. For example, the molecular pathways through which N regulates chlorophyll synthesis, enzyme activities, and leaf senescence are not fully understood. The role of N in enhancing photosynthetic efficiency under abiotic stresses (e.g., drought and salinity) is underexplored. Future research should address these questions to refine N management strategies and improve wheat productivity.

4.2. Effects of Nitrogen Fertilizer Basal and Topdressing Mode on Yield Formation in Wheat

Increasing the topdressing rate enhances the net photosynthetic rate and chlorophyll fluorescence parameters of flag leaves post-anthesis, promoting post-flowering dry matter accumulation, and ultimately increasing grain yield [54]. The appropriate nitrogen application method can significantly increase spike number, grain per spike, and yield of wheat [47,55,56]. However, there are differences in the response of wheat varieties from different regions to nitrogen fertilizer management modes [57]. Research has shown that a nitrogen fertilizer base-to-topdressing ratio of 3:7 can promote the accumulation and transport of dry matter in ‘Bainong Short resistance 58’ and increase yield [58]. Studies have illustrated that wheat yield and compositions can be effectively improved under N use of 225 kg·hm−2 and basal and a topdressing mode of 6:4 (topdressing: jointing stage, flowering stage, and filling stage) [50]. In our study, the direct effects of yield compositions on yield were spike number > grain per spike > 1000-grain weight. Thus, maximizing yield potential hinges on optimizing the spike number and grains per spike under drip irrigation in arid regions. A nitrogen fertilization regime with a 3:7 basal-to-topdressing ratio proved most conducive to yield formation in this system. Excessive topdressing (e.g., N28) increased biomass accumulation, but led to excessive vegetative growth at the expense of reproductive growth, ultimately reducing yield. In contrast, the 3:7 ratio balanced population growth and facilitated the translocation of photosynthetic assimilates to reproductive organs. This ratio also aligns with wheat’s N demand dynamics—stable early, promoted mid-season, and controlled late—ensuring efficient N utilization. Under consistent irrigation, N application rates, and agronomic management, the N37 achieved the highest net photosynthetic rate (Pn), resulting in greater yield and economic returns. Specifically, N37 increased economic benefits by 11.51–16%, 1.38–3.33%, and 4.995–10.34% compared to N28, N46, and N55, respectively. These findings underscore the importance of optimizing basal and topdressing ratios for cost-effective yield improvement. Correlation analysis showed that yield and its compositions showed significantly and highly significant positive correlations with LAI, SPAD, Pn, Gs, Tr, Fv/Fm, ΦPSⅡ, RuBPC, and PEPC, and highly significant negative correlations with Ci, respectively. In addition, the direct effects of photosynthetic indexes on the spike number, grain per spike, and 1000-grain weight were LAI > RuBPC > Pn > Ci, and the direct effects on the yield were Fv/Fm> Ci, which indicated that increasing LAI, RuBPC, and Pn and decreasing Ci can increase spike number, grains per spike, and the 1000-grain weight, and increasing Fv/Fm and decreasing Ci can increase the yield. Therefore, the key to boosting wheat yield is to improve wheat’s ability to intercept and absorb light energy, enhance photosynthesis and the photosynthetic enzyme activity of wheat leaves, and thereby increase wheat dry matter accumulation. Compared with the two varieties, the yield and constitutive factors of XC37 were higher than XC6. Perhaps due to the high protein content in strong-gluten wheat, the conversion rate of protein is high in the later stage, which can produce more protein. Therefore, XC37 allowed it to accumulate higher photosynthetic products [59,60,61]. In summary, it is recommended to implement a 3:7 basal chasing mode for two wheat varieties in Xinjiang wheat-growing areas. However, further research is needed to understand the underlying molecular mechanisms of the effects of different nitrogen fertilizer base ratios on the yield of different wheat varieties. In addition, extensive experimental research is needed on how to effectively utilize different nitrogen fertilizer base application modes in different regions.

5. Conclusions

Under a drip irrigation cropping system for spring wheat in arid areas, a nitrogen fertilizer basal and topdressing ratio of 3:7 is beneficial for improving LAI and SPAD, enhancing and maintaining the activity of RuBPC and PEPC, thereby upgrading the net photosynthetic rate. Ultimately, it promotes the proportion of dry material accruing and allocation of photosynthetic products to reproductive organs in spring wheat. Therefore, a nitrogen fertilizer basal and topdressing mode of 3:7 is suitable for application in drip irrigation spring wheat production areas in arid regions. For arid region spring wheat production under drip irrigation, we recommend using the standard nitrogen fertilizer 37 as a benchmark and adjusting it to the specific situation (cultivar characteristics, soil fertility status, climatic conditions). Based on this study, we can continue to expand our research by modeling gene–nitrogen fertilizer interactions for wider applications, leading to efficient photosynthesis and high yields of spring wheat in the future.

Author Contributions

Conceptualization, Y.Z., R.W., J.X., G.J. and H.W.; methodology, Y.Z., J.X., G.J., F.H. and H.W.; software, Y.Z. and F.H.; validation, Y.Z., H.W. and R.W.; formal analysis, Y.Z., G.J. and F.H.; investigation, F.H.; resources, R.W. and H.W.; data curation, Y.Z., J.X., H.W., R.W. and F.H.; writing—original draft preparation, Y.Z. and H.W.; writing—review and editing, Y.Z., R.W., J.X., G.J. and F.H.; visualization, Y.Z.; supervision, R.W.; project administration, F.H.; funding acquisition, R.W. and G.J. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the National Natural Science Foundation of China (31760346) and the Support Program for Innovation and Development of Key Industries in Southern Border (2021DB010).

Data Availability Statement

The datasets generated for this study are available upon request to the corresponding authors.

Acknowledgments

The authors would like to thank the reviewers for their valuable comments and suggestions for this work. And the authors expressed their most sincere gratitude to the teachers and classmates at Shihezi University for their technical support during the experiment.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

AbbreviationsMeaning
CiIntercellular CO2 concentration
Fv/FmMaximum photochemical efficiency
GsStomatal conductance
LAILeaf region index
PEPCPhosphoenolpyruvate carboxylase
PnNet photosynthetic rate
RuBPCRibulose-1,5-diphosphate ribulose carboxylase
SPADRelative chlorophyll content
TrTranspiration rate
ΦPSⅡActual photochemical efficiency

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Figure 1. The temperature and precipitation for the spring wheat in 2020 and 2021.
Figure 1. The temperature and precipitation for the spring wheat in 2020 and 2021.
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Figure 2. Schematic diagram of drip tape layout.
Figure 2. Schematic diagram of drip tape layout.
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Figure 3. The effect of nitrogen fertilizer basal and topdressing mode on LAI of spring wheat leaf under drip irrigation. Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, TS: tillering stage, JS: jointing stage, BS: booting stage, FS: flowering stage, MS: milky maturity stage, DS: dough stage. Different lowercase letters indicate significant differences in treatment between the two varieties at the 0.05 level.
Figure 3. The effect of nitrogen fertilizer basal and topdressing mode on LAI of spring wheat leaf under drip irrigation. Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, TS: tillering stage, JS: jointing stage, BS: booting stage, FS: flowering stage, MS: milky maturity stage, DS: dough stage. Different lowercase letters indicate significant differences in treatment between the two varieties at the 0.05 level.
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Figure 4. The effect of nitrogen fertilizer basal and topdressing mode on SPAD of spring wheat leaf under drip irrigation. Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, TS: tillering stage, JS: jointing stage, BS: booting stage, FS: flowering stage, MS: milky maturity stage, DS: dough stage. Different lowercase letters indicate significant differences in treatment between the two varieties at the 0.05 level.
Figure 4. The effect of nitrogen fertilizer basal and topdressing mode on SPAD of spring wheat leaf under drip irrigation. Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, TS: tillering stage, JS: jointing stage, BS: booting stage, FS: flowering stage, MS: milky maturity stage, DS: dough stage. Different lowercase letters indicate significant differences in treatment between the two varieties at the 0.05 level.
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Figure 5. The effect of nitrogen fertilizer basal and topdressing mode on RuBPC (A) activity and PEPC (B) activity of spring wheat leaf under drip irrigation. Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, TS: tillering stage, JS: jointing stage, BS: booting stage, FS: flowering stage, MS: milky maturity stage, DS: dough stage. Different lowercase letters indicate significant differences in treatment between the two varieties at the 0.05 level.
Figure 5. The effect of nitrogen fertilizer basal and topdressing mode on RuBPC (A) activity and PEPC (B) activity of spring wheat leaf under drip irrigation. Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, TS: tillering stage, JS: jointing stage, BS: booting stage, FS: flowering stage, MS: milky maturity stage, DS: dough stage. Different lowercase letters indicate significant differences in treatment between the two varieties at the 0.05 level.
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Figure 6. The effect of nitrogen fertilizer basal and topdressing mode on Pn (A), Gs (B), Ci (C), and Tr (D) of spring wheat leaf under drip irrigation. Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, TS: tillering stage, JS: jointing stage, BS: booting stage, FS: flowering stage, MS: milky maturity stage, DS: dough stage. Different lowercase letters indicate significant differences in treatment between the two varieties at the 0.05 level.
Figure 6. The effect of nitrogen fertilizer basal and topdressing mode on Pn (A), Gs (B), Ci (C), and Tr (D) of spring wheat leaf under drip irrigation. Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, TS: tillering stage, JS: jointing stage, BS: booting stage, FS: flowering stage, MS: milky maturity stage, DS: dough stage. Different lowercase letters indicate significant differences in treatment between the two varieties at the 0.05 level.
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Figure 7. The effect of nitrogen fertilizer basal and topdressing mode on Fv/Fm (A) and ΦPSⅡ (B) of spring wheat leaf under drip irrigation. Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, TS: tillering stage, JS: jointing stage, BS: booting stage, FS: flowering stage, MS: milky maturity stage, DS: dough stage. Different lowercase letters indicate significant differences in treatment between the two varieties at the 0.05 level.
Figure 7. The effect of nitrogen fertilizer basal and topdressing mode on Fv/Fm (A) and ΦPSⅡ (B) of spring wheat leaf under drip irrigation. Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, TS: tillering stage, JS: jointing stage, BS: booting stage, FS: flowering stage, MS: milky maturity stage, DS: dough stage. Different lowercase letters indicate significant differences in treatment between the two varieties at the 0.05 level.
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Figure 8. Correlation of parameters under nitrogen fertilizer transport planning. Notes: Y1: yield, Y2: spike number, Y3: grain per spike, Y4: 1000-grain weigh. * and ** demonstrate remarkable differences at the 0.05 and 0.01 standard.
Figure 8. Correlation of parameters under nitrogen fertilizer transport planning. Notes: Y1: yield, Y2: spike number, Y3: grain per spike, Y4: 1000-grain weigh. * and ** demonstrate remarkable differences at the 0.05 and 0.01 standard.
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Figure 9. Component analysis under nitrogen fertilizer management (PCA). Notes: Y1: yield, Y2: spike number, Y3: grain per spike,Y4: 1000-grain weigh, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing.
Figure 9. Component analysis under nitrogen fertilizer management (PCA). Notes: Y1: yield, Y2: spike number, Y3: grain per spike,Y4: 1000-grain weigh, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing.
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Table 1. The major chemical characteristics of the experimental soil.
Table 1. The major chemical characteristics of the experimental soil.
YearTotal N (g·kg−1)Avail. N (mg·kg−1)Avail. P (mg·kg−1)Avail. K (mg·kg−1)Organic (g·kg−1)Soil Capacity
(g·cm−3)		Electrical Conductivity
(ds·m−1)		PH
20201.2058.3014.24137.0113.431.270.447.8
20211.2755.7115.96132.0212.841.290.447.7
Table 2. Application mode of nitrogen fertilizer basal and topdressing in each treatment.
Table 2. Application mode of nitrogen fertilizer basal and topdressing in each treatment.
TreatmentsBasal Nitrogen Fertilizer
(kg·hm−2)		Topdressing Nitrogen Fertilizer (kg·hm−2)
Tillering StageJointing StageBooting StageFlowering Stage
N00 (0:0)00000
N28 (2:8)5040804040
N37 (3:7)7535703535
N46 (4:6)10030603030
N55 (5:5)12525502525
Table 3. Effect of basal and topdressing mode of nitrogen fertilizer on wheat yield and its components.
Table 3. Effect of basal and topdressing mode of nitrogen fertilizer on wheat yield and its components.
Years20202021
Variety (V)Treatment (N)1000-Grain Weight·(g)Spike Number (×104·hm−2)Grain per SpikeGrain Yield
(kg·hm−2)		1000-Grain Weight·(g)Spike Number (×104·hm−2)Grain per SpikeGrain Yield
(kg·hm−2)
XC37N0043.08 d389.08 d33.38 cd5444.33 e44.67 bc389.95 de34.49 c5664.87 de
N2846.22 bc409.27 c35.25 bc6558.08 c46.09 abc410.21 bc35.64 bc6509.64 bc
N3748.55 a433.93 a37.94 a7269.17 a47.29 a436.20 a38.36 ab7302.18 a
N4647.72 ab418.80 bc38.06 a7206.61 ab47.08 a415.62 bc39.01 a7166.48 a
N5546.41 bc414.52 bc36.26 ab6956.17 b46.99 a412.77 bc36.04 abc6923.64 ab
XC 6N0042.76 d384.07 d32.91 d5364.23 e44.24 c382.15 e34.07 c5383.47 e
N2845.51 c406.14 c34.98 bcd6208.25 d45.93 abc403.55 cd35.61 bc6198.44 cd
N3747.51 ab426.05 ab36.08 ab7194.45 ab47.02 a425.18 ab37.89 ab7218.51 a
N4646.89 abc413.33 bc37.74 a7000.70 ab46.61 ab410.84 bc38.23 ab6948.61 ab
N5546.08 bc408.83 c36.03 ab6554.42 c46.42 ab408.65 bcd36.67 abc6508.78 bc
FVns*ns*ns*ns*
N****************
N × V*ns*******
Notes: XC37: Xinchun37, XC6: Xinchun6, N00: no nitrogen fertilizer, N28 (2:8): 20% basal fertilizer and 80% topdressing, N37 (3:7): 30% basal fertilizer and 70% topdressing, N46 (4:68): 40% basal fertilizer and 60% topdressing, N55 (5:5): 50% basal fertilizer and 50% topdressing, V: variety, N: nitrogen fertilizer basal and topdressing mode. Different lowercase letters indicate that different treatments of the variety have remarkable differences at 0.05 level. * and ** of the F indicates remarkable differences at the 0.05 and 0.01 levels, ns indicates no remarkable difference.
Table 4. Path coefficient analysis on diurnal variation in various photosynthetic parameters and yield factors.
Table 4. Path coefficient analysis on diurnal variation in various photosynthetic parameters and yield factors.
Dependent VariableAction FactorCorrelation CoefficientPath CoefficientIndirect Path Coefficients
X1X3X6X7X8Total
Y1X6−0.3140−0.5580 −0.2584 −0.2584
X70.53200.7650 0.3542 0.3542
Y2X10.23801.3990 1.12760.1945 1.30812.6301
X30.49701.01500.8181 0.3735 0.98562.1772
X6−0.3320−0.4820−0.0670−0.1774 −0.2208−0.4651
X90.50301.04700.97891.01660.4795 2.4751
Y3X10.23201.6190 1.30490.2250 1.5300
X30.48500.89500.7214 0.3294 1.0507
X6−0.3170−0.3850−0.0535−0.1417 −0.1952
X90.50501.32701.24071.28850.6078 3.1370
Y4X10.23001.3360 1.07680.1857 1.2625
X30.47800.80500.6488 0.2962 0.9451
X6−0.3300−0.4890−0.0680−0.1800 −0.2479
X90.49101.18301.10611.14870.5418 2.7966
Notes: X1: LAI, X2: SPAD, X3: Pn, X4: Gs, X5: Tr, X6: Ci, X7: Fv/Fm, X8: ΦPSⅡ, X9: RuBPC, Y1: yield, Y2: spike number, Y3: grain per spike,Y4: 1000-grain weigh.
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Zhang, Y.; Wang, H.; Wang, R.; He, F.; Jiang, G.; Xu, J. Improvement of the Photosynthetic Characteristics and Yield of Wheat by Regulating the Proportion of Nitrogen Fertilizer Base and Topdressing. Agronomy 2025, 15, 899. https://doi.org/10.3390/agronomy15040899

AMA Style

Zhang Y, Wang H, Wang R, He F, Jiang G, Xu J. Improvement of the Photosynthetic Characteristics and Yield of Wheat by Regulating the Proportion of Nitrogen Fertilizer Base and Topdressing. Agronomy. 2025; 15(4):899. https://doi.org/10.3390/agronomy15040899

Chicago/Turabian Style

Zhang, Yaoyuan, Haiqi Wang, Rongrong Wang, Fangfang He, Guiying Jiang, and Jianwei Xu. 2025. "Improvement of the Photosynthetic Characteristics and Yield of Wheat by Regulating the Proportion of Nitrogen Fertilizer Base and Topdressing" Agronomy 15, no. 4: 899. https://doi.org/10.3390/agronomy15040899

APA Style

Zhang, Y., Wang, H., Wang, R., He, F., Jiang, G., & Xu, J. (2025). Improvement of the Photosynthetic Characteristics and Yield of Wheat by Regulating the Proportion of Nitrogen Fertilizer Base and Topdressing. Agronomy, 15(4), 899. https://doi.org/10.3390/agronomy15040899

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