Thermal indices as indicators of soybean phenological development under different sowing windows

Introduction

Crop productivity heavily relies on the specific climate conditions required by each crop. Soybean (Glycine max (L.) Merrill) is predominantly cultivated in tropical, subtropical, and warm-temperate regions worldwide. It is an important rainfed oilseed crop in central India, where its productivity is closely associated with the timing and distribution of monsoon rainfall and the prevailing thermal environment. Throughout various phenological stages, soybeans encounter diverse weather conditions, leading to significant fluctuations in growth and yield. Being a short-day and photosensitive crop, soybeans, thrive in warm and moist climates and responds to changes in the thermal and photoperiodic environment, which consequently influence the duration of vegetative and reproductive phases, flowering, pod development and maturity. Temperature, a crucial weather factor, profoundly influences crop growth, development, and yield. Soil temperature of 15.5 °C and above is ideal for rapid germination and robust seedling growth (Patil et al., 2014) [1] . For most soybean varieties, optimal germination temperature ranges between 26 °C and 30 °C, with a base temperature of 10 °C (Chavan et al., 2018) [2] . Temperature and photoperiod jointly dictate sowing timing and subsequently affect the duration of phenophases, influencing pre-and post-flowering stage development rates. Given these factors, understanding the precise duration of different phenophases and their correlation with yield determinants is paramount for achieving high crop yields. The thermal unit method, commonly used for assessing crop thermal relationships, was employed in this study to investigate the impact of temperature on soybean phenology and yield (Ramteke et al., 1996) [3] . This investigation aims to analyze how accumulated heat units, such as growing degree days (GDD), photothermal units (PTU), and helio-thermal units (HTU), at various phenophases, influence soybean crop performance under different sowing dates. terized by dry winters, hot and dry summers, and moderate rainfall during the monsoon season, with an average annual rainfall of 916.0 mm. The experiment was conducted under rainfed conditions. A split-plot design with three replications was used. Four sowing windows were considered: D 1 = 25 th SMW, D 2 = 26 th SMW, D 3 = 27 th SMW and D 4 = 28 th SMW. The subplot treatments consisted of soybean cultivars MAUS-612, MAUS-725 and JS-20-116. Standard recommended crop management practices were followed and the crop was harvested at physiological maturity.

Materials and Methodology

Experimental site and crop establishment

Weather observations and phenology

Daily weather data throughout the experiment were collected from the agrometeorological observatory of the Department of Agricultural Meteorology, VNMKV, Parbhani. Five plants were selected randomly from each replication (three) to study the different phenological events. The occurrence of different phenophases (number of days required for various growth stages) of the crop was observed on the basis of external morphological characteristics under different dates of sowing. The crop life cycle was divided into ten phenophases: P 1sowing to emergence; P 2 -emergence to seedling; P 3 -seedling to branching; P 4 -branching to flowering; P 5 -flowering to pod formation; P 6 -pod formation to grain formation; P 7 -grain formation to pod development; P 8 -pod development to pod containing full-size grain; P 9 -full-size grain to dough stage; and P 10 -dough stage to physiological maturity.

Computation of agrometeorological indices

Growing degree days (GDD)

The daily GDD would be calculated by taking the mean daily temperature (T max + T min )/2 and subtracting the base temperature of the crop. The GDD was calculated by considering a base temperature of 10 ℃ (Chavan et al., 2018) [2] . The method used for calculating the accumulated growing degree days (GDD) is expressed by the following formula (Rajput, 1980) [4] 𝐴𝑐𝑐𝑢𝑚𝑢𝑙𝑎𝑡𝑒𝑑

Helio thermal units (HTU)

The HTU is defined as the product of total GDD and average daily bright sunshine hours. The sum total of HTU for each phenophase was calculated using the following formula (Rajput, 1980) [4] HTU = ( GDD × BSS )

Where, GDD-Growing degree day BSS - Bright sunshine hours

Photo thermal units (PTU)

The photothermal unit is defined as the product between GDD and day length. Total photothermal units for different phenophases were calculated by using the following equation (Rajput, 1980) [4] :

Photo thermal units (°C day hrs) = ( GDD × D )

Where, GDD-Growing degree days D-Day length in hours

Result and Discussion

Phenological duration under different sowing windows

Sowing time markedly altered the duration of soybean development in both seasons (Table 1). During kharif 2023, the crop required 101 days to reach maturity under D 1 (25 th SMW), compared with 99, 95 and 88 days under D 2 , D 3 and D 4 , respectively. Thus, a delay from the 25 th to the 28 th SMW shortened the crop cycle by 13 days (12.9%). The shortening was distributed across several phenophases. The seedling phase (P 2 ) declined from 23 days under D 1 to 19 days under D 4 , while the branching-to-flowering phase (P 4 ) declined from 5 to 4 days. The later reproductive phases also showed moderate compression.

A similar response was observed in kharif 2024, although the overall crop duration was longer than in kharif 2023. The maturity period decreased from 106 days under D 1 to 96, 94 and 91 days under D 2 , D 3 and D 4 , respectively. The difference between D 1 and D 4 was therefore 15 days (14.2%). The longer duration of D 1 was associated with extended time available for completion of successive phenological stages, while delayed sowing compressed the crop cycle. This response is consistent with the thermal-time concept that crop development progresses according to the prevailing temperature environment rather than calendar date alone. Neog et al. (2008) [5] similarly reported substantial variation in soybean maturity duration and thermal requirements among cultivars and growing seasons. Bhan et al. (2019) also showed that changes in the thermal environment were associated with differences between actual and normal crop duration in soybean.

Weather environment during crop growth

The weather environment associated with each sowing window differed mainly in rainfall amount and distribution and, to a lesser extent, in sunshine duration (

Growing degree days

The accumulated GDD differed clearly among sowing windows in both seasons (Tables 3 and4). The consistent decline in total GDD with delayed sowing was closely associated with shortening of the crop duration. Earlier sowing gave the crop a longer period to accumulate temperatures above the 10 °C base temperature before reaching physiological maturity. Therefore, the higher GDD recorded under D 1 does not mean that the crop required more heat to complete its growth. Instead, it was mainly due to the longer duration of crop growth under the earlier sowing condition. This interpretation agrees with Kaushik et al. (2015) [6] , who observed a progressive reduction in soybean GDD with delayed sowing, and Nath et al. (2017) [7] also found higher GDD under earlier sowing in the Vidarbha region of Maharashtra.

Table 2. Seasonal weather environment associated with the four sowing windows during soybean crop growth. [8] and Kaushik et al. (2015) [6] , who reported greater helio-thermal accumulation under earlier soybean sowing environments.

Photo-thermal units

PTU combines accumulated thermal time with day length and therefore represents the combine influence of temperature and photoperiod. In kharif 2023, PTU was highest under D 1 (20390 °C day h), followed by D 2 (19928), D 3 (19113) and D 4 (17070 °C day h). The reduction from D 1 to D 4 was 16.3%. The largest phenophase-wise PTU under D 1 occurred during P 2 (4139 °C day h), while relatively high values were also observed during P 7 and P 10 . In kharif 2024, total PTU followed the same descending trend as GDD: D 1 (21911), D 2 (19507), D 3 (18986) and D 4 (18325 ℃ days h). The difference between D 1 and D 4 was 16.4%. The highest phenophase-wise PTU was recorded during P 2 under D 1 (5095 °C day h). The greater PTU accumulation under earlier sowing indicates that the crop experienced a longer period of combined thermal and photoperiodic exposure before maturity. The phenophase-wise differences demonstrate that day length modified the amount of PTU accumulated within individual development stages. The present results are comparable with Kaushik et al. (2015) [6] .

Integration of agrometeorological indices with soybean productivity

Considering crop duration, thermal-time accumulation and yield together gives a better understanding of the suitability of different sowing windows than considering any single parameter alone. In the present study, delayed sowing shortened the crop duration in both seasons. Crop duration decreased from 101 to 88 days in kharif 2023 and from 106 to 91 days in kharif 2024 between the 25 th and 28 th standard meteorological weeks. The reduction in crop duration was also associated with lower accumulation of GDD, HTU and PTU, indicating that late-sown crops completed their growth and development within a shorter thermal and photothermal period. The yield results were in agreement with these observations. Pooled seed yield was 1701 kg ha -1 under the 25 th SMW and 1725 kg ha -1 under the 26 th SMW, while it decreased to 1521 and 1353 kg ha -1 un-der the 27 th and 28 th SMW, respectively. Compared with the 28 th SMW, seed yield was 27.5% higher under the 26 th SMW and 25.7% higher under the 25 th SMW. Since the 25 th and 26 th SMW were statistically at par for seed yield, these results indicate that both weeks provided a favourable sowing window rather than suggesting a single fixed sowing date. The response of the varieties also showed that genotype influenced productivity under the prevailing thermal conditions. MAUS-725 recorded the highest pooled seed yield (1823 kg ha -1

), followed by MAUS-612 (1641 kg ha -1 ), while JS-20-116 recorded the lowest yield (1261 kg ha -1 ). MAUS-725 also recorded the highest pooled biological yield (4071 kg ha -1 ) and harvest index (44.8%). However, the interaction effect for seed yield was non-significant. Therefore, the results do not indicate the need to recommend a separate optimum sowing week for each of the varieties tested.

Conclusion

The two-year study clearly showed that sowing time had a major influence on soybean development and productivity under the semi-arid conditions of Parbhani. The delayed sowing from the 25 th to the 28 th SMW progressively reduced crop duration and the accumulation of GDD, HTU and PTU, resulting in lower seed yield. The 25 th and 26 th SMW performed similarly, with pooled seed yields of 1701 and 1725 kg ha -1 , respectively, whereas yield declined considerably under the 28 th SMW (1353 kg ha -1 ). Thus, the 25 th -26 th SMW can be considered a favourable sowing window under the conditions of the present study. Among the tested varieties, MAUS-725 showed better overall productivity, recording the highest pooled seed yield (1823 kg ha -1 ), biological yield (4071 kg ha -1 ) and harvest index (44.8%). The results also showed that thermal indices were useful in explaining the changes in crop duration and development associated with different sowing times.

References

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