Screening groundnut (Arachis hypogaea) genotypes for sulphur efficiency

Oil seeds are one of the major sources of fat and protein for human diet and groundnut is the major source of protein (25%) and oil (48%). In India, groundnut is grown in an area of 39.19 lakh ha with the average production of 71.78 lakh tones. In Tamil Nadu, it is cultivated in an area of 346 lakh ha with the production of 8.92 lakh tones. Globally, 50% of the groundnut is used for oil extraction, 37% for confectionary and 12% for seed purpose (Nurezannat et al, 2019). Application of sulphur to all crops particularly to oilseeds increased the pod yield and quality besides the oil content. It is essential for many physiological processes such as synthesis of sulphur containing amino acids viz., cystine, cystein and methionine, vitamins, chlorophyll and few co-enzymes (Coates and Howe, 2007; Tejeswara Rao et al., 2013; Sisodiya et al., 2017; Mahipal Choudhary et al., 2019).
       
It also helpful for the synthesis of glycosides in oilseed crops, promotes nodulation in legumes, N fixation and seed development hence sulphur addition is essential for improving the yield and quality of oilseed crops (Hossain et al., 2006; Jamal et al., 2010). Many researchers reported the positive role of sulphur in increasing the pod yield, nutrient absorption and its availability in soils (Jamal et al., 2006; El-Kader and Mona, 2013; Sisodiya et al., 2017; Abdul Manaf et al., 2017). Wider variability for the application of various sources of sulphur like single super phosphate, sulphate of potash and gypsum was observed in many crops and concluded that all were equally effective in increasing the pod yield, sulphur content and uptake, protein and oil content, oil yield in groundnut (Ramdevputra et al., 2010; Chattopaddhyay, 2012; Abdul Manaf et al., 2017).
       
Sulphur availability to crops more specifically depends on free CaCO₃, pH and organic matter content (Hilal and Abd-Elfattah, 1987; Ramdevputra et al., 2010; Assefa et al., 2014) in the soils. Further the gap between supply and demand for sulphur requirement of crop is also widening due to intensive farming practices with high yielding and high analysis fertilizers, inadequate supply of organic manures and imbalanced application of nutrients which leads to sulphur deficiencies in soils and crops. Applied sulphur also turns into unavailable forms due to unfavourable soil conditions and the use efficiency of applied S hardly exceeds 8-12% in different crops. In this context, screening and selection of sulphur efficient groundnut genotypes having higher yield and sulphur utilization potentials is of practical significance. The S efficient crops posses the ability to utilize applied S effectively and maintain better growth even under sulphur deficient situations while the inefficient genotypes are not able to utilize the applied sulphur sufficiently and resulted in yield loss. It was reported that S content in plants showed 6 to 8 fold differences among the genotypes (Ahmad et al., 2005; Balint et al., 2008; Halil Erdem and Mustafa Bulent Torun, 2017). The differential ability of the crop genotypes to extract sulphur from soil can be exploited for increasing the sulphur use efficiency hence, the present paper deals with how the groundnut genotypes differs in utilizing the applied sulphur and to identify sulphur efficient genotypes by assessing the variation in growth, yield, S efficiency, S content and uptake.

A field experiment was conducted with 19 groundnut genotypes to screen sulphur efficient genotypes on a sandy clay loam soil with two levels of sulphur as gypsum (0 and 40 kg S). The seeds of groundnut genotypes viz., VRI 2, VRI 3, VRI 4, VRI 5, VRI 6, VRI7, VRI 8, CO1, CO2, CO3, CO 6, CO7, TMV 2, TMV 7, TMV 10, TMV 13, BSR 1, ALR3, Local were sown and grown with and without sulphur application. The treatments were replicated thrice in a randomized block design with a plot size of 5 m x 3 m. The recommended NPK fertilizers (25:50:75 kg NPK ha^-1) were applied as urea, DAP and muriate of potash. The experimental soil was neutral in reaction (7.33) with low salt status (0.36 dS m^-1) and organic carbon content (3.8 g kg^-1). The available major nutrient status showed low N (188 kg ha^-1), medium P (13 kg ha^-1) and K (211 kg ha^-1) availability. The soil was deficient in sulphur (8.18 mg kg^-1) and Zn (0.48 mg kg^-1) availability while rests of the nutrients were sufficient in status (Table 1). Necessary plant protection and production measures were carried out, grown to maturity and harvested. Plant growth and yield parameters such as plant height, root length, number of pods plant^-1 and 100 kernal weight were recorded besides registering pod and haulm yield.
 

       
The post-harvest soil samples were collected and analyzed for 0.15 % CaCl₂ extractable S availability using BaCl₂ turbidity method outlined by Williams and Steinbergs (1959). The plant samples collected after the harvest were washed, shade dried then oven dried at 70°C for 48 hours and the dry weight was determined. The dried samples were powdered using Willey mill and digested with 15 ml acid mixture (Nitric acid: Perchloric acid at 5:2 ratio) and used for S analysis spectrophotometrically. The sulphur uptake was computed by multiplying sulphur content in plants and DMP. The sulphur efficiency indices were also calculated using the following formula:
 
Sulphur efficiency (SE %) =   [Pod yield at S₀ conditions /Pod yield at S₄₀ conditions] x100
 
Yield efficiency index =  [Pod yield in S₀ conditions /Mean pod yield at S₀] x [Pod yield at S₄₀ conditions / Mean pod yield at S₄₀]
 
S uptake efficiency index = [S uptake at S₀ conditions / S uptake S₄₀ conditions] x100
 
S harvest index =  [S content in kernal / S content in plant]
 
        (Ahmad et al., 2005; Halil Erdem and Mustafa Bulent Torun, 2017).
       
Based on various indices computed, the genotypes were grouped as S efficient and inefficient responsive and non responsive genotypes. The data generated for all the parameters were subjected to analysis of variance to find out the significance as suggested by Gomez and Gomez (1984). The statistical differences between the means were tested using the least significance differences (LSD) at 0.05 level of probability. Simple relationship between various parameters were also studied and presented.

Growth and yield attributes
 
The response of groundnut genotypes for sulphur addition in terms of growth and yield attributes were recorded at harvest stage and furnished in Table 2. Addition of sulphur increased the plant height, root length, number of pods / plant and 100 kernel weight of all the groundnut genotypes. The values varied from 46.3 to 66.8 cm, 12.5 to 22.7 cm, 12 to 27 nos. and 35.3 to 54.7 g respectively for plant height, root length, number of pods / plant and 100 kernel weight. Higher growth and yield attributes were associated with VRI 6 = CO7 > TMV 7 while the lowest values of all the attributes were observed with local variety. Application of 40 kg sulphur as gypsum along with recommend dose of fertilisers increased the growth and yield attributes of all the groundnut genotypes. Better response of the genotypes for the sulphur application might be due to its involvement in photosynthesis and translocation of photosynthetic products from source to sink. Further increase in root traits with sulphur addition might have also facilitated the absorption of nutrients and improved the metabolic activity within the plants (Subhendu et al., 2005; Patel et al., 2009; Taejeswara Rao et al., 2013; Lali et al., 2017; Sisodiya et al., 2017).
 

 
Pod and haulm yield
 
Significant and positive pod and haulm yield responses for the addition of sulphur were observed with all the eighteen groundnut genotypes which differed widely (Table 3). The mean pod and haulm yield of genotypes varied from 1269 to 2807 kg and 2229 to 2904 kg ha^-1 respectively. Higher mean pod and haulm yield was recorded with CO7 (2807 and 2904 kg ha^-1) followed by TMV 13 (2513 and 2726 kg ha^-1) and VRI 6 (2422 and 2863 kg ha^-1). Application of 40 kg sulphur as gypsum significantly increased the pod and haulm yield of groundnut genotypes by 16.0 and 18.2 per cent respectively over no sulphur addition. Improvement in both pod and haulm yield of genotypes might be attributed to the positive effect of sulphur on growth, partitioning and translocation of metabolites to reproductive structures (Tathe et al., 2008; Badawy et al., 2011 and Sarangi and Lama, 2013; Abdul Manaf et al., 2017). The lowest pod yield was noted in the local variety (1269 kg ha^-1) and haulm yield was registered with VRI 4 (2229 kg ha^-1), which might be due to lesser photosynthetic rate as well as poor nutrient acquisition and utilization by these genotypes. The variations among different genotypes might be due to genetic makeup as well as the environmental conditions.
 

 
Yield efficiency indices
 
The yield and S uptake efficiency indices were worked out for all the genotypes to identify the S efficient genotypes and furnished in Fig 1 and Table 4. The yield efficiency varied from 71.7 to 97.1 per cent and more than 90 per cent yield efficiency was noticed with the genotypes: VRI 6 > TMV 7 > CO7. With regard to sulphur uptake efficiency, higher uptake efficiency was recorded with VRI 6> TMV 7 > VRI 5 which varied from 62.8 to 82.2 per cent and the lowest yield and S uptake efficiency was registered with the local variety. The relationship between sulphur content in kernals, yield and uptake efficiency of groundnut genotypes also varied significantly and showed 75.9 per cent variation in yield and 65.3 per cent variation in sulphur uptake by the genotypes (Fig 2).
 

 

 

       
Based on higher yield and S uptake efficiencies, the genotypes viz., VRI 6, CO 7, TMV 7, TMV 13 were grouped as efficient and responsive to sulphur fertilization, CO2, VRI 8, VRI 3, VRI 5, ALR 3, BSR 1, CO 6 were inefficient but responsive to S addition and local, CO 1, VRI 4, CO3, VRI 7, TMV 2, TMV 10 were inefficient and non-responsive genotypes for S addition. The efficient genotypes miight have modified their root environment by the secretion of root exudates to utilize the soil sulphur which depends mainly on sulphur use efficiencies, its absorption, transport and concentration in kernals. Similar response was reported by Ahmad et al., (2005) in mustard, Abdul Manef et al., (2017) in groundnut and by Halil Erdem and Mustafa Bulent Torun (2017) in wheat genotypes.
 
Sulphur uptake and availability
 
Application of sulphur as gypsum significantly increased the S content and uptake by all the genotypes and reported in Table 3. The sulphur content varied from 0.19 to 0.87 per cent in kernel and 0.23 to 0.67 per cent in haulm while the S uptake values ranged from 1.73 to 14.9 kg ha^-1 in kernel and 5.10 to 22.1 kg ha^-1 in haulm. The highest sulphur content and uptake was observed in the genotypes: VRI 6 > TMV 7 > CO 7 > TMV 13 for kernel and TMV 7 > VRI 6 > CO 7 > TMV 13 for haulm. The lowest sulphur content and uptake in kernel and haulm was registered in local variety followed by VRI 4. This trend might be due to increased growth and yield attributes, total dry matter production and yield due to sufficient supply of sulfur which helped in better absorption and translocation (Singh and Chaudhari, 1995,1997; Sisodiya et al., 2017). The increased sulphur availability might have also influenced the photosynthetic rate, increased protein content which leads to greater synthesis of sulphur containing amino acids and might have resulted in more uptake of sulphur and higher sulphur use efficiency (Abd EL-Kader and Mona, 2013; Assefa et al., 2017; Mahipal Choudhary et al, 2019).
       
The post- harvest soil samples were also analyzed for available sulphur status and reported in Table 3. Application of 40 kg S as gypsum increased the sulphur availability in soil and the values varied from 7.32 to 14.8 mg kg^-1. Higher S availability was observed with TMV 7 (9.45 and 14.8 mg kg^-1) in the soils applied with and without sulphur which was on par with CO 7 (9.15 and 14.3 mg kg^-1) and VRI 6 (9.25 and 13.6 mg kg^-1). The increased soil availability of sulphur might be due to external S supply through gypsum and concurrent increase in soil which was observed by Pandya and Bhatt (2008), Ramdevputra et al., (2010) and Humair Ahmed (2017) in groundnut. The lowest S availability was noted in control plots.

Screening of groundnut genotypes for sulphur efficiency on a sandy clay soil showed considerable variation in growth, yield, sulphur uptake and use efficiency. Based on higher kernel yield, sulphur absorption and uptake efficiency, VRI 6, CO 7, TMV 7, TMV 13, VRI 5, VRI 3, CO 2 and CO6 were categorized as sulphur efficient and responsive genotypes while the genotypes  Local, VRI 4, CO 3, VRI 2, VRI 7, TMV 2, ALR 3, BSR 1 were found inefficient and non responsive. The efficiency was confirmed with the positive and significant relationship exists between sulphur absorption and translocation efficiency and higher kernel yield. From the results, it can be concluded that, the genotypes VRI6, CO7, TMV7, TMV13, VRI5, VRI3, CO2 and CO6 were categorized as sulphur efficient and responsive hence can be used for low S soils and S bio-fortification.