Combined Effects of Mycorrhiza and Rhizobium

It is difficult to isolate the effect of mycorrhiza on the growth of A. mangium without also considering the effect of Rhizobium. Thus, many reports in the literature include dual effects of mycorrhiza and Rhizobium.

Reddell and Warren (1987) reported that A. mangium forms associations with VAM fungi, and subsequent research suggests that this is the tree's dominant mycorrhizal association. Yantasath (1989) and Poonsawad and Yantasath (1991) reported that Acaulospora spp. and Glomus spp. were some of the dominant strains of VAM found in large numbers in acid sandy soils (pH 4.3-5.0) associated with A. mangium. Among ectomycorrhizal fungi, fruiting bodies of Thelephora ramariodes have been observed under plantations of A. mangium in Malaysia (Gibson 1981; Lee 1990) and the Philippines. However, this fungus probably does not form a true ectomycorrhizal association because it does not show typical symptoms, including fungal sheath and Hartig net. Nor are there demonstrated growth increases from inoculation with Tlwlepliora. More studies of possible ectomycorrhizal associations in A. mangium are warranted.

Only plants dependent on mycorrhiza for their growth show positive responses to inoculation with VAM fungi in growth and nutrient uptake (Yantasath and Poonsawad 1991). Chemical fertilizers can improve tree growth, but once the fertilizer is exhausted, growth will cease. Fortunately, native VAM populations where trees are planted (especially in logged-over areas and degraded grasslands) are often adequate for at least minimal growth. Tambalo-Zarate and dela Cruz (1991), in screening 18 crops (6 agronomic, 6 fruit species, and 6 reforestation species) for their responses to VAM inoculation, found A. mangium to be mycor-rhizal-dependent; that is, it would not survive in soils of marginal fertility with out forming VAM associations. Introduction of A. mangium in some parts of the tropics has thus been successful partly due to the presence of native VAM fungi. VAM fungi are not specific in their associations with crops.

The major factor limiting the large-scale use of mycorrhiza in the establishment of A. mangium has been the scarcity of cheap but effective mycorrhizal inoculants. Unlike ectomycorrhizal fungi, VAM fungi have not been isolated in vitro in the laboratory, making mass production of inoculants for nursery use difficult. The standard practice is to isolate the spores of VAM fungus by wet-sieving and decanting techniques, and inoculating these to the roots of trap plants (grown in soil) such as corn, sorghum, or other grasses. The spores germinate around the roots of the trap plant, sending out numerous mycelia which later invade the roots where they proliferate and sporulate. After 4-5 months, the potting soil—together with spores and infected roots—are harvested and used as inoculants.

Commercial VAM inoculants have been available on the market. Earlier, an important inoculant was NUTUI-LINK. which was produced in Salt Lake City, Utah, USA. and contained a single VAM fungus species. However, this product was phased out due to failed attempts to enhance growth of target crops, and contamination problems. Other inoculant technologies incorporated VAM into expanded clays, but use of this, too. has been limited due to contamination problems. Limited quantities of inoculants are produced by the nutrient film technique in UK. aerophonics in Florida, USA, and by root tissue culture in the USA.

In the Philippines, two VAM inoculants are produced by BIOTECH at UPLB under the commercial names MYKOVAM 1 and MYKOVAM 2. For M YKOVAM 1, three species of VAM fungi are grown separately in trap plants (Pensacola Bahia grass) using sterilized 3:1 soil:sand mixtures. After 4-5 months, the roots are harvested and Uie soil mixtures of the 3 VAM species are mixed thoroughly. The three species of VAM fungi in MYKOVAM 1 were earlier screened for effectiveness in promoting growth and yield of target crops. Spore counts determine the number of VAM propagules. MYKOVAM 1 is then added to target plants to give an inoculum potential of 50 spores/plant for container-grown seedlings and 200 spores/plant lor lield-grown seedlings. Most study results presented below are responses lo inoculations with MYKOVAM 1.

MYKOVAM 2 is a granulated form of MYKOVAM 1 which resembles granular chemical fertilizers. It is easier to handle than MYKOVAM 1 and has a longer shelf life (6-8 months, as opposed to 3-4 months for MYKOVAM 1). MYKOVAM 2 can also be stored at room temperature.

Some VAM fungi are more effective than others in promoting A. mangium growth. Dela Cruz et al. (1988) found that Glomus fasciculatus + Rhizobium and Gigaspora margarita + Rhizobium were most effective. These promoted higher N concentration and N uptake of seedlings, higher P content and uptake, and higher nodule weight and nitrogen fixation rates. Treatments yielding consistently poor growth included inoculation with the VAM fungus Sclcrocystis

Table 6.1. Periodic height growth (cm) of Acacia mangium inoculated with three VAM species in the nursery and outplanted in a degraded grassland.

Nursery Field (months)

Nursery Field (months)

Table 6.1. Periodic height growth (cm) of Acacia mangium inoculated with three VAM species in the nursery and outplanted in a degraded grassland.

VAM species

2 mo

% incr.

2.5

5

7

10

15

24

Uninoculated

5.0

___

10.73

11.10

11.89

15.40 c

26.73

b

All dead

Glomus etunicaium

14.0

180

23.17

30.51

33.69

45.40 a

82.90

a

136.30

Gl. macrocarputn

13.5

170

23.52

31.96

37.73

49.20 a

78.59

a

126.78

Gigaspora margarita

11.11

22

22.93

31.46

37.50

42.20 a

75.22

a

111.70

Numbers in the same column followed by the same letter are not significantly different at P = 0.05. Source: Dela Cruz et al. (1991)

clavispora + R, inoculation by Rhizobium alone, and uninoculated control.

Table 6.1 summarizes the results of another study testing inoculation of A. mangium with three VAM species in the nursery, followed by outplanting in a degraded grassland. Inoculation witfi any of the three VAM species resulted in very significant increases in both height and diameter. Uninoculated seedlings were shortest and showed symptoms of nutrient deficiency. Seedlings inoculated with any of the 3 VAM species grew significantly taller than the uninoculated seedlings. After 24 months in the field, all uninoculated seedlings were dead.

Lorilla et al. (1992) tested inoculation of A. mangium in the nursery using Glomus etunicaium, G. margarita, or Scuicllispora calospora. After 5 months, seedlings were outplanted in a degraded grassland. Five levels of chemical fcnilizer (14-14-14) were applied (0, 5, 10, 25, and 50 g per plant) at planting. In the nursery, seedlings inoculated with S. calospora performed best of all the treatments (Table 6.2). In the licld. Lorilla et al. (1992) reported that mycorrhizal treatments significantly increased seedling growth in terms of height and diameter. The most efficient VAM fungus remained S. calospora (Table 6.3). Interaction between VAM and fertilizer levels proved significant, with best growth obtained by seedlings inoculated with S. calospora in the nursery and fertilized with 50 g complete fertilizer per plant in the field. G. margarita replaced 549f,

Table 6.2. Height of A mangium inoculated with VAM after 5 months in the nursery.

Mycorrhizal

Height

Increase

treatment

(cm)

m

Uninoculated

19.9 b

___

Gl. etunicaium

16.3 b

-18

G. nuirgarila

13.6 c

-32

S. calospora

30.7 a

54

Numbers in (he same column followed by the same letter are not significantly different ai P = 0.05. Source: Lorilla el al. (1991)

Table 6.3. Height growth (cm) of A. mangium in the field following inoculation with VAM fungi in the nursery and different chemical fertilizer treatments at outplanting.

Fertilizer levels (g per plant)

VA mycorrhiza --Treatment treatments 0 5 10 25 50 means

IJninoculated

38.4

h

64.2

ph

91.6

efR

151.8

cd

147.0

cd

98.6

b

Gl. eiunicatum

102.2

ef

146.6

cd

149.4

cd

170.4

c

216.5

b

157.0

ab

G. margarita

72.6

fp

120.2

de

121.3

de

212.4

b

218.8

b

149.1

ab

S. calospora

112.2

e

167.2

c

178.4

c

216.4

b

249.7

a

184.8

a

Fertilizer means

81.4

c

124.6

b

135.2

b

187.8

a

208.0

a

Numbers in the same column followed by the same letter are not significantly different at p = 0.05. Source: Lorilla ci al. (1992)

Gl. eiunicatum 88%. and S. calospora 100% of the chemical fertilizers required for growth in the field.

Research in the Philippines is now incorporating VAM into traditional silvicultural technologies for growing A. mangium. In direct-seeding experiments. Dela Cruz et al. (1992b) placed A. mangium seeds into fabricated direct-seeding blocks (DSB) and inoculated them with either Gl. macrocarpum, G. margarita or combinations of the two fungi. Seeds received one of three pre-soaking treatments (soaked for 1 minute in hot water, 90°C; soaked overnight in tap water; soaked for 1 minute in hot water, then soaked overnight in tap water) to overcome seed dormancy. The DSB tested various combinations of agricultural waste materials (sugarcane bagasse, coconut coir dust, and sawdust) compressed into blocks about 2.5 cm high x 2.5 cm diameter. At one end, a 1.2 cm hole is punched in which the seeds and inoculants are placed. The DSB was directly seeded into pots containing soils collectcd from a degraded grassland. Seeds inoculated with G. margarita alone attained best height and diameter growth (Tabic 6.4).

Seeds inoeulated with G. margarita alone attained best height and diameter growth. A. mangium seeds need to be pre-soaked for 1 minute in hot water (90 C) and then soaked overnight in tap water for best germination. Inoculation of the DSB with Gl. macrocarpum or combinations of Gl. macrocarpum + G. margarita also gave better height and diameter growth responses than the uninoculatcd control or DSB without mycorrhizal treatments.

MYKOVAM 2, the granulated form of VAM inoculant containing G. margarita, has also increased height growth of A. mangium , with inoculated seedlings averaging 10.6 cm taller than uninoculatcd seedlings after 4 months (17.5 cm compared to 6.9 cm, respectively)(Dcla Cruz ct al. 1992a). Nursery workers accustomed to handling chemical fertilizers appreciate the granular nature of MYKOVAM 2.

Table 6.4. Height and diameter growth of Acacia mangium in response to seed pre-soaking and DSB-VAM treatments.

DSB-VAM treatments* fte-soaking

Pre-soaking - treatment treatments IJ DSB-M DSB+M1 DSB+M2 DSB+M12 means

Height growth (an)

Hot water 13.5 cf 19.2 de 18.0 def 42.3 b 15.3 ef 21.6 b

Soaked overnight 0.0 g 0.0 g 0.0 g 0.0 g 0.0 g 0.0 c

Hoi water + 12.9 f 14.2 ef 22.1 d 49.8 a 38.0c 26.6a soaked overnight

DSB-VAM Treatment 13.2 d means

Diameter growth (mm)

Hut water 170 d 230 d 220 d 455 ab 200 d 255 b

I lot water + 175 d 180d 305 c 525 a 425 b 322 a soaked overnight

DSB-VAM treatment 172 d means

205 cd 262 be 490 a

312b

*l! = Uninoculatcd with VAM; DSB-M = Direct-seeding block without mycorrhiza; DSB+ Ml = Direct-seeding block with (it. macrocarpum: DSB+M2 = Direct-seeding block with O. margarita. DSB+M12= Dircel-secding block with Gl. tnacrocarpum andG. margarita Numbers in the same column followed by the same letter are not significantly different at p = 0.05. Source: Déla Cruz el al. (1992b)

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