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Frontiers in Agronomy ·

Volume 8 - 2026 | https://doi.org/10.3389/fagro.2026.1924091

Optimizing physiological performance, assimilate partitioning and productivity of late-sown wheat (Triticum aestivum L.) through mepiquat chloride application under nitrogen enrichment

  • Kavita Devi1*corresponding author
  • Jasdev Singh Deol1
  • Rajni Sharma1
  • Amit Kaul1
  • Arun Kumar2
  • Rahul Sharma1
  1. 1Department of Agronomy, Punjab Agricultural University, Ludhiana, Punjab, India
  2. 2Department of Agronomy, Rajasthan University of Veterinary and Animal Sciences, Bikaner, Rajasthan, India
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Abstract

Late sowing constrains grain yield formation in wheat by shortening the duration of post-anthesis assimilate accumulation and limiting the effective utilization of source-derived assimilates during grain filling. Optimizing nitrogen nutrition and growth regulation could help maintain physiological activity, sink development and grain productivity under late-sown conditions. This two-year field study evaluated the effects of nitrogen fertilization and mepiquat chloride application on physiological responses, assimilate partitioning and grain yield of late-sown wheat in north-western India. Nitrogen treatments comprised 100% (M1), 125% (M2) and 150% (M3) of the recommended dose of nitrogen (RDN), while growth regulation treatments included an untreated control (S1), mepiquat chloride at 750 ppm at maximum tillering (S2), flag leaf (S3) and both stages (S4), and 1000 ppm at maximum tillering (S5), flag leaf (S6) and both stages (S7). Nitrogen fertilization significantly increased grain yield compared with 100% RDN (M1), while 125% RDN (M2) and 150% RDN (M3) produced statistically comparable grain yields during both years. Grain yield under M2 was 48.1 and 49.1 q ha -1, compared with 43.7 and 45.7 q ha -1 under M1, representing increases of 10.1 and 7.4%, respectively, whereas M3 recorded 50.4 and 51.9 q ha -1, corresponding to increases of 15.3 and 13.6%, respectively, over M1 during 2023–24 and 2024–25. Among growth regulation treatments, dual-stage application of mepiquat chloride at 750 ppm (S4) recorded grain yields of 50.2 and 51.8 q ha -1 during 2023-24 and 2024-25, respectively, and remained statistically comparable with the corresponding 1000 ppm treatment (S7). S4 also increased harvest index by 11.7-13.2% compared with the untreated control (S1), with values similar to S7 in both years. Higher nitrogen supply and dual-stage mepiquat chloride application were associated with improved physiological activity, sink capacity and assimilate partitioning toward grains during grain filling. These findings indicate that appropriate nitrogen management and dual-stage mepiquat chloride application can help sustain wheat productivity under delayed sowing.

Keywords

1 Introduction

The Indo-Gangetic Plains (IGP) constitute the principal wheat-producing region of India, occupying nearly 61% of the national wheat area and contributing approximately 67% of total wheat production (Government of India, 2024). Sustaining assimilate production and efficient partitioning during grain filling is fundamental to maintaining wheat productivity under intensive production systems because this stage largely determines the extent of grain development and final yield (Sharma et al., 2023). Late harvesting of rice in north-western India compels farmers to sow wheat after the recommended date which reduces the time available for crop growth and development (Dhanda et al., 2022). Delayed sowing adversely affects physiological activity, dry matter partitioning and grain yield of wheat (Jarecki, 2024). Under late-sown conditions, the grain filling duration decreases from approximately 47 days to 34 days which restricts post-anthesis assimilate accumulation and results in considerable yield losses (Sattar et al., 2023).

Nitrogen is a critical nutrient that regulates canopy development, physiological processes and grain productivity in wheat (Ding et al., 2015; Gawdiya et al., 2025). Appropriate nitrogen management has the potential to extend the grain filling period and improve resource use efficiency in late-sown wheat crops (Zhang et al., 2022; Paccioretti et al., 2025). Field studies have reported that optimized nitrogen application maintains higher SPAD values, photosynthetic activity and sucrose metabolism during grain filling, extending its duration and increasing grain yield by 6.8% to 11.2% over suboptimal nitrogen treatments (Li et al., 2025). Maintaining physiological activity during the reproductive stage is therefore critical to achieving acceptable yield levels in late-sown wheat. A shorter growth period can limit the production and accumulation of resources, which affects grain filling and final yields (Fan et al., 2017).

Effective movement of resources to developing grains is vital for maintaining efficiency and yield in wheat (Zhang et al., 2010). “Therefore, strategies that support physiological activity and encourage efficient resource distribution during grain filling can greatly help sustain yields in late-sown wheat (Fan et al., 2017). Other plant growth regulators, particularly chlormequat chloride and trinexapac-ethyl, have also been investigated in wheat for modifying plant architecture, reducing excessive vegetative growth and influencing yield formation under different nitrogen regimes (Knapp and Harms, 1988; Berti et al., 2007; Yan et al., 2020; Li et al., 2025). Accordingly, mepiquat chloride application has been increasingly investigated as a strategy for improving physiological efficiency, assimilate partitioning, and grain productivity in wheat. Mepiquat chloride, an anti-gibberellin growth regulator, has been reported to enhance chlorophyll retention, delay leaf senescence and improve assimilate translocation toward developing grains in wheat (Abbas et al., 2022). Studies have reported that foliar application of mepiquat chloride at 500–1000 ppm increased chlorophyll content and delayed leaf senescence during grain filling in wheat (El-Nwehy and Afify, 2023). Furthermore, Previous studies reported that foliar application of mepiquat chloride at 500 ppm increased grain yield by 8-15% and improved harvest index under elevated nitrogen fertilization conditions in wheat (Miziniak and Matysiak, 2019). Although the individual effects of nitrogen fertilization and plant growth regulators on wheat have been investigated extensively, much less is known about their interactive influence on crop physiology, assimilate partitioning and grain productivity under late-sown conditions. Evidence describing how mepiquat chloride modifies these processes across different nitrogen levels is particularly scarce for the north-western Indo-Gangetic Plains. Addressing this knowledge gap is important for developing management strategies that improve wheat performance under delayed sowing. It was hypothesized that nitrogen fertilization would influence crop growth and physiological activity, whereas mepiquat chloride would regulate plant growth and assimilate partitioning in late-sown wheat. Therefore, the present study was undertaken to assess the effects of different nitrogen levels and mepiquat chloride application on physiological responses, assimilate partitioning and grain yield of late-sown wheat under north-western Indian conditions.

2 Material and methods

2.1 Experimental site and climate

Field experiments were conducted during the rabi seasons of 2023–24 and 2024–25 at the Student’s Research Farm, Department of Agronomy, Punjab Agricultural University, Ludhiana, Punjab, India (30°56′N, 75°52′E; 247 m above mean sea level). The geographic location of the experimental site is shown in Figure 1. The region experiences a subtropical semi-arid climate characterized by hot, dry summers, a humid monsoon season, and cool winters. Weekly meteorological observations, including maximum and minimum air temperatures, relative humidity, and rainfall during both crop seasons, are presented in Supplementary Tables S1, S2. The experimental soil was well-drained loamy sand with pH ranging from 7.74 to 7.81, organic carbon content from 0.17 to 0.19%, and available nitrogen, phosphorus, and potassium ranging from 173.5-174.4, 20.6-21.8 and 185.4-193.5 kg ha-1, respectively. The experiment was established using a split-plot design with four replications. Three nitrogen levels, comprising 100, 125 and 150% of the recommended dose of nitrogen (RDN), were assigned to the main plots, whereas seven growth regulation treatments were allocated to the subplots. The growth regulation treatments consisted of an untreated control and foliar application of mepiquat chloride at 750 or 1000 ppm at the maximum tillering (MT), flag leaf (FL), or MT + FL stages. Mepiquat chloride was applied at the maximum tillering stage (40–45 DAS) and flag leaf stage (60–70 DAS), either singly or at both stages, as per the treatment schedule. A total of 21 treatment combinations (three × seven) were evaluated. The main plot measured 30.0 m × 10.0 m, while individual sub-plots measured 10.0 m × 1.2 m (12.0 m2). The details of the nitrogen levels and growth regulation treatments applied during both crop seasons are presented in Table 1.

Geographical location of the experimental site at Punjab Agricultural University, Ludhiana, Punjab, India. **(A)** Location of Punjab in India; **(B)** location of Ludhiana district in Punjab; and **(C)** detailed location of the experimental site at the Student’s Research Farm, Department of Agronomy, Punjab Agricultural University, Ludhiana.
Figure 1. Geographical location of the experimental site at Punjab Agricultural University, Ludhiana, Punjab, India. (A) Location of Punjab in India; (B) location of Ludhiana district in Punjab; and (C) detailed location of the experimental site at the Student’s Research Farm, Department of Agronomy, Punjab Agricultural University, Ludhiana.

Table 1. Description of nitrogen levels and growth regulation treatments imposed during both crop seasons.

FactorTreatment codeTreatment description
Nitrogen levelsM1100% recommended dose of nitrogen (RDN)
M2125% recommended dose of nitrogen (RDN)
M3150% recommended dose of nitrogen (RDN)
Growth regulationS1Control
S2Mepiquat chloride @ 750 ppm at maximum tillering stage
S3Mepiquat chloride @ 750 ppm at flag leaf stage
S4Mepiquat chloride @ 750 ppm at maximum tillering and flag leaf stages
S5Mepiquat chloride @ 1000 ppm at maximum tillering stage
S6Mepiquat chloride @ 1000 ppm at flag leaf stage
S7Mepiquat chloride @ 1000 ppm at maximum tillering and flag leaf stages

2.2 Crop management

Late-sown wheat (Triticum aestivum L.) cultivar ‘PBW 752’ was sownon December 11, 2023, and December 10, 2024 during the 2023–24 and 2024–25 growingseasons, respectively. The crop was established by the kera method behind amanually operated plough at a sowing depth of 4–6 cm using a seed rate of 100 kg ha-1. Row and plant spacing were maintained at 22.5 and 10 cm, respectively. All plots received uniform weed, pest, disease and irrigation management according to the recommended package of practices for wheat cultivation prescribed by the Punjab Agricultural University, except for the treatment factors under investigation. Nitrogen and phosphorus were supplied through urea and single superphosphate (SSP), respectively. The recommended fertilizer dose (RDF) for wheat was 125:62.5:0 kg ha-1 of N:P2O5:K2O, respectively (PAU, 2024). Accordingly, the nitrogen treatments representing 100, 125 and 150% of the recommended nitrogen dose received 125.0, 156.25 and 187.50 kg N ha-1, respectively. Nitrogen was applied in three equal splits, with one-third applied at sowing and the remaining two-thirds top-dressed in two equal applications at the first and second irrigations. The second and third nitrogen splits were applied on 5 January and 17 February 2024 in 2023-24, corresponding to 25 and 68 days after sowing (DAS), respectively, and on 29 January and 13 February 2025 in 2024-25, corresponding to 50 and 65 DAS, respectively. Phosphorus was applied uniformly to all treatments as a basal dose through SSP at the time of sowing. Growth regulation treatments consisted of foliar application of mepiquat chloride at 750 and 1000 ppm at the maximum tillering, flag leaf and maximum tillering + flag leaf stages. Foliar sprays were applied using a thoroughly cleaned knapsack sprayer during the afternoon hours after 1400 h under clear, sunny, and wind-free conditions. Irrigation was applied uniformly to all treatment combinations. A pre-sowing irrigation (rauni) was applied on 1 December 2023 and 28 November 2024, followed by three post-sowing irrigations during 2023–24 on 5 January, 17 February and 20 March 2024, and four post-sowing irrigations during 2024–25 on 14 January, 12 February, 10 March and 26 March 2025. Thus, the second and third nitrogen applications were associated with the first and second post-sowing irrigations in 2023–24, whereas in 2024–25 the nitrogen applications on 29 January and 13 February were made separately from the first and second post-sowing irrigations on 14 January and 12 February, respectively. Irrigations were applied using the same water supply across the experimental treatments.

2.3 Physiological attributes

At the booting and physiological maturity stages, gas-exchange measurements were recorded from the flag leaves of five randomly selected and tagged plants per plot using an ADC BioScientific LCi-SD portable infrared gas analyzer (IRGA; ADC BioScientific Ltd In 2023-24, measurements were taken at 70 DAS (19 February 2024) during the booting stage (Zadoks GS 45) and at 132 DAS (21 April 2024) during physiological maturity (Zadoks GS 89), whereas in 2024-25, the corresponding measurements were taken at 70 DAS (18 February 2025) and 131 DAS (20 April 2025), respectively. Measurements were conducted under ambient field conditions on clear, sunny days. During measurements, photosynthetically active radiation (PAR) ranged from 103 to 888 µmol m-2 s-1, reference CO2 concentration ranged from 418 to 564 µmol mol-1, gas flow rate was maintained at approximately 200 mL min-1 (199.4-200.6 mL min-1), and chamber temperature ranged from 33.4 to 42.0 °C. The corresponding reference water vapor pressure ranged from 14.6 to 21.5 mbar. Net photosynthetic rate (Pn, µmol CO2 m-2 s-1), transpiration rate (E, mmol H2O m-2 s-1), and stomatal conductance (gs, mol H2O m-2 s-1) were recorded after stabilization of the readings. Measurements from the five tagged plants within each plot were averaged to obtain a single plot-level value, which was used as the experimental-unit value for statistical analysis. Chlorophyll content was measured using a chlorophyll content meter (CCM-300; Opti-Sciences Inc., Hudson, USA).

2.4 Assimilate partitioning

For assimilate partitioning, above-ground plant material was sampled from two randomly selected 50-cm row lengths within the central portion of each plot at the respective sampling stages, avoiding border rows. The sampled material was separated into leaves, stems and grains, and each component was oven-dried at 65 ± 2 °C under forced-air circulation until a constant weight was attained. Each fraction, including leaves, stems and grains, was then weighed independently. Dry matter accumulation within these fractions was quantified both as absolute values (q ha-1) and as a proportion of total above-ground biomass, allowing direct comparison of how assimilates were distributed between vegetative and reproductive organs at different growth stages.

2.5 Yield attributes, yield and sink capacity

The crop was harvested at physiological maturity on April 27, 2024 and April 25, 2025, during the 2023–24 and 2024–25 growing seasons. We estimated grain weight per spike by collecting five randomly selected spikes from each plot. We manually threshed and weighed them, then calculated the average grain weight per spike. To determine thousand-grain weight, we counted and weighed 1000 grains from a representative sample, and expressed it in grams (g). Grain and biological yields were determined from the net plot area (9.0 m × 1.0 m). The harvested produce from each plot was weighed to record the biological yield (kg plot-1). Following threshing, the grains were cleaned, weighed separately, and converted to grain yield (q ha-1), whereas straw yield (q ha-1) was obtained as the difference between the biological and grain yields. The harvest index (HI) was calculated as the ratio of grain yield to biological yield and expressed as a percentage. To assess sink capacity, representative crop samples were collected at physiological maturity from a known area (1 m2 quadrat) or from a representative number of plants/spikes within each plot. The sampled material was manually threshed to determine the number of grains per unit area (grains m-2). A representative grain sample was oven-dried to a constant weight to obtain the mean individual grain weight (g grain-1). Sink capacity was calculated as the product of grain number per unit area and mean grain weight (SC = grains m-2 × g grain-1), expressed as g m-2 and subsequently converted to q ha-1.

2.6 Weather conditions

Meteorological data were obtained from the Agrometeorological Observatory, School of Climate Change and Agricultural Meteorology, Punjab Agricultural University, Ludhiana, India. During the 2023–24 growing season, the maximum, minimum, and mean air temperatures varied between 11.4-32.0 °C, 5.0-17.0 °C, and 8.6-23.6 °C, respectively. In 2024-25, the corresponding temperature ranges were 15.2-35.8 °C, 4.1-19.2 °C, and 11.9-26.3 °C. The morning and evening relative humidity ranged from 76-95% and 26-80%, respectively, during 2023-24, compared with 66-95% and 20-79%, respectively, during 2024-25. Seasonal rainfall totalled 138.5 mm and 75.0 mm during 2023–24 and 2024-25, respectively, with rainfall distributed across 14 rainy days in each season. Thus, the 2024–25 season was characterized by lower rainfall and relatively higher temperatures, indicating a comparatively drier growing environment than of 2023-24.

2.7 Statistical analysis

All observations recorded during the 2023–24 and 2024–25 crop seasons were subjected to separate statistical analyses using R statistical software (v4.6.0). Analysis of variance (ANOVA) appropriate for a split-plot design was employed, considering nitrogen levels as the main-plot factor, growth regulation treatments as the subplot factor, and replication as a random effect. Treatment effects were evaluated using the F-test, and differences among treatment means were separated using the least significant difference (LSD) test at P ≤ 0.05. Associations among physiological attributes, assimilate partitioning, sink capacity, yield-related traits and grain yield were quantified through Pearson’s correlation analysis. For Pearson correlation analysis, principal component analysis (PCA) and hierarchical cluster analysis (HCA), observations from the 2023–24 and 2024–25 crop seasons were combined at the experimental-unit level. The multivariate dataset comprised 168 observations (21 treatment combinations × 4 replications × 2 crop seasons) and 8 variables: chlorophyll content at physiological maturity (Chl_PM), stomatal conductance at physiological maturity (Gs_PM), net photosynthetic rate at physiological maturity (Pn_PM), sink capacity (SC), spike weight (SW), thousand-grain weight (TGW), grain yield (GY) and harvest index (HI). Replicate observations were retained as individual observations and were not averaged before the multivariate analyses. The observations from the two crop seasons were pooled by combining the corresponding experimental-unit observations, without averaging the seasons. Thus, the data matrix used for correlation, PCA and HCA was 168 × 8. Grain dry matter (GDM) was excluded from the multivariate dataset because of its near-mathematical redundancy with grain yield (GY), while transpiration rate at physiological maturity (Tr_PM) was excluded because complete experimental-unit observations were unavailable. Pearson correlation coefficients were calculated using the Pearson method and visualized using the corrplot() function. PCA was performed on the standardized 168 × 8 dataset using the prcomp() function. Prior to PCA, each variable was standardized to mean zero and unit variance to account for differences in measurement scales. PCA scores were used to represent individual observations, and treatment centroids were subsequently calculated from the PCA scores for visualization of treatment-level patterns; treatment centroids were not used as input observations for PCA. HCA was performed on the same standardized 168 × 8 dataset using Euclidean distance and Ward’s minimum-variance method with the hclust() function. Correlation matrices, PCA plots and hierarchical clustering outputs were generated using the corrplot, factoextra and ggplot2 packages in R statistical software (v4.6.0).

3 Results

3.1 Physiological parameters of flag leaves at booting and physiological maturity stages

The physiological characteristics of flag leaves responded significantly to nitrogen fertilization at both the booting and physiological maturity stages over the two experimental years. Among the nitrogen levels tested, chlorophyll content increased steadily with higher nitrogen supply, peaking at 150% recommended dose of nitrogen (RDN; M3) during both growth stages in both years (Figure 2). The 125% RDN treatment (M2) showed intermediate values, while the lowest chlorophyll content was consistently linked to 100% RDN (M1 Stomatal conductance (gs), transpiration rate (Tr), and net photosynthetic rate (Pn) also responded to nitrogen supply, although the magnitude and direction of change between booting and physiological maturity varied between the two growing seasons. Across the two stages and growing seasons, M3 recorded the highest values of these physiological parameters, followed by M2 and M1 (Figure 2). The simultaneous increase in chlorophyll content, stomatal conductance and transpiration under higher nitrogen availability improved the photosynthetic performance of the flag leaves. Growth regulation treatments also affected the physiological responses during both years of the study (Figure 2). The dual-stage application of mepiquat chloride at 750 ppm, applied at the maximum tillering and flag leaf stages (S4), consistently maintained the highest values for chlorophyll content, stomatal conductance, transpiration rate, and net photosynthetic rate, followed closely by the 1000 ppm treatment (S7). In contrast, the untreated control (S1) generally had the lowest values for these traits. Enhanced chlorophyll retention along with improved stomatal regulation and gas exchange characteristics under S4 and S7, contributed to improved photosynthetic activity during crop growth. The interaction between nitrogen fertilization and growth regulation treatments was non-significant for all physiological parameters during both years of the study.

Effects of nitrogen fertilization and mepiquat chloride application on chlorophyll content, stomatal conductance, transpiration rate and net photosynthetic rate of late-sown wheat at booting and physiological maturity during the 2023–24 and 2024–25 growing seasons. Points represent means ± standard error (SE). Different lowercase letters indicate significant differences among treatments within each growth stage (P ≤ 0.05), whereas ns indicates a non-significant difference. BS, booting stage; PM, physiological maturity.
Figure 2. Effects of nitrogen fertilization and mepiquat chloride application on chlorophyll content, stomatal conductance, transpiration rate and net photosynthetic rate of late-sown wheat at booting and physiological maturity during the 2023–24 and 2024–25 growing seasons. Points represent means ± standard error (SE). Different lowercase letters indicate significant differences among treatments within each growth stage (P ≤ 0.05), whereas ns indicates a non-significant difference. BS, booting stage; PM, physiological maturity.

3.2 Assimilate partitioning

Nitrogen fertilization significantly influenced assimilate partitioning among plant organs during both years of the study. Increasing nitrogen supply enhanced grain dry matter accumulation and its proportion of total biomass (Table 2). The highest grain dry matter was recorded under 150% recommended dose of nitrogen RDN (M3), reaching 49.9 and 51.3 q ha-1 during 2023–24 and 2024-25, respectively, which represented increases of 15.2 and 12.5% over 100% RDN (M1). Similarly, the proportion of total biomass partitioned to grains increased from 42.24 and 42.78% under M1 to 43.59 and 43.20% under M3 in the respective years. However, M3 remained statistically comparable to 125% RDN (M2) for grain dry matter accumulation. In contrast, M1 exhibited lower grain dry matter production and a smaller proportion of assimilates partitioned toward grains. The greater allocation of dry matter to grains under M2 and M3 indicates a more efficient utilization of assimilates for reproductive growth and grain development.

Table 2. Effects of nitrogen fertilization and growth regulation treatments on assimilate partitioning among stems, leaves and grains of late sown wheat during the 2023–24 and 2024–25 growing seasons.

TreatmentsStemLeavesGrain
Weight (q ha-1)Proportion (%)Weight (q ha-1)Proportion (%)Weight (q ha-1)Proportion (%)Weight (q ha-1)Proportion (%)Weight (q ha-1)Proportion (%)Weight (q ha-1)Proportion (%)
2023-242023-242024-252024-252023-242023-242024-252024-252023-242023-242024-252024-25
Nitrogen levels
M140.1b39.0341.2b38.5719.2b18.7719.9b18.6543.3b42.2445.6b42.78
M242.1a38.1143.7a38.4720.7a18.7121.1a18.6447.7a43.1448.7a42.89
M343.6a37.9245.8a38.3721.1a18.4921.9a18.4349.9a43.5951.3a43.2
Growth regulation
S143.340.7446.141.4819.6d18.5119.7d17.7443.5d40.7545.2d40.78
S242.139.0843.638.8620.1c18.6721.0b18.9845.5c42.2547.1c42.16
S34338.5944.638.9120.4b18.4220.4c17.9147.3b42.9948.8b43.17
S441.136.7141.936.2621.0a18.8322.1a19.2249.8a44.4751.1a44.52
S541.238.7443.238.7820.1c18.7921.0b19.0445.3c42.4847.0c42.17
S642.738.444.438.9120.4b18.5220.3c17.8947.1b43.0948.7b43.2
S740.236.2541.236.0620.8a18.8821.9a19.2449.5a44.8750.9a44.7

Values followed by different lowercase letters within a column differ significantly at P ≤ 0.05. M, nitrogen levels; S, growth regulation treatment.” and ‘ns’ indicate significant and non-significant effects, respectively.

Growth regulation treatments also significantly altered the assimilate partitioning patterns (Table 2). Dual-stage application of mepiquat chloride at 750 ppm at the maximum tillering (MT) and flag leaf (FL) stages (S4) resulted in the highest grain dry matter accumulation, recording 49.8 and 51.1 q ha-1 during 2023–24 and 2024-25, respectively, while remaining statistically at par with the corresponding 1000 ppm treatment applied at both stages (S7), which produced 49.5 and 50.9 q ha-1. Compared with the untreated control (S1), S4 increased grain dry matter accumulation by 14.5 and 13.1% during the two years of the study. The proportion of total biomass partitioned to grains was also highest under S4 (44.47 and 44.52%) and S7 (44.87 and 44.70%), whereas the untreated control recorded comparatively lower values (40.75 and 40.78%). Conversely, single-stage applications and the untreated control generally exhibited lower grain dry matter accumulation and partitioning. The superiority of S4 and S7 indicates enhanced partitioning of assimilates toward reproductive structures, thereby favoring grain development during the reproductive phase. The interaction between nitrogen fertilization and growth regulation treatments was not significant for any of the assimilate partitioning parameters during both years of investigation.

3.3 Yield attributes, sink capacity and yield

Nitrogen fertilization had a significant influence on yield attributes and sink capacity during both years of the study (Table 3). Spike weight, an important indicator of sink development, increased with increasing nitrogen supply and attained its highest values under 150% recommended dose of nitrogen (RDN; M3), reaching 1.75 and 1.85 g during 2023–24 and 2024-25, respectively. These values were 48.3 and 45.7% higher than those recorded under 100% RDN (M1), although M3 remained statistically comparable to 125% RDN (M2). Thousand-grain weight followed a comparable trajectory, climbing from 33.3 and 35.5 g recorded under M1 to 37.3 and 38.8 g under M3, which translated into percentage gains of 12.0 and 9.3% across the two experimental years. Sink capacity showed similar trends, reaching peak values of 52.0 and 57.3 q ha-1 under M3. It surpassed M1 by 36.8 and 29.6%, respectively. However, M3 and M2 were statistically equal. These figures suggest that higher nitrogen availability supported a larger active sink during the post-anthesis period. This strengthened the ability of developing grains to draw upon and use available assimilates throughout the grain filling phase.

Table 3. Effect of nitrogen fertilization and growth regulation on spike weight, 1000-grain weight, and sink capacity of late sown wheat during 2023–24 and 2024-25.

TreatmentsSpike weight (g)1000-grain weight (g)Sink capacity (q ha-1)
2023-242024-252023-242024-252023-242024-25
Nitrogen levels
M11.18b1.27b33.3b35.5b38.0b44.2b
M21.58a1.69a35.9a37.9a46.4a52.5a
M31.75a1.85a37.3a38.8a52.0a57.3a
Growth regulation
S11.23d1.31d33.4d35.8d40.3c47.1c
S21.58b1.67b36.1b37.8b46.4ab52.5ab
S31.40c1.49c34.6c36.9c42.8bc49.7bc
S41.73a1.85a37.1a38.6a50.8a55.2a
S51.52b1.65b36.0b37.7b45.6ab51.5ab
S61.36c1.45c34.3c36.6c42.5bc49.0bc
S71.71a1.81a36.9a38.5a49.8a54.2a

Similar letters within a column indicate a non-significant difference at the 0.05 probability level using Fisher’s LSD test. RDN, recommended dose of nitrogen; MC, mepiquat chloride. s, significant; ns, non-significant.

Growth regulation treatments further influenced yield attributes and sink capacity (Table 3). The dual-stage application of mepiquat chloride at 750 ppm at the maximum tillering (MT) and flag leaf (FL) stages (S4) recorded the highest spike weight (1.73 and 1.85 g), 1000-grain weight (37.1 and 38.6 g), and sink capacity (50.8 and 55.2 q ha-1) during 2023–24 and 2024-25, respectively, while remaining statistically at par with the corresponding 1000 ppm treatment applied at both stages (S7). Compared with the untreated control (S1), S4 increased spike weight by 40.7 and 41.2%, 1000-grain weight by 11.1 and 7.8%, and sink capacity by 26.1 and 17.2% during 2023–24 and 2024-25, respectively. In contrast, comparatively lower values for these parameters were observed under single-stage applications and the untreated control (S1) treatment. The superiority of S4 and S7 indicates enhanced sink establishment and a greater capacity to utilize assimilates during the reproductive phases. The interaction between nitrogen fertilization and growth regulation treatments was non-significant for all yield attributes and sink capacity parameters during both years of investigation. Consistent with these responses, the untreated control (S1) exhibited the poorest sink expression throughout the study.

3.4 Yield and harvest index

Differences in physiological performance, assimilate partitioning, and sink development were ultimately reflected in grain yield, straw yield, and harvest index. Grain yield was higher under 150% recommended dose of nitrogen (RDN; M3) than under 100% RDN (M1) and remained statistically comparable with 125% RDN (M2), reaching 50.4 and 51.9 q ha-1 during 2023–24 and 2024-25, respectively. These values represented yield advantages of 15.3 and 13.6% over 100% RDN (M1), although M3 remained statistically comparable to 125% RDN (M2). A similar trend was observed for straw yield, where M3 produced the highest biomass and exceeded M1 by 9.3 and 8.9% during the respective years while remaining at par with M2. The harvest index showed only marginal variation among the nitrogen levels, with numerically higher values recorded under M2 and M3; however, the effect of nitrogen was non-significant during both years. The interaction between nitrogen fertilization and growth regulation treatments was non-significant for grain yield, straw, and harvest index during both years of the study.

Growth regulation treatments markedly influenced the crop productivity (Table 4). The dual-stage application of mepiquat chloride at 750 ppm at the maximum tillering (MT) and flag leaf (FL) stages (S4) produced the highest grain yield (50.2 and 51.8 q ha-1 during 2023–24 and 2024-25, respectively), representing increases of 14.4 and 14.3% over the untreated control (S1). Grain yield under S4 remained statistically comparable to the corresponding 1000 ppm treatment applied at MT and FL stages (S7). In contrast, the untreated control produced the highest straw yield, whereas dual-stage mepiquat chloride applications (S4 and S7) produced the lowest straw biomass. Straw yield under S4 dropped by 8.9% and 7.6% compared to the untreated control during 2023–24 and 2024-25, respectively. This change in biomass allocation was clear from the harvest index, which reached its highest values under S4 and S7 at 41.2% in both years. In contrast, the untreated control had the lowest values at 35.7% and 36.1%. Overall, the results indicate that applying mepiquat chloride in two stages mainly improved grain productivity by shifting the focus of assimilate partitioning and biomass allocation toward the grain, rather than increasing total vegetative biomass.

Table 4. Effect of nitrogen fertilization and growth regulation on grain yield, straw yield and harvest index of late sown wheat during 2023–24 and 2024-25.

TreatmentsGrain yield (q ha-1)Straw yield (q ha-1)Harvest index (%)
2023-242024-252023-242024-252023-242024-25
Nitrogen levels
M143.7b45.7b70.9b73.1b38.238.5
M248.1a49.1a75.4a76.6a3939.1
M350.4a51.9a77.5a79.6a39.439.5
Growth regulation
S143.9d45.3d78.7a79.9a35.7c36.1c
S245.9c47.4c73.9bc75.8bc38.4b38.5b
S348.2b49.7b76.2ab78.0ab38.7b38.9b
S450.2a51.8a71.7c73.8c41.2a41.2a
S545.7c47.2c73.9bc75.8bc38.2b38.4b
S648.0b49.5b76.2ab77.9ab38.6b38.8b
S750.0a51.6a71.6c73.7c41.2a41.2a

Values within a column that share different lowercase letters are statistically different(P ≤ 0․05) based on the LSD test. MC‘ mepiquat chloride; RDN‘ recommended dose of nitrogen․ s and ns denote significant and non-important effects‘ respectively․.

3.5 Correlation of physiological traits‘ assimilate partitioning‘ sink capacity‘ yield attributes and yield

Pearson correlation analysis (Figure 3) showed positive associations among the physiological, sink and yield-related traits. Chlorophyll content at physiological maturity (Chl_PM) was positively correlated with stomatal conductance (Gs_PM; r = 0.78), net photosynthetic rate (Pn_PM; r = 0.74), spike weight (SW; r = 0.84), thousand-grain weight (TGW; r = 0.74) and grain yield (GY; r = 0.77). Stomatal conductance showed strong positive correlations with SW (r = 0.90) and TGW (r = 0.80). The strongest correlation was observed between Pn_PM and SW (r = 0.91), followed by Gs_PM and SW (r = 0.90). Sink capacity (SC) was positively correlated with TGW (r = 0.71), SW (r = 0.62) and GY (r = 0.53), whereas its correlation with HI was relatively weak (r = 0.33). Grain yield was positively associated with HI (r = 0.78), Chl_PM (r = 0.77), SW (r = 0.75), Gs_PM (r = 0.74), Pn_PM (r = 0.66), TGW (r = 0.63) and SC (r = 0.53).

Pearson correlation matrix depicting the relationships among physiological traits, sink capacity, yield attributes and grain yield in late-sown wheat based on pooled experimental-unit data across the 2023–24 and 2024–25 crop seasons.
Figure 3. Pearson correlation matrix depicting the relationships among physiological traits, sink capacity, yield attributes and grain yield in late-sown wheat based on pooled experimental-unit data across the 2023–24 and 2024–25 crop seasons.

3.6 Principal component analysis

Principal component analysis revealed structured relationships among the measured physiological, assimilate partitioning, sink capacity and yield-related traits of late-sown wheat across the two crop seasons (Figure 4). PC1 accounted for 72.3% of the total variation, while PC2 and PC3 accounted for 10.5% and 6.3%, respectively. The first two principal components together explained 82.8% of the total variation. Chl_PM, Gs_PM, Pn_PM, SW and GY showed positive loadings on PC1, with particularly strong contributions from SW, GY, Chl_PM and Gs_PM. SC and HI also showed positive loadings on PC1, although their contributions were comparatively lower. Along PC2, HI showed the strongest positive loading, followed by GY, whereas SC and TGW showed negative loadings. Pn_PM showed a relatively small loading on PC2 but a comparatively stronger positive loading on PC3, while SW also contributed positively to PC3. The PCA score plot showed that treatment combinations were distributed across the ordination space, with M3S4 positioned at the positive end of PC1 and treatment combinations such as M1S1, M1S2, M1S3 and M1S6 positioned toward the negative side of PC1, while the remaining treatments occupied intermediate positions. The three-dimensional loading plots further showed close positioning of Chl_PM and Gs_PM along PC1, reflecting their similar contribution to the major variation, whereas SW and GY also showed strong positive associations with PC1. SC and TGW were positioned in the negative direction of PC2, whereas HI was oriented positively along PC2. Pn_PM showed a comparatively distinct contribution along PC3.

Principal component analysis (PCA) depicting the relationships among physiological traits, sink capacity, yield attributes and grain yield in late-sown wheat based on pooled experimental-unit data across the 2023–24 and 2024–25 crop seasons. **(A)** PCA score plot showing individual experimental-unit observations and treatment centroids, with trait vectors indicating the direction and magnitude of their contributions to the principal components. **(B)** Three-dimensional PCA loading plot showing the physiological traits. **(C)** Three-dimensional PCA loading plot showing sink capacity and yield-related traits.
Figure 4. Principal component analysis (PCA) depicting the relationships among physiological traits, sink capacity, yield attributes and grain yield in late-sown wheat based on pooled experimental-unit data across the 2023–24 and 2024–25 crop seasons. (A) PCA score plot showing individual experimental-unit observations and treatment centroids, with trait vectors indicating the direction and magnitude of their contributions to the principal components. (B) Three-dimensional PCA loading plot showing the physiological traits. (C) Three-dimensional PCA loading plot showing sink capacity and yield-related traits.

3.7 Hierarchical clustering analysis

Hierarchical cluster analysis separated the observations into three clusters based on their multivariate similarity (Figure 5). Cluster I contained the largest number of observations (95), followed by Cluster III (59), whereas Cluster II comprised 14 observations. Cluster II was predominantly associated with the M1 treatment combinations, particularly M1S1, M1S3 and M1S6, while Cluster I was mainly represented by M2 and M3 treatment combinations. The M3 treatment combinations from M3S2 to M3S7 were entirely represented in Cluster I. The M1 treatment combinations were distributed mainly between Clusters II and III, whereas several M2 combinations showed a greater representation in Cluster I. The dendrogram indicated greater similarity among observations within clusters and progressively larger Euclidean distances between the major clusters.

Hierarchical clustering dendrogram showing the grouping of observations based on standardized physiological traits, sink capacity, yield attributes and grain yield in late-sown wheat using pooled experimental-unit data from the 2023–24 and 2024–25 crop seasons.
Figure 5. Hierarchical clustering dendrogram showing the grouping of observations based on standardized physiological traits, sink capacity, yield attributes and grain yield in late-sown wheat using pooled experimental-unit data from the 2023–24 and 2024–25 crop seasons.

4 Discussion

The maintenance of photosynthetic activity during the reproductive phase is particularly important in late-sown wheat because accelerated crop development shortens the period available for assimilate production and grain filling (Fang et al., 2024). Under such conditions, the capacity of the crop to sustain chlorophyll content and gas exchange activity largely determines the supply of assimilates available for reproductive growth (Mu et al., 2016). In the present investigation, increasing nitrogen fertilization enhanced chlorophyll content, stomatal conductance, transpiration rate and net photosynthetic rate across the evaluated growth stages and growing seasons. The observed improvement in these physiological traits may be attributed to enhanced carbon assimilation under late-sown conditions (Guo et al., 2019). These responses are consistent with the fundamental role of nitrogen as a structural component of chlorophyll, photosynthetic proteins and enzymes involved in carbon fixation (Mu et al., 2016).

Higher nitrogen supply strengthened sink development in late-sown wheat, as reflected in heavier spikes, greater 1000-grain weight and improved sink capacity recorded across nitrogen treatments. Mepiquat chloride at 750 ppm applied at both maximum tillering and flag leaf stages produced comparable gains in sink-related traits among growth regulation treatments. Nitrogen adequacy determines how long flag leaves remain active and how effectively sugars are produced during grain filling, both of which matter more when late sowing shortens the available filling period (Sharma et al., 2023). The higher physiological performance observed under M3 may be attributed to improved nitrogen availability supporting sustained photosynthetic activity. Thus, this outcome is consistent with reports that higher nitrogen nutrition increases photosynthate supply to developing sinks and sustains grain development (Rodriguez et al., 2024).

Mepiquat chloride acted through gibberellin inhibition which checked internode elongation and produced a more compact plant structure (Rangan et al., 2016; Rademacher, 2015). Within a compact canopy, radiation reached deeper leaf layers, mutual shading declined and the photosynthetically active area remained larger into the reproductive phase (Liu et al., 2022). The delayed chlorophyll degradation may have resulted from the improved canopy architecture created by mepiquat chloride (El-Nwehy and Afify, 2023). Dual-stage application at 750 ppm during maximum tillering and flag leaf stages produced the highest values for chlorophyll content, stomatal conductance, transpiration rate and net photosynthetic rate among all growth regulation treatments. The simultaneous improvement in gas exchange and chlorophyll parameters under S4 and S7 may be due to prolonged maintenance of source activity during grain filling (Chhokar et al., 2024). Leaf water status and stomatal behavior also appeared to benefit from canopy restructuring under these treatments with improved carbon uptake and photosynthetic efficiency as the net result (Wang et al., 2024). Cotton crops treated with mepiquat chloride showed net photosynthetic rate increases of 3.4% to 23.2% over untreated plants, a finding consistent with the physiological responses recorded in the experiment (Gao et al., 2019). Similar improvements in chlorophyll retention and photosynthetic performance following mepiquat chloride application have been reported in other crops as well (Wu et al., 2023; Noor et al., 2023).

Physiological improvements recorded across treatments were reflected in biomass production and assimilate utilization patterns. Continuous photoassimilate availability under higher nitrogen supply supported concurrent growth of vegetative and reproductive sinks throughout the cropping season (Noor et al., 2023). The greater biomass accumulation may have resulted from sustained photo assimilate availability under higher nitrogen nutrition (Khan et al., 2022; Gill and Walia, 2025). These responses resulted in a stronger source-sink relationship and greater overall biomass productivity. Mepiquat chloride introduced a distinct dimension to assimilate dynamics by altering how resources were partitioned between vegetative and reproductive organs. Suppression of excessive vegetative growth reduced the assimilate demand from stems and leaves, effectively releasing a larger share of newly synthesized photosynthates for reproductive structures during the acquisition phase (Rademacher, 2015). The preferential allocation of assimilates to spikes and developing grains was likely owing to reduced assimilate demand from vegetative organs (Fang et al., 2024; Slafer et al., 2014).

Grain filling is inherently competitive, and any management practice that shifts sink priority toward reproductive organs tends to improve dry matter accumulation in kernels during the filling stage (EL Sabagh et al., 2021). Application of mepiquat chloride at 750 ppm at both maximum tillering and flag leaf stages produced the highest grain dry matter percentage among all treatments evaluated, which is consistent with this partitioning mechanism. It indicates a measurable redirection of assimilate flow from vegetative structures to reproductive organs. Evidence from other crops supports this mechanism, with growth retardants applied to soybeans resulting in similar gains in allocating assimilates to economic sinks along with improved yield formation (Jaidka et al., 2020). Since grain yield in wheat relies largely on sink size and its ability to accumulate assimilates, yield traits and sink capacity are key indicators of productive potential (Zhang et al., 2025). The greater spike weight, 1000-grain weight and sink capacity observed with higher nitrogen supply suggest that improved nitrogen availability enhanced the formation and functioning of reproductive sinks (Slafer et al., 2014; Zhang et al., 2026). Such responses were likely associated with improved spike fertility throughout the reproductive phase and resulted in greater grain set and ultimately higher grain yield. Improved spikelet survival and grain retention in the evaluated treatments seem to have increased the functional sink size of the spike, leading to the higher spike weight recorded in both years (Ershadimanesh et al., 2024; Li et al., 2025). Higher assimilate deposition during the grain-filling period increased the dry matter accumulated by individual kernels which was reflected in the higher 1000-grain weight. This response suggests that developing grains received an adequate assimilate supply throughout the filling phase, enabling greater realization of their sink potential (EL Sabagh et al., 2021; Li et al., 2025). Since sink capacity depends on both the number of grains produced and the extent of grain filling, improvements in these two yield components together explain the greater sink capacity observed following mepiquat chloride application (Slafer et al., 2014; Vicentin et al., 2024).

Late sowing shortens grain filling duration and restricts the time available for assimilate production and transport to developing grains. Under these conditions, sustained photosynthetic activity and uninterrupted assimilate movement are the primary determinants of grain yield. The higher grain yield obtained with 125% and 150% RDN compared with 100% RDN, with the two higher nitrogen levels remaining statistically comparable across both experimental years, indicates that increased nitrogen availability supported these processes and favored grain production (Fang et al., 2024; Gawdiya et al., 2025; Kaur et al., 2016). Straw yield increased progressively with nitrogen level and confirmed that greater nitrogen availability promoted dry matter deposition in vegetative tissues during crop growth (Li et al., 2025). Harvest index values differed only marginally among nitrogen treatments, though M2 and M3 recorded slightly higher values than M1, indicating that biomass produced under higher nitrogen supply was converted into grain somewhat more efficiently (Tiwari et al., 2025).

Mepiquat chloride influenced yield through a mechanism distinct from nitrogen, acting primarily on assimilate partitioning rather than on vegetative biomass production. Application of 750 ppm at both peak tillering and flag leaf stages raised grain yield and harvest index while reducing straw yield relative to the untreated control. This shift in the balance between vegetative and reproductive dry matter indicates that a greater proportion of available assimilates was directed toward grain development. Concurrent improvements in spike weight, 1000-grain weight and sink capacity under this treatment confirm that sink development was enhanced and grain filling became more effective without stimulating excessive vegetative growth (EL Sabagh et al., 2021; Tiwari et al., 2025). Earlier studies have reported the same pattern, where mepiquat chloride reduced structural biomass and improved the proportion of total biomass converted into economic yield (Tiwari et al., 2025; Cigel et al., 2023; Ali et al., 2025).

Multivariate analysis separated treatments according to their physiological and yield performance under late-sown conditions. Treatments occupying positions in the ordination space associated with higher chlorophyll content, stomatal conductance, transpiration rate, net photosynthetic rate, sink capacity, spike weight and grain yield generally represented more favorable profiles of physiological and yield-related traits (Rademacher, 2015; Zhang et al., 2025). Treatments with lower productivity were distributed away from these traits, reflecting lower physiological activity and weaker sink development. The association of grain yield with multiple physiological, assimilate partitioning and sink-related traits indicated that yield variation was related to the combined behavior of these traits rather than to a single trait, which aligns with the agronomic benefits observed under adequate nitrogen nutrition and targeted growth regulation. The improved physiological activity and grain productivity observed with nitrogen fertilization and mepiquat chloride application indicate their potential to sustain wheat productivity under delayed sowing conditions. However, the present study did not quantify nitrogen-use efficiency, nitrogen losses, greenhouse gas emissions, or economic/resource-use efficiency indicators.

5 Conclusion

Late sowing imposes substantial constraints on wheat productivity by limiting the duration available for assimilate accumulation and grain filling. The present study demonstrated that both nitrogen fertilization and mepiquat chloride application play important roles in mitigating these limitations through improvements in physiological activity, assimilate partitioning and reproductive sink development. Increasing nitrogen supply enhanced chlorophyll content, gas-exchange characteristics, sink capacity and grain yield, while foliar application of mepiquat chloride further improved the efficiency of assimilate partitioning toward developing grains and strengthened sink realization during the reproductive phase. Improved physiological performance, assimilate partitioning and yield-forming traits were associated with higher grain productivity under late-sown conditions. Based on the results obtained over two consecutive years, application of 125% and 150% recommended dose of nitrogen produced statistically comparable grain yields under late-sown conditions, and foliar application of mepiquat chloride @ 750 ppm at maximum tillering and flag leaf stages, which remained statistically comparable with the corresponding 1000 ppm treatment, were found effective for enhancing grain yield and productivity of late-sown wheat under north-western Indian conditions.

6 Limitations and future research

Although the present study provides important insights into the physiological responses, assimilate partitioning and productivity of late-sown wheat under varying nitrogen and mepiquat chloride treatments, the findings are based on a single cultivar evaluated under a specific agro-climatic environment. Furthermore, while clear improvements in physiological performance, assimilate partitioning and grain yield were observed, the biochemical and molecular mechanisms underlying these responses were not directly investigated. Future research involving diverse wheat genotypes across contrasting environments, together with advanced physiological and biochemical approaches, would help to further elucidate the mechanisms governing crop responses to nitrogen fertilization and mepiquat chloride application under late-sown conditions.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fagro.2026.1924091/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Jiban Shrestha, Nepal Agricultural Research Council, Nepal

Reviewed by: Éva Babett Ábrahám, University of Debrecen, Hungary

Koushik Bag, The ICAR Research Complex for North Eastern Hill Region (ICAR RC NEH), India

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

KD: Data curation, Formal Analysis, Investigation, Methodology, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. JD: Conceptualization, Investigation, Methodology, Project administration, Supervision, Validation, Writing – review & editing. RNS: Conceptualization, Data curation, Investigation, Supervision, Validation, Visualization, Writing – review & editing. AMK: Data curation, Investigation, Supervision, Validation, Visualization, Writing – review & editing. ARK: Data curation, Software, Visualization, Writing – review & editing. RHS: Data curation, Formal Analysis, Visualization, Writing – review & editing.

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Citation

Devi, K., et al. (2026). Optimizing physiological performance, assimilate partitioning and productivity of late-sown wheat (Triticum aestivum L.) through mepiquat chloride application under nitrogen enrichment. Frontiers in Agronomy, Volume 8. https://doi.org/
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8 - 2026

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Copyright © 2026 Devi, Deol, Sharma, Kaul, Kumar and Sharma.

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*Correspondence: Kavita Devi

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