The root system is a key driver of overall plant growth and development, playing a vital role in maintaining plant health and productivity. The design of the root system affects how well plants adjust to drought stress. A variety of structural characteristics make up the root system architecture (RSA), including the density and length of root hairs as well as the number and length of main and lateral roots. Under water-limited conditions, these characteristics show flexibility and may be essential for creating crops with effective root systems that can withstand drought (Ranjan et al., 2022). Under drought conditions, the root system becomes crucial for enabling water uptake and maintaining the requisite water status in plant tissues, serving as a vital mechanism for drought avoidance. Root plasticity is governed by morphological and architectural traits that allow plants to adapt to water scarcity, directly influencing the maintenance of grain yield (Lucob-Agustin et al., 2021).

Rice performs optimally in irrigated environments but is highly vulnerable to various stresses, among which drought poses serious threats. Drought significantly hampers growth of root and shoot of rice that drastically reduces yield (Reddy et al., 2023). Creating rice cultivars that can thrive in less-than-ideal environments has grown more crucial as climate change worsens. Plants react to drought stress through intricate processes that control oxidant balance, membrane integrity, and water relations (Schneider and Asch 2020). To cope with drought, rice activates adaptive strategies, including drought avoidance through stomatal closure, and developmental plasticity to escape drought. The ability to tolerate drought is executed through the accumulation of protective compounds such as major macromolecular osmolytes and functional proteins, stress specific amino acids and other antioxidant scavengers (Tiwari et al., 2017). Moreover, root architecture is essential for efficient water and nutrient absorption and acts as a critical defense mechanism against both abiotic and biotic stresses (Bisht et al., 2019; Hu et al., 2018). Water productivity (WP) is a crucial factor in addressing the high water requirements of a flood-dependent crop like rice. Efforts to enhance WPaim to reduce water consumption and promote more sustainable rice cultivation practices.

Understanding the incidence of water shortage events, particularly their intensity, duration, and timing in relation to rice phenology, is crucial for addressing drought. Plant growth and development are significantly influenced by the root system, and the architecture of the root system is thought to be the primary characteristic associated with drought tolerance in plants (Guimarães et al., 2020). The dynamics of root development are essential for plant adaptability in situations of severe water scarcity. The adaptability of the root system to changing environmental circumstances is similarly significant (Lu et al., 2020). Polyethylene glycol (PEG) is widely used osmolyte to mimic drought stress in different plant systems (Murillo-Amador et al., 2002). The ability to regulate the osmotic potential in hydroponic nutrient solutions allows for the controlled production of water scarcity, which makes it perfect for experimental studies. Under PEG6000-induced osmotic stress, rice plants exhibit changes in root system architecture, including enhanced root elongation and deeper root growth, as an adaptive mechanism to improve water uptake and sustain productivity during drought conditions (Matsunami et al., 2020). PEG is effectively employed in hydroponic systems to lower osmotic potential at relatively low temperatures. When incorporated into hydroponic solutions, PEG induces osmotic stress, reducing plant tissue water content and ultimately results in reduced growth and productivity (Robin et al., 2015). Hence, the current study aims at a comparative introspection into the responses of a set of selected rice genotypes under simulated osmotic and field-level drought stress conditions.

A. Screening of rice genotypes in hydroponics

A total of 24 rice genotypes were germinated separately in two hydroponic setups (Fig. 1, 2). The experiment was laid out in a Completely Randomized Design (CRD) with two treatments, each replicated three times. After 2 to 3 days of germination, the seedlings were transferred into two treatment solutions: one containing Yoshida solution (unstressed medium) and the other with Yoshida solution supplemented with 10% PEG6000 (to simulate osmotic stress). The seedlings were grown in the hydroponic setup for 21 days. On the 21st day seedling images were recorded, and root traits, including total root length, total root volume and mean root diameter were analyzed using WinRHIZO software (Regent Instruments Inc., Canada). Depth of rooting was measured manually as the distance from the crown root to the tip of the root bulk.

B. Screening of rice genotypes in root structure

The experiment was laid in CRD with two treatments namely irrigated (<-10 kPa) and drought (~-70 kPa). Twenty four genotypes were exposed to cycles of drought stress and recovery in root structure (Fig. 1, 2) from active tillering to booting stage. The matric potential of soil was measured using tensiometer. At booting stage, the plants were uprooted, washed, and imaged. Depth of rooting was measured as the distance from the crown root to the tip of the longest root. Total root volume was estimated by water displacement method.

Canopy temperature depression (CTD) was measured as the difference between the temperature of plant canopy and air temperature. CTD was recorded in the plants grown in root structure at the early booting stage (Jackson et al., 1981). Using an IR thermometer (FLUKE, 62 Max) from each genotype, leaf temperatures were recorded from topmost fully opened leaves.

CTD was calculated using the formula:

CTD = Leaf temperature – Ambient temperature

A. Osmotic stress-induced changes in root traits of rice genotypes raised under hydroponics system

(i) Depth of rooting (cm). Treatment with 10% PEG6000 at 21 DAG generally stimulated root elongation compared to the control. Under non-stress conditions, root depth ranged from 5.2 cm in Kumkumasali to 13.0 cm in Chomala. In contrast, under stress conditions, the range was 10.7 cm in Gandhakashala to 16.1 cm in Shakti. Notably, genotypes such as Mullan Channa, Thekkan Chitteni, and Kuttadan demonstrated superior rooting depth, whereas Jeerakalsala, Kumkumasali, and Neycheera exhibited a comparatively weaker response (Table 1). Similarly, main root axis length was induced under mild osmotic stress in genotype Bina Dhan-7, a drought tolerant genotype. Certain genotypes exhibited a reduced rooting depth which is indicative of their susceptibility to water deficit stress. Similar results were obtained for the genotype Bina Dhan-11, a drought susceptible genotype (Hannan et al., 2020).

(ii) Total root length (cm). In control conditions, root lengths ranged from 8.5 cm in Gandhakasala to 37.5 cm in Orpandi, whereas under stress conditions, the range was 17.9 cm in Champavu to 115.1 cm in Mullan Channa. Genotypes such as Thekkan Chitteni, Kodu Veliyan, and Orpandi demonstrated greater root lengths, while Neycheera, Krishnakamodh, Jeerakasala, and Gandhakasala exhibited comparatively lower responses. Interestingly, Gandhakasala and Jaya recorded the highest percentage increase in total root length (Fig. 3), suggesting their potential for enhanced root growth under drought conditions, enabling them to access water from deeper soil layers. These findings align with the results of Patmi and Pitoyo (2020), who reported reduced root lengths under drought stress compared to irrigated conditions in the mutant rice strain 51 of the Cempo Ireng cultivar.

(iii) Total root volume (cm³). Under non-stress conditions, root volume ranged from 0.04 cm³ in Neycheera to 0.24 cm³ in Kodu Veliyan. Under stress conditions, the range extended from 0.06 cm³ in Mullan Channa to 0.65 cm³ in Neycheera. Noteworthy genotypes such as Orpandi, Kodu Veliyan, and Thekkan Chitteni exhibited superior root volume, whereas Ponmani, Kuttadan, and Mallikuruva showed a comparatively weaker response. Genotypes such as Chomala and Champavu recorded the highest incremental total root volume (Fig. 4). Our results corroborate with the findings of Hannan et al. (2020), where maximum volume of root was exhibited by Bina Dhan-7, a drought tolerant genotype under mild stress. Similar trend was also reported by Syamsia et al. (2018) where Impari 20 genotype showed reduced total root surface area under osmotic stress.

(iv) Mean diameter of root (mm). The mean root diameter ranged from 0.7 mm in Orpandi to 1.1 mm in Ponmaniunder control conditions. Under stress conditions, the range was from 0.6 mm in Orpandi to 1.0 mm in Karutha Njavara. Notably, genotypes such as Jeerakasala, Gandhakasala, and Krishnakamodh exhibited higher root diameter, while Shakti, Sahbhagi Dhan, Chomala, and Thowan showed a comparatively lower response. Genotypes such as Karutha Njavara and Jeerakasala recorded the highest percent change in mean root diameter (Fig. 5). A similar outcome was observed in a study by Swapna and Shylaraj (2017), where Swarnaprabha, a drought-tolerant variety of rice, exhibited the highest root thickness under mild osmotic stress.

B. Water deficit stress-induced changes in root traits of rice genotypes raised in root structure

(i) Depth of rooting (cm). Under irrigated conditions, root depth ranged from 27.1 cm in Aathira to 39.8 cm in Sreyas. Under stress conditions, the range was from 17.8 cm in Krishnakamodh to 38.7 cm in Thekkan Chitteni. Notably, genotypes such as Sreyas, Sahbhagi Dhan, and Shakti exhibited higher root depth, whereas Chomala, Kuruva, and Champavu showed a lower response. Genotypes such as Champavu and Krishnakamodh showed the highest percentage increase in total root volume (Table 1). Similar trends were reported by Nasrin et al. (2020) where the root depth of BRRI-Dhan 56 was found to be greater under drought stress conditions.

(ii) Total root volume (cm³). The total root volume recorded by water displacement method exhibited significant variation across genotypes and among treatments. Under irrigated conditions, root volume ranged from 3.0 cm³ in Karutha Njavara to 21 cm³ in Kuttadan. Under drought conditions, the range was from 2.3 cm³ in Valiya Thondi to 14.3 cm³ in Karutha Njavara. Additionally, genotypes like Kumkumasali and Shakti showed superior total root volume values, while Krishnakamodh and Kuruva exhibited comparatively inferior responses. The genotypes Aathira and Kumkumasali exhibited the highest increase in root volume (Fig. 6). Comparable findings were reported by Nahar et al. (2018), where the SN-14 genotype showed a decrease in root volume under drought stress conditions.

(iii) Canopy Temperature Depression (^oC). Under irrigated conditions, CTD values ranged from 5.8 cm in Champavu to 9.2 cm in Chomala. Under drought conditions, the values ranged from 2.1 cm in Thowan to 4.3 cm in Aathira. Additionally, genotypes such as Sahbhagi Dhan, Chomala, and Sreyas displayed superior CTD, while Kuruva, Kothambalarikaima, and Neycheera showed weaker responses.Thowan and Neycheera exhibited the highest percentage increase in canopy temperature depression (Fig. 7). These findings align with those of Tripathi et al. (2023), who reported that genotypes such as Sukhkha Dhan 6 and IR 74371-70-1-1 had lower canopy temperatures, whereas Sabitri, being drought sensitive showed higher canopy temperature under stress at reproductive stage.

Fig. 1. The list of genotypes employed in the present study. Both grain and duhusked seeds are kept for detailed identification.

Fig. 2. The experimental setups usedin the study. A. Hydroponics; B. Root structure. In hydroponics 10 % PEG 6000 was used as treatment, while in root structure, drought stress was imposed as cycles of stress (~-70 kPa) and recovery (~-10 kPa).

Table 1: Response of rice genotypes to osmotic stress and drought stress on depth of rooting. The data shown are mean ± SE, along with percent change over control conditions.

Fig. 3. Effect of osmotic stress on total root length (cm) of rice genotypes grown under hydroponics. Vertical bars show mean ± SE. DMRT analysis was done separately for control and stress treatments. Vertical bars with same alphabets are not statistically significant.

Fig. 4. Effect of osmotic stress on total root volume (cm3) of rice genotypes grown under hydroponics. Vertical bars show mean ± SE. DMRT analysis was done separately for control and stress treatments. Vertical bars with same alphabets are not statistically significant.

Fig. 5. Effect of osmotic stress on mean root diameter (mm) of rice genotypes grown under hydroponics. Vertical bars show mean ± SE. DMRT analysis was done separately for control and stress treatments. Vertical bars with same alphabets are not statistically significant.

Fig. 6. Effect of water deficit stress on total root volume (cm3) of rice genotypes grown under root structure. Vertical bars show mean ± SE. DMRT analysis was done separately for control and stress treatments. Vertical bars with same alphabets are not statistically significant.

Fig. 7. Effect of water deficit stress on Canopy Temperature Depression (ºC) of rice genotypes grown under root structure. Vertical bars show mean ± SE. DMRT analysis was done separately for control and stress treatments. Vertical bars with same alphabets are not statistically significant.

Identification of elite donors of RSA can pave way for targeted genetic manipulation, and related studies such as GWAS and genome editing.