Plant growth promoting rhizobacteria alleviates drought stress in potato in response to suppressive oxidative stress and antioxidant enzymes activities

Abstract

Maintenance of plant physiological functions under drought stress is normally considered a positive feature as it indicates sustained plant health and growth. This study was conducted to investigate whether plant growth-promoting rhizobacteria (PGPR) Bacillus subtilis HAS31 has potential to maintain potato growth and yield under drought stress. We analyzed trends of chlorophyll concentration, photosynthesis process, relative water content, osmolytes, antioxidants enzymes and oxidative stress, relative growth rate, tuber and aboveground biomass production in two potato varieties, Santae (drought-tolerant) and PRI-Red (drought-sensitive). Plants of both genotypes were treated with 100 g of HAS31 inoculant at 10 days after germination and exposed to different soil relative water contents (SRWC), including 80 ± 5% (well watered), 60 ± 5% (moderate stress) and 40 ± 5% SRWC (severe stress) for 7 days at tuber initiation stage (30 days after germination). The drought stress reduced plant relative growth rate, biomass production, leaf area, number of leaves and tubers, tuber weight, and final yield. The drought-stressed plants showed decline in chlorophyll contents, membrane stability, leaf relative water contents and photosynthetic rate. Under drought stress, enzymatic activity of catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD), contents of total soluble sugars, soluble proteins and proline increased. The application of PGPR reduced the impact of drought and maintained higher growth and physio-chemical traits of the plants. The plants with PGPR application showed higher relative growth rate, dry matter production, leaf area, number of tubers, tuber weight and yield as compared to plants without PGPR. The PGPR-HAS31 treated plants maintained higher photosynthetic process, contents of chlorophyll, soluble proteins, total soluble sugars, and enzymatic activities of CAT, POD and SOD as compared to plants without PGPR. The results of the study suggest that plant growth regulators have ability to sustain growth and yield of potato under drought stress by maintaining physiological functions of the plants.

Figure 1

Effect of drought stress and PGPR application on tuber yield (a,b) and drought index (c,d) in two potato cultivars. Plants of both genotypes were treated with 100 g of bio-fertilizer at 10 days after germination and exposed to different soil relative water contents (SRWC), including 80 ± 5% (well watered; CK), 60 ± 5% (moderate stress; MS) and 40 ± 5% SRWC (severe stress; SS) for 7 days at tuber initiation stage (30 days after germination). The data is mean of 5 replications. Lowercase letters above bars indicate significant difference between the treatments at P < 0.05.

Figure 2

Effect of drought stress and PGPR application on leaf relative water content (a,b) and membrane stability index (c,d) in two potato cultivars. Plants of both genotypes were treated with 100 g of bio-fertilizer at 10 days after germination and exposed to different soil relative water contents (SRWC), including 80 ± 5% (well watered; CK), 60 ± 5% (moderate stress; MS) and 40 ± 5% SRWC (severe stress; SS) for 7 days at tuber initiation stage (30 days after germination). The data is mean of 5 replications. Lowercase letters above bars indicate significant difference between the treatments at P < 0.05.

Figure 3

Effect of drought stress and PGPR application on contents of chlorophyll a (a,b), chlorophyll b (c,d) and carotenoids (e,f) in two potato cultivars. Plants of both genotypes were treated with 100 g of bio-fertilizer at 10 days after germination and exposed to different soil relative water contents (SRWC), including 80 ± 5% (well watered; CK), 60 ± 5% (moderate stress; MS) and 40 ± 5% SRWC (severe stress; SS) for 7 days at tuber initiation stage (30 days after germination). The data is mean of 5 replications. Lowercase letters above bars indicate significant difference between the treatments at P < 0.05.

Figure 4

Effect of drought stress and PGPR application on photosynthetic rate (Pn; a,b), stomatal conductance (gs; c,d), transpiration rate (Tr; e,f), and intercellular CO₂ (Ci; g,h) in two potato cultivars. Plants of both genotypes were treated with 100 g of bio-fertilizer at 10 days after germination and exposed to different soil relative water contents (SRWC), including 80 ± 5% (well watered; CK), 60 ± 5% (moderate stress; MS) and 40 ± 5% SRWC (severe stress; SS) for 7 days at tuber initiation stage (30 days after germination). The data is mean of 5 replications. Lowercase letters above bars indicate significant difference between the treatments at P < 0.05.

Figure 5

Effect of drought stress and PGPR application on photosynthetic efficiency of PSII (Fv/Fm; Fig. 3a,b), quantum yield of PSII (ΦPSII; c,d) and non-photosynthetic quenching (NPQ; e,f) in two potato cultivars. Plants of both genotypes were treated with 100 g of bio-fertilizer at 10 days after germination and exposed to different soil relative water contents (SRWC), including 80 ± 5% (well watered; CK), 60 ± 5% (moderate stress; MS) and 40 ± 5% SRWC (severe stress; SS) for 7 days at tuber initiation stage (30 days after germination). The data is mean of 5 replications. Lowercase letters above bars indicate significant difference between the treatments at P < 0.05.

Figure 6

Effect of drought stress and PGPR application on the production of superoxides (a,b), hydrogen peroxide (c,d) and malondialdehyde (e,f) in two potato cultivars. Plants of both genotypes were treated with 100 g of bio-fertilizer at 10 days after germination and exposed to different soil relative water contents (SRWC), including 80 ± 5% (well watered; CK), 60 ± 5% (moderate stress; MS) and 40 ± 5% SRWC (severe stress; SS) for 7 days at tuber initiation stage (30 days after germination). The data is mean of 5 replications. Lowercase letters above bars indicate significant difference between the treatments at P < 0.05.

Figure 7

Effect of drought stress and PGPR application on enzymatic activities of catalase (CAT; a,b), superoxide dismutase (SOD; c,d) and peroxidase (POD; e,f) in two potato cultivars. Plants of both genotypes were treated with 100 g of bio-fertilizer at 10 days after germination and exposed to different soil relative water contents (SRWC), including 80 ± 5% (well watered; CK), 60 ± 5% (moderate stress; MS) and 40 ± 5% SRWC (severe stress; SS) for 7 days at tuber initiation stage (30 days after germination). The data is mean of 5 replications. Lowercase letters above bars indicate significant difference between the treatments at P < 0.05.

Figure 8

Effect of drought stress and PGPR application on total soluble sugars (Fig. 4a,b), free amino acids (Fig. 4c,d), soluble proteins (Fig. 4e,f), and proline contents (Fig. 4g,h) in two potato cultivars. Plants of both genotypes were treated with 100 g of bio-fertilizer at 10 days after germination and exposed to different soil relative water contents (SRWC), including 80 ± 5% (well watered; CK), 60 ± 5% (moderate stress; MS) and 40 ± 5% SRWC (severe stress; SS) for 7 days at tuber initiation stage (30 days after germination). The data is mean of 5 replications. Lowercase letters above bars indicate significant difference between the treatments at P < 0.05.