​​Evaluation of Cluster Bean [Cyamopsis tetragonoloba (L.) Taub] Genotypes for Drought Stress Adaptation and its Effects on Yield

Cluster bean [Cyamopsis tetragonoloba (L.) Taub] is one of the potent arid legumes and versatile crop majorly cultivated in India. The crop is primarily grown in the dry habitats of Indian province i.e. Rajasthan, Haryana, Gujarat and Punjab and to limited extent in Uttar Pradesh and Madhya Pradesh (Anonymous, 2014).
       
It is generally grown for seed purpose but the pods are also used as a green vegetable or as a cattle feed after extraction of guar gum. It thrives well on light texture, sandy to sandy loam soils receiving 300-500 mm annual rainfall. It grows upright, reaching a height of 2 to 3 meters with well-developed tap root system. The guar bean is an important source of gum and it has contains galactomannan gum (30-35% extraction) (Wankhade et al., 2017). Thus, guar is one of the potential vegetable crops. However, around the globe, it is mainly cultivated for its seeds.Tender pods of cluster bean are nutritionally very rich and their gum is used in dairy products like ice cream and as a stabilizer in cheese and cold meets processing.
       
Drought is crucial abiotic factor which adversely effects on crop production by affecting one or a combination of biological processes (Fathi and Tari, 2016). Being a drought tolerant crop with a better growth in warm climate, cluster bean is more popular in semi-arid and arid regions of the tropics where other food legumes do not perform as well. To increase the productivity and there is an urgent need to develop high yielding and drought tolerant varieties that can cope up with vagaries of climate fluctuations. On this context, this experiment was framed and it could be definitely help in genome improvement and future breeding program of cluster bean.

The experimental materials comprised of nine accessions of cluster bean (IC116865, IC116869, IC421848, IC311417, IC329038, HG1-2-20, IC369860, IC116866 and IC6869) procured from Central Arid Zone Research Institute (CAZRI), Jodhpur and one cultivar (Pusa Navbahar) from Main Vegetables Research Station, Anand Agricultural University, Anand. The experiment was carried out under the rain out shelter facility available at Department of Agricultural Biotechnology, AAU, Anand and seed were sown during 1^st week of July, 2018 with the maintaining of 45×10 cm spacing. The soil used in the experiment is representative soil of the middle Gujarat region and is locally known as “Goradu”. The texture of the soil is sandy loam with the pH = 7.6. The experiment was laid out in completely randomized design (Factorial) with three replicates.
               
Non-stress (NS) treatment was given by irrigating until maturity whenever soil moisture was depleted to 30% field capacity. And, drought stress (DS), in which all the genotypes were irrigated up to 50% flowering followed by withdrawal of irrigation was imposed. Leaves samples were collected at 15 days after 50% flowering for the assessment. Leaves dried weight (LDW, g), relative water content (RWC,%), chlorophyll content (CC, SPAD value), leaf area (LA, cm²) and stomata per unit area of leaf (STO) were used for the judgment of drought stress of the selected cluster bean genotypes.
 
Morpho-physiological parameters
 
LDW was measured after hot air oven drying at 50^oC until the constant weight was achieved. RWC was estimated by adopting the procedure and equation described by Morgan (1984). The fully expanded leaf sample was used to measure the CC (SPAD 502 meter) and expressed in SPAD value. LA was measured from randomly selected plant in each replication from the third tri leaf from the top with the help of leaf area meter instrument. STO were counted from lower side of top third position leaf from each replication of randomly selected plants using a Neubauer-Improved chamber (0.0625 mm²) in Microscope (Bastidas, 2013).
 
Yield and yield attributing parameters
 
Various yield and yield attributing parameters were recorded at the time harvesting which included plant height (cm) (PH), leaves per plant (LPP), secondary branches per plant (SBPP), primary and secondary root per plants (PRPP, SRPP), length of primary root (LPR, cm), pods per plant (PPP), pod length and width (PL, PW, cm), seeds per pod (SPP), test weight (TW, g) and seed weight per plant (SWPP, g).

Morpho-physiological parameters
 
Data from non-stress and drought stress treatments were compared to assess the effect of drought stress on morpho-physiological parameters and it is depicted in Table 1. The mean square values for morpho-physiological parameters were highly significant (P≤0.01) between the treatments, the genotypes and their interaction (Table 2). Significantly maximum LDW (0.095 g), RWC (80.09%), CC(69.75 SPAD), LA (24.41 cm²) and STO(11.12) was represented in NS condition as compared to DS. With respect to the selected cluster bean genotypes, all the morpho-physiological parameters were recorded significant difference. Maximum LDW (0.308 g), RWC (82.32%), CC (86.92 SPAD), LA (31.18 cm²) and STO (14.56) was found in Pusa Navbahar genotype followed by genotype IC116866.
 

 

       
LDW ranged from 0.038(IC369860) to 0.334(Pusa Navbahar) with mean value of 0.10 for the NS treatment and from 0.033 (IC369860) to 0.282 (Pusa Navbahar) with a mean of 0.08 for the DS treatment. The mean LDW was 16.30% reduced in the DS than the NS treatment. Exposure to drought stress caused reduction in RWC (7.49%), CC (28.50%), LA (32.07%) and STO (36.21%) in the DS treatment. Genotypes Pusa Navbahar and IC369860 showed the maximum and minimum RWC, CC, LA and STO in both the conditions (NS and DS). The significant effect of the treatments, genotypes and their interaction for the various traits indicated that the expressions of the genotypes across the drought stress conditions was not static and non-responsive but rather adaptable. 
       
The acclimation of plants to water deficit is the result of many different physiological and biochemical mechanisms. Under the drought stress condition, the chlorophyll content is reduced due to photo-oxidation, rupture of chloroplast, disintegration of chlorophyll molecules, or impaired chlorophyll biosynthesis. As a result of this, net photosynthesis rate is decreased (Gebeyehu, 2006). In drought stress condition, there is an increase in ion. Available water and water potential are the two main components affecting the water translocation to the actively cell dividing sites (Haider et al., 2018) which reflects on the behaviour of plant growth. Relative water content is a crucial indicator of relative water amount in plant. Under abiotic drought stress condition, the properties and structure of cell altered, unable to maintain the turgor as well as osmotic pressure. As a result, plant root cell are unable to absorb the soil water through osmosis. The reduction of RWC under drought stress was also revealed by Haider et al., (2018). Leaf area is directly facilitated to the amount of photosynthesis. Decreased in leaf area is an early adaptive response in water deficit conditions. This decrease in cell volume results in lower turgor pressure and the subsequent concentration of solutes in the cells and finally plasma membrane becomes thicker and more compressed (Taiz and Zeiger, 2002). This indicates that selection of genotypes under the stress conditions could be more productive for the future breeding and crop improvement programmes.
 
Impact of drought stress on yield and yield attributing parameters
 
Among yield and yield attributing parameters (Table 3), highly significant differences (P≤0.01) between the treatments, genotypes and their interactions were also perceived in all the variables except SPP and SWPP. Average value of PH was 135.21 cm and ranges between 100.68 to 191.72 cm among the genotypes. The yield related parameters like PPP, PL, PW and TW were found significantly maximum in Pusa Navbahar (126.4, 7.23 cm, 0.80 cm and 14.57 g) and lowest was observed in genotype IC369860 (51.90, 4.90 cm, 0.60 cm and 5.76 g) respectively. SBPP ranged from 13.36 to 4.76 with an average of 7.17 among the genotypes. The genotype Pusa Navbahar also showed the highest growth parameters PRPP (2.06), SRPP (19.10), LPR (23.05 cm) and LPP (178.16) as compared to the other genotypes. With respect to the NS and DS plants, NS plant was found significantly maximum growth, yield and yield attributing parameters.
 

       
The mean squares of treatment-by-genotype interaction (Table 4) also showed highly significant differences (P≤0.01). The PH was reduced 26.6% in the DS treatment as compared to the NS with mean value of 156.0 cm for NS and 114.5 cm for DS. Among the genotypes maximum plant height was obtained in Pusa Navbahar 194.9 cm under the NS and 188.6 cm in DS treatment. SBPP ranged from 7.5 to 13.7 with mean of 9.4 for the NS treatment while in DS treatment ranged from 2.0 to 13.0 with mean of 4.9. Due to the drought stress, there was reduction in PPP (34.9%), PL (17.2%), PW (14.7%) and TW (35.9%) than the non-stress conditions. As well as the other growth parameters (PRPP, SRPP, LPR and LPP) were found to reduce in the DS treatment.
 

       
The influence of drought stress on trait expression had been observed variation among the genotypes. Some parameters were found more prominent to drought stress effects than others. This difference could be attributed to differences between genotypes or to the nature of the traits. Under drought stress condition there is less availability of soil moisture which ultimately reflect on photosynthesis and other metabolism in plants which favors the luxurious vegetative growth (Ange et al., 2016). There may be decreased in photosynthesis assimilation and less photosynthate partitioning to the developing grain under the drought stress regimes which thought to be decreased in yield and yield associated parameters (Asfaw et al., 2012). The strong association between photosynthate assimilation and better remobilization of carbohydrates by drought-tolerant genotypes permits them to maintain high test weight irrespective of the moisture content of the soil (Singh, 1995). Therefore, maintenance of higher photosynthesis rates is very much necessary under the water stress conditions which led to significantly increase in higher total dry matter accumulation and seed yield which was reported in groundnut (Nageswara et al., 2001). Genotypic variations in root traits showed the adaptability under the drought stress condition. This indicated that genotypes disclosed a mechanism of dehydration avoidance by increasing or maintaining of water absorption through deep and extensive root systems (Garg, 2004). In chickpea also showed that there was a positive correlation between root length and root density with the seed yield under the water stress condition (Kashiwagi et al., 2006). Therefore, this is a clear indication of selection of genotypes having the deep and extensive root system would be very helpful to increase in seed yield under the drought stress conditions.  

Cluster bean genotypes exhibited well adaptation under drought stress conditions were associated with maximum chlorophyll content, higher relative water content and number of stomata which reflect on the higher dry matter accumulation. As a consequence, genotypes showing drought resistance were recorded significantly higher yield and yield attributing traits and Pusa Navbahar was well suited under the drought stress conditions among the selected genotypes followed by genotype IC116866, which may be good source of drought resistance lines in future breeding and crop improvement programmes.

None.