Fungi As Plant Growth Promoter 21 Pgpf

Many fungi isolated from soil can inhibit plant growth. Generally, the inhibition of plant growth is mostly caused by plant-pathogenic fungi, however, some fungi that are not normally considered as pathogens can also inhibit plant growth. These fungi have been termed indefinite pathogenic fungi, and in one study, isolates of Eupenicillium javanicum, Penicillium janthinellim, P. citreonigrum, and P. citrinum obtained from roots of zinnia plant caused a 23-57% inhibition of the growth of the same plant (Yuen and Schroth 1986). Gamliel and Katan (1991) reported that almost all the fungi isolated from the rhizosphere and roots of tomato inhibited the growth of the plant. In contrast other soil-borne fungi, such as Trichoderma sp., Rhizoctonia solani, and others, can promote significant plant growth. Most of these PGPF have a high rhizosphere competence as a character. Because the genera found to be PGPF are common soil-borne fungi, there is a possibility that fungi having a similar role of PGPF exist widely in natural ecosystems. Some examples of plant growth promotion by PGPF are shown in Table 1. Most of these studies were quantified from the relative dry weights of root or above-ground part of treated plant seedlings with PGPF compared to nontreated ones over periods as short as 4 weeks. In some cases, significant growth promoting effects of PGPF were observed as increased yield of plants grown in fields over longer periods of 14 weeks or more (Shivanna et al. 1994).

2.1.1 PGPF in Trichoderma

Isolates of Trichoderma harzianum and T. koningii have been shown to enhance seedling emergence in tomato with increased shoot and root dry weights when compared to nontreated control plants (Table 1) (Windham et al. 1986). These species also gave rise to increased shoot and root dry weights in tobacco (Table 1) (Windham et al. 1986). Isolates of T. viride have been reported to increase tomato plant height (Windham et al. 1986). Chang et al. (1986) have shown that isolates of T. harziamum enhanced seedling emergence in chilli pepper and promoted growth of tomato, chilli pepper,

Table 1 Growth promotion on plants treated with plant growth-promoting fungi (PGPF)

Fungus

Crop

Growth promoting effect

References

T. harzianum

Tomato

2.1-2.8 times

Increased dry weight, enhanced germination

Windham et al. (1986)

Tobacco

7.9 times

Increased dry weight, enhanced germination

Windham et al. (1986)

Red pepper

Enhanced germination

Chang et al. (1986)

Periwinkle

Enhanced germination

Chang et al. (1986)

Bentgrass

9.1 times

Increased dry weight and plant height

Hyakumachi (1994)

T. koningii

Tomato

2.6-3.2 times

Increased dry weight, enhanced germination

Windham et al. (1986)

Tomato

5.1 times

Increased dry weight

Hyakumachi (1994)

Tobacco

2.7 times

Increased dry weight, enhanced germination

Windham et al. (1986)

Ryegrass

4.4 times

Increased dry weight and plant height

Hyakumachi (1994)

T. ciride

Tomato

Increased plant height

Windham et al. (1986)

Sterile black fungus

Wheat

40%

Increased shoot dry weight

Speakman and Kruger (1984)

Rye

40%

Increased shoot dry weight

Speakman and Kruger (1984)

Sterile dark fungus

Wheat

30%

Increased shoot dry weight

Narita and Suzui (1991)

Sterile red fungus

Wheat

10-60%

Increased shoot fresh weight

Dewan and Sivasithamparam (1989)

Rye

10-60%

Increased shoot fresh weight

Dewan and Sivasithamparam (1989)

Ryegrass

10-60%

Increased shoot fresh weight

Dewan and Sivasithamparam (1989)

R. solani AG4

Radish

13.4-19.8%

Increased shoot fresh weight

Sneh et al. (1986)

28.4-36.0%

Increased shoot dry weight

Sneh et al. (1986)

Carrot

80.0-97.7%

Increased shoot fresh weight

Sneh et al. (1986)

55.0-150.5%

Increased shoot dry weight

Sneh et al. (1986)

Lettuce

58.4%

Increased shoot fresh weight

Sneh et al. (1986)

Cotton

28.7%

Increased yield

Sneh et al. (1986)

R. nigricans

Tomato

42%

Increased shoot dry weight

Lindsey and Baker (1967)

F. roseum

Tomato

54%

Increased shoot dry weight

Lindsey and Baker (1967)

Phoma sp.

Wheat

46-77%

Number of grain

Shivanna et al. (1994)

Bean

23 -25%

Increased yield (Green house)

Shivanna et al. (1994)

11-52%

Increased yield (Field)

Shivanna et al. (1994)

Cucumber

2.1 times

Increased fresh weight (6 weeks)

Hyakumachi (unpublished)

1.8 times

Increased fresh weight (10 weeks)

Hyakumachi (unpublished)

and cucumber. Isolates of T. harziamum have also been used to enhance flowering of periwinkle and to increase the number of flowers per plant in chrysanthemum.

2.1.2 PGPF in Mycelial Fungi

Plant growth promotion has been obtained from isolates of mycelial fungi that do not produce any spores. These unidentified fungi have been termed sterile black fungus (SBF), sterile dark fungus (SDF), and sterile red fungus (SRF), and have been isolated from corn roots, wheat roots, and wheat and rye grass roots, respectively. Unidentified isolates considered to be SBF (Speakman and Kruger 1984) and SDF (Narita and Suzuki 1991) were shown to increase shoot dry weight in wheat and similarly isolates termed to SRF increased shoot wet weight in wheat (Dewan and Sivasithamparam 1989). Growth promotion by these unidentified fungi has been reported in other plants and isolates of SBF have been reported to increase shoot dry weight in barley (Speakman and Kruger 1984). Fungi considered as SRF have been reported to promote plant growth of rye, brome grass, chick pea, lupine, medic, pea, ryegrass, and clover, all of which are used as typical rotation crops with wheat, and resulted in an increased shoot fresh weight (Dewan and Sivasithamparam 1989). The mycelial isolates used in these studies have not been identified, although isolates of SRF are thought to be Basidiomycetes because of the presence of clamp connection. Strains of SBF and SDF are easily isolated from herbal plants as well as woody plants and their relationships to endophytic fungi are being currently considered.

2.1.3 PGPF in Rhizoctonia

A particular nonpathogenic strain of R. solani has shown growth promotion and significantly increased yield for various crops in field experiments (Sneh et al. 1986). These included increased wet and dry weights of radish roots and carrot roots, and increased weights of cotton fiber and wheat grains. In similar experiments with potato, although increases were observed in shoot and tuber weight until 63-70 days after transplanting, there was no increase in yield at the time of harvest. Some isolates of binucleate Rhizoctonia have been found to be PGPF (Harris et al. 1993; Villajuan-Abgona et al. 1996).

2.1.4 Other PGPF

Isolates of Rhizopus nigricans and Fusarium roseum have been reported to increase shoot dry weight in tomato (Lindsey and Baker 1967). Soil conditions such as pH, water, nutrient and organic content, together with the presence of other micro-organisms are important considerations for the introduction of beneficial micro-organisms into soil. Hyaku-machi (1994) reported the plant growth promotion effect of PGPF occurred in sterilized or nonsterilized nutrient-deficient and rich soils, potting soil, and most conspicuously in the nutrient-deficient one. The effect of PGPF was also observed in soil that had been converted to nutrient-rich by amendment with NPK fertilizer.

The duration of the plant-growth promoting effect of PGPF in the treated plants is an important factor for the earlier application. Increased growth responses were observed in wheat treated with PGPF isolates during the seedling stage (2 weeks after sowing), vegetative stage (4 weeks), pre-flowering stage (10 weeks), and seed maturation stage (14 weeks) (Hyakumachi 1994; Shivanna et al. 1994). All of the isolates used increased plant height, and also significantly increased the ear-head length, weight, seed number, and biomass (Table 1) (Shivanna et al. 1994).

In order to develop applications, it is important to isolate PGPF strains that (a) have high affinity for plants and can colonize their rhizosphere, (b) show high levels of plant growth promotion, and (c) offer consistent performance in field trials. Although an isolate with a wide host range is an ideal candidate, there is a requirement for isolates that show high specific effectiveness with an individual host plant.

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