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Current IssueEditorial BoardOnline First ArticlesArchive

Full Research Article

Impact of elevated CO2 and Temperature on Nodulation and Crop Weed Competition in Groundnut (Arachis hypogaea L.)

T
T. Ram Prakash1,*
0000-0001-6992-7769
B
B. Padmaja1
0000-0002-2594-7554
M
M. Vanaja2
0000-0002-6634-4055
T
T. Bala Srikanth Reddy1
0009-0007-3912-0332
S
Shobha Sondhia3
0000-0002-0776-5546
M
M. Prabhakar2
0000-0002-6899-3390
J
J.S. Mishra3
1AICRP on Weed Management, Professor Jayashankar  Telangana State Agricultural University, Hyderabad-500 030, Telangana, India.
2ICAR-Central Research Institute for Dryland Agriculture, Hyderabad-500 059, Telangana, India.
3ICAR-Directorate of Weed Research, Jabalpur-482 004, Madhya Pradesh, India.

  • Submitted27-10-2025|

  • Accepted13-01-2026|

  • First Online 31-01-2026|

  • doi 10.18805/LR-5593

ABSTRACT

Background: Rising atmospheric CO2 and temperature due to climate change pose significant threats to agricultural productivity, particularly in semi-arid tropics. These changes can alter weed distribution, population dynamics and crop-weed interactions. Assessing groundnut’s adaptability to changing climates, weed competition and herbicide efficacy is crucial for developing resilient cultivars and effective herbicide management systems to mitigate yield and quality losses, thereby enhancing global food security.

Methods: A two-year (2022-23 and 2023-24) experiment was conducted in a Carbon Dioxide and Temperature Gradient Chamber (CTGC) at the Central Research Institute for Dryland Agriculture, Hyderabad. The study evaluated groundnut under four climate variables: ambient CO2  + ambient temperature (aCO2 + aT) (396-405ppm), elevated CO2 (eCO2, 550 ± 50 ppm), elevated temperature (eT, +2°C) and combined eCO2 + eT (550 ± 50 ppm and +2°C) , three crop-weed combinations (groundnut + C3 weeds, groundnut + C4 weeds, groundnut + C3 + C4 weeds) and four herbicide rates (0X, 1X, 1.5X, 2X) using a completely randomized design. Growth parameters, soil rhizobium population and nodulation were assessed at 60 days after sowing (DAS).

Result: Pooled data revealed that eCO2 significantly enhanced groundnut growth and nodulation, with increases in plant height (25%), leaf area (26%), chlorophyll content (12%), Rhizobium population (22%), nodulation (58%) and dry matter production (22%) compared to ambient conditions. Combination of elevated CO2 and temperature also showed positive effects, though less pronounced, while et alone reduced all parameters. Herbicide application at various rates showed suppressive effect on Rhizobium population, nodulation and chlorophyll content compared to no application. However, plant height, leaf area and dry matter production were significantly higher at 1.5 X rate over no application under eCO2 conditions in contrast to 1.0X rate under aCO2 + aT, eCO2 + eT and eT conditions. Groundnut exhibited better competitiveness against C4 weeds compared to C3 weeds or their combination. The findings revealed that groundnut growth and nodulation were significantly enhanced in treatments eCO2 and eCO2 + eT but reduced under eT. Herbicide efficacy declined under eCO2, suggesting the need for higher application rate under future climate scenarios. These findings highlight the importance of adaptive strategies to sustain groundnut productivity under climate change scenario.

INTRODUCTION

The global population is projected to reach 8.6 billion by 2030 and 9.6 billion by 2050, placing increasing pressure on agriculture to ensure food security, address climate change and improve soil health (Sharma et al., 2025). Groundnut (Arachis hypogaea L.), is an annual legume with high quality edible oil and easily digestible protein of its seeds (Vasanthi et al., 2015). The crop is cultivated mainly for its seed and it comprises 40-50% oil, 20-30% protein (Naik et al., 2022). It is cultivated on 7.74 million hectares in India, producing 7.8 million tonnes annually (FAO, 2023). It is vital for nutritional security and livestock fodder, especially as global demand for protein-rich diets grows. However, rising atmospheric CO2 levels, driven by human activities, threaten agricultural productivity through climate change, altering weather patterns, increasing temperatures and intensifying pest and disease pressures. Since 1850, CO2 concentrations have risen from 280 ppm to 407 ppm in 2017 (Asha et al., 2017), with projections reaching 421-936 ppm by 2100 (IPCC, Climate change, 2013).
       
Carbon dioxide in the C3 pathway of photosynthesis promotes growth and productivity and is an example of a positive effect on C3 plants (Pawar, 2025). C3 plants like groundnut generally benefit from elevated CO2, showing improved growth and CO2 assimilation (Laza et al., 2021), but associated temperature rises can reduce yields (Vanaja et al., 2019). Weeds, with their diverse gene pools and physiological plasticity, may adapt better to changing climates, intensifying crop-weed competition (Ziska, 2001; Varanasi et al., 2016). Elevated CO2 can also delay herbicide efficacy (Naidu et al., 2023), while higher temperatures may reduce herbicide effectiveness (Rudell et al., 2023). Understanding these interactions is crucial for optimizing weed control and ensuring food security under future climates. Hence, this study investigates the effects of climate change on groundnut growth, weed competition and herbicide efficacy.

MATERIALS AND METHODS

Location of the experimental site

The experiment was conducted in Carbon dioxide and Temperature Gradient Chamber (CTGC) facility located at ICAR-CRIDA, Hayathnagar Farm, Hyderabad (17°34’54.65"N latitude and 78°59’42.85"E longitude).
 
CTGC facility
 
The Carbon Dioxide and Temperature Gradient Chamber (CTGC) facility consists of four chambers, aT + aCO2: Maintains ambient temperature and CO2 levels (396-405 ppm), eT: Elevates temperature by 2°C above ambient using infrared heaters, eCO2: Elevates CO2 concentration to 550± 50 ppm) andeT + eCO2: Combined elevated temperature (+2°C) and CO2 (550±50 ppm).The facility uses solar radiation and heaters to regulate temperature, with cooling pads and exhaust fans maintaining airflow. A Programming Logical Controller (PLC) monitors and controls temperature and CO2 levels, ensuring precise environmental conditions.
 
Treatment details
 
The experiment studied groundnut response under three factors:
 
Climate variables (4 levels)
 
• aT + aCO2: Ambient temperature and CO2 (396-405 ppm).
• eT: Elevated temperature (+2°C).
• eCO2: Elevated CO2 (550 ± 50 ppm).
• eT+eCO2: Elevated temperature (+2°C) and CO2 (550±50 ppm).
 
Herbicide rates (4 levels)
 
• 0X: No herbicide.
• 1X: Recommended dose (75 g/ha Imazethapyr + 50 g/ha  Propaquizafop).
• 1.5X: 150% recommended dose (112.5 g + 75 g/ha).
• 2X: 200% recommended dose (150 g + 100 g/ha).
 
Crop-weed combinations (3 levels)
 
• Groundnut + C3 weeds (Parthenium hysterophorus and Alternanthera sessilis).
• Groundnut + C4 weeds (Cyperus rotundus and Cynodon dactylon).
• Groundnut + C3 + C4 weeds.
       
The present experiment was taken under pot culture and popular groundnut cultivar in Telangana K6 was sown on 7-12-2022 and 6-12-2023 during both the years of study. Pot size of (12" diameter, 14" height) were filled with 18-19 kg red sandy loam soil. C3 weed seeds and C4 weed rhizomes were sown/transplanted alongside. Fertilizers (30 kg N + 40 kg P2O5 + 50 kg K2O + 40 kg S/ha) and vermicompost (2 tons/ha) were applied. 75 g/ha Imazethapyr + 50 g/ha Propaquizafop herbicidew as sprayed post-emergence at 4 ml/L (1X), 6 ml/L (1.5X) and 8 ml/L (2X).
 
Sampling
 
At 60 DAS, parameters on crop growth and nodule number was recorded, Plant height was measured from the base to the top leaf tip. SPAD readings (chlorophyll content) were taken using a SPAD-502 meter. Rhizosphere soil samples were collected to assess Rhizobium population using Yeast Mannitol Agar Media (Al-Mujahidy et al., 2013). Plants were carefully uprooted, washed, air-dried for 30 minutes and nodules counted. Leaves were separated and leaf area was measured using a LICOR 3100 leaf area meter. Stem, root and leaf dry weights were recorded after oven-drying at 60°C and total biomass was calculated by summing them.
 
Statistical analysis
 
Data were analyzed using three-way ANOVA at 5% significance to assess variability among climate variables, herbicide rates, crop-weed combinations and their interactions using WINDOSTAT software.

RESULTS AND DISCUSSION

Plant height
 
Pooled data from two years (Fig 1) showed that groundnut height increased significantly by 25% under elevated CO2  (eCO2, 550±50 ppm) compared to ambient conditions, followed by eT + eCO2 (12.4%) and eT (3.8%). The tallest plants (39.4 cm) were recorded with a 1.5X herbicide dose (Imazethapyr 3.75% + Propaquizafop 2.5% ME), similar to 1.0X but superior to 0X and 2X doses. Plants grew taller with C4 weeds than with C3 weeds or their combination. Naidu et al., (2023) noted that C4 weeds like Sorghum halepense were less competitive than C3 weeds like Euphorbia geniculata under eCO2. The eCO2 + 1.5X herbicide combination produced the tallest plants, outperforming other combinations except eCO2 + 2.0X. Under ambient and eT conditions, plant height increased at 1X but declined at 1.5X and 2X, while under eCO‚  and eT + eCO2, height increased up to 1.5X before declining at 2X. Other interactions were non-significant. Geethalakshmi et al., (2017) and Chander et al., (2023) also reported positive effects of elevated CO2 and temperature on plant height, leaf area and nodulation in soybean.

Fig 1: Growth parameters of groundnut as influenced by the climate variables, rates of herbicide application and crop weed interaction.



Leaf area per plant
 
Leaf area was significantly influenced by all three factors independently, as well as by climate × herbicide and climate × crop-weed interactions (Fig 1). Maximum leaf area was observed under eCO2, being 3%, 10% and 26% higher than eT, eT + eCO2 and aT + aCO2, respectively. Laza et al., (2021) found that long-term exposure to eCO2 (650 ppm) enhanced CO2 assimilation, transpiration efficiency, biomass and pod yield in peanuts. Leaf area was highest at the 1.5X herbicide rate, 10% greater than no application. C4 weeds resulted in significantly higher leaf area compared to C3 weeds or their combination. The interaction effect showed that leaf area with C4 weeds was highest under eT + eCO2. Climate × herbicide interactions revealed that the recommended dose increased leaf area over no application, but higher rates were ineffective under aT + aCO2, eT and eT + eCO2. Under eCO2, 1X did not enhance leaf area, but 1.5X and 2X rates achieved maximum leaf area, suggesting higher herbicide rates may be needed for effective weed control und elevated CO2 and temperature Varanasi et al. (2016) noted that herbicide efficacy is reduced under high temperature and CO‚  conditions.
 
Leaf chlorophyll content (SPAD readings)
 
Chlorophyll content in groundnut leaves was significantly influenced by individual factors and climate × herbicide interactions (Table 1). eCO2 resulted in the highest SPAD readings (43.3), significantly surpassing other climate variables, while eT recorded the lowest (35.9). Enriched CO2 increased leaf area, photosynthetic rate and chlorophyll content, as reported by Dey et al., (2017). Similar trends were observed in maize, with eCO2 showing the highest physiological parameters, comparable to eT + eCO2. Herbicide application reduced chlorophyll content, with the highest SPAD (41.4) under no herbicide treatment, significantly higher than 1X, 1.5X and 2X rates. Makarian et al. (2016) also noted high concentrations of herbicide caused a significant decline in leaf chlorophyll content in maize and barley. Chlorophyll content was higher with C4 weeds (41.1) in comparison to the C3 weeds or their combination (40.2 and 39.1, respectively). weeds under elevated temperatures revealed that the C3 weeds compete better in cooler conditions, while C4 weeds adapt to higher temperatures (Upasani and Barla, 2018).

Table 1: Groundnut SPAD values as influenced by the climate variables, rates of herbicide application and crop weed interaction at 60 DAS.


 
Soil Rhizobial population
 
The Rhizobium population (CFU/g soil) was significantly influenced by all three factors and due to their interactions (Table 2). eCO2 increased Rhizobium colonies by 22%, while eT reduced them by 30% compared to ambient conditions. The combination (eT + eCO2) partially offset this loss, showing an 11% increase. Herbicide application negatively impacted Rhizobium colonization, with higher rates causing greater declines. Groundnut with C3 weeds had higher Rhizobium populations (19.7 CFU/g soil) than with C4 weeds (16.4 CFU/g soil) or their combination (18.1 CFU/g soil). Vu et al., (2023) noted that soil microbial populations decreased with increased weed biomass. The highest Rhizobium population was observed under eCO‚  with C3 weeds and in treatments with no herbicide application. Ruan et al., (2023) reported that CO2 enrichment alone accelerated microbial growth, while combined temperature increases delayed it. Raghavendra et al. (2017) reported that herbicide application reduced Rhizobium populations in the soil, primarily because many herbicides are directly toxic to these beneficial microorganisms. In addition, herbicides can alter rhizosphere conditions by affecting root exudation, soil microbial balance and nutrient availability, creating a toxic or  unfavorable environment for Rhizobium survival and activity.

Table 2: Rhizobium population (CFU/g soil) in soil rhizosphere as influenced by climate variables, rates of herbicide application and crop weed interaction at 60 DAS.


 
Nodule number
 
Nodulation, a key indicator of groundnut’s nitrogen-fixing ability, was significantly influenced by climate variables, herbicide doses, crop-weed competition and their interactions (Table 3). Nodulation increased by 58% under eCO2 and 39% under eT + eCO2 compared to ambient conditions but decreased by 23% under eT. Libault (2014) noted similar trends, with eCO2 enhancing nodulation and eT reducing it due to soil moisture changes. Growth, nodule dry weight and nitrogenase activity are inversely related to temperature (Day et al., 1978). Herbicide application reduced nodulation by 7%, 15% and 22% at 1.0X, 1.5X and 2.0X rates, respectively, consistent with Zaidi et al., (2005). The highest nodulation (440 nodules/plant) was observed under eCO‚  with C4 weeds and no herbicide application while the lowest was under ambient conditions with 2X herbicide and C3 weeds. Elevated CO‚  may boost plant growth, but weed competition for resources can affect nodulation outcomes. Climate change modifies plant–herbicide interactions by altering crop physiology, herbicide uptake and effectiveness and weed competitiveness. These changes can either suppress or stimulate nodulation, depending on crop species, environmental conditions and the intensity and type of weed competition (Burul et al., 2022).

Table 3: Nodulation (No. of nodules/plant) in groundnut as influenced by climate variables, rates of herbicide application and crop weed interaction at 60 DAS.


 
Dry matter production
 
Dry matter accumulation in groundnut was significantly influenced by climate variables, herbicide doses and their interactions (Fig 1). The highest dry matter (16.9 g/plant) was recorded under eCO2 (550±50 ppm), followed by eT + eCO2 (15.8 g/plant), while eT resulted in the lowest (13.2 g/plant). Similar increases in biomass under eCO2 were reported by Saha et al., (2012) in pigeon pea and by Vanaja et al., (2019) and Guna et al., (2024) in groundnut and black gram, respectively. Dey et al., (2017) noted a 20.7% and 17.3% increase in photosynthetic rate at 25 and 45 DAS in mung bean under eCO2, enhancing biomass production. Among herbicide doses, the highest dry matter (15.6 g/plant) was observed at 0X, significantly higher than 1.0X and 2.0X but similar to 1.5X. Interaction effects were non-significant. While herbicides improve growth by reducing weed competition, excessive doses can cause phytotoxicity, reducing productivity (Galon et al., 2015).

CONCLUSION

Elevated CO‚  (eCO2) significantly enhanced groundnut growth, increasing plant height, leaf area, chlorophyll content, nodulation and dry matter production, while elevated temperature (eT) reduced these parameters. The combination of eCO2  + eT partially offset the negative effects of eT. Herbicide application improved growth at moderate rates but suppressed Rhizobium populations and nodulation at higher doses. Groundnut competed better with C4 weeds than with C3 weeds. Under future climate scenarios, higher herbicide rates may be needed for effective weed control, as herbicide efficacy is reduced under elevated CO‚  and temperature. These findings highlight the need for adaptive management strategies to sustain groundnut productivity under changing climatic conditions.

ACKNOWLEDGEMENT

The present study was supported by ICAR NICRA project under Competitive Grants. Sincere acknowledgements for the financial and infrastructure extended in conducting the project.
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.
 
Informed consent
 
All animal procedures for experiments were approved by the Committee of Experimental Animal care and handling techniques were approved by the University of Animal Care Committee.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.

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    Full Research Article

    Impact of elevated CO2 and Temperature on Nodulation and Crop Weed Competition in Groundnut (Arachis hypogaea L.)

    T
    T. Ram Prakash1,*
    0000-0001-6992-7769
    B
    B. Padmaja1
    0000-0002-2594-7554
    M
    M. Vanaja2
    0000-0002-6634-4055
    T
    T. Bala Srikanth Reddy1
    0009-0007-3912-0332
    S
    Shobha Sondhia3
    0000-0002-0776-5546
    M
    M. Prabhakar2
    0000-0002-6899-3390
    J
    J.S. Mishra3
    1AICRP on Weed Management, Professor Jayashankar  Telangana State Agricultural University, Hyderabad-500 030, Telangana, India.
    2ICAR-Central Research Institute for Dryland Agriculture, Hyderabad-500 059, Telangana, India.
    3ICAR-Directorate of Weed Research, Jabalpur-482 004, Madhya Pradesh, India.

    • Submitted27-10-2025|

    • Accepted13-01-2026|

    • First Online 31-01-2026|

    • doi 10.18805/LR-5593

    ABSTRACT

    Background: Rising atmospheric CO2 and temperature due to climate change pose significant threats to agricultural productivity, particularly in semi-arid tropics. These changes can alter weed distribution, population dynamics and crop-weed interactions. Assessing groundnut’s adaptability to changing climates, weed competition and herbicide efficacy is crucial for developing resilient cultivars and effective herbicide management systems to mitigate yield and quality losses, thereby enhancing global food security.

    Methods: A two-year (2022-23 and 2023-24) experiment was conducted in a Carbon Dioxide and Temperature Gradient Chamber (CTGC) at the Central Research Institute for Dryland Agriculture, Hyderabad. The study evaluated groundnut under four climate variables: ambient CO2  + ambient temperature (aCO2 + aT) (396-405ppm), elevated CO2 (eCO2, 550 ± 50 ppm), elevated temperature (eT, +2°C) and combined eCO2 + eT (550 ± 50 ppm and +2°C) , three crop-weed combinations (groundnut + C3 weeds, groundnut + C4 weeds, groundnut + C3 + C4 weeds) and four herbicide rates (0X, 1X, 1.5X, 2X) using a completely randomized design. Growth parameters, soil rhizobium population and nodulation were assessed at 60 days after sowing (DAS).

    Result: Pooled data revealed that eCO2 significantly enhanced groundnut growth and nodulation, with increases in plant height (25%), leaf area (26%), chlorophyll content (12%), Rhizobium population (22%), nodulation (58%) and dry matter production (22%) compared to ambient conditions. Combination of elevated CO2 and temperature also showed positive effects, though less pronounced, while et alone reduced all parameters. Herbicide application at various rates showed suppressive effect on Rhizobium population, nodulation and chlorophyll content compared to no application. However, plant height, leaf area and dry matter production were significantly higher at 1.5 X rate over no application under eCO2 conditions in contrast to 1.0X rate under aCO2 + aT, eCO2 + eT and eT conditions. Groundnut exhibited better competitiveness against C4 weeds compared to C3 weeds or their combination. The findings revealed that groundnut growth and nodulation were significantly enhanced in treatments eCO2 and eCO2 + eT but reduced under eT. Herbicide efficacy declined under eCO2, suggesting the need for higher application rate under future climate scenarios. These findings highlight the importance of adaptive strategies to sustain groundnut productivity under climate change scenario.

    INTRODUCTION

    The global population is projected to reach 8.6 billion by 2030 and 9.6 billion by 2050, placing increasing pressure on agriculture to ensure food security, address climate change and improve soil health (Sharma et al., 2025). Groundnut (Arachis hypogaea L.), is an annual legume with high quality edible oil and easily digestible protein of its seeds (Vasanthi et al., 2015). The crop is cultivated mainly for its seed and it comprises 40-50% oil, 20-30% protein (Naik et al., 2022). It is cultivated on 7.74 million hectares in India, producing 7.8 million tonnes annually (FAO, 2023). It is vital for nutritional security and livestock fodder, especially as global demand for protein-rich diets grows. However, rising atmospheric CO2 levels, driven by human activities, threaten agricultural productivity through climate change, altering weather patterns, increasing temperatures and intensifying pest and disease pressures. Since 1850, CO2 concentrations have risen from 280 ppm to 407 ppm in 2017 (Asha et al., 2017), with projections reaching 421-936 ppm by 2100 (IPCC, Climate change, 2013).
           
    Carbon dioxide in the C3 pathway of photosynthesis promotes growth and productivity and is an example of a positive effect on C3 plants (Pawar, 2025). C3 plants like groundnut generally benefit from elevated CO2, showing improved growth and CO2 assimilation (Laza et al., 2021), but associated temperature rises can reduce yields (Vanaja et al., 2019). Weeds, with their diverse gene pools and physiological plasticity, may adapt better to changing climates, intensifying crop-weed competition (Ziska, 2001; Varanasi et al., 2016). Elevated CO2 can also delay herbicide efficacy (Naidu et al., 2023), while higher temperatures may reduce herbicide effectiveness (Rudell et al., 2023). Understanding these interactions is crucial for optimizing weed control and ensuring food security under future climates. Hence, this study investigates the effects of climate change on groundnut growth, weed competition and herbicide efficacy.

    MATERIALS AND METHODS

    Location of the experimental site

    The experiment was conducted in Carbon dioxide and Temperature Gradient Chamber (CTGC) facility located at ICAR-CRIDA, Hayathnagar Farm, Hyderabad (17°34’54.65"N latitude and 78°59’42.85"E longitude).
     
    CTGC facility
     
    The Carbon Dioxide and Temperature Gradient Chamber (CTGC) facility consists of four chambers, aT + aCO2: Maintains ambient temperature and CO2 levels (396-405 ppm), eT: Elevates temperature by 2°C above ambient using infrared heaters, eCO2: Elevates CO2 concentration to 550± 50 ppm) andeT + eCO2: Combined elevated temperature (+2°C) and CO2 (550±50 ppm).The facility uses solar radiation and heaters to regulate temperature, with cooling pads and exhaust fans maintaining airflow. A Programming Logical Controller (PLC) monitors and controls temperature and CO2 levels, ensuring precise environmental conditions.
     
    Treatment details
     
    The experiment studied groundnut response under three factors:
     
    Climate variables (4 levels)
     
    • aT + aCO2: Ambient temperature and CO2 (396-405 ppm).
    • eT: Elevated temperature (+2°C).
    • eCO2: Elevated CO2 (550 ± 50 ppm).
    • eT+eCO2: Elevated temperature (+2°C) and CO2 (550±50 ppm).
     
    Herbicide rates (4 levels)
     
    • 0X: No herbicide.
    • 1X: Recommended dose (75 g/ha Imazethapyr + 50 g/ha  Propaquizafop).
    • 1.5X: 150% recommended dose (112.5 g + 75 g/ha).
    • 2X: 200% recommended dose (150 g + 100 g/ha).
     
    Crop-weed combinations (3 levels)
     
    • Groundnut + C3 weeds (Parthenium hysterophorus and Alternanthera sessilis).
    • Groundnut + C4 weeds (Cyperus rotundus and Cynodon dactylon).
    • Groundnut + C3 + C4 weeds.
           
    The present experiment was taken under pot culture and popular groundnut cultivar in Telangana K6 was sown on 7-12-2022 and 6-12-2023 during both the years of study. Pot size of (12" diameter, 14" height) were filled with 18-19 kg red sandy loam soil. C3 weed seeds and C4 weed rhizomes were sown/transplanted alongside. Fertilizers (30 kg N + 40 kg P2O5 + 50 kg K2O + 40 kg S/ha) and vermicompost (2 tons/ha) were applied. 75 g/ha Imazethapyr + 50 g/ha Propaquizafop herbicidew as sprayed post-emergence at 4 ml/L (1X), 6 ml/L (1.5X) and 8 ml/L (2X).
     
    Sampling
     
    At 60 DAS, parameters on crop growth and nodule number was recorded, Plant height was measured from the base to the top leaf tip. SPAD readings (chlorophyll content) were taken using a SPAD-502 meter. Rhizosphere soil samples were collected to assess Rhizobium population using Yeast Mannitol Agar Media (Al-Mujahidy et al., 2013). Plants were carefully uprooted, washed, air-dried for 30 minutes and nodules counted. Leaves were separated and leaf area was measured using a LICOR 3100 leaf area meter. Stem, root and leaf dry weights were recorded after oven-drying at 60°C and total biomass was calculated by summing them.
     
    Statistical analysis
     
    Data were analyzed using three-way ANOVA at 5% significance to assess variability among climate variables, herbicide rates, crop-weed combinations and their interactions using WINDOSTAT software.

    RESULTS AND DISCUSSION

    Plant height
     
    Pooled data from two years (Fig 1) showed that groundnut height increased significantly by 25% under elevated CO2  (eCO2, 550±50 ppm) compared to ambient conditions, followed by eT + eCO2 (12.4%) and eT (3.8%). The tallest plants (39.4 cm) were recorded with a 1.5X herbicide dose (Imazethapyr 3.75% + Propaquizafop 2.5% ME), similar to 1.0X but superior to 0X and 2X doses. Plants grew taller with C4 weeds than with C3 weeds or their combination. Naidu et al., (2023) noted that C4 weeds like Sorghum halepense were less competitive than C3 weeds like Euphorbia geniculata under eCO2. The eCO2 + 1.5X herbicide combination produced the tallest plants, outperforming other combinations except eCO2 + 2.0X. Under ambient and eT conditions, plant height increased at 1X but declined at 1.5X and 2X, while under eCO‚  and eT + eCO2, height increased up to 1.5X before declining at 2X. Other interactions were non-significant. Geethalakshmi et al., (2017) and Chander et al., (2023) also reported positive effects of elevated CO2 and temperature on plant height, leaf area and nodulation in soybean.

    Fig 1: Growth parameters of groundnut as influenced by the climate variables, rates of herbicide application and crop weed interaction.



    Leaf area per plant
     
    Leaf area was significantly influenced by all three factors independently, as well as by climate × herbicide and climate × crop-weed interactions (Fig 1). Maximum leaf area was observed under eCO2, being 3%, 10% and 26% higher than eT, eT + eCO2 and aT + aCO2, respectively. Laza et al., (2021) found that long-term exposure to eCO2 (650 ppm) enhanced CO2 assimilation, transpiration efficiency, biomass and pod yield in peanuts. Leaf area was highest at the 1.5X herbicide rate, 10% greater than no application. C4 weeds resulted in significantly higher leaf area compared to C3 weeds or their combination. The interaction effect showed that leaf area with C4 weeds was highest under eT + eCO2. Climate × herbicide interactions revealed that the recommended dose increased leaf area over no application, but higher rates were ineffective under aT + aCO2, eT and eT + eCO2. Under eCO2, 1X did not enhance leaf area, but 1.5X and 2X rates achieved maximum leaf area, suggesting higher herbicide rates may be needed for effective weed control und elevated CO2 and temperature Varanasi et al. (2016) noted that herbicide efficacy is reduced under high temperature and CO‚  conditions.
     
    Leaf chlorophyll content (SPAD readings)
     
    Chlorophyll content in groundnut leaves was significantly influenced by individual factors and climate × herbicide interactions (Table 1). eCO2 resulted in the highest SPAD readings (43.3), significantly surpassing other climate variables, while eT recorded the lowest (35.9). Enriched CO2 increased leaf area, photosynthetic rate and chlorophyll content, as reported by Dey et al., (2017). Similar trends were observed in maize, with eCO2 showing the highest physiological parameters, comparable to eT + eCO2. Herbicide application reduced chlorophyll content, with the highest SPAD (41.4) under no herbicide treatment, significantly higher than 1X, 1.5X and 2X rates. Makarian et al. (2016) also noted high concentrations of herbicide caused a significant decline in leaf chlorophyll content in maize and barley. Chlorophyll content was higher with C4 weeds (41.1) in comparison to the C3 weeds or their combination (40.2 and 39.1, respectively). weeds under elevated temperatures revealed that the C3 weeds compete better in cooler conditions, while C4 weeds adapt to higher temperatures (Upasani and Barla, 2018).

    Table 1: Groundnut SPAD values as influenced by the climate variables, rates of herbicide application and crop weed interaction at 60 DAS.


     
    Soil Rhizobial population
     
    The Rhizobium population (CFU/g soil) was significantly influenced by all three factors and due to their interactions (Table 2). eCO2 increased Rhizobium colonies by 22%, while eT reduced them by 30% compared to ambient conditions. The combination (eT + eCO2) partially offset this loss, showing an 11% increase. Herbicide application negatively impacted Rhizobium colonization, with higher rates causing greater declines. Groundnut with C3 weeds had higher Rhizobium populations (19.7 CFU/g soil) than with C4 weeds (16.4 CFU/g soil) or their combination (18.1 CFU/g soil). Vu et al., (2023) noted that soil microbial populations decreased with increased weed biomass. The highest Rhizobium population was observed under eCO‚  with C3 weeds and in treatments with no herbicide application. Ruan et al., (2023) reported that CO2 enrichment alone accelerated microbial growth, while combined temperature increases delayed it. Raghavendra et al. (2017) reported that herbicide application reduced Rhizobium populations in the soil, primarily because many herbicides are directly toxic to these beneficial microorganisms. In addition, herbicides can alter rhizosphere conditions by affecting root exudation, soil microbial balance and nutrient availability, creating a toxic or  unfavorable environment for Rhizobium survival and activity.

    Table 2: Rhizobium population (CFU/g soil) in soil rhizosphere as influenced by climate variables, rates of herbicide application and crop weed interaction at 60 DAS.


     
    Nodule number
     
    Nodulation, a key indicator of groundnut’s nitrogen-fixing ability, was significantly influenced by climate variables, herbicide doses, crop-weed competition and their interactions (Table 3). Nodulation increased by 58% under eCO2 and 39% under eT + eCO2 compared to ambient conditions but decreased by 23% under eT. Libault (2014) noted similar trends, with eCO2 enhancing nodulation and eT reducing it due to soil moisture changes. Growth, nodule dry weight and nitrogenase activity are inversely related to temperature (Day et al., 1978). Herbicide application reduced nodulation by 7%, 15% and 22% at 1.0X, 1.5X and 2.0X rates, respectively, consistent with Zaidi et al., (2005). The highest nodulation (440 nodules/plant) was observed under eCO‚  with C4 weeds and no herbicide application while the lowest was under ambient conditions with 2X herbicide and C3 weeds. Elevated CO‚  may boost plant growth, but weed competition for resources can affect nodulation outcomes. Climate change modifies plant–herbicide interactions by altering crop physiology, herbicide uptake and effectiveness and weed competitiveness. These changes can either suppress or stimulate nodulation, depending on crop species, environmental conditions and the intensity and type of weed competition (Burul et al., 2022).

    Table 3: Nodulation (No. of nodules/plant) in groundnut as influenced by climate variables, rates of herbicide application and crop weed interaction at 60 DAS.


     
    Dry matter production
     
    Dry matter accumulation in groundnut was significantly influenced by climate variables, herbicide doses and their interactions (Fig 1). The highest dry matter (16.9 g/plant) was recorded under eCO2 (550±50 ppm), followed by eT + eCO2 (15.8 g/plant), while eT resulted in the lowest (13.2 g/plant). Similar increases in biomass under eCO2 were reported by Saha et al., (2012) in pigeon pea and by Vanaja et al., (2019) and Guna et al., (2024) in groundnut and black gram, respectively. Dey et al., (2017) noted a 20.7% and 17.3% increase in photosynthetic rate at 25 and 45 DAS in mung bean under eCO2, enhancing biomass production. Among herbicide doses, the highest dry matter (15.6 g/plant) was observed at 0X, significantly higher than 1.0X and 2.0X but similar to 1.5X. Interaction effects were non-significant. While herbicides improve growth by reducing weed competition, excessive doses can cause phytotoxicity, reducing productivity (Galon et al., 2015).

    CONCLUSION

    Elevated CO‚  (eCO2) significantly enhanced groundnut growth, increasing plant height, leaf area, chlorophyll content, nodulation and dry matter production, while elevated temperature (eT) reduced these parameters. The combination of eCO2  + eT partially offset the negative effects of eT. Herbicide application improved growth at moderate rates but suppressed Rhizobium populations and nodulation at higher doses. Groundnut competed better with C4 weeds than with C3 weeds. Under future climate scenarios, higher herbicide rates may be needed for effective weed control, as herbicide efficacy is reduced under elevated CO‚  and temperature. These findings highlight the need for adaptive management strategies to sustain groundnut productivity under changing climatic conditions.

    ACKNOWLEDGEMENT

    The present study was supported by ICAR NICRA project under Competitive Grants. Sincere acknowledgements for the financial and infrastructure extended in conducting the project.
     
    Disclaimers
     
    The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.
     
    Informed consent
     
    All animal procedures for experiments were approved by the Committee of Experimental Animal care and handling techniques were approved by the University of Animal Care Committee.

    CONFLICT OF INTEREST

    The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.

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