Water stress mitigation in upland rice through bacterial seed priming

Abstract

The study entitled ‘Water stress mitigation in upland rice through bacterial seed priming’ was conducted at the College of Agriculture, Vellayani, during 2023-2025. The objective of the study was to assess the influence of bacterial seed priming on upland rice under different levels of moisture stress during summer taken up as lab study, pot culture and field experiment. in vitro screening of beneficial bacteria for imparting drought tolerance in rice variety Prathyasa was conducted by a roll-towel assay laid out in completely randomized design (CRD) with 6×4 treatments, replicated four times. The factors were biopriming (b0- without biopriming, b1- B. pumilus VLY 17, b2- B. amyloliquefaciens VLY 24, b3- B. velezensis PCSE 10, b4- consortium and b5- hydropriming) and moisture stress levels (m₀ – without stress, m₁ – PEG 6000 at 5% (mild stress), m₂ – PEG 6000 at 10% (moderate stress), and m₃ – PEG 6000 at 20% (severe stress).The seeds were primed by soaking in respective bacterial cultures at concentration of 108 CFU mL-1 for 12 hours followed by shade drying. Both biopriming and moisture stress levels significantly influenced the germination indices, root parameters and growth attributes of rice seedlings. Under moderate stress, germination percentage and shoot length were significantly higher when bioprimed with B. pumilus, B. amyloliquefaciens, and the bacterial consortium; and were superior to the unprimed control. However, under severe stress, seeds primed with B. pumilus recorded the highest root-to-shoot ratio compared to the unprimed and hydroprimed control. The second experiment was a pot culture on ‘Screening of beneficial bacteria for drought tolerance in rice’ for identifying the most effective bacterial culture conferring drought tolerance. The experiment was laid out in CRD with two factors viz., biopriming (b0-without biopriming, b1 - B. pumilus, b2 - B. amyloliquefaciens, b3 - B velezensis, b4- consortium) and irrigation intervals [i1 - irrigated to field capacity, i₂ - irrigation completely withdrawn at 40 days after sowing (DAS)] with the rice variety Prathyasa. Seed primed with B. pumilus produced the tallest plants (85.22 cm at 60 DAS), the highest number of leaves (16.08 at 50 DAS) and dry matter production (39.23 g per plant at 50 DAS). At 50 DAS and 60 DAS, the highest rooting depth (27.65 and 32.48 cm), root volume (14.25 and 16.01 cm³), and root dry weight (13.25 and 16.29 g) were observed in B. pumilus primed plants, outperforming the unprimed control under complete irrigation withdrawal at 40 DAS. Lower malondialdehyde content, and the highest superoxide dismutase activity at 60 DAS were also noted in plants primed with B. pumilus. Based on PCA with a cut-off score of 0.75, B. pumilus was identified as the promising bioinoculant for priming and was subsequently selected for field validation under different levels of moisture stress. A field experiment was laid out in a randomized complete block design with 2 × 2 × 2 treatments replicated thrice, with the upland rice variety Prathyasa during March to June 2025. The factors included biopriming (b₀-without biopriming and b₁-with B. pumilus), irrigation interval (i₁ - at 30 mm CPE and i₂ - at 40 mm CPE), and depth of irrigation (d₁ -to a depth of 1.5 cm and d₂ - to a depth of 3.0 cm). The stress levels were categorized as ideal (i1d2), moderate (i2d2), severe (i1d1) and extremely severe (i2d1). The crop was raised as per KAU Package of Practices Recommendations for short-duration rice. Among the growth parameters, B. pumilus priming resulted in the tallest plants (83.47 cm), highest leaf area index (2.78) at 60 DAS, and the highest dry matter accumulation (3538 kg ha⁻¹) at harvest under extremely severe moisture stress compared to the control. Plants of bioprimed seeds produced the highest productive tillers (198.53 m⁻²), grains per panicle (82.33), grain yield (2163 kg ha⁻¹), and straw yield (2445 kg ha⁻¹), along with the lowest spikelet sterility (16.03%) under severe moisture stress, outperforming the unprimed control. Under extremely severe moisture stress, plants of bioprimed seeds recorded the highest proline content (90.04 and 65.08 μg g⁻¹ FW), and the highest relative leaf water content (78.93 and 73.70%) at panicle initiation (PI) and flowering, respectively, compared to the unprimed control. Similarly, under moderate stress, plants of bioprimed seeds maintained the highest cell membrane stability index and chlorophyll stability index at PI and flowering, respectively. Bioprimed plants exhibited the highest water use efficiency and water productivity at all stress levels compared to the unprimed control. They recorded the highest nutrient uptake under extremely severe stress, with nitrogen, phosphorus, and potassium uptake values of 51.75 kg ha⁻¹, 11.45 kg ha⁻¹, and 85.12 kg ha⁻¹, respectively. The study identified seed biopriming with the bacterial isolate Bacillus pumilus VLY 17 as an effective strategy for mitigating moisture stress in upland rice during summer as assessed based on growth, yield, physiological parameters and water-use efficiency.