Rice (Oryza Sativa) is staple food for more than half of the world’s population.  . About 90% of the global rice is produced as well as consumed in Asia  . Rice supplies more than 80% of the calorie and 75% of the protein consumed by people residing in Asia  . Drought is considered as the major yield reducing factor in rain-fed areas of Southeast Asia, affecting more than 23 million ha area  . In Asia, around 50% of the rice cultivated area is rainfed. Although rapid progress has been attained in breeding of irrigated rice, there is little gain in rainfed rice ecosystem over the last few decades with significantly low yield potential of 1.5 to 2.5 t·ha−1  . Rice is highly sensitive to soil moisture deficit during its critical growth stages, mostly during pre-flowering and grain filling stages. Losses due to drought stress are most severe in south Asian countries mainly in Nepal, India, Bangladesh and Myanmar. In Nepal, rainfed rice accounts for 57% of the total area under rice cultivation in the country. Rice was grown in about 1.42 million hectors of land; approximately 0.8 million hectors are estimated to be drought-prone  . Most of the traditional as well as high yielding varieties cultivated in Nepal are highly susceptible to drought. Farmers are seeking for better cultivars that can perform well under stress condition, and in the eyes of farmers, drought resistant cultivars are the ones that have better yield potential and stability than various other available cultivars specifically under water scarce conditions  . The higher frequency and intensity of drought spells necessitates development of rice cultivars, which can survive under water deficit at critical growth stages and quickly recover after drought, by rapid growth upon the availability of soil moisture  .
Under severe drought stress, yield declines in rice range between 65% - 85% as compared to non-stress conditions  . There are various indices to determine drought tolerance such as stress tolerance level (TOL), stress tolerance index (STI), stress susceptibility index (SSI) and drought tolerant efficiency (DTE) that can be used as an indicator to identify superior cultivars that can perform better in stress-prone environments   . Mean yield and relative yield performance are the most extensively used parameters for selecting genotypes in stress-prone environments comparing the yield and yield potential under the stressed and irrigated conditions   . Pinter et al. (1990)  , and Kumar et al. (2014)  found relative grain yield to be a useful criterion for assessing drought response of wheat genotypes. The ability of any crop cultivars to perform reasonably well in drought stressed environments is noted as a key indicator for crop stability. Different authors have proposed that combined results of high yield stability and high relative yield can be reliable selection criteria for characterizing genotypic performance under varying degrees of moisture stress  . The objective of present study was to identify rice cultivars having high yield potential and stability under drought stress conditions by analyzing drought tolerance indices during its critical growth stages under rainfed condition of western Nepal.
2. Methods and Methodology
2.1. Experimental Details
Research was conducted in Western Nepal at the Farmer’s field around the vicinity of Regional Agriculture Research Station (RARS) Nepalgunj. Geographically, it is located between at 81˚37"E longitudes and 28˚06"N latitude and an altitude of 181 meters above mean sea level. Soil of the research site is sandy to silty loam, poor in organic carbon and available nitrogen but medium in available phosphorus (P2O5) and potassium (k2O) pH varies from 7.2 - 7.5. The field experiment was conducted in Completely Randomized Block Design (RCBD). There were two conditions, fully irrigated and drought. Each condition was replicated 3 times. Each replication consists of ten treatments of plot size 3.6 × 5 m2. Each plot consisted of 18 rows with spacing of 20 cm between rows and 20 cm spacing between plants. There was 0.5 m between blocks and plots.
Irrigated Condition: Ten varieties, Sukha-3 as positive check and Samba Mansuli Sub-1 as negative check were used
Unirrigated (Drought stress) Condition: Ten varieties, Samba Mansuli sub-1 as positive check and Sukha-3 as negative checks were used. The pedigree and yield potential of tested cultivars are presented in Table 1.
Table 1. Pedigree and characteristics of tested cultivars.
*The six generations of selection from F2 bulk IR 55072 (Mahsuri/IR4547-6-2-2). #Samba Mahsuri/IR4983-7-1-2-3//2*samba Mahsuri (Samba Mahsuri*3IR48390-7-1-2-3). Source: CDD and ASoN, 2017)  .
2.2. Selection of Drought Tolerant Indices
Several drought indices have been suggested based on various mathematical relationship between yield under drought stress and non-stressed conditions. These indices vary with resistance and susceptibility of genotypes  . If (Yi)S denotes yield of ith genotype under stress condition, (Yi)NS denotes Yield of ith genotype under non-stress condition, Ys denotes total mean yield under stress condition and YNS denotes total mean yield under non-stress condition. Rosielle and Hamblin (1981)  defined stress tolerance (TOL) as the differences in yield between stress and non-stress environments. i.e. TOL = (Yi)NS − (Yi)S. Higher value of TOL indicates, susceptibility of a given cultivar. Hosssain et al. (1990)  defined mean productivity index MPI as the average of (Yi)NS and (Yi)S. Ramirez, Vallejo and Kelly (1998) defined geometric mean productivity (GMP) as the square root of product of yield under stress and non-stress environment: . GMP is useful and more efficient when breeding objective is directed toward testing performance under favorable vs unfavorable environment, in account with variability in drought intensity and years (Raman et al., 2012). Bouslama and Schapaugh, (1984)  defined, yield stability index, (YSI) as the ratio of mean yield of genotypes under stress environment to that of non-stress environment as, YSI = (Yi)s/(Yi)NS. The value close to 1 indicates higher stability of genotypes under the stress environment as compared to non-stress environment. Fernandez (1992)  defined a stress
tolerance index (STI) as, , this can be used to identify
genotypes that perform well under both stress and non-stress conditions. Fisher and Maurer (1978)  proposed stress susceptibility index (SSI), that assess the reduction in yield caused by unfavourable vs favourable environments, SSI is expressed as [1 − Yi)S/(Yi)NS]/SI where, SI is the stress intensity, estimated as, SI = [1 − (Yi)S/(Yi)NS]. Lower SSI values indicate lower difference in yield across stress levels, i.e. more tolerance to drought. SSI is exclusively used to identify genotypes with yield stability in moisture deficit environment   .
2.3. Temperature and Precipitation
During last few years, the precipitation has substantially reduced. In absence of artificial irrigation and uneven rainfall, rice cultivated in western Nepal faces mid-season drought and late season drought. The monthly temperature and precipitation of Regiional Agriculture Research Center (RARS), Nepalgunj is presented in Figure 1. The weather data of RARS, Nepalgunj reflects the micro climate condition of research site since, research field is located at the close vicinity of research station. During the crop growth period (June to October 2017) the mean maximum monthly temperature was 37.04 during June and the mean minimum monthly temperature was 18.8˚C during October. The rice crop cultivated in western terai belt of Nepal during year 2017 received maximum precipitation during the month of August i.e. 497.8 with gradual decline in temperature and rainfall during Sept. and Oct.
Figure 1. Weather data of RARS Nepalgunj during crop growing period. (Source: RARS Banke, 2017).
2.4. Statistical Analysis
Statistical analysis was carried out using, R-3.3.0. Analysis of variance was performed for randomized complete block design (RCBD). Duncan’s Multiple Range Test (DMRT) was used to compare the differences among key parameters of drought tolerance indices at P < 0.05 level of significance.
3. Result and Discussion
3.1. Drought Tolerance Indices
The drought tolerant indices for ten different rice cultivars are presented in Table 2. Significant difference was observed between the mean grain yield under stress and non-stress condition for all cultivars indicating, considerable difference in performance between two different conditions. The production was substantially higher in non-stress environment for every cultivar as compared to the yield obtained under stress environment. The mean yield ranged from 9.18 t·ha−1 to 6.17 t·ha−1 under the non-stress condition, while it ranged from 5.70 t·ha−1 to 3.71 t·ha−1 under the stress environment. Sukha Dhan-3 was the top performer under the stress condition followed by Sukha Dhan-6 and Radha 4. Although, Samba Mansuli Sub-1 had exceptionally higher yield under non-stress, its yield declined by 60% under stress environment. Loktantra and Bideshwori, other two superior performers under the non-stress environment had 2-fold decline in yield under the stress-environment. The cultivars performing best under non-stress condition performed poor in terms of mean yield under the stress condition, which is agrees with the findings reported by (Kumar et al., 2014 and Raman et al., 2012)   .
Table 2. Mean grain yield and drought tolerance indices of rice cultivars in response to drought stress and non-stress condition.
Least TOL value was recorded in Radha-4 (1.42) followed by Sukha Dhan-1, Sukha Dhan-3 and Sukha Dhan-6, whereas, Samba Mansuli Sub-1 (5.47) and Loktantra (4.76) exhibited higher TOL value. Lower value of TOL (stress tolerance) indicates, higher stress tolerant ability of given cultivar. The results of this study are in good agreement with the earlier findings reported by (Raman et al., 2012; Singh et al., 2018)   .
Stress Susceptibility index (SSI) precisely evaluates the yield reduction caused by unfavorable conditions as compared to favorable environments. Lower the value of SSI, higher will be the yield stability. Cultivars with less fluctuation in mean yield show lower SSI and higher stability in different environment. Sukha Dhan-4 exhibited least SSI with value 0.38 followed by Radha-4 (0.39) and Sukha Dhan-2 (0.40) while, Samba Mansuli Sub-1 had higher SSI (1.09). The SSI of Loktantra and Bindeshwori were close to 1. Cultivars with low SSI values can be considered as drought tolerant (Chauhan et al., 2007) since, they exhibit smaller yield reductions under water stress as compared to irrigated conditions. Based on the value and direction of desirability ranking was done for different cultivars as highly drought tolerant (SSI < 0.5), moderately drought tolerant (SSI: 0.5 - 0.75), drought tolerant (SSI: 0.75 - 0.1) and drought susceptible (SSI > 1). Drought indices: TOL and SSI are useful parameters for identifying genotypes that perform well in stress environment (Kumar et al., 2014; Singh et al., 2018)   . In this current research, cultivars with SSI close to 0.5 showed remarkably better performance for yield and stability under drought stress.
Stress tolerance index (STI) is used to compare the consistency and relative change in yield under stress and non-stress condition. Sukha series of rice and Radha-4 showed higher values of STI as compared to other three varieties. Samba Mansuli sub-1 showed lowest STI value (0.15), which implies that it is highly susceptibility to drought stress. Similar result was obtained from the assessment of yield stability index (YSI) and Yield Index (YI).
3.2. Correlation among Drought Indices and Yield
Grain Yield under the stress condition (Ys) and non-stress condition (Yp) (Table 3) were found non-significant and negatively correlated indicating that cultivars with high yield potential under favorable environment does not anticipate superior yield under the stress environment. The weak association between Yp and Ys was reported in wheat by Anwar et al. (2011)  . Grain yield under the non-stress (Yp) had positive and significant correlation with MP and TOL whereas, association of Yp was significantly negative for STI, YSI and SSI. The grain yield under the stress (Ys) exhibited positive and significant association with GMP, STI, YSI, YI, and SSI. Similar findings for Ys with GMP, STI and MP were reported by Mau et al. (2019)  . Negative correlation was observed for TOL with YSI, STI, GMP and SSI. Raman et al. (2012)  reported similar results for correlation of TOL with YSI, STI and GMP. A perfect positive correlation was exhibited by YSI, STI and SSI among each other. This offers a choice for researcher to choose one among these drought indices and can be used interchangeably during selection. Mau et al. (2019)  had also reported for interchangeable drought indices in rice.
Table 3. Correlation among drought indices and yield (Yp and Ys).
*Correlation is significant at the 0.05 level (2-tailed). **Correlation is significant at the 0.01 level (2-tailed).
This study concludes that, water stress during critical growth stage of rice significantly reduces rice yield in all cultivars. Further, it also indicates that selection based on drought tolerance indices can be efficient tool in identification of superior drought tolerant cultivars with higher yield potential and stability. Based on results assessed from various parameters of drought tolerance indices, it revealed that Sukha series of rice cultivars, and Radha-4 possessed high level of drought tolerance since these cultivars exhibited low SSI and TOL with high STI values. These cultivars showed the highest yield under irrigated environment as well as performed better in drought stress conditions. Thus, these drought-tolerant rice cultivars can be better substitute of traditional, primitive and un-tested varieties in rainfed and drought-prone belts for rice cultivation.