Biochemical responses associated with drought tolerance in rice genotypes under rainfed and irrigated conditions

Introduction

Drought is one of the major abiotic stresses limiting rice productivity, particularly in rainfed production systems. Water deficit affects several physiological processes, including photosynthesis, cellular water balance and metabolic activity, ultimately reducing plant growth and yield. The ability of rice genotypes to maintain physiological functions under water stress is therefore an important component of drought tolerance. Biochemical responses play an important role in the adaptation of rice plants to drought. Chlorophyll content is closely associated with photosynthetic capacity, whereas proline accumulation contributes to osmotic adjustment and protection of cellular structures under water deficit. Drought stress also induces the formation of reactive oxygen species (ROS), which can cause oxidative damage when their production exceeds the plant's protective capacity. Plants counteract this oxidative stress through antioxidant enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POD) and catalase (CAT). Considerable genotypic variation exists in the biochemical response of rice to water stress. Genotypes capable of maintaining higher chlorophyll content and accumulating compatible solutes while enhancing antioxidant enzyme activity may possess better mechanisms for coping with drought. In the present study, these biochemical traits were evaluated in rice genotypes under rainfed and irrigated conditions to identify responses associated with better drought adaptation. Therefore, the present study was undertaken to evaluate chlorophyll content, proline accumulation and antioxidant enzyme activities (APX, SOD, POD and CAT) in rice genotypes under rainfed and irrigated conditions and to identify genotypes showing superior biochemical responses to water stress.

Materials and Methods

The present study was conducted at the Research Cum Instructional Farm, Indira Gandhi Krishi Vishwavidyalaya, Raipur, Chhattisgarh, during the Kharif season of 2019-20. Thirty rice (Oryza sativa L.) genotypes obtained from IIRR, Hyderabad, under the AICRP on Rice programme, along with the check variety Poornima (LC), were evaluated under rainfed and irrigated conditions. The experiment was laid out in a randomized block design with three replications. The plot size was 3.80 × 1.40 m, with a spacing of 20 × 20 cm. A fertilizer dose of 100:45:60 kg N:P₂O₅:K₂O ha⁻¹ was applied.

Under rainfed conditions, no artificial irrigation was provided after crop establishment, whereas irrigation was supplied according to crop requirement under irrigated conditions. Biochemical observations were recorded for chlorophyll content, proline content, ascorbate peroxidase (APX), superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities. Chlorophyll content was measured using a SPAD meter, while proline and antioxidant enzyme activities were estimated using the respective biochemical procedures followed in the original study.

The biochemical data were analyzed statistically to determine differences among genotypes and moisture regimes. The significance of treatment differences was assessed at the 5% probability level. The responses of individual genotypes under rainfed conditions were compared with those under irrigated conditions to identify genotypes showing better biochemical adaptation to water stress.

Results and Discussion

Chlorophyll content

Chlorophyll content differed significantly among rice genotypes under rainfed and irrigated conditions. ( [1] and Sahoo et al. (2020) [7] .

Ascorbate peroxidase activity

Ascorbate peroxidase (APX) activity increased significantly under rainfed conditions compared with irrigated conditions, reflecting activation of the antioxidant defense system in response to water stress. ( [4] , Kumari et al. (2019) [3] and Sahoo et al. (2019) [6] .

Superoxide dismutase activity

Superoxide dismutase (SOD) activity showed a significant increase under rainfed conditions. (Table 2) The genotype mean SOD activity ranged from 1.28 to 3.55 U mg⁻¹ under rainfed conditions, compared with 0.10 to 0.43 U mg⁻¹ under irrigation. The highest SOD activity under rainfed conditions was recorded in Poornima (LC) (3.55 U mg⁻¹), followed by RFU 329 (3.33 U mg⁻¹), whereas RFU 310 recorded the lowest value (1.28 U mg⁻¹). Increased SOD activity indicates enhanced capacity to detoxify superoxide radicals generated during drought stress. Similar results were reported by Sahoo et al. (2019) [6] .

Peroxidase activity

Peroxidase (POD) activity was significantly higher under rainfed conditions than under irrigation. (Table 2). The genotype mean POD activity ranged from 5.33 to 11.28 U mg⁻¹ protein under the reported moisture conditions, with Poornima (LC) recording the maximum activity (11.28 U mg⁻¹ protein), followed by RFU 329 (10.77 U mg⁻¹ protein).

RFU 319 recorded the minimum value (5.33 U mg⁻¹ protein). The enhanced POD activity under water stress suggests an increased enzymatic antioxidant response to oxidative stress. Similar results were reported by Sahoo et al. (2020) [7] .

Catalase activity

Catalase (CAT) activity also increased significantly under rainfed conditions. (Table 2) The genotype mean CAT activity ranged from 0.33 to 0.65 U mg⁻¹ protein under rainfed conditions, compared with 0.07 to 0.21 U mg⁻¹ protein under irrigation. Poornima (LC) recorded the highest CAT activity under rainfed conditions (0.65 U mg⁻¹ protein), followed by RFU 329 (0.58), whereas RFU 315 recorded the lowest value (0.33). The higher CAT activity under water stress indicates an enhanced capacity for detoxification of hydrogen peroxide and protection against oxidative damage. Similar findings were reported by Kumari et al. (2019) [3] and Sahoo et al. (2020) [7] .

Conclusion

The study demonstrated that rainfed conditions substantially altered the biochemical responses of rice genotypes compared with irrigated conditions. Water stress generally reduced chlorophyll content but enhanced proline accumulation and the activities of APX, SOD, POD and CAT, indicating activation of osmotic adjustment and antioxidant defense mechanisms. Among the evaluated genotypes, Poornima (LC) and RFU 329 consistently showed higher biochemical responses under rainfed conditions, particularly for proline accumulation and antioxidant enzyme activities. These genotypes may therefore possess better biochemical adaptation to water stress and could be useful for further evaluation in droughttolerance breeding programmes.

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