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Comparative Effectiveness of Enterobacter aerogenes and Pseudomonas fluorescens for Mitigating the Depressing Effect of Brackish Water on Maize.

Byline: ZAHIR A. ZAHIR, SAQIB SALEEM AKHTAR, MAQSHOOF AHMAD, SAIFULLAH AND SAJID MEHMOOD NADEEM

ABSTRACT

Experiments were conducted under axenic and natural conditions to evaluate the comparative effectiveness of Enterobacter aerogenes and Pseudomonas fluorescens for improving maize growth and yield irrigated with synthetic brackish water [EC, 5 dS m-1; SAR, 10 (mmol L-1)1/2]. In the first experiment under axenic conditions, normal water, brackish water and two PGPR strains (S14, E. aerogenes and S20, P. fluorescens) were tested. In the second experiment in pots, normal and brackish water irrigations at different stages of maize growth along with the same PGPR strains were tested. Brackish water significantly decreased the maize growth, whilst bacterial strains significantly reduced its adverse effects. Interestingly, inoculation was even more effective in case of brackish water as compared to normal water irrigation. In pot trial, PGPR inoculation reduced the adverse effects of salinity on maize growth and yield.

The maximum improvement in growth and yield parameters was observed with S20 inoculation under brackih water treatment. The chlorophyll content and K+/Na+ ratio of maize were also improved by PGPR strain S20. It is concluded that P. fluorescens could be used as an effective tool to minimize the inhibitory effects of brackish water on the growth and yield of maize. (c) 2012 Friends Science Publishers

Key Words: Brackish water; PGPR; ACC-deaminase; Maize; Yield

INTRODUCTION

The limited supply of good quality canal water for irrigation has compelled the farmers to use water from alternate sources like sewage and ground water pumped by tube wells. This situation prevails in most of the countries situated in arid to semi arid regions of the world. Pakistan is no exception to this. According to an estimate, about 50-60% of the total water that is being pumped up is unfit for irrigation due to its high electrical conductivity (EC), sodium adsorption ratio (SAR) or residual sodium carbonate (RSC) (Ashfaq et al., 2009).

The continuous use of such water could result in an accumulation of soluble salts and/or exchangeable Na somewhere in the soil profile (Ahmed et al., 2008). The presence of excessive soluble salts and/or exchangeable Na could result in severe yield losses by altering the soil physical or chemical properties (Nadeem et al., 2006). The primary effect of high EC water on crop productivity is the development of physiological drought i.e inability of the plants to compete with ions in the soil solution for water. Sodium in irrigation water can also cause toxicity problems for some crops (Bauder et al., 2006). Water potential and osmotic potential of shoot became more negative with an increase in brackish water salinity which is associated with an accumulation of ions in leaves (Gulzar et al., 2003).

Moreover, salinity is one of the most detrimental stresses that affect the plant growth by influencing a number of physiological processes positively or negatively. It is a serious production problem for crops as saline conditions are known to suppress plant growth, particularly in arid and semiarid regions (Parida and Das, 2005). This implies that plants, growing in saline conditions come under stress and often suffer from more physiological disorders as compared to those growing in normal environment (Bernstein, 1975) and ultimately the yield is reduced.

Ethylene is one of the important growth hormones produced by almost all the plants, which mediates a wide range of plant responses (Arshad and Frankenberger Jr.,2002). The production of ethylene in most plant tissues is normally low; however, its accelerated production can be induced by various developmental and experimental cues,including seed germination, fruit ripening, leaf and fruit senescence and a number of other biotic and abiotic stresses (Theologis, 1992). It has been well documented that ethylene synthesis is accelerated in the presence of various environmental stresses (Glick et al., 1997) including salt stress and it has been reported to increase significantly in many plant species subjected to salt stress (Zapata et al.,2004; Nadeem et al., 2010).

The production of ethylene may be inhibitory or stimulatory depending upon its concentration (Arshad and Frankenberger Jr., 2002), the nature of the physiological process (Johnson and Ecker, 1998) and the growth phase of the plant (Abeles et al., 1992). Higher levels of ethylene ( greater than 25 ug L-1) can be damaging for plants, leading to epinasty, shorter roots and premature senescence (Holguin and Glick, 2001). Metabolizing ACC, the immediate precursor of ethylene, is an established approach to reduce ethylene level in plants. Some PGPR strains possess the enzyme ACC-deaminase, which can cleave the ACC and thereby lower the level of ethylene in a developing seedling or stressed plant (Mayak et al., 2004). A decreased level of ACC results in a lower level of endogenous ethylene, which eliminates the inhibitory effect of high ethylene concentrations (Glick et al., 1999).

Plants inoculated with PGPR containing ACC- deaminase are more resistant to the deleterious effects of stress ethylene that is synthesized as a consequence of stressful conditions such as high salt concentrations (Mayak et al., 2004; Nadeem et al., 2010). Such inoculation was also reported to reduce the drastic effect of salinity stress in plants and increase the shoot fresh weight, root fresh weight, shoot dry weight, root dry weight, number of leaves per plant, leaf relative water content, emergence percentage and chlorophyll content but decrease electrolyte leakages in the plants (Yildirim et al., 2008). Inoculation of wheat with P. putida carrying ACC-deaminase activity under salinity stress resulted in increased plant height, root length, grain yield, 100-grain weight and straw yield up to 52, 60, 76, 19 and 67%, respectively over un-inoculated control at 15 dS m[?]1 (Zahir et al., 2009).

Similarly, chlorophyll content and K+/Na+ ratio of leaves were also increased by P.fluorescens over control when the maize plants were inoculated under salinity stress (Nadeem et al., 2007). It is also reported that inoculation of maize under salinity stress with P. fluorescens carrying ACC-deaminase activity increased the nutrient uptake and plant yield efficiently over un-inoculated control by lowering the endogenous level of ethylene (Nadeem et al., 2009). Inoculation with the same bacteria has also been reported to increase the wheat growth and yield (Nadeem et al., 2010). Pseudomonas fluorescens increased the K+/Na+ ratio, relative water content and chlorophyll content in wheat (Nadeem et al., 2010).

Plant growth promoting rhizobacteria containing ACC-deaminase are not only effective for reducing stress- induced ethylene, but are also helpful for the absorption of nutrients necessary for better growth. The Klebsiella oxytoca (Rs-5) containing ACC-deaminase mitigate the negative effects of salt stress and promote the plant growth. Inoculation also enhances the absorption of major nutrients such as N, P, K and Ca and decreases the uptake of the Na+ (Yue et al., 2007).

There is repeated evidence available in literature indicating that the microbial inoculation can play an important role to improve crop yields in salt affected soils. However, the prospects of using such PGPR in crops grown with brackish water have not been studied. Continued use of brackish water results in the development of secondary salinity in the long run. But it could also raise an immediate adverse effect on plant growth by disturbing the composition of soil solution. The unfavorable conditions in the soil solution through application of brackish water may upset the plant uptake of water and other essential nutrient elements. Therefore, it is imperative to explore all such possible ways, which may reduce the depressing effect of brackish water on plant growth. The present study was conducted to study the comparative effectiveness of E. aerogenes and P. fluorescens for mitigating/ eliminating the stress, induced by irrigating maize with brackish water.

MATERIALS AND METHODS

Basic experimental details: Two trials were conducted under axenic and natural conditions to test the comparative efficacy of two pre-isolated (Nadeem et al., 2007) bacterial strains (S14, Enterobacter aerogenes) and (S20, Pseudomonas fluorescens). A mixture of salts NaCl, Na2SO4, MgSO4 and CaCl2 in a specific ratio was used to produce brackish water containing almost all the major cations and anions found in ground water. Synthetic brackish water of EC, 5 dS m-1; SAR, 10 (mmol L-1)1/2 prepared with the aforementioned salts was used to grow crops in axenic and natural conditions.

Pouch trial: Maize seeds were surface-sterilized by momentarily dipping them in 95% ethanol solution and then in 0.2% HgCl2 solution for 3 min. and were subsequently washed thoroughly with sterilized distilled water (Russell et al., 1982). Petri dishes with filter paper sheets were autoclaved, surface-sterilized seeds were arranged in them and were placed in an incubator at 25oC for 4 days. Sterilized distilled water was used to maintain optimum moisture for germination. Germinating seeds were inoculated by dipping for 10 min. in culture media (107-108 cfu mL-1) of strain S14 or S20. Inoculated seedlings were transplanted in sterilized growth pouches. In case of un- inoculated control, sterilized broth was used for seed dipping.

Four treatments of synthetic brackish water treatments were applied: N0 (normal water throughout the growth period), B0 (brackish water throughout the growth period), C (alternate normal and brackish water) and D (2 normal water irrigations followed by 1brackish water). In the growth room, the temperature was maintained at 25+-1oC and 14 h of light (intensity, 275 umol m-2s-1) alternated with 10 h darkness. The growth parameters were measured after 25 days of planting. Root length was measured by using Delta T-Scan (Win DIAS 3, England).

Pot trial: A pot experiment was conducted under natural conditions to evaluate the comparative efficacy of two pre- selected strains (S14 and S20) of PGPR to minimize the effect of salinity stress induced by brackish water on maize crop at vegetative and reproductive stages. For this purpose, synthetic brackish water of EC, 5 dS m-1 and SAR, 10 (mmol L-1)1/2 was used. The soil was sandy loam with pH, 7.9; electrical conductivity (ECe), 1.4 dS m-1; organic matter, 0.75 %; total N, 0.07 %; available (Olsen) P, 7.12 mg kg-1 and extractable K, 89 mg kg-1. Surface-disinfected seeds were coated with PGPR strains S14 and S20 by using slurry prepared with sterilized peat, broth culture (107-108 cfu mL-1) and sterilized sugar solution 10% in the ratio 5:4:1 (w/w). To the un-inoculated control, plain sterilized (autoclaved) broth was used in the slurry. Ten inoculated seeds of maize were sown in each pot containing 12 kg soil and one plant was maintained in each pot after germination.

Four different treatments of brackish water were applied: N (all irrigations with normal water), B (all irrigations with brackish water), NB (normal water during vegetative growth and brackish water during reproductive growth) and BN (brackish water during vegetative growth and normal water during reproductive growth) and each treatment was repeated thrice. Pots were arranged in a wire house at ambient light and temperature using completely randomized design (CRD). Recommended dose of N, P, K fertilizers (220: 125: 125 kg ha-1) were applied in each pot as urea, diammonium phosphate and sulphate of potash, respectively. Data regarding plant height, no. of leaves plant-1, fresh shoot weight, dry shoot weight, root length, fresh root weight, dry root weight, cob length, fresh cob weight, dry cob weight, no. of rows cob-1, no. of grains row-1 and 100-grain weight were recorded after harvesting.

The chlorophyll contents were measured in the leaf samples through chlorophyll meter. Leaf samples were collected prior to harvesting and stored in polypropylene centrifuge tubes at freezing temperature (Akhtar et al.,1998). The sap was collected and analyzed for sodium (Na+) and potassium (K+) concentrations by flame photometer as described by US Salinity Laboratory Staff (1954).

Statistical analysis: The Analysis of variance (ANOVA) techniques (Steel et al., 1980) was applied to analyze the data using complete randomized factorial design and means were compared by Duncan's Multiple Range Tests (Duncan, 1955).

RESULTS

Pouch trial: In growth pouch trial under axenic conditions, the application of brackish water always significantly reduced the maize growth but the alternate irrigation of normal and brackish water or two normal water irrigations followed by, one brackish water had a significantly less negative effect than application of brackish water throughout the growth. Furthermore, the bacterial strains significantly reduced the inhibitory effect of brackish water on maize growth but P. fluorescens was more efficient.

The data regarding root length revealed that the brackish water significantly reduced the root length but the alternate irrigation of normal and brackish water (C) or two normal water irrigations followed by1 brackish water (D) equally reduced this inhibitory effect though the root length was still significantly less than with normal water treatment (Table I). Moreover, the bacterial strains reduced the inhibitory effect of brackish water on root length, with the PGPR strain S20 being more efficient one in all the treatments. But the results were more pronounced in case of brackish water as compared to normal water treatment. The maximum increase in root length (124%) compared with the respective un-inoculated control was observed with the S20 inoculation in the treatment (B) where brackish water was applied throughout the growing period.Data (Table I) showed that the PGPR inoculation significantly improved the root fresh weight of maize seedlings, which was otherwise reduced by brackish water application.

The maximum increase in root fresh weight was observed with S14 inoculation in the brackish water treatment B, where increase in root fresh weight was about 5 fold over the respective un-inoculated control. As in the case of root fresh weight, the root dry weight of maize seedlings was also improved by both PGPR strains in all the treatments, and strain S20 gave the maximum increase in root dry weight in the treatment B over the respective un- inoculated control. But the increase was statistically non- significant in all the treatments.

It is evident from the data (Table II) that the shoot length of the maize seedlings was adversely affected by brackish water. However, it was less affected by alternate irrigation of normal and brackish water (C) or two normal water irrigations followed by, one brackish water (D). The inoculation of maize seeds with PGPR strains (S14 and S20) on the other hand significantly reduced the inhibitory effect of brackish water. Both the bacterial strains increased the shoot length (7 to 50%) of maize seedlings. However, the bacterial isolate S20 was more efficient and it increased the shoot length up to 50% over the respective un-inoculated control in the treatment B where brackish water was applied.Shoot fresh weight and shoot dry weight (Table II) were also significantly reduced by the application of brackish water and the minimum shoot fresh weight (0.993 g) and shoot dry weight (0.058 g) were observed in the treatment where brackish water was applied throughout the growth period.

The bacterial inoculation significantly reduced the adverse effect of brackish water. The PGPR strain S14 increased the shoot fresh weight and shoot dry weight of maize seedlings grown with brackish water by 103 and 206%, respectively compared with the corresponding un-inoculated control.Pot trial: The data from the pot trial revealed that brackish water significantly reduced the growth (Table III and IV) and yield (Table V) of maize crop, and decreased the chlorophyll content (Fig. 1) and K+/Na+ ratio (Fig. 4) in

Table I: Comaprarive effect of bacterial strains on roo growth of maize grown with brackish water unde axenic conditions (Average of three replicates)

PGPR Strain###N0###Bo###C###D

Root length (mm)

Control###2450 ab###941 c###1351 bc###1446 bc

###2972 a###1612 bc###2967 a###1673 bc

###2908 a###2104 ab###2224 ab###2863 a

LSD value###967.5

Root fresh weight (g)

Control###0.448 e###0.137 f###0.382 e###0.174 f

S14###0.583 cd###0.667 ab###0.616 bc###0.584 ed

S20###0.734 a###0.529 d###0.587 cd###0.734 a

LSD value###0.0754

Root dry weight (g)

Control###0.095 ab###0.036 b###0.057 b###0.065 ab

S14###0.092 ab###0.065 ab###0.075 ab###0.066 ab

S20###0.120 a###0.069 ab###0.088 ab###0.079 ab

Table II: Comaprarive effect of bacterial strains on shoot growth of maize grown with brackish water under axenic conditions (Average of three replicates)

PGPR Strain###N0###B0###C###D

Shoot Ien2th (cm)

Control###32.43 ef###24.83 g###30.67 f###31.33 f

###38.50 b###36.67 be###34.58 ede###33.50 def

###42.33 a###37.33 be###38.33 b###36.17 bed

LSD value###2.819

Shoot fresh weight (g))

Control###2.685 ab###0.993 e###1.590 d###1.707 d

ST4###2.655 a1###2.217 be###1.868 ed###1.903 cd

S70###3.047 a###1.868 cd###2.232 bc###2.560 b

LSD value###0.4296

Shoot dry weight (g)

Control###0.198 be###0.058 f###0.0780 ef###0.143 ed

514###0.213 b###0.177 bed###0.117 de###0.150 cd

S20###0.280 a###0.150 cd###0.186bc###0.175bcd

LSD value###0.0533

Table III: Comaprarive effect of bacterial strains on shoot growth of maize grown with brackish water in pot trial (Average of three replicates)

PGPR Strain###N0###B0###NB###BN

Plant hei2ht (cm)

Contml###140.3 a###108.0 d###139.0 ab###113.7 cd

###142.7 a###114.5 cd###140.7 a###126.0 be

###142.0 a###125.3 be###136.7 ab###136.0 ab

LSD value###12.89

Shoot fresh weight (g)

Control###147.8 b###59.33 I###89.60 f###80.33 g

###147.4 b###73.67 h###116.4 d###96.67 e

###155.3 a###87.00 f###137.3 c###117.7 d

LSD value###5.041

Shoot dry weight (g)

Control###53.11 b###23.73 g###46.55 c###32.13 f

Si4###56.52 b###26.13 g###35.84 de###38.67 d

###62.67 a###33.13 ef###54.93 b###47.07 c

LSD value###3.256

Table IV: Comaprarive effect of bacterial strains on root growth of maize grown with brackish water in pot trial (Average of three replicates)

PGPR Strain###N###B###NB###BN

Root length (cm)

Control###61.33 ab###53.33 b###59.00 ab###58.67 ab

###62.50 ab###57.00 ab###68.33 a###61.67 ab

###65.67 ab###59.33 ab###69.67 a###61.33 ab

LSIJ value###10.87

Root fresh weight (g)

Control###32.24 ab###10.80 f###13.50 ef###12.74 f

S14###31.56 abc###11.78 f###14.24 ef###18.98 def

S20###38.11 a###19.58 def###24.99 bcd###22.82 ede

LSD value###8.572

Root dry weight (g)

Contml###7.217 abc###2.660 d###5.973 bcd###5.070 bcd

S14###7.430 abc###3.963 cd###6.567 abc###6.867 abc

S20###10.20 a###5.107 bcd###7.377 abc###8.310 ab

LSD value###3.352

the maize leaves. However, the bacterial strains significantly reduced the inhibitory effect of brackish water on maize growth and yield, and increased the chlorophyll content and K+/Na+ ratio but with different degrees of efficacy. Moreover, brackish water application at the vegetative stage was more deleterious than at the reproductive stage.

The results (Table III) showed that the minimum plant height was found in the case of treatment B, where all irrigations were with brackish water. Where that was not the case, treatment NB, where brackish water was applied at the reproductive stage, inhibited significantly less the shoot growth than the treatment BN, where brackish water was applied at the vegetative stage. This inhibitory effect was again reduced by inoculation with both PGPR strains (S14 and S20). The bacterial strains increased the plant height up to 16% but the isolate S20 was more efficient in the treatment B.

Shoot fresh weight (Table III) of maize was improved by inoculation with PGPR strains. Both the PGPR strains reduced the inhibitory effect of brackish water at vegetative as well as reproductive stages. These strains increased the shoot fresh weight up to 24 to 47%, in the treatment B; where brackish water was applied throughout the growth period, as compared with the respective un-inoculated control. However, the maximum increase in shoot fresh weight was observed with the strain S20, which was 47% more than the respective un-inoculated control. As in the case of shoot fresh weight, the bacterial inoculation was also equally efficient in increasing the shoot dry weight of maize plants and the results were similar to those of shoot fresh weight in case of S20 but S14 was non-significant in case of brackish water treatment. A 6 to 40% increase in shoot dry weight was observed in case of inoculated plants compared.

Table V: Comaprarive effect of bacterial strains on yield parameters of maize grown with brackish water in pot trial (Average of three replicates)

PCPR Strain###N###B###NB###BN

Cob length (cm)

Control###13.00 b###6.333 f###9.000 de###7.667 ef

###13.50 b###8.000 e###12.67 b###11.00 e

###15.80 a###9.667 cd###14.00 b###10.83 c

LSD value###1.442

No. of grains row1

Control###12.83 ab###8.00 d###11.00 abed###8.333 cd

Sj4###12.82 ab###8.887 bcd###12.17 abe###10.48 abcd

S20###13.61 a###9.220 bed###12.56 ab###10.61 abcd

LSD value###3.537

100-grain weight (g)

Control###22.98 ab###12.41 e###19.96 abed###17.98 bede

S14###22.71 ab###14.76 de###21.76 abe###20.27 abed

S:0###24.97 a###16.01 ede###24.58 a###22.44 ab

LSD value###5.722

Grain yield planf' (g)

Control###29.22 ede###19.15 h###24.17 g###20.08 h

###31.24 bed###24.95 fg###31.63 be###23.74 g

###34.92 a###27.70 ef###34.09 ab###28.21 de

LSD value###2.93

with the respective un-inoculated control in all the treatments but the maximum increase in shoot dry weight (40%) over the respective un-inoculated control was observed in the case of S20 inoculation in treatment B, where all irrigations were applied with brackish water.

The root length of maize plants (Table IV) was inhibited by the application of brackish water and the minimum root length was observed in the un-inoculated plants with treatment B, where all irrigations were applied with brackish water. This was followed by BN, where brackish water was applied at the vegetative stage.

The maximum increase in root length was observed in case of S 20 inoculation in NB treatment, where brackish water was applied at reproductive stage; it increased the root length up to 18% over respective un-inoculated control but the increase was statistically non significant in all the treatments.

Root fresh and dry weight of maize plants was significantly reduced by brackish water application both at vegetative as well as reproductive stages but the effect was more pronounced at vegetative stage. However, PGPR inoculation reduced the inhibitory effect of brackish water on the root fresh and dry weight but the difference with the respective un-inoculated control was statistically non- significant. The strain S 20 was more efficient in increasing the root fresh as well as dry weight of maize plants under normal and brackish water application at vegetative as well as reproductive stages (Table IV).

The results (Table V) showed that bacterial inoculation reduced the inhibitory effect of brackish water on the cob length of maize both at vegetative as well as reproductive stages. Brackish water reduced the cob length 51% over the normal water treatment. However, PGPR inoculation increased the cob length (4 to 53 %) over the respective un-inoculated control. The maximum increase (53%) in cob length over the respective un-inoculated control was observed with S 20 inoculation in B, where throughout brackish water was applied. The number of grains per row (Table V) was also improved by PGPR inoculation with normal as well as brackish water applications at vegetative and reproductive stages. But the results were statistically non-significant compared with the respective un-inoculated controls.

Brackish water significantly reduced 100-grain weight (Table V) but a lesser reduction in 100-grain weight was observed in the treatments NB and BN, where brackish water was applied at reproductive and vegetative stage, respectively. The PGPR inoculation improved the 100-grain weight in all the treatments but the increase was statistically non-significant compared with the respective un-inoculated control. PGPR inoculation also improved the grain yield of maize, which otherwise was drastically reduced by the brackish water application (Table V). Again the strain S 20 was the more efficient one in all the brackish water treatments. The maximum increase in grain yield (45%) over the respective un-inoculated control by S 20 was observed in treatment B, where all irrigations were applied with brackish water.

The PGPR inoculation also significantly improved the grain yield in treatment NB and BN, where brackish water was pplied at vegetative and reproductive stage, respectively. However, non significant results were observed by S 14 inoculation in N; where throughout normal water was applied. It is evident from the data (Fig. 1) that the PGPR inoculation increased the chlorophyll contents of maize plant, which otherwise were decreased by brackish water application and the increase was more in the treatment B, where all irrigations were applied with brackish water.

The PGPR strain S20 was more efficient in increasing the Na+ contents from 2 to 10% and the maximum decrease chlorophyll contents and the maximum increase (28%) over the respective control was observed in case of B.

The data regarding Na+ content (Fig. 2) showed that the Na+ content increased in all the treatments with brackish water application. However, inoculation with PGPR strains significantly decreased the Na+ content as compared to un-inoculated control. It was also observed that the effect of strains was more pronounced in the treatments, where brackish water was applied than with the normal water treatment where the results were non-significant. The PGPR inoculation when brackish water was used decreased the

Na+ contents from 2 to 10% and the maximum decrease (10%) was observed with the S20 strain in treatment B, where all irrigations were with brackish water.

In contrast to Na+, K+ contents of maize plant decreased under salinity stress. However, plants treated with PGPR strains showed a higher K+ content compared with un-inoculated control at the respective brackish water treatments (Fig. 3). The PGPR strain S20 was more efficient and it increased the K+ content up to 5% in the treatment BN, where brackish water was applied at vegetative stage. Under salinity stress due to brackish water, Na+ concentration in the leaf sap increased in all the treatments and K+ concentration decreased, however inoculation with PGPR strains decreased the Na+ content and enhanced the K+ uptake thus improving the K+/Na+ ratio in maize.

The maximum increase (15%) in K+/ Na+ ratio (Fig. 4) was observed in NB with the bacterial strain S20 over the respective un-inoculated control, which otherwise was reduced up to 17% by the brackish water application. After harvesting, the EC of the soil in the treatments N, B, NB and BN was 1.88, 17.06, 8.01 and 6.67 dS m-1, respectively.

DISCUSSION

This study conducted under axenic and natural conditions, demonstrates the potential of plant growth promoting rhizobacteria (PGPR) containing ACC- deaminase along with some other plant growth promoting characteristics (Nadeem et al., 2007) for improving growth and yield of maize irrigated with synthetic brackish water. It was observed that PGPR strains (E. aerogenes and P. fluorescens) reduced the depressing effect of brackish water on the growth and yield of maize but P. fluorescens was more efficient.

The application of brackish water significantly reduced the maize growth and yield, which might be attributed to increased ethylene production. It is well documented that ethylene synthesis is accelerated in response to various environmental stresses, including salinity (Mayak et al., 2004) and its high concentration inhibits the plant growth. It is well established that brackish water results in the buildup of secondary salinity which induces osmotic stress by limiting absorption of water from the soil (Mayak et al., 2004) and ionic stress resulting from high concentrations of potentially toxic ions within the plant cells, which may reduce plant growth. It also affects the crop yield as it has pronounced adverse effects on reproductive growth, and the number and fresh weight of pods plant-1 in mung bean (Vigna radiata L.) decreased with increasing salinity (Elahi et al., 2004).

In our study, inoculation with PGPR strains, showingACC-deaminase activity significantly enhanced the root, shoot and other growth and yield contributing parameters of maize grown with brackish water under axenic and pot conditions. It was observed that inoculation was even more effective in the treatment B, where brackish water was applied throughout the growth period. It is very likely that PGPR strains promoted root growth by lowering the endogenous ethylene levels due to their ACC-deaminase activity which metabolizes ACC, the immediate precursor of ethylene thus lowering the ethylene level in plants and eliminating the inhibitory effect of stress-induced high ethylene concentrations (Glick et al., 1999; Zhenyu et al.,2007). Very recently, Nadeem et al. (2010) have also reported a positive correlation between ACC deaminase activity and root elongation in maize seedlings grown under salt-stressed axenic conditions.

The better root growth due to inoculation with PGPR containing ACC-deaminase activity was postively correlated with shoot growth. Our results are also supported by the findings of Principe et al. (2007) who reported that plants inoculated under saline conditions produced more shoot dry weight as compared to un-inoculated control (Nadeem et al., 2009; Ahmad et al.,2011) have confirmed the ability of PGPR containing ACC- deaminase to reduce the salinity-induced classical triple response in maize, a typical assay for the confirmation of ethylene production.

In the present study that inoculation with PGPR strains increased the chlorophyll contents of maize. Increased stress-induced ethylene causes senescence (Arshad and Frankenberger Jr., 2002) thus decreasing chlorophyll content. So this may be due to the inhibition of ethylene synthesis by PGPR, thus decreasing the chlorophyll decay. The increased chlorophyll content may also be due to increased leaf area index due to decreased inhibitory effect of ethylene. These results are supported by the work of Nadeem et al. (2010) who reported significantly increased chlorophyll contents in maize plants inoculated with PGPR strains grown under salinity stress.

It was observed in the present study that the brackish water treatment changed the Na+ and K+ concentration and thus induced nutritional imbalance in plants which may be one of the main damages caused by the salt stress (Greenway and Munns, 1980). It was reported that an increase in Na+ content in the rooting medium caused an increase in Na+ uptake and a decrease in K+ content of plant (Pervaiz et al., 2002). Na+ inhibits the uptake of K+ and results in toxic accumulation of Na+ (Saqib et al., 2000). In our study, inoculation markedly altered the selectivity of Na+ and K+ uptake by plants. Inoculation restricted the Na+ uptake and enhanced the uptake of K+, consequently increasing K+/Na+ ratio. The lower uptake of Na+ by inoculated roots might be due to the decreased apoplasmic flow of Na+ into the vascular tissues, caused by a higher proportion of root zone being covered with soil sheath due to inoculation.

Nadeem et al. (2010) also reported that maize plants treated with PGPR having ACC-deaminase activity showed low Na+ contents and high K+ than un- inoculated plants. This decrease in Na+ concentration may also be due to the production of exopolysaccharides (EPSs) by these bacteria (Nadeem et al., 2007). Such bacterial strains have the ability to bind cations including Na+ (Geddie and Sutherland, 1993). These EPS-producing bacteria under salt-stressed conditions have been found to restrict Na+ uptake by roots (Ashraf et al., 2004).

It was also observed in our studies that the application of brackish water at the vegetative stage was more deleterious than at reproductive stage and it is in line with the findings of Maas et al. (1983) who reported that maize was more sensitive during the vegetative growth stage than during the germination and reproductive stages. They also concluded that brackish water could be used during and after tasseling without reducing the yield.

The variable efficacy of strains was observed in reducing the effect of brackish water and thus improving plant growth and yield. This variation in growth promotion might be due to variation in their efficacy to colonize the germinating roots and hydrolyze ACC along with some other mechanisms which have already been established in our previous study (Nadeem et al., 2007).

In conclusion, adverse effects associated with the application of brackish water could effectively be reduced through bacterial inoculation and the strain P. fluorescens was more effective in this regard, which may be tested further to develop a biofertilizer. Moreover, application of brackish water at reproductive stage had less inhibitory effect on maize growth and yield, thus the salt tolerance by maize crop during the later stages of growth was much higher than during the seedling stage. So if normal water is available, it may be applied at vegetative stage followed by brackish water at reproductive stage with less yield loss. The approach has very good prospects for using brackish water for sustainable maize production particularly in the scenario deteriorating/decreasing water resources. Acknowledgement: We are thankful to Dr. Maria L.W. Sels for editing this manuscript.

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Author:Zahir, Zahir A.; Akhtar, Saqib Saleem; Ahmad, Maqshoof; Nadeem, Sajid Mehmood; Nadeem, Saifullah
Publication:International Journal of Agriculture and Biology
Article Type:Report
Geographic Code:9PAKI
Date:Jun 30, 2012
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