INTRODUCTION Rice is considered as a prime most food intake for about 2.5 billion of World population. It plays a vital role in the Indian agriculture as it is a staple food for more than 70% of population (Devi et al., 2022). It was predicted that 15–20 million hectares of irrigated rice will face water crisis by 2025 (Venkateshwarlu et al., 2022; Wu et al., 2017). In Cauvery delta zone rice occupies a pivotal place as it is acting as a rice bowl for Tamilnadu and Puducherry regions. Here rice is cultivated in irrigated lowland under puddled flooded condition using Cauvery River water. Karaikal, U.T of Puducherry falls under tail end region of Cauvery delta zone. The rice crop cultivated in this region faces late receipt and inadequate supply of Cauvery River water, which is the major irrigation source of this region and further deficit, irregular and frequent failures of monsoons resulted in water shortage leading to steady reduction or decrease in area under rice cultivation in this highly productive region. Therefore, water stress is a major factor limiting rice production that causes a great threat to food security (Fellahi et al., 2013). To reduce yield losses of rice crops in water deficient areas and to increase the overall rice production, rice varieties with greater adaptation to drought stress are essential. Although plenty of studies are reported on drought tolerance of crops, crop improvement in this part is hampered due to several unknown mechanisms involved in respond to drought stress (Aghaei et al., 2017; Zu et al., 2017; Zhu et al., 2016). Drought tolerance is a complex trait associated with number of morphophysiological traits (Ahmed et al., 2021). A judicious phenotypic evaluation may be helpful in direct selection of drought tolerant genotypes with good yield potential. Keeping the known fact in the mind that crop improvement depends on the magnitude of genetic variability and the extent to which the desirable traits are heritable, the present study was aimed to assess the variability parameters among the rice genotypes under normal and drought environments. MATERIALS AND METHODS The plant material includes forty-eight rice genotypes in which 32 advanced breeding lines are medium duration from the AICRIP - Initial variety Trial – irrigated medium (Kharif, 2018). The aim of including advanced breeding lines is to study their performance among each other (i.e., between genotypes) under drought and normal environments. Varieties included in the trial are popularly cultivated in Cauvery delta region. Here the main aim is to compare these varieties between normal and drought environment for their relative performance. Along with these genotypes six drought tolerant lines were also planted. The details of these genotypes are presented in Table 1. The experiments viz., normal and drought environment were conducted simultaneously in two adjacent plots of 20 cents field area at Pandit Jawaharlal Nehru College of Agriculture and Research Institute (PAJANCOA & RI), Karaikal. Forty-eight rice genotypes were sown in three lines per entry under raised bed nursery. Twenty-five days old seedlings were planted in the experimental blocks, where they were equally partitioned to two separate experiments one under normal environment and other under drought environment in randomized block design (RBD) with three replications. Each genotype was planted in three rows with the spacing of 20 × 10 cm within genotype and 30 cm spacing between two genotypes. Both the fields were in puddled condition during transplanting of seedlings. The total amount of rainfall during the crop period was 96.9 cm (IMD, 2018) with dry spell of 4 weeks. The trial is under sufficient water stress during the vegetative period. Vegetative stage drought is more experienced in Cauvery delta zone, in which Karaikal region is most vulnerable for transplanted seedlings stage during late samba because of late receipt and inadequate supply of Cauvery River water. Hence, water stress is imposed after 15 days of transplanting in the drought field while the normal field was irrigated for 5 cm of water depth at frequent intervals. The drought environment was allowed for drying for the disappearance of water till the formation of fine cracks or hairline cracks indicating the moisture level below the soil surface (>15cm) and this condition was maintained up to peak tillering phase (20 days) until the drought symptoms appeared over the crop as reported by Manickavelu et al. (2006) (Fig. 1) while the normal field was kept flooded (Fig. 2). In rice once the plants attain 70% RWC, it indicates real physiological stress of the plant irrespective of environment (Manickavelu et al., 2006). Hence the RWC was taken at 5 days intervals after 2 weeks of draining water. When most of the recorded entries reach RWC of 70% on clay loam soil, then drought scores related traits were recorded. Here we have taken RWC as criteria to predict physiological stress occurrence. Observations were recorded on five randomly selected plants of each genotype per replication in both the experiments for yield component traits viz., days to 50% flowering (DF), plant height (PH), productive tillers (PT), panicle length (PL), grains per panicle (GP), grain weight (GW) and grain yield per plant (GY). Additionally, when most of the genotypes attained 70% RWC level, the scoring of leaf rolling (LR), leaf drying (LD) and leaf senescence (LS) were observed according to Standard Evaluation System adopted for rice (IRRI, 1996) in drought environment. Statistical analysis. Mean, variance and standard deviation were worked out by adopting the standard method suggested by Panse and Sukhatme (1967). The analysis of variance was carried out individually for each environment (Table 2). Pooled analysis of variance was also performed for normal and drought environment to assess the significance of genotypes across the environments, between the environments and interaction of genotypes with environments as suggested by Singh and Chaudhary (1977). The phenotypic and genotypic variances were estimated as per Lush (1940). The phenotypic and genotypic coefficient of variations were estimated using the formula suggested by Burton (1952) and expressed in percentage. Heritability in broad sense was calculated according to Lush (1940) and expressed in percentage. Genetic advance as per cent of mean was worked out based on the formula given by Johnson et al. (1955).RESULTS AND DISCUSSION Mean Performance. Forty-eight genotypes were evaluated for their mean performance of twelve characters viz., seven characters under both normal and drought environment and additional five characters under drought environment. The genotypes Moroboreken and N-22 had exhibited early flowering in normal and drought environments respectively. The shortest genotypes observed are IVT-129/1519 and IVT-132/1535 while tallest genotypes are Moroboreken and Dular under normal and drought environments respectively. Maximum number of productive tillers was produced by TKM 13 and IVT-137/1523 under normal and drought environments respectively. The genotype IVT-134/1560 under normal environment and IVT-138/1502 under drought environment had enlisted for maximum panicle length. Grains per panicle for genotype IVT-130/1530 under normal environment and for genotype IVT-141/1537 under drought environment were recorded high. The genotypes IW Ponni and Moroboreken had registered the maximum grain yield under normal and drought environments respectively. The genotype IVT-105/1528 had shown the highest relative water content under drought environment. The overall mean of these traits of the genotypes were greatly influenced by drought stress. Mean performance data were shown in Table 5 & 6. In the present study, significant grain yield reduction was noticed under drought environment over normal environment condition by considering the overall mean performance of all the forty-eight genotypes. Venkateshwarlu et al. (2022) reported the same trend of yield decline under water deficit condition compared to irrigated situation. The stress prevailed in drought environment reduced greatly the number of productive tillers, grains per panicle, panicle length and grain weight. Days to 50 per cent flowering, plant height and relative water content also reduced under drought environment. This poor performance of the yield contributing traits was responsible for yield reduction realized under drought environment compared to normal environment as reported by earlier workers (Kamoshita et al., 2008; Ndjiondjop et al., 2010; Sandhu and Kumar, 2017; Bhattarai and Subudhi, 2018). However, genotypes which flowered and matured earlier may favored by partial escape from drought and have an ability to complete their life cycle. The decrease in plant height in response to drought stress may be due to decreased relative water content (Arnon, 1972). Sinclair and Ludlow (1985) proposed that RWC was better measure for plant’s water status than thermodynamic state. The visual symptoms which show that the plant is under stress condition are leaf senescence, leaf rolling and leaf drying which differed significantly among the genotypes under drought environment. The genotype DRR DHAN-44 have recorded the highest score of 3 under drought stress and the genotype IVT-102/1557 and IVT-123/1521 have recorded the lowest score of 0 (Table 6). Mitchell et al. (1998) had reported that mean drought score changes with time as a result of the development of plant water deficit and using drought score measured as an indirect selection criterion for grain yield, it is possible to achieve a positive response to selection for grain yield under drought environment. Variability analysis. Variability parameters such as phenotypic variance (PV), genotypic variance (GV), phenotypic coefficient of variance (PCV), genotypic coefficient of variance (GCV), heritability (h2) and genetic advance (GA) were calculated for the traits under study separately for each environment (Table 3 & 4). Large genotypic and phenotypic variation was observed for traits such as productive tillers, grains per panicle and drought scores which indicated that these traits would respond for effective selection programme for their improvement in both the environments. This was in accordance with Mini and Mohanan (2009); Abarshahr et al. (2011). Henderson et al., 1995 have reported inconsistency of genotypic drought score measurements across environments. These results suggest that the genotypes will respond to drought stress differently, as measured by drought score, when the pattern of development of soil-plant water deficit is different. Thus, the genotypic differences in drought score are strongly influenced by the growing environment. Therefore, the environment used for screening genotypic variation of drought score at vegetative state must correspond to the target environment’s wet season. The traits such as plant height and grain weight have recorded moderate genetic variation and this was same as given by Lakshmi et al. (2016). Heritability estimates in broad sense alone do not serve as the true indicator of genetic potential of the genotypes since the scope is restricted by their interaction with the environment. The heritability estimates along with genetic advance would be more useful and valid for phenotypic selection than heritability estimates alone. Further, the heritability in broad sense includes both additive and epistatic gene effects and hence it is reliable to ascertain the worthiness of the trait only if it is accompanied with the genetic advance. High heritability accompanied with low genetic gain indicates the presence of dominance or epistatic effects. In the present investigation, estimates of genetic variability were quantified by the broad sense heritability and genetic advance as per cent mean among other genetic parameters. High heritability combined with high genetic advance was exhibited by traits such as days to 50 per cent flowering, productive tillers, grains per panicle, grain weight, grain yield, leaf senescence, leaf drying and stress percentage under drought environment. This is in accordance with Mahto et al. (2003) for days to 50 per cent flowering, Mini and Mohanan (2009) for productive tillers, Sharma and Sharma (2007) for grain weight, for grain yield and Manickavelu et al. (2006) for leaf senescence, leaf drying and stress percentage. High heritability accompanied with high genetic advance indicates that most likely the heritability is due to additive gene effects and selection may be effective for these traits. The trait plant height had shown high heritability with moderate genetic advance under drought condition which was similar as said by Kumar et al. (2014). Low heritability with low genetic advance was registered for relative water content. The condition of low heritability accompanied with low genetic advance indicates that the character is highly influenced by environmental effects. The heritability for the drought score measurements were moderate to high which was in accordance with Pantuwan et al. (2004). Heritability for overall stress percentage which was derived from drought score in present study was found to be lower than grain yield as reported by Pantuwan et al. (2004).