Screening of groundnut (Arachis hypogaea L.) varieties for drought tolerance using polyethylene glycol

Groundnut (Arachis hypogea L.)  stands as a crucial oilseed crop that contains 47-53% oil and 25-36% protein (Prasad et al., 2010). China ranks first in total annual production (18.3 million tons) and area of (4.4 million ha) whereas India stands first in area (5.7 million ha) and ranks second in production (10.1 million tons) other significant countries in the order of production are Nigeria, United States of America, Sudan and Myanmar. This could be attributed to the effect of drought on crop productivity (Benga, 2020 and Sen et al., 2012) and the crucial role of rainfall in Groundnut production in many countries (Boote and Ketrind, 1990). Although the global area and production of groundnut have seen growth, productivity levels have largely remained unchanged.

Groundnut is grown in rainfed conditions within the semi-arid tropics and is exposed to many abiotic stresses among which drought stress is the major yield-limiting factor. Yield decline caused by insufficient soil moisture has been documented on a global scale (Vorosoot et al., 2003 and Songsri, 2009). The identification and selection of drought-tolerant genotypes are crucial for sustainable agricultural production under water-deficit conditions. Screening techniques using osmotic agents like polyethylene glycol (PEG) provide a controlled environment to simulate  drought stress, enabling the evaluation of genotypic responses to water deficit (Michel, 1973) . 

Polyethylene glycol (PEG) has emerged as an effective tool for simulating drought stress in a controlled manner. PEG, which exists in various forms from viscous liquids to waxy solids, is widely used in plant research to create osmotic stress by lowering cell water potential (Govindaraj et al., 2010). Increasing concentrations of PEG, particularly PEG-6000, have been shown to adversely affect critical growth parameters such as germination rate, root and shoot length, and seed vigor in many crop species (Khodarhmpour, 2011). This makes PEG an essential medium for evaluating drought tolerance and studying plant responses under water-limiting conditions. PEG-induced drought stress screening has been widely used in crop research due to its ability to create consistent and reproducible osmotic conditions without causing toxicity to plants (Hohl and Schopfer, 1991).

In this study a total of 28 groundnut genotypes, including cultivars and advanced breeding lines, were screened for drought tolerance. Key physiological traits such as germination percentage, germination velocity index, and root length were measured, and their percent reduction over control conditions was calculated to identify drought-tolerant and drought-susceptible genotypes. The results provide critical insights into the drought adaptation potential in groundnut, contributing to the development of resilient varieties for drought-prone regions. 

Experimental material

The study was conducted using 28 groundnut genotypes, including both cultivars and advanced breeding lines. TMV 1, a drought-tolerant variety, was used to determine the lethal dose 50 (LD50) concentration of polyethylene glycol (PEG) for subsequent screening.

PEG Screening for LD 50 determination

PEG-6000 was prepared at concentrations of 5%, 10%, 15%, and 20% (w/v) to simulate varying levels of drought stress. Seeds of TMV 1 were surface-sterilized and subjected to these PEG solutions under laboratory conditions. Germination percentage were monitored to determine the LD50, which was identified as 15% PEG-6000.

Screening of genotypes

Following LD50 determination, the 15% PEG-6000 concentration was used to evaluate drought tolerance in the 28 groundnut genotypes. Seeds of each genotype were surface sterilized and placed in a petri plate containing blotter paper imbibed in the 15% PEG solution. At the same time, all the accessions were germinated in distilled water were maintained as the control group. The study was conducted in a completely randomized design (CRD) with three replications per genotype. Each replication consisted of ten seeds per treatment (PEG and control). Germination Percentage (GP), Germination Velocity Index (GVI), and Root Length (RL) were observed in both control and treatment. The percent reduction over the control was calculated for each trait.

The genotypes were classified as drought-tolerant or drought-susceptible based on the mean value of percent reduction for each trait over control. Genotypes with the lowest mean performance for the percent reduction over control were considered as drought tolerant, while those with the highest reductions were considered as drought susceptible.

Poly Ethelene Glycol (PEG) is the most commonly used osmotic agent for simulating drought in different crops. The evaluation of 28 groundnut genotypes under simulated drought conditions using 15% PEG 6000 revealed significant variations in germination percentage (GP), germination velocity index (GVI), and root length (RL) reductions compared to control conditions as presented in table 1. These differences highlight the genotypes variability in drought responses aligns with previous findings, which demonstrate the effectiveness of PEG 6000 in simulating drought stress by lowering cell water potential (Govindaraj et al., 2010).

Germination Percentage (GP)

Traits such as GP serve as critical indicators of early-stage drought tolerance, with lower reductions reflecting enhanced physiological and biochemical adaptations to water scarcity (Khodarhmpour, 2011). The percent reduction in germination percentage ranged from 10.00% in VRI 5 to 100.00% in VRI 3. VRI 5, along with CO 7 (12.50%) and ALR 1 (14.29%), displayed minimal reductions, indicating their potential to maintain seed viability under water deficit conditions. Conversely, VRI 3 exhibited complete failure in germination, making it the most drought-susceptible genotype.

Germination Velocity Index (GVI)

The percent reduction in germination velocity index varied between 19.34% in CO 7 and 100.00% in VRI 3. CO 7 showed the least reduction, reflecting its ability to sustain seedling vigor. Moderate reductions were observed in genotypes like VRI 5 (56.07%) and BSR 2 (59.31%). In contrast, VRI 3 (100.00%) and TMV 7 (97.38%) recorded the highest reductions, emphasizing their vulnerability to drought stress.

Root Length (RL)

Root traits, in particular, play a pivotal role in accessing deeper water reserves, contributing to better drought resilience (Biswasbet al., 2002). Maximum   root   length was observed in the control  treatment  (medium  devoid  of PEG). Root length reduction ranged from 4.00% in CO 2 to 100.00% in VRI 3. Genotypes such as CO2 (4.00%), CO7 (13.91%) VRI 4 (25.00%) exhibited better root growth under simulated drought conditions, suggesting their ability to adapt by promoting root elongation. CO 2 recorded the lowest reduction in root length as represented in Fig.1, emphasizing its superior drought tolerance in maintaining root development. Meanwhile, COG17007 (95.77%) suffered significant reductions as presented in fig. 2, underscoring their susceptibility. Similar results were observed in groundnut genotypes (Abdulmalik et al., 2018).

Table 1. Percent reduction of physiological traits over control under simulated drought conditions in groundnut

(GP- Germination percentage, GVI- Germination velocity index, RL- Root length)

In combination of all the parameters, VRI 5, CO 7 and CO 2 were identified as the most drought-tolerant genotypes, showing the least reduction in germination percentage, germination velocity index, and root length, respectively. Suggesting these  varieties  to  be  more  drought  tolerant  as  they  had better  rooting,   which   could   have   superior   their capability  to  absorb  water  even  under  PEG  induced water stress. Water deficit influenced mostly the number of lateral roots and the variety with a greater increase of its lateral root numbers could  be  considered  a  drought tolerant   variety   (Badiane   et   al.,   2004). Genotypic variation  under  PEG simulated  drought  has  also  been reported   in   tomato,   sunflower   and   cactus   cultures (Mengesha et al ., 2016). This serves the deployment of this current procedure for drought particularly with the groundnut genotypes used in this study.

This study demonstrated that PEG (6000) at a concentration of 15% effectively simulates water stress in groundnut, allowing for the identification of drought-tolerant genotypes. Among the 28 evaluated varieties, VRI 5, CO 7, and CO 2 exhibited the lowest reduction in germination percentage, germination velocity index, and root length respectively, making them the most drought-tolerant varieties. Their ability to sustain seedling vigor and maintain root growth under water stress conditions suggests their potential for cultivation in drought-prone environments. The findings confirm that PEG-induced screening is a reliable approach for assessing drought tolerance in groundnut and can serve as a baseline for future drought tolerance studies. Further validation of these results under field conditions will enhance breeding strategies aimed at improving drought resilience in groundnut.

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