Study of agrometeorological indices in different cotton genotypes

Introduction

Cotton (Gossypium spp.) stands as one of the most important commercial crops in India, with substantial economic significance. In India area under cotton during 2024-25 was 112.94 lakh ha as against 123.70 lakh ha in 2023-24 during the previous year. Among the states, Maharashtra is leading in cotton acreage with 40.86 lakh ha followed by Gujarat (23.66 lakh ha), Telangana (17.70 lakh ha), Karnataka (6.84 lakh ha) and Madhya Pradesh (6.14 lakh ha). (Anonymous, 2025) [1] . Cotton crop is estimated at 299.26 lakh bales as compared to 325.22 lakh tonnes in 2023-24. Among the states, Maharashtra is leading in cotton production with 84.80 lakh bales followed by Gujarat (80.01 lakh bales), Telangana (48.95 lakh bales), Rajasthan (20.42 lakh bales) and Karnataka (18.56 lakh bales). (Anonymous, 2025) [1] . Its growth and development are highly sensitive to temperature, which plays a critical role in regulating physiological processes and determining the timing of phenological events. To evaluate the influence of thermal conditions on crop growth and yield, agrometeorological indices such as accumulated heat units are commonly employed. In Maharashtra, where cotton is cultivated under both rainfed and irrigated conditions, productivity is often affected by climatic variability, soil characteristics, and management practices (Kadam and Dakhore., 2019) [5] . The current study was undertaken to evaluate the performance of Bt and non-Bt cotton genotypes in response to climatic factors, particularly in terms of yield outcomes. Canopy temperature, a key physiological indicator, serves as a useful tool in identifying genotypic tolerance to drought stress. It is influenced by transpiration driven cooling and provides insight into plant water status. A lower canopy temperature relative to ambient air temperature quantified as canopy temperature depression (CTD) is generally associated with better hydration and greater stress resilience (Karunakar et al., 2018) [6] . Understanding how climatic parameters such as temperature, growing degree days (GDD), heliothermal units (HTU), photothermal units (PTU), and canopy temperature affect the performance of different cotton genotypes can enhance selection for thermal use efficiency, particularly under dryland farming systems.

Materials and Methods

A field investigation was carried out at Research field of AICRP for Dryland Agriculture, Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola, during kharif season of 2024-25. Six treatments (cotton genotypes) such as V 1 (Yugank Bt), V 2 (Suraj Bt), V 3 (PKV 081 Bt), V 4 (NH 615), V 5 (AKH 9916) and V 6 (AKH 09-5) were laid out in randomised block design with four replications. The soil of experiment site was clayey in texture. Cotton crop was sown on 3 rd July, 2024. Total rainfall of 881.4 mm was recorded during the crop growing season. Meteorological data viz, rainfall, maximum and minimum temperature, bright sunshine hours and day length were recorded from Agrometeorological observatory, Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola. The Agrometeorological indices such as growing degree days (GDD), photothermal units (PTU), heliothermal units (HTU), thermal use efficiency (TUE) were calculated using following formula:

GDD across each phenophase were added together so as to obtain GDD i.e. thermal requirement in 0 C day during that phenophase in each treatment. Further total GDD or thermal unit requirement over crop period under each treatment was calculated by summation (Waghmare and Singh., 2017). [13] The GDD was computed by summing the daily mean temperature recorded during period of crop growth stages by following formula, Base temperature is the temperature below which the physiological activities in the plant practically ceases and as a result plant does not show any growth. It is considered as 15 0 C for cotton crop (Kadam and Dakhore., 2019) [5] .

Photothermal units were accumulated from date of sowing to particular date of phenophase and from that accumulated PTU for each phenophase and total PTU over crop period in each treatment were computed. Photothermal unit required was computed by formula, (Deshmane et al., 2022) [2] .

Photothermal unit = GDD x day length (hrs)

Heliothermal units were accumulated from date of sowing to particular date of phenophase and from that accumulated HTU for each phenophase and total HTU over crop period in each treatment were computed. Heliothermal unit required was computed by formula, (Prakash et al., 2015) [11] .

Heliothermal unit = GDD x bright sunshine hours (hrs)

Thermal use efficiency (kg ha -1 0 C day -1 ) Thermal use efficiency (kg ha -1 0 C day -1 ) of crops in terms of seed cotton yield and biological yield was computed treatment wise by dividing seed cotton yield and biomass yield by corresponding accumulated thermal units of the treatment (Karunakar et al., 2018) [6] .

Canopy temperature (°C) and canopy temperature depression

Infrared thermometer was used to record the canopy temperature (CT) of crop. The observations were recorded at different phenophases of crop. Canopy temperature depression (CTD) is difference between air temperature and canopy temperature and it is positive (more) when the canopy is cooler than the air (Karunakar et al., 2018) [6] .

Results and Discussion

Growing degree days (GDD)

The data presented in Table 1 indicate that cotton genotypes accumulated an average of 1499 °C days (GDD) during their life cycle, calculated at a base temperature of 15.0 °C. The accumulation of GDD for each phase varied among different genotypes, reflecting genotypic differences in thermal time requirements. Among the different cotton genotypes evaluated, the maximum thermal units were accumulated by Suraj Bt (V 2 ) (1532 0 C day) to complete all phenophases, indicating a longer growth duration compared to other cotton genotypes and this is followed by PKV 081 Bt (V 3 ) (1525 0 C day) and AKH 09-5 (V 6 ) (1525 0 C day). Genotype Yugank Bt (V 1 ) accumulated the least growing degree days. These differences suggest that Suraj Bt (V 2 ) had relatively longer developmental durations or slower physiological progress under similar temperature regimes, possibly contributing to differences in growth and yield outcomes. Genotypic differences in GDD also reported by Waghmare and Singh (2017) [13] and Deshmane et al. (2022) [2] . [12] , Waghmare and Madho Singh (2017) [13] and Kadam and Dakhore (2019) [5] reported that the photothermal units were significantly influenced by different Bt hybrids. These results indicate that Suraj Bt (V 2 ) may require more photothermal energy for completing their phenological stages, which could be associated with extended growth duration, possibly supporting higher biomass accumulation or yield under favorable conditions. The differences in PTU accumulation among the genotypes also reflect their adaptability to specific environments and provide insights for genotype selection based on thermal and light energy use efficiency.

Heliothermal unit (HTU)

Heliothermal units (HTU) required to attain different phenological stages presented in Table 3. Among genotypes, Suraj Bt (V 2 ) required more heliothermal units (7068 0 C day hrs) for the entire crop growth period. It was followed by PKV 081 Bt (V 3 ) (7028 0 C day hrs) and AKH 09-5 (V 6 ) (7028 0 C day hrs).

Comparatively longer growth duration of crop coupled with greater sunshine hours for the respective genotype caused higher accumulation of HTU. Genotype Yugank Bt (V 1 ) accumulated the least heliothermal units. This result is in line with Prakash et al. (2017) [8] and Premdeep et al. (2017) [9] . Thermal use efficiency Thermal use efficiency was computed with respect to seed cotton yield and biological yield production. Thermal use efficiency (TUE), is the conversion of heat energy into dry matter and economic yield (e.g., seed cotton yield or lint yield) and depends on crop type and genetic factors. Relevant data presented in Table 4. The mean thermal use efficiency was 1.06 and 2.82 (kg ha -1 0 C day -1 ), respectively in terms of seed cotton yield and biological yield. Thermal use efficiency was found to be higher in genotype Suraj Bt (V 2 ) with respect to both seed cotton yield and biological yield. It was followed by genotype PKV 081 Bt (V 3 ) and AKH 09-5 (V 6 ). Thermal use efficiency was found to be least in genotype Yugank Bt (V 1 ). This result is in line with Prakash et al. (2017) [8] , Nandini and Sridhara (2019) [7] and Engonopoulos et al. (2025) [4] . Different cotton genotypes exhibit varying abilities to utilize thermal time. High TUE indicates superior physiological efficiency and better adaptation to the thermal environment. This efficiency is critical for understanding the genotypic adaptability and yield potential of cotton under diverse agroclimatic conditions. The use of thermal use efficiency indices offers a foundation for assessing the combined impact of temperature, radiation, and photoperiod on the phenological development of crops. These indices help to determine how efficiently crops utilize heat energy under varying environmental conditions. Total heat energy available to any crop is never completely converted to dry matter even under most favourable agro climatic conditions.

Canopy temperature and Canopy temperature depression

Canopy temperature measurements have been widely used in recent years to study genotypic response to drought. The disparity between canopy and ambient air temperatures can serve as an indirect indicator of plant hydration levels. Cooler CT is favourable for higher yield and this association is stronger and more consistent under heat/moisture stress. Data on canopy temperature (CT) recorded periodically are presented in Table 5. Among the genotypes studied, Suraj Bt (V 2 ) consistently exhibited the lowest canopy temperature (CT) and the highest canopy temperature depression (CTD) across all stages of observation. This indicates a comparatively lower degree of plant water stress at the field scale, suggesting greater environmental stress tolerance in Suraj Bt (V 2 ) compared to other genotypes. These findings align with Karunakar et al.

(2018) [6] , who reported that crop varieties with lower canopy temperatures tend to have higher yields. The superior performance of Suraj Bt (V 2 ) in maintaining cooler canopies under stress conditions highlights its potential for improved adaptation and productivity in challenging environments. Saeed et al. (2025) [10] reported significant differences among genotypes for CTD at all three stages.

Conclusion

Among the genotypes, genotype Suraj Bt took highest GDD, PTU, HTU and Thermal use efficiency for maturity and lowest canopy temperature with high canopy temperature depression and greater tolerance to environment stress with comparative to genotypes PKV 081 Bt and AKH 09-5.

References

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