Impact of weather variables on rice growth and development in special zone of problem areas

Introduction

Rice serves as the main food source for more than half of the global population, establishing itself as the world's most vital food crop. In Asia, which accounts for 60% of the global population, over 90% of the world's rice is both cultivated and consumed (Fukagawa and Ziska, 2019) [1] . Rice being the staple food for a sizable section of the world's population, climate change has the potential to have a big impact on rice production. According to Krishnan et al. (2020) [2] , it is expected that the average temperature in India will rise by approximately 4.4 degrees Celsius by the end of the twenty-first century. In the medium emission scenario, climate change in India will result in 3-5% reduction in total rice yield, and in the high emission scenario, 3.5-10% reduction. With global rice cultivation projected to increase by only 0.4% in the coming years, the primary factor determining total rice production will be productivity levels across different regions (Palanisami et al., 2017) [3] . Despite advancements in production systems, rice growing remains a risky endeavour in unfavourable environmental conditions due to its susceptibility to weather fluctuations. During the reproductive phase, excessively high temperatures can affect fertilization, the production of grains or fruits, and the viability of pollen (Hatfield et al., 2008; Hatfield and Prueger, 2011) [4,5] . Prolonged exposure to extreme temperatures during the pollination period or early stages of grain or fruit development can lead to a decrease in yield potential. Kuttanad, located in the Indian state of Kerala, spans across the districts of Alappuzha, Kottayam, and Pathanamthitta is renowned for its vast paddy fields and distinctive geological characteristics. This region is one of the rare areas globally where agriculture takes place at elevations ranging from 1.2 to 3.0 meters below sea level. The region has the lowest altitude all over India and is highly vulnerable to floods during monsoon season due to it's peculiar geographic features. To mitigate the negative impacts of high-water table and frequent flooding, rice growing in north Kuttanad is mainly timed to coincide with the Puncha season (Oct/Nov -Feb/Mar), where sowing is initiated after the cessation of south west monsoon. Hence, there are high chances for exposure of the crop cultivated in the area to high temperature during it's reproductive stage. High temperature negatively affects the crop duration, particularly during critical stages viz., panicle initiation and development, anthesis, and seed set, resulting in lower grain yield under heat stress conditions. Further, crop physiological characteristics and yield-attributing parameters are also negatively impacted, which results in substantial decrease in yield. Comprehending the unique meteorological conditions in Kuttanad, located in the designated special zone of problem area of Kerala, it is highly essential to formulate climate smart approaches to alleviate their influence on rice phenology. Many studies have been conducted to understand the effect of meteorological factors on rice growth and development in Kerala. However, limited research exists on the widely grown rice variety, Uma (MO-16), and the new promising variety from KAU, Pournami (MO-23). Therefore, this study was undertaken to examine the influence of weather parameters on rice phenology in special zone of problem areas during the Puncha season, using these promising varieties.

Materials and Methods

A study was carried out at the Regional Agricultural Research Station, Kumarakom, Kerala Agricultural University, using two medium-duration rice varieties, Uma and Pournami, during the Puncha season of 2022-23. The experiment followed a Randomized Complete Block Design. The research station is situated 0.6 meters below mean sea level (MSL) at a latitude of 9°3' N and longitude of 76°3' S, within the Special Zone of Problem Areas. The treatments consisted of five dates of planting for the two varieties, viz., D1-5 th September, D2-20 th September, D3-5 th October, D4-20 th October, D5-5 th November, with four replications. Uma is the leading rice variety in Kerala, accounting for over 60% of the state's rice cultivation area and more than 80% of the acreage in the Kuttanad region. Pournami, the newest variety developed by KAU, is known for its resilience to high temperatures and acidic conditions. In the experimental field, crop management and pest control practices were carried out as per KAU package of practices recommendations. Observations on plant characters were recorded at critical stages. Weather data was collected from the Agromet Observatory at RARS, Kumarakom. The data collected was statistically analyzed to establish a relationship between weather, various growth stages and yield attributes of rice.

Results and Discussion

The study on phenology revealed that crop duration decreased from early planting to late planting in both the varieties. The crop duration decreased from 113 to 102 days in Uma and 115 to 105 days in Pournami (Table 1). September 20 th planting had the highest crop duration, while planting on November 5 th had the lowest crop duration, in both the varieties. Generally, it was found that crop duration tends to decrease from early planting to late planting in both the varieties. The height of the plant was found to be significantly higher for September 5 th planting and October 5 th planting in Uma, while in Pournami, no significant difference was found among crops of different dates of planting (Table 2). This might be due to the low amount of rainfall received during whole growth period of the late planted crop (Fig. 1). Total amount of rainfall received during the entire growing period of September 5 th planting was 683.8 mm for both the varieties whereas, it was only 259.1 mm and 266.4 mm during the November 5 th planting for both the varieties. The findings were consistent with those of Singh et al. (2012) [6] and Safdar et al. (2013) [7] , who reported that plant height was significantly influenced by sowing dates, with early sowing resulting in taller plants compared to delayed sowing.

The leaf area index remained relatively unaffected by varying planting dates in both varieties (Table 3). However, dry matter accumulation showed a decrease in late-planted crops in both varieties (Table 4). Bright sunshine hours, showed significant negative influence on dry matter (Fig. 2) whereas rainfall showed positive correlation with dry matter accumulation (Fig. 3.) The results, indicating decreased dry matter accumulation in late planted crops, align with the findings of Reedy and Reedy (1992) [8] . They asserted that delayed sowing leads to accelerated development stages, with each stage's duration decreasing exponentially as planting is delayed. Studies by Singh et al. (2012) [6] supported the observation of higher dry matter accumulation in early-sown crops. Akram et al. (2007) [9] also reported a reduction in biomass production associated with a delay in planting.

Phenophase duration

Uma exhibited a shorter duration for reaching active tillering compared to Pournami. The number of days required for active tillering increased between the September 5 th and September 20 th plantings but decreased towards later planting dates in both the varieties. Uma took 24-34 days from transplanting to active tillering, while the Pournami variety required 27-37 days for the covering the same period. The minimum temperature, mean temperature, wind speed, rainy days, and evaporation had significant positive effect, whereas the maximum temperature, forenoon relative humidity, afternoon relative humidity, and mean relative humidity had significant negative effect on the duration of phenophases for Uma variety (Table 5). In Pournami, the minimum temperature and rainy days had significant positive influence, while, the maximum temperature, mean temperature, mean relative humidity, and rainfall had significant negative influence on the phenophase duration (Table 6.) Number of days for panicle initiation for Uma and Pournami initially increased from the September 5 th to September 20 th planting and then decreased towards the later planting dates. The number of days for panicle initiation ranged from 47-53 for Uma and 48-55 for Pournami. Both the varieties exhibited significant positive correlation with minimum temperature, mean temperature, wind speed, afternoon relative humidity, and rainfall. Additionally, significant negative correlation was observed with forenoon relative humidity for Uma variety. The number of days taken for attaining booting stage in Uma ranged from 54 to 63 days, while for Pournami, it was 57 to 66 days. Days for booting increased from the September 5 th to September 20 th planting and decreased towards the last planting, for both the varieties. Significant positive correlation with afternoon relative humidity, number of rainy days and evaporation, while, significant negative correlation with forenoon relative humidity and BSS were observed in Uma variety. In Pournami variety, minimum temperature and afternoon relative humidity had significant positive correlation whereas wind speed, forenoon relative humidity and BSS had significant negative correlation. The time taken for heading ranged from 63 to 72 days for Uma and from 64 to 73 days for Pournami. Uma took fewer days than Pournami for attaining heading stage. In Uma and Pournami, days for heading initially increased in the September 20 th planting and then reduced towards the last planting. Uma variety exhibited significant positive correlation with minimum temperature, RH 1, RH 2, and RH mean, in contrast, for the Pournami variety, factors such as minimum temperature, RH 1, RH 2, mean RH, rainfall, and the number of rainy days had a significant positive impact. Meanwhile, maximum temperature, wind speed, and BSS significantly negatively affected the number of days to heading in both varieties. Uma reached 50 per cent flowering in a shorter period compared to Pournami. It took Uma 73 to 80 days after transplanting, while Pournami took 75 to 83 days. The influence of weather parameters on the number of days taken for 50 per cent flowering showed the same trend in Uma and Pournami varieties. Both the varieties exhibited an initial increase followed by a decreasing trend. The two varieties under investigation showed a significant positive correlation with minimum temperature, mean temperature, RH 1, RH 2, RH mean, rainfall, and rainy days, whereas maximum temperature, wind speed, BSS, and evaporation had a significant negative correlation. The duration for Uma to reach physiological maturity after transplanting varied between 102 and 113 days, while for Pournami, it ranged from 105 to 115 days. Both the varieties exhibited an initial increasing trend up to the second planting, followed by a subsequent decrease towards the last planting date. Significant positive correlation was noticed with minimum temperature, mean temperature, RH1, RH 2, RH mean, rainy days and rainfall, while, significant negative correlation was noticed with wind speed and BSS to days for attaining physiological maturity in Uma variety. In Pournami variety, rainfall and rainy days had significant positive correlation, While, wind speed and BSS had significant negative influence on number of days taken for physiological maturity, in both varieties. Maximum temperature showed negative correlation during active tillering to panicle initiation period on duration of phenophases (Fig. 4). The findings aligned well with the study conducted by Rani and Maragatham (2013) [10] . Their research indicated that higher temperature impacts the duration of rice crops, causing them to reach phenological stages earlier, while, accumulating fewer growing degree days. The number of rainy days significantly decreased from September 5 th to November 5 th during the planting to active tillering stage (Fig. 2). This reduction in rainy days may the reason for the shorter growing period observed in late-planted crops. These results are consistent with the research by Sujariya et al. (2020) [11] , who demonstrated that the distribution of rainfall during the rice cultivation growth phase had direct impact on both the growing period and the crop yield.

In general, studies to analyse the relationship between weather variables and crop duration revealed that maximum temperatures, WS (Wind speed), BSS (Bright Sunshine Hours), and evaporation during booting (P3) to physiological maturity (P6) stage had significant negative impact on both the varieties. Minimum temperature, rainfall, and rainy days showed significant positive influence during heading (P4) to physiological maturity (P6) stages.

Yield and yield attributes

The planting dates had a significant impact on grain yield in both Uma and Pournami varieties. September 5 th planting resulted in the highest grain yield for both Uma (4388.8 kg ha -1 ) and Pournami (3715 kg ha -1 ), whereas November 5 th planting recorded the lowest yield in Uma (2118.8 kg ha -1 ) and Pournami (2313.8 kg ha -1 ) (Table 7). This might be due to the slight increase of maximum temperature during critical period of rice like the vegetative stage and maturity stage (Fig. 6). Increasing temperature may speed up the growing period in the late planted crops and lead to accumulation less growing degree days compared to early planted crops. This finding was similar to the result observed by Peng et al. (2004) [12] , who stated that decreased yield is due to increased temperature during critical periods. Jalota et al. (2009a) [13] also reported similar outcomes. The higher grain yield observed in the early planting on September 5 th for both varieties may be due to a favourable relationship with the number of productive tillers per hill, spikelets per panicle, and filled grains per panicle (Table 7). This aligns with the research by Hassan et al. (2003) [14] , which emphasizes that grain yield is influenced by the interaction of different yield components, including the number of productive tillers, spikelets per panicle, and filled grains. The results are consistent with earlier research by Iqbal et al. (2008) [15] and Ram et al. (2004) [16] , indicating that optimal rice yield occurs when the crop is planted early in the season. Varied sowing dates can alter growth periods, impacting biomass accumulation by changing the duration of photosynthetic activity (Yao et al., 2012) [17] . Consequently, the timing of sowing significantly affects grain yield by influencing the overall rice cultivation processes (Mo et al., 2016) [18] .

Conclusion

The study revealed that different planting dates had significant effect on rice phenology. The phenophase duration significantly reduced from early planting to delayed planting in both Uma and Pournami, rice varieties. The significant negative influence of maximum temperature, BSS and evaporation rate during panicle initiation to physiological maturity periods affected the crop duration in both the varieties under study, viz., Uma and Pournami. The reduction in rainfall and rainy days during critical stages of growth also influenced the duration of rice. It was observed that Uma (MO16) variety performed better than the new variety Pournami (MO 23) during the field experiment. The study on phenology revealed that crop duration decreased from early planting to late planting in both varieties. Grain yield was significantly affected by the planting dates in both varieties. September 5 th planting produced the highest grain yield in both Uma (4388.8 kg ha -1 ) and Pournami (3715 kg ha -1 ) varieties, while November 5 th planting produced the lowest yield in Uma (2118.8 kg ha -1 ) and Pournami (2313.8 kg ha -1 ). Planting dates also had a significant impact on yield attributes, following a similar pattern. The study's findings demonstrated that the ideal time to plant the most popular variety Uma and promising new variety Pournami is crucial for any crop to reach its maximum yield potential because it is impacted by a number of meteorological factors. In order to allow the crop to better exploit the weather factors, the study advises early sowing of the crop starting on September 5th instead of late sowing in October/ November.

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