Soybean [Glycine max (L.) Merrill], a major legume crop, is valued for its high protein (30-45%) and moderate oil (15-24%) content. In addition to being a source of essential amino acids, vitamins and nutraceutical compounds, soybean contributes to sustainable agriculture through nitrogen fixation and adaptability to diverse cropping systems. Globally, soybean was cultivated on 130 million hectares with a production of 372 million tonnes during 2021-22. Brazil, USA, Argentina, China and India are the top producers, with India contributing 3% of global production. In India, Maharashtra and Madhya Pradesh states dominate soybean production.
       
Soybean growth is highly sensitive to environmental factors, particularly photoperiod and temperature. As a quantitative short-day plant, soybean’s flowering, morphology and yield are influenced by day length and thermal regimes, which vary with season and location (Han et al., 2006). Soybean crop growth is restricted to particular season especially rainy season in India due to this photo-thermo sensitiveness. To overcome these environmental barriers and extend adaptability, identifying specific genetic traits that modulate flowering time under varying day lengths  are important in legume breeding (Sapey et al., 2021). Due to high protein content and good quality oil, there is ever increasing demand for soybean. This can be achieved by increasing the crop area in the main season of its cultivation, or extending its cultivation to non-traditional areas. Both these are not practically achievable and hence the better alternative will be extending its cultivation to non-traditional seasons i.e., rabi and summer in Indian context. Genotypes differ in their response to different environmental factors existing in different growing seasons (Deshmukh et al., 2019; Mattos et al., 2020), making genotype selection for photoperiod and temperature adaptability essential for stable performance across diverse environments (Tiwari et al., 2016).
       
The global demand for vegetable oils is increasing with limited arable land, expanding soybean cultivation into new regions and seasons are critical. This requires identifying genotypes with stable yield across diverse environments without much effect on the protein and oil content. This study aims to evaluate the response of diverse soybean genotypes to varying photoperiod and temperature conditions grown under three different seasons for phenological, physiological, yield, yield components and quality traits. Further the study helps in identification of soybean genotype(s) with superior adaptability and yield stability across seasons.

The study consisted fifteen soybean genotypes, including MACS 330 (photoperiod-insensitive; Singh et al., 2008), obtained from Agharkar Research Institute, Pune, while the remaining genotypes (Local black soybean, JS 335, JS 93-05, DSb 21, DSb 23, DSb 28, DSb 33, DSb 34, DSb 39, DSb 40, DLSb 1, DLSb 3, DLSb 5 and DLSb 6) were sourced from All India Coordinated Research Project on Soybean, University of Agricultural Sciences (UAS), Dharwad. The experiment was conducted at the Main Agricultural Research Station, University of Agricultural Sciences, Dharwad, during kharif 2022, rabi 2022-23 and summer 2023. Meteorological data (Rainfall, temperature, humidity) were recorded at the station and day length data was obtained from NASA-GISS.
 
Experimental design and crop management
 
A randomized complete block design with three replications was used. Each plot measured 4 m × 1.2 m, consisting of four rows of 4 meter length per genotype, with spacing of 30 cm and 10 cm between rows and plants, respectively. During kharif, genotypes were sown on 17^th June 2022 while, during rabi, the sowing was taken up on 29^th October 2022 and during summer, sowing was on 11^th January, 2023. Recommended agronomic practices were followed to raise good crop. Irrigation was given both under rabi and summer seasons while during kharif season only protective irrigation was provided.
 
Physiological traits
 
SPAD chlorophyll meter reading (SCMR) was measured at 45 DAS using SPAD-502 Plus on third trifoliate leaf from top and mean of five plants was considered for analysis. Chlorophyll a, b and total were estimated adopting DMSO method at 45 DAS using Arnon (1949) method. Growing degree days (GDD) was calculated at flowering and maturity stages using the following formula given by Monteith (1984).

 
Where, 
Tbase =10°C.
 
Phenological and morphological traits
 
Days to initiation and fifty per cent flowering was recorded as days from sowing to first flowering and flowering in 50% of plants in the plot. Days to Maturity was taken as days from sowing to 90% pod maturity. Plant height in cm was measured at maturity in five tagged plants per replication.
 
Quality parameters
 
Seed Protein Content was estimated via Kjeldahl method wherein the nitrogen content was multiplied by 6.25. Seed oil content was measured in each genotype using NMR (MCQ+) at University of Agricultural Sciences Raichur, Karnataka.
 
Yield and yield components
 
Branches per plant, pods per plant were taken at maturity in five randomly tagged plants. Seeds per pod was counted in 10 pods from five tagged plants.100-seed weight (g) was measured from each replication taking random 100 seeds. Seed yield (kg/ha) was recorded from plot yield converting with suitable conversion factor and expressed in kg/ha.
 
Statistical analysis
 
The data on various traits was analyzed season-wise using R software and mean performance of each soybean genotype was compared among the three seasons with assessment of reduction in rabi and summer season over kharif season.

Fifteen soybean genotypes were evaluated to assess their response to different photoperiods and temperature regimes (kharif 2022, rabi 2022-23 and summer 2023). The choice of the experimental time frame was based on the analysis of day length during the crop period for Dharwad location which indicated that kharif (17^th June- 2^nd October) had highest day length (12 h 33 minutes) followed by summer (11^th January-2^nd May, 11 h 56 minutes) and rabi (29^th October-7^th February, 11 h 20 minutes). The variation in the photoperiods during kharif 2022, rabi 2022-23 and summer 2023 has been depicted in the Fig 1. The maximum temperature in the location also varied during kharif (28.3°C), rabi (30.1°C) and summer (33.8°C, Table 1). This is a pre-requisite to assess the role of varityping photoperiod and temperature on the soybean growth and reproductive traits.




       
Significant mean sum of squares attributable to genotypes was observed for physiological traits (SPAD chlorophyll meter reading at 45 DAS, chlorophyll ‘a’, ‘b’ and total at 45 DAS), phenological traits (Days to initiation of flowering, days to 50 per cent flowering and days to maturity), yield components (Plant height, number of branches per plant, number of pods per plant, number of seeds per pod, hundred seed weight), seed yield and seed quality traits (Protein content and oil content) during kharif 2022, rabi 2022-23 and summer 2023 (Table 1). This signifies the existence of genetic variability among genotypes for different seasons (Photoperiods and temperature). The availability of genetic variability is a pre-requisite for further analysis and gives scope for the selection of stable genotypes across seasons. Earlier, Ibrahim (2012); Kandil et al., (2012); Deshmukh et al., (2019); Mandić et al. (2020) and Saicharan et al., (2022) reported significant differences among soybean genotypes for different traits under different seasons.
       
The analysis of per cent change for various traits across the studied genotypes during rabi over kharif season revealed that the highest reduction occurred for seed yield followed by plant height, number of pods per plant, chlorophyll ‘b’ and chlorophyll ‘a’ while, least was exhibited by number of seeds per pod across genotypes (Table 2). The reduction in seed yield can be attributed to insufficient vegetative growth (Fatichin et al., 2009), decreased light interception (Umburanas et al., 2019; Nleya et al., 2020) and reduced growth cycle (Mattos et al., 2020) and reduced photoperiod during rabi (11 h 20 min) when compared to kharif (12 h 33 min). Reduced photoperiod duringrabi caused early flowering which resulted in shorter plants that had lower vegetative growth, fewer pods per plant, smaller seed size and ultimately lower seed yield (Ibrahim, 2012; Umburanas et al., 2019; Mattos et al., 2020). Number of seeds per pod was consistent across seasons implying less effect of photoperiod and temperature on this trait which could be due to no effect of prevailing environmental conditions on ovule fertilization success or embryo retention. The genotype MACS 330 attained maturity at 69 DAS during kharif, 70 DAS during rabi and summer  indicating its photoperiod and temperature insensitivity with respect to days to maturity. Earlier, Kumar et al., (2005) observed similar results in MACS 330 while studying under different photoperiods.


       
During summer, highest per cent reduction was observed for seed yield followed by number of pods per plant, hundred seed weight, plant height and chlorophyll ‘a’ whereas, least reduction was observed for days to maturity and protein content across genotypes during summer over kharif (Table 2). The decrease in seed yield can be ascribed to the prevailing photoperiod (11 h 56 min) and elevated temperatures (average: 33.8°C) between 32 and 37°C during summer, which had a major influence on soybean seed yield (Zheng et al., 2009; Mishra and Cherkauer, 2010; Hu and Wiatrak, 2012; Mandić et al., 2020) and number of pods per plant (Hu and Wiatrak, 2012; Puteh et al., 2013). The reduction in hundred seed weight is attributed to increased temperature, plant’s response to the increased temperature and reduction in seed reserve (Yari et al., 2013; Mandić et al., 2020).
       
Among the genotypes studied, DSb 34 exhibited low per cent reduction during rabi (51.6 %) and summer (50.6 %) over kharif with high seed yield of 1697 kgha^-1 during rabi and 1731 kgha^-1 during summer when compared tokharif(3503 kg/ha). Therefore, DSb 34 might be considered as stable genotype across the three seasons as it is high yielding and exhibited low per cent reduction during rabi and summer over kharif.
       
As far as oil content is concerned, there was per cent increase (4.3%) during rabi over kharif season. The oil content increased due to reduced photoperiod (11 h 20 min) in rabi (Junior et al., 2017). Similarly, there was increased SCMR (3.7 %) during rabi over kharif. Highest per cent increase was exhibited by chlorophyll ‘b’ followed by days to initiation of flowering, days to 50 per cent flowering, SCMR and total chlorophyll during summer over kharif (Table 2). The increase in chlorophyll content could be attributed to higher temperature during summer (33.8°C) over kharif (28.3°C).

The present study demonstrated significant genotypic variability in response to seasonal photoperiod and temperature variations across kharif, rabi and summer seasons. Photoperiod sensitivity notably influenced phenological, physiological and yield-related traits in soybean. Seed yield declined substantially under rabi and summer conditions, primarily due to reduced photoperiod and elevated temperatures, which adversely affected plant height, pod number and seed weight. However, traits such as number of seeds per pod and days to maturity in some genotypes showed minimal seasonal variation, indicating partial or complete photoperiod insensitivity. Genotype MACS 330 exhibited photoperiod insensitivity for maturity duration, while DSb 34 demonstrated yield stability across seasons, making it a promising candidate for cultivation under diverse environmental conditions. Overall, the findings highlight the importance of evaluating soybean genotypes under contrasting photoperiod regimes to identify stable, high-yielding and photoperiod and thermo-resilient genotypes for year-round cultivation.

The present study was supported by University of Agricultural Sciences, Dharwad and National Soybean Research Institute, Indore. We acknowledge the help by these institutes in conduct of the present research.
 
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
 
No animal was involved in the conduct of the research.

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.