Synergistic Effects of Rhizobacteria and Salicylic Acid on Maize Salt-Stress Tolerance

Abstract

Maize (Zea mays L.) is a salt-sensitive plant that experiences stunted growth and development during early seedling stages under salt stress. Salicylic acid (SA) is a major growth hormone that has been observed to induce resistance in plants against different abiotic stresses. Furthermore, plant growth-promoting rhizobacteria (PGPR) have shown considerable potential in conferring salinity tolerance to crops via facilitating growth promotion, yield improvement, and regulation of various physiological processes. In this regard, combined application of PGPR and SA can have wide applicability in supporting plant growth under salt stress. We investigated the impact of salinity on the growth and yield attributes of maize and explored the combined role of PGPR and SA in mitigating the effect of salt stress. Three different levels of salinity were developed (original, 4 and 8 dS m^-1) in pots using NaCl. Maize seeds were inoculated with salt-tolerant Pseudomonas aeruginosa strain, whereas foliar application of SA was given at the three-leaf stage. We observed that salinity stress adversely affected maize growth, yield, and physiological attributes compared to the control. However, both individual and combined applications of PGPR and SA alleviated the negative effects of salinity and improved all the measured plant attributes. The response of PGPR + SA was significant in enhancing the shoot and root dry weights (41 and 56%), relative water contents (32%), chlorophyll a and b contents (25 and 27%), and grain yield (41%) of maize under higher salinity level (i.e., 8 dS m^-1) as compared to untreated unstressed control. Moreover, significant alterations in ascorbate peroxidase (53%), catalase (47%), superoxide dismutase (21%), MDA contents (40%), Na⁺ (25%), and K⁺ (30%) concentration of leaves were pragmatic under combined application of PGPR and SA. We concluded that integration of PGPR and SA can efficiently induce salinity tolerance and improve plant growth under stressed conditions.

Keywords: PGPR; ascorbate peroxidase; malondialdehyde; relative water contents; salicylic acid.

Figure 1

Effect of combined application of PGPR and SA on (A) shoot length (cm), (B) shoot fresh weight (g), (C) shoot dry weight (g), (D) root length (cm), (E) root fresh weight (g), and (F) root dry weight (g) of maize under saline conditions. Error bars show the standard deviation from three independent replications. Variations in lowercase show significant difference between PGPR and SA treatments at p ≤ 0.05. SA: salicylic acid; PGPR: salt-tolerant rhizobacterial strain; SA + PGPR: combination.

Figure 2

Effect of combined application of PGPR and SA on (A) cob length (cm), (B) cob fresh weight (g), (C) cob dry weight (g), (D) 100 grain weight (g), (E) grain yield (g), and (F) relative water content (%) of maize under saline conditions. The error bars show the standard deviation from three independent replications. Variations in lowercase show significant difference between PGPR and SA treatments at the p ≤ 0.05. SA: salicylic acid; PGPR: salt-tolerant rhizobacterial strain; SA + PGPR: combination.

Figure 3

Effect of combined application of PGPR and SA on (A) chlorophyll a (mg g⁻¹ f. wt.), (B) chlorophyll b (mg g⁻¹ fresh weight.), (C) carotenoid (mg g⁻¹ fresh weight.), (D) sodium content in leaves (%), (E) potassium content in leaves (%), and (F) K⁺/Na⁺ ratio of maize under saline conditions. Error bars show the standard deviation from three independent replications. Variations in lowercase show significant difference between PGPR and SA treatments at the p ≤ 0.05. SA: salicylic acid; PGPR: salt-tolerant rhizobacterial strain; SA + PGPR: combination.

Figure 4

Effect of combined application of PGPR and SA on (A) ascorbate peroxidase (units g⁻¹ FW), (B) catalase (units g⁻¹ FW), (C) superoxide dismutase (units g⁻¹ FW), and (D) malondialdehyde content (µg g⁻¹ FW) of maize under saline conditions. The error bars show the standard deviation from three independent replications. Variations in lowercase show statistical significance between PGPR and SA treatments at the p ≤ 0.05. SA: salicylic acid; PGPR: salt-tolerant rhizobacterial strain; SA + PGPR: combination.

Figure 5

Correlation matrix SL (shoot length), chl a (chlorophyll a), chl b (chlorophyll b), GY (grain yield), RWC (relative water content), RL (root length), K/Na (K⁺/Na⁺ ratio), cob length, MDA (malondialdehyde), APX (ascorbic peroxidase), CAT (catalase), SOD (Superoxide dismutase), HGW (100 grain weight).