Solar radiation use efficiency of pigeon pea (Cajanus cajan L.) in relation to crop geometry and varieties

Pigeon pea (Cajanus cajan L.) is commonly known as red gram or arhar or tur. This pulse crop is dominantly grown during the kharif season both as a sole and intercrop under wide range of agro-ecological situations. It is known to produce reasonable quantities of food even under unfavorable production conditions mainly due to its qualities such as drought tolerance, nitrogen fixation, and deep root system (Saxena et al., 2010). Among the different agronomic practices limiting the yield, choice of a suitable geometry and population for a particular genotype is one of the important factors (Mallikarjun et al., 2014). The potential yield of genotypes within its genetic limit is set by environment where it is grown. Genotypes are different in their yield potential depending on many complex physiological processes taking place in different parts of the plant, which are controlled by both genetic makeup of plant and the environments (Chandrakar et al., 2015). Adaptation of proper planting geometry to a particular genotype will go a long way in making efficient use of limited growth resources and thus to stabilize yield. However, yield of pigeon pea is attributed to the non-availability of improved cultivars that are sensitive to the crop and land management practices, in addition to pests and diseases with change in the climate for a short period. There is need to diversify the production systems of pigeon pea which requires knowledge of its yield potential, production risks and variation in climatic environments (Robertson et al., 2001). Planting density and row spacing are powerful management factors. This study emphasises on response of pigeon pea varieties under different crop geometry in accounts of its growth, solar radiation and radiation use efficiency.
        
A field experiment was carried out during kharif season (2015) at Punjab Agricultural University, Ludhiana. The experiment was taken of four replications with two varieties viz. AL 201 (determinate) and PAU 881 (indeterminate) and five row spacings. The treatment allotted plots were arranged under factorial randomised block design (RBD). The experimental data was analysed out by using the CPCS 1 software. The experimental area had sandy loam soil, low in organic carbon and nitrogen, medium in available phosphorus and low in potassium. The gross plot size was 6 m x 5 m. The recommended and uniform seed rate of both the varieties (AL 15 and PAU 881) was used for the sowing of crop. The seed treatment was given before sowing and irrigation was applied at time of flowering. Accumulation of rainfall during crop season was about 587.2 mm. The data on number of days to attain various phenological stages was determined visually through selection of five plants in all the plots from emergence to maturity.
        
 
To calculate the radiation use efficiency (Kumar et al., 2008) following formula was used:

 
            RUE = yield/accumulated PAR (kg ha^-1Mj^-1)

 
The canopy temperature and photosynthetically active radiations were recorded with the help of infra red thermometer and line quantum sensor.
        
Acummulated PARI calculated by the summation of PARI (Cumulated Radiations absorbed by the crop) during growth period of the crop upto the harvest.
 
 
Canopy temperature
 
The data presented in Table 1 depicts that crop with narrow row (45 cm) spacing showed higher canopy temperature than all other wider row spacings (50 cm, 60 cm, 75 cm, 90 cm). Narrow row spacings (45 cm x 25 cm) recorded significantly higher canopy temperature at 60, 90, 120 DAS and at harvest followed by row spacing 50 cm where canopy temperature was found statistically significant up to 60 DAS, but were at par with other row spacings. The narrow row spacing (60 cm) at 60, 90 and 120 DAS had higher canopy temperature than other wider row spacings i.e. 75 cm and 90 cm and less canopy temperature than narrow row spacing of 50 cm which was significantly better than all other row spacings. In genotypes, the canopy temperature observed in PAU 881 was lower than genotype AL 15 except at 30 DAS of the crop. The genotypic behaviour of both the genotypes PAU 881 and AL 15 (indeterminate and determinate) of pigeon pea during initial stage was different i.e. up to 60 days of sowing and later on determinate genotype (AL 15) developed better canopy structure, resulted in higher canopy temperature than indeterminate genotype (PAU 881). The increase in canopy temperature due to more competition of resources like soil moisture, nutrient and light by the plants for its growth and development which further effect the assimilation of resources in plants.  Similar results were reported by Patel et al., (2001).
 
Seed and stover yield
 
60 cm row spacing (60 cm) recorded higher value of vegetative and reproductive growth parameter of individual plants over narrow spacing of 45 cm and 50 cm. 60 cm row spacing (1492 kg ha^-1) produced significantly higher seed yield than 45 cm (1286 kg ha^-1), 50 cm (1324 kg ha^-1), 75 cm (1206 kg ha^-1) and 90 cm (1107 kg ha^-1). Genotype AL 15 (determinate) resulted more yield than the genotype PAU 881(indeterminate) due to uniform growth behaviour during vegetative and reproductive phase of pigeon pea. Each genotype has its own yield potential which expressed in shape of plant growth and ultimately to seed yield. Similar findings have also been reported by Pahwa et al., (2013).
        
Among the planting geometry, significantly higher stover yield was recorded from the 60 cm (4441 kg ha^-1) planting geometry, while the lowest stover yield (4039 kg ha^-1) was recorded in wider spacing 90 cm planting geometry. Narrow spacing was statistically found superior over wider spacing might be due to the fact that optimum plant population over wider spacing and this optimum plant population neutralized the effect of vegetative and reproductive parameter registered in wider spaced crops. Similarly, due to better and uniform plant stand and growth habits of determinate genotype AL 15 gave significantly higher stover yield than indeterminate genotype PAU 881.
 
Seed and stover solar radiation use efficiency
 
The concept of radiation use efficiency (RUE) has great potential for the predictions of crop productivity. The incoming solar radiation and PAR are practically easy to measure. Table 2 represents the highest seed (RUE) was found in crop geometry; 60 cm x 21 cm (2.11 kg ha^-1 Mj^-1) while 50 cm x 25 cm (1.87 kg ha^-1 Mj^-1), 75 cm x 17 cm (1.84 kg ha^-1 Mj^-1) and 45 cm x 27 cm (1.82 kg ha^-1 Mj^-1) was almost similar with each other. The lowest seed (RUE) was seen in 90 cm x 14 cm (1.76 kg ha^-1 Mj^-1). The trend variation was due to more spacing adjustments were available to plants to flourish themselves without any kind of hard competition for available resources. Likewise, higher stover radiation use efficiency (6.41kg ha^-1 Mj^-1) was found in wider spacing 90 cm x 14 cm while the lowest stover radiation use efficiency was shown by 45 cm x 27 cm ( 5.88 kg ha^-1 Mj^-1). 75 cm x 17 cm (6.30 kg ha^-1 Mj^-1) and 60 cm x 21 cm (6.41 kg ha^-1 Mj^-1) accounted for nearly same radiation use efficiency. Similar results were found by Yahuza (2011). As the row spacing wides, the stover (RUE) had moderately increased than narrow row spacing. The reason for more stover radiation use efficiency was full interception of light at this spacing which ultimately enhances more green area as compared to rest of the treatments.

The crop varieties having different genetic make-up as well as growth behaviour plays a significant role in the translocation of photosynthates from source to sink i.e. formation of dry matter which cause yield variation. Optimum crop geometry (60 cm x 21 cm) may also supple ments the crop plants by efficient solar radiation interception.