Introduction

Agricultural productivity is influenced mainly by cultural practices followed and prevailing weather conditions. Rice is the predominant cereal crop in east India including the state of West Bengal. The total cultivated area of rice in India is 43.2 million ha and the country produced 110.2 million tons of rice in 2016-17. West Bengal is the largest rice producing state in India. Almost half of its arable land is under rice cultivation. In the fiscal year 2016, the state produced about 15.75 million tonnes of rice over 5.46 million hectare cultivable area (Anonymous, 2019). Rice requires a mean temperature range of 17 to 33 °C and solar radiation range of 300 to 600 calories cm^-2 day^-1 in its growing areas and seasons. Rice is a water loving plant requiring a heavy rainfall of 125 cm during its vegetative period and an annual rainfall of 100-200 cm (Prasada Rao, 2008). To improve production it is imperative to understand the weather at both macro and micro-levels as weather directly influences the crop growth and development. For higher productivity, rice requires higher light intensity. For enhancing crop productivity its exposure to sunlight and utilization efficiency are important so as to produce dry matter. The rate of photosynthesis is dependent upon the availability of photosynthetically active radiation intercepted by the leaves. The ultimate capacity of a plant to produce dry mater depends on degree of exposure to sunlight and its radiation use efficiency. There is a need to know the microclimate variations particularlyair temperature, canopy temperature, radiation in rice crop for optimum crop production. Hence, the present investigation was carried out to screen suitable planting dates, varieties and irrigation regimes considering optimum microclimatic parameters.

Materials and Methods

2.1.  Study site          

Field experiment was conducted during the kharif (July-October) seasons of 2017 in the ‘C’ Block Farm of Bidhan Chandra Krishi Viswavidyalaya, Kalyani, (Latitude 22^o59’13’’ N, Longitude 88^o27’20’’ E and altitude 10.8 m above mean sea level), West Bengal, India. The experimental site characterized as hot, sub-humid climate. The study site receives an average annual rainfall of 1600 mm out of which 1300 mm (81%) occurs during monsoon period (June to September). May is the hottest with 27.6 to 31.1 ^oC average temperature, whereas the coldest day experiences during January (15.5 to 21.3 ^oC). The soil of the experimental site is Entisol with sandy loam texture.

2.2.  Experimental design and treatments

The experiment was arranged in a split-split plot design with four dates of transplanting [DOT, D₁: 16.06.2017, D₂: 30.06.2017, D₃: 14.07.2017 and D₄: 28.07.2017) as main plot treatment, three rice cultivars (V₁: Nayanmani, V₂: Satabdi (IET4786) and V₃: Swarna (MTU 7029) in sub-plot treatment and two irrigation management (I₁: Optimum and I₂: Deficit irrigation) as sub sub-plot treatment. Size of the individual plot was 4.5×5.0 m² with spacing 20 cm (RxR) and 15 cm (PxP).

In I₁, a continuous water level of 3 cm was maintained up to milking stage. However, in I₂, 150-200 mm water was applied only during transplanting process. Thereafter no irrigation was applied and the crop was grown with the help of rainwater only.

During land preparation, synthetic fertilizer was applied @ 80N (Urea), 60P₂O₅ (SSP), 60K₂O (MOP) kg ha^-1. Full dose of phosphate, potassium and 50% nitrogen were applied at land preparation and rest 50% nitrogen was applied in two equal splits; 21 DAT and before panicle initiation stage.

2.3.  Harvesting

The crop was harvested at physiological maturity stage. The grain yield was recorded after proper sun drying and threshing of the crop.

2.4.  Observation and measurement

2.4.1.  Measurement of microclimatic parameters of the crop stand

The microclimate of a crop stand is largely influenced by photosynthetically active radiations (PAR) interception, relative humidity and canopy temperature within and above crop canopy. Air temperature within canopy has direct effect on physiological process such as photosynthesis, transpiration and respiration. It also determines thermal environment within the crop stand. The measurements of microclimatic parameters were done at 08.00, 10.00, 12.00, 14.00 and 16.00 hours during various phenological stages (tillering, panicle initiation, 1^st Anthesis, flowering, milking, dough and maturity).

2.4.1.1.  Photosynthetically active radiation

The diurnal cycles of photosynthetically active radiation (PAR) was recorded with the help of a line quantum sensor (Model: APOGEE/MQ-301). Measurements of PAR were made placing the line quantum sensor across the row. The incident PAR (PAR₀) was measured placing the sensor horizontally 100 cm above the crop height and the PAR reaching to the ground through canopy (transmitted PAR) was measured by placing the sensor 50 cm above the soil surface horizontally. The reflected PAR from canopy and ground surface was measured by just inverting the sensor towards the canopy and ground surface respectively. The output of the instrument was given in µmol m^-2 s^-1. The output was converted to W m² by a conversion factor of 4.57.

2.4.1.1.1.  Component of the photosynthetic active radiation

The following components of the PAR were calculated as Follows:

(i) Intercepted photosynthetically active radiation (IPAR) was computed as

IPAR=Incident PAR- Reflected PAR-Transmitted PAR ……. (i)

(Dhaliwal et al., 2007)

IPAR (%)=(IPAR×100)/ PAR₀ ……………… (ii)                                       TPAR (%)=(IPAR×100)/ PAR₀……………… (iii)

R_CPAR (%)=(R_CPAR×100)/ PAR₀ …………... (iv)

R_SPAR (%)=(R_SPAR×100)/ TPAR …………… (v)

Where

PAR₀=Incident PAR above the rice canopy

TPAR=PAR transmitted through the rice canopy to the ground

R_CPAR=Reflected portion of PAR from the canopy

R_SPAR=Reflected portion of PAR from the soil

(ii) Absorbed photosynthetically active radiation (APAR)

Absorbed PAR (APAR) was calculated by the equation proposed by Gallo and Daughtry (1986)

APAR=[PAR₀+ PARR_S] – [TPAR+ PARR_C]……….(vi)

APAR (%) (APAR×100)/PAR₀   ^{.............} (vii)

2.4.1.2.  Radiation use efficiency

RUE=(Total grain yield of rice/Total accumulated absorbed PAR)

2.4.1.3.  Canopy temperature

The canopy temperature was measured with the help of infrared thermometer (MODEL: R-tek, RT/SRG550). The infrared thermometer was targeted to the canopy from east, west, north and south side of the each plot at an angle of 45^o. The mean value of the four values considered as the canopy temperature value for each plot. Unit of the recorded temperature was °C.

2.4.1.4.  Statistical analysis

Statistical analysis was done by using the standard procedures of split-split plot design (Gomez and Gomez, 1984) to draw a valid conclusion on sole and interaction effect of all the variables.

Results and Discussion

3.1.  Photosynthetic active radiation

Flowering stage is the most important stage among different crop growth stages of rice, during which is flowering stage, weather elements along with photosynthetic active radiation plays pivotal role towards seed formation, development and finally to yield. In the present manuscript diurnal variation of PAR is being discussed only for flowering stage and variation of other micrometeorological parameter along with PAR is also being discussed considering all the phenological stages.

3.1.1.  Diurnal variation

3.1.1.1.  Flowering stage

Irrespective of all the treatment combination, the higher R_CPAR (4.8%) was recorded at 8 hrs which decrease gradually and reached its lowest level (4.0%) at 12 hrs followed by a steady increase upto 16 hrs (Table 1). Higher angle of incident radiation resulted at higher reflection at morning and afternoon time. R_CPAR from the crop canopy indirectly indicates the physiological condition of the crop. In Table 1, the diurnal variation of R_CPAR clearly shows that it ranged from 3.8 to 5.4% across DOTs. Gates (1981) recorded that reflectance from the ‘high sun’ is lower than the ‘low sun’ in different crop species. Jean et al. (2009); Biswas et al. (2011) and Maji et al. (2015) reported PAR reflection from mung bean, sesamum and potato during early and late hours of the day respectively.On an average the highest magnitude was recorded in D₄ (5.4%) followed by D₁ (4.6%), D₃ (4.4%) and the lowest in D₂ (3.8%). Among the three varieties, the average value of R_CPAR was in the order of V₃ (5.0%)>V₂(4.6 %)>V₁(4.0%). Stress plant under I₂ reflected 20.83% more PAR compare to non-stress plant (I₁).

Table 1: Variation of reflected PAR from crop and soil, transmitted PAR and absorbed PAR during flowering stage of rice under different dates of transplanting, different cultivars and irrigation regimes

Irrespective of all the treatment combination, the diurnal variation of R_SPAR showed that the highest magnitude was observed at 16 hrs (9.0%) and the lowest magnitude was seen at 12 hrs (5.0%) (Table 1). This is due to increases solar elevation angle and Jean et al. (2009), Biswas et al. (2011) and Maji et al. (2015) recorded similar observation in different crops. Variation of reflected soil (R_SPAR) from soil among the different dates of transplanting showed that the peak value was obtained by D₁ (7.6%) followed by D₄ (6.4%), D₃ (6.0%) and the lowest value was seen under D₂ (5.8%). The highest magnitude of R_SPAR among the different varieties was found under V₃ (8.2%) followed by V₂ (6.4%) and lowest in V₁ (5.6%). Among the irrigation regimes, it showed a similar pattern as that of R_CPAR where highest magnitude was recorded in I₂ (6.6%) and the lowest in I₁ (6.4%). Dry soil under I₂ result more reflection of PAR (Table 1).

The diurnal variation showed the highest magnitude of TPAR was recorded at 12 hrs (21.3%) and the lowest magnitude was recorded at 8 hrs (10.1%) (Table 1). Similar observation also recordedin rice by Chakraborty et al. (2017), in mung bean by Jean et al. (2009a, 2010b), in sesamum by Biswas et al. (2011) and in potato by Maji et al. (2015). The transmitted PAR (TPAR) shows an opposing pattern as portrayed by R_CPAR among the different dates of transplanting. The highest (18.6%) magnitude of TPAR was recorded under D₂ followed by D₃ (15.2%), D₄ (11.6%) and the lowest value in D₁ (10.2%).The highest magnitude (18.6%) and lowest magnitude (7.6%) of TPAR was recorded respectively in V₁ and V₃. Similar trend was observed in the values of TPAR as that of R_CPAR and R_SPAR where it peak value was seen in I₂ (15.4%) and lowest value in I₁ (11%) (Table 1).

The diurnal pattern of APAR showed a highest value at 8 hrs (85.7%) and the lowest value at 12 hrs (75.9%) (Table 1). Absorbed PAR determines the amount of radiation absorbed by the canopy. The results in Table 1 shows that the highest APAR was arranged in decreasing order among the different dates of planting as D₁ (86%)>D₄ (83.8%)>D₃ (81.2%)>D₂ (78.6%). Among the different varieties, peak value of APAR was seen under V₃ (88.2%) followed by a decrease in V₂ and V₁ by 7.8% and 17.9% respectively. Amidst the irrigation regimes the peak value was found in I₁ which was decreased by 6.8% in I₂.

3.1.1.2.  Stage wise variation

Temporal variation of absorbed PAR shows an increasing trend from tillering to panicle initiation stage and remain static afterwards till harvesting. The maximum magnitude of APAR (49-88%) was found under D₂ followed by D₁, D₃ and D₄ (Figure 1). Mukherjee et al. (2013) also observed that highest PAR absorption in rice was noted under D₂ (i.e. transplanting in end of July) followed by D₃ and D₁. In general, APAR under deficit irrigation (I₂) showed superiority over optimum irrigation (I₁). The maximum magnitude of APAR (33-87%) was found in deficit irrigation condition (I₂) compare to optimum irrigation condition (I₁). The maximum magnitude of APAR (22-89%) was found under V₃ followed by V₂ (21-84%) and lowest magnitude was noted under V₁ (26-79%).

Figure 1: Variation of absorbed PAR during different crop growth growth stages (P1: Tillering; P2: Panicle Initiation; P3: 1st Antheis; P4: Flowering; P5: Milking); P6: Dough) and P7: Maturity) under different (a) DOTs (b) irrigation regimes and (c) rice cultivar

3.2.  Absorbed photosynthetically active radiation

Photosynthetically active radiation (PAR) is the most important natural resource, which is assimilated through photosynthesis process, and the energy is stored in grain as starch material. More the absorption means more production of photosynthates thus more the dry mater production. Absorption of PAR directly regulates the total biomass production as well as economic yield of any crop. In this study, it has been observed that due to variation in crop duration and magnitudinal difference in radiation intensity, the total amount of PAR absorbed by rice crop during its entire life cycle, varied under different dates of transplanting, variety and irrigation regimes. The amount of APAR was at the lowest level (479 MJ) under D₁, which increased gradually with progress of transplanting time. It was 19 and 20 MJ more under D₂ and D₃ respectively. The magnitude of APAR was at its highest peak (529 MJ) under D₄ (Table 2).

Table 2: Impact of date of transplanting, cultivars and irrigation regimes on total absorbed PAR, grain yield, straw yield and radiation use efficiency (RUE) of rice

The duration of crop did not vary significantly between optimum and deficit irrigation regimes as the crop received good amount of rainfall during entire cropping period. However, the water stress which encountered by the crop grown under deficit irrigation in general matured 3 to 4 days earlier than optimum irrigation condition. Thus, the total APAR value was 14 MJ less under I₂, compared to I₁ (506 MJ). Different crop varieties are distinct from each other in-terms of their varietal traits, growing duration and adaptability to different resources. Among the variety, the lowest (411 MJ) APAR value was noted under V₁ followed by V₃ (454 MJ). Being a short duration crop Nayanmani got minimum days to mature thus it accumulated lowest APAR value. The highest magnitude of PAR was absorbed (633 MJ) by Swarna, which is predominantly a long duration variety (Table 2).

3.3.  Radiation use efficiency

Radiation use efficiency is an important index, which represent the capacity of a crop to utilize the solar radiation. In this study, the RUE value under all the treatment combination ranged from 0.48 to 1.10 g MJ^-1. Irrespective of irrigation regimes and variety, the highest magnitude of RUE (0.98 g MJ^-1) was noted under D₁, which decrease gradually with delay in transplanting time. The RUE value arranged for other DOT is as D₂ (0.79 g MJ^-1)>D₃ (0.64 g MJ^-1)>D₄ (0.58 g MJ^-1) (Table 2). Similar findings have been reported by Chakraborty et al. (2018), Dutta et al. (2011), Ahmed et al. (2008) in rice, Tabarzad et al. (2016) in barley and Sun et al. (2013); Reynolds et al. (2005) in wheat.The total APAR value was gradually increasing with date of planting. Incontrast, completely reverse trend in grain resulted decreasing trend in RUE with delay in transplanting. In general, the crop grown under delayed transplanting faced more daily maximum and minimum temperature during its critical stages compared to early date of transplanting. Exposure to such higher temperature in late transplanting compelled the crop to consume more photosynthates through photorespiration. Thus, even though the APAR was more, but accumulation of photosynthates to the grain was less in delayed transplanting treatments.The highest magnitude (0.82 g MJ^-1) of RUE was noted under optimum irrigation regimes and it was 0.15 g MJ^-1 less under deficit condition.In conformity Chakraborty et al. (2018), Sun et al. (2013), Reynolds et al. (2005) and Gomez- Macpherson and Richards (1995) also found reduction in RUE due to late sowing not only caused by high temperature but also the moisture content in soil and demand for photosynthates during grain filling. Among the varieties, the highest magnitude (1.10 g MJ^-1) of RUE was recorded under Nayanmani, which was 0.40 g MJ^-1 less under Satabdi. The lowest RUE (0.48 MJ^-1) was noted under Swarna.

3.4.  Canopy-air temperature difference

During flowering and milking stage, crop under I₁, was under more water condition which is inferred from lowest negative CATD value compare to I₂. Similarly, Dalil and Ghassemi-Golezani (2012), recorded the largest difference in leaf and air temperature (CATD) under the lowest water supply in maize crop.The CATD value from the Figure 2 showed that except tillering there were always negative CATD value for V₁. In terms of negative CATD value the varieties are arranged in the order V₁>V₂>V₃, which indicate that V₁ was in more comfortable condition compared to V₂ and V₃. In terms of negative CATD value of the DOTs are arranged in the order D₁>D₄>D₂>D₃.

Figure 2: Variation of canopy temperature (CT) and canopy air temperature difference (CATD) under different (a) DOTs (b) rice cultivar and (c) irrigation regimes

3.5.  Grain and straw yield

Irrespective of irrigation regimes and variety, the highest grain yield (4365 kg ha^-1) was obtained when the crop was transplanted on middle of June (D₁). A significant reduction in yield by 644 and 1322 kg ha^-1 were recorded when transplanting was delayed by 15 days (D₂) and 30 days (D₃) respectively (Table 2). The lowest (3023 kg ha^-1) grain yield was recorded when the crop was transplanted during end of July (D₄). Temperatures above 35 ^oC cause different types of heat injury to rice crop, depending on the cultivar and growth stage. Anthesis is the most sensitive stage of rice to high temperatures (Yoshida, 1981) and the heat-sensitive processes of anthesis are anther dehiscence, pollination, pollen germination, and to a lesser extent pollen tube growth (Ekanayakeet al., 1989). Temperatures above 35 ^oC at anthesis can result in up to 40–60% sterility (Krishnan et al., 2011). In our experimentation, the maximum temperature under D₃ and D₄ was higher level up (>37 ^oC) to flowering stage and it was at lower level under D₂ and D₁. Crop grown under D₄ and D₃ produce more number of chaffy grain compare to D₁ and D₂ treatment. Higher air temperature during panicle imitation, flowering stage and grain filling may be the probable causes for having such result. Zhang et al. (2007) have same observation in rice.

Rai and Kushwaha (2008) reported 15.3% more grain yield under 15 June sown crop than 15 July sowing, which might be due to optimum conditions available for growth and development resulted in more storage of photosynthates in the grain in early sown crop. Reduction in grain yield under delayed transplanting owed to reduction in favorable growing period (low air temperature) leading to poor grain filling and low yield. (Gill et al., 2006). Statistically significant variation in yield was recorded between water stress and non-stress treatments. The highest productivity of rice (3915 kg ha^-1) was observed when crop was grown under optimum irrigation condition, which reduced by 21.73% under water deficit condition. In I₂, average CATD (-0.1 ^oC), CADT at tillering (+6.2 ^oC) and flowering stage (-0.8 ^oC) which were more compare to I₁ (Figure 2) and this is the probable cause of lower grain yield in stress condition. Water stress at critical crop growth stages (tillering, flowering) is responsible for achieving lower grain yield under water stress treatment (Basha et al., 2017). Significantly, higher (4495 kg ha^-1) grain yield was recorded under Nayanmani compared to this 33.0% and 34.0% lower grain yield was produced by Swarna and Satabdi respectively because of Satabdi grown under more stress (Average CADT +0.4, tillering (-1.1) and Flowering stage (+8.2) than Swarna and Nayanmani. The effect of CADT on important growth stages showed a negative correlation and great yield reduction with the increased CADT. If the magnitude of CADT is positive the crop in stress. Patel et al. (2001) also reported significant negative correlation with CADT and different growth stages in pigeon pea. Erdem et al. (2006) noticed similar relation in potato.

Irrespective of irrigation regimes and variety, the highest straw yield (7700 kg ha^-1) of rice was obtained under D₄, which reduced significantly and gradually with early dates of transplanting. The lowest (6500 kg ha^-1) straw yield during middle of June (D₁). Straw yield was significantly governed by irrigation pattern. The highest productivity of straw (7570 kg ha^-1) was observed when crop was nurture under optimum irrigation condition (I₁). Scarcity of water forced to lower down the production of straw biomass by almost 13.3%. Due to varietal differences, crops took various time periods to mature. Thus, the total amount of PAR interception is different for every cultivar which determines the crop biomass. Due to this, Swarna produced highest (10,500 kg ha^-1) straw biomass. Rest of the two varieties produced 37% (Satabdi) and 62% (Nayanmani) less straw biomass throughout the crop period.

Conclusion

Rice should not be transplanted beyond 15^th of July in eastern Gangetic plains of West Bengal for better productivity based on CATD, absorbed and reflected pattern of PAR by the rice canopy.

Acknowledgement

The author duly acknowledge the support received from AICRP on Agrometeorology, Director of Research, BCKV.

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