Botanical Studies (2007) 48: 147-154.

3

Qiang-Sheng WU: E-mail: wuqiangsh@163.com; Tel:

+86-27-87284181.

*

Corresponding author: E-mail: renxuexia@mail.hzau.edu.

cn; Tel: +86-27-87286913.

INTRODUCTION

Arbuscular mycorrhiza symbiosis, a natural association

between the roots of higher plants and arbuscular

mycorrhizal fungi (AMF), are rather important in

horticultural crops, because AMF are believed to improve

host plants growth, water relations and acquisition of

nutrients especially P from soil (Maronek et al., 1981).

There is a role played by AMF in alleviating drought

stress of higher plants as it appears that drought resistance

is enhanced (Auge, 2001; 2004). Fidelibus et al. (2001)

showed that four Glomus species isolated from arid,

semiarid and mesic areas stimulated the root growth

(dry weight and length) of Citrus volkameriana, an d

leaf P concentration were 12-56% higher in arbuscular

mycorrhizal (AM) plants than in non-AM plants under

well-watered conditions. Mycorrhizal infection appeared

to improve establishment of citrus into transplant situations

by improving P uptake and reducing plant stress (Johnson

and Hummel, 1985). Most effects of the mycorrhizal

association were on stomatal regulation rather than on root

resistance (Levy and Krikun, 1980). However, the precise

mechanisms underpinning changes in water relations are

still in doubt.

In higher plants, metabolism of reactive oxygen

species (ROS), such as superoxide, hydrogen peroxide

and hydroxyl radicals is kept in dynamic balance under

well-watered conditions. Drought stress often induces

cellular damage and photo-oxidative damage, through the

accumulations of ROS. As a consequence, higher plants

evolve cellular responses like up-regulation of oxidative

phySIOlOgy

Five Glomus species affect water relations of Citrus

tangerine during drought stress

Qiang-Sheng WU

1,2,3

, Ying-Ning ZOU

2

, Ren-Xue XIA

1,

*, and Ming-Yuan WANG

1

1

College of Horticulture and Forestry, Huazhong Agricultural University, Wuhan, Hubei Province, 430070, P. R. China

2

College of Horticulture and Gardening, Yangtze University, Jingzhou, Hubei Province, 434025, P. R. China

(Recieved June 12, 2006; Accepted November 22, 2006)

ABSTRACT

. The efficacy of five Glomus species, Glomus mosseae, G. geosporum, G. versiforme, G.

etunicatum and G. diaphanum was studied for the ability to improve water relations of Citrus tangerine Hort.

ex Tanaka under well-watered and drought stress conditions in terms of growth, carbohydrate, photosynthetic

characteristic and antioxidant enzymes activities. The ranking of five Glomus species for mycorrhizal

dependency of C. tangerine was as follows: G. mosseae . G. geosporum > G. versiforme > G. etunicatum

> G. diaphanum. In general, the arbuscular mycorrhizal fungi used in this study showed benefical effects in

these parameters. The colonization by G. geosporum showed the highest plant height, leaf number per plant,

stem diameter, relative water content, soluble sugar, starch and total non-structural carbohydrates under well-

watered and drought stress conditions and G. etunicatum colonization the least effects. G. mosseae seedlings

showed the highest soluble protein concentration and catalase activity in leaves, G. diaphanum seedlings

showed the highest superoxide distumase activity, and G. versiforme seedlings showed the highest guaiacol

peroxidase activity. Both G. mosseae and G. geosporum colonization showed greater transpiration rates and

stomatal conductance. In addition, five Glomus species significantly decreased leaf temperature of mycorrhizal

seedlings. The different arbuscular mycorrhizal fungal species differed in their ability to improve water

relations of C. tangerine. Both G. mosseae and G. geosporum were more efficient fungi in improving water

relations of C . tangerine, and G. etunacatum was less efficient fungi. Arbuscular mycorrhizal symbiosis

improved water relations of C. tangerine in part due to increases of antioxidant enzymes.

Keywords: Arbuscular mycorrhizal fungi; Citrus; Drought; Glomus; Water relations.

Abbreviations: AM, arbuscular mycorrhizal; AMF, arbuscular mycorrhizal fungi; C AT, catalase; DS, drought

stressed; E, transpiration rates; G-POD, guaiacol peroxidase; g

s

, stomatal conductance; Lt, leaf temperature;

NSC, total non-structural carbohydrates; Pn, photosynthetic rates; ROS, reactive oxygen species; RWC, leaf

relative water content; SOD, superoxide distumase; WW, well-watered.

148

Botanical Studies, Vol. 48, 2007

stress protectors (Reddy et al., 2004). Antioxidant

defense enzymes, such as superoxide distumase (SOD),

catalase (CAT), ascorbate peroxidase, guaiacol peroxidase

(G-POD), and glutathione reductase, are designed to

minimize concentrations of superoxide

and hydrogen

peroxide. The antioxidants containing ascorbate and

glutathione are involved in scavenging ROS primarily

via the Halliwell-Asada pathway (Apel and Hirt, 2004).

Ruiz-Lozano (2003) reported that AM symbiosis might

increase the drought resistance of higher plants by

promoting antioxidant enzymes. However, the relationship

between AMF and antioxidants is poorly known. Although

antioxidants in bean (Lambais et al., 2003), red clover

(Palam et al., 1993) and some shrub species (Alguacil et

al., 2003) inoculated with AMF have been investigated,

the differences in antioxidant enzymes due to drought

and AMF have not been reported in citrus. We wanted to

determine whether the presence of AMF would influence

the antioxidant enzymes activities of citrus.

Citrus is one of the most important commercial fruit

crops grown in wide areas of China. Most citrus species

are fairly dependent on AMF as they have a positive

growth response to AMF (Graham and Syversten, 1985)

that are mostly Glomus species (Davies and Albrigo,

1994). Moreover, C. tangerine is one main citrus species

cultivated in southwest regions of China and is a fine

rootstock. Hence, basing on the positive effects of AMF

on citrus, we also investigated the responses to different

Glomus species on growth, carbohydrate, antioxidant

enzymes activities and photosynthesis characteristics of C.

tangerine, in order to select an efficient AM fungus from

Glomus.

MATERIAlS AND METhODS

plant culture and biological materials

Seeds of Citrus tangerine Hort. ex Tanaka were

surface sterilized in 70% alcohol for 5 min, subsequently

rinsed with sterilized water and germinated on wet filter

paper in Petri dishes in dark at 28¢XC. Seven-day-old

uniform seedlings were transplanted into plastic pots

(15¡Ñ20 cm) containing 4.010 kg autoclaved (0.11 MPa,

121¢XC, 2 h) growing mixture of yellow soil (from Fruit

Sample Garden, Huazhong Agricultural University), plus

vermiculite and perlite (6:2:1, v/v/v), with pH 5.6, 0.9%

organic matter, 9.99 mg kg

-1

available phosphorus, 84.53

mg kg

-1

alkali hydrolyzable nitrogen and 84.13 mg kg

-1

available potassium. The experimental pots were placed

in a plastic greenhouse under natural light conditions from

March to September, which had no controlling temperature

equipment. The midday photosynthetically active photon

flux density ranged from 550 to 900 £gmol m

-2

s

-2

. The

average day/night temperature was 25/18¢XC and the

relative humidity was 60-95%.

The AMF used in this study were Glomus versiforme

(Karsten) Berch, G. mosseae (Nicol. & Gerd.) Gerdemann

& Trappe, G. geosporum (Nicol. & Gerd.) Walker, G.

diaphanum Morton & Walker, and G. etunicatum Becker

& Gerdemann obtaining from Institute of Plant Nutrition

and Resources, Beijing Academy of Agriculture and

Forestry Sciences. The inoculated dosage was approx.

1100 spores per pot. Non-AMF treatments (control

seedlings) received the same weight of autoclaved growth

mixture. The inocula were placed 5 cm below citrus at

transplanting time.

Drought treatments began 90 days after transplant.

Well-watered (WW) pots were maintained at 75% of the

relative soil water content (corresponding to field capacity)

by weighing the substrate before and after drying at 105¢XC

for 24 h), and drought stressed (DS) pots were maintained

at 55% of the relative soil water content (corresponding

to 73% of field capacity). The water status in the substrate

was determined daily and the amount of water loss was

added to each pot in order to maintain the designed soil

water content.

Experimental design

The experimental treatments were made up of two soil

water regimes (WW and DS) and six AM inoculations (G.

versiforme, G. mosseae, G. geosporum, G. diaphanum,

G . etunicatum and non-AMF) and were arranged in a

randomized complete block design. Three replicates of

each AMF treatment were performed, totaling 36 pots with

four seedlings per pot.

parameter measurement

Eighty days after drought stress treatment were begun,

plant height, stem diameter and leaf number per plant

were recorded and the AM and non-AM seedlings were

harvested. Half of seedlings were separated into roots

and shoots, dried in hot-air oven at 75¢XC for 2 d, and dry

weights of shoots and roots were recorded.

Part of fresh roots was carefully washed, cut into

1-cm long root segments and fixed by FAA at least 24 h.

The root samples were cleared with 10% KOH solution,

stained with 0.05% trypan blue in lactophenol (Phillips

and Hayman, 1970), and examined microscopically for

root colonization. At the same time, the number of entry

points, vesicules and arbuscules were calculated in the

infected root. The AMF infected percentage was calculated

by the following formula: AM colonization (%) = 100¡Ñ

root length infected/root length observed. Mycorrhizal

dependency was defined as the ratio of the dry weight (dry

wt.) of the AM seedlings and non-AM seedlings (Graham

and Syvertsen, 1985).

Fresh leaf samples were homogenized in 5 mL of

phosphate buffer (0.1 mol L

-1

, pH 7.8), centrifuged at 4,200

¡Ñg for 10 min at 4¢XC, and the supernatant was used for

assays of soluble protein, SOD, G-POD and CAT. SOD

activity was analyzed using the methods of Giannopolitis

and Ries (1987) and was expressed as Unit g

-1

fresh weight

(fwt.). One SOD unit was defined as the amount of enzyme

that inhibited 50% nitro blue tetrazolium by light. CAT

activity was measured according to Aebi (1984). G-POD

WU et al. ¡X Mycorrhizal effects on

Citrus tangerine

149

activity was determined using the method

of Chance

and Maehly (1955). Soluble protein was evaluated using

bovine serum albumin

as the standard (Bradford, 1976).

Soluble sugar and starch contents were determined

by the anthrone method (Wu and Xia, 2006). Total non-

structural carbohydrates (NSC) were the sum of soluble

sugars and starch.

Stomatal conductance (g

s

), transpiration rates (E ),

photosynthetic rates (Pn) and leaf temperature (Lt) were

measured using TPS-1 Photosynthesis System (USA) on

four replicated leaves randomized from three replicate

pots of each treatment from 10:00 to 11:00 am. The

fifth full leaf from the apices of these seedlings was

measured in the place where they grew. The reference

CO

2

concentration ranged from 354 to 368 £gmol mol

-1

;

the cuvette air temperature ranged from 30 to 34¢XC; the

natural photosynthetically active radiation ranged from

175 to 381 £gmol m

-2

s

-1

.

The fifth full leaf from the apices of these seedlings was

used for leaf relative water content (RWC) assays (Wu and

Xia, 2006).

Statistical analysis

The experimental data were subjected to analysis of

variance (ANOVA) with Statistical Analysis System (SAS)

8.1 software (SAS Institute Inc., Cary, N.C.) and Fisher¡¦s

Protected least significant difference (LSD) (p=0.05) was

used to compare treatment means. CORR program was

used to analyze the correlation of two variables.

RESUlTS

No mycorrhizal structure was found in any of the non-

AM seedlings (Table 1). The roots of C. tangerine were

infected by each Glomus species as shown by the presence

of entry points, vesicules and arbuscules. Drought stress

strongly reduced AM infection but the reduction in AM

infection was the least pronounced with G. versiforme.

Entry points, vesicules, arbuscules and AM colonization

were highest in seedlings colonized by G. mosseae than

in those colonized by other Glomus species under WW

conditions. The mycorrhizal developments in the roots

of AM seedlings inoculated with G . mosseae and G .

versiforme were higher than with other Glomus species

under DS conditions.

The highest shoot dry wt. was reached in seedlings

inoculated with G . mosseae under WW conditions

and G. geosporum under DS conditions (Table 2). The

enhancement of root dry wt. and plant dry wt. growth

under WW and DS conditions was highest by G. mosseae

and lowest by G. diaphanum. The ranking of five Glomus

species for the mycorrhizal dependency under WW

conditions was as follows: G. mosseae > G . geosporum

Table 2. Effects of five Glomus species inoculation and drought stress on shoot dry wt., root dry wt., plant dry wt. and mycorrhizal

dependency in Citrus tangerine.

AMF status Shoot dry wt. (g plant

-1

) Root dry wt. (g plant

-1

) Plant dry wt. (g plant

-1

) Mycorrhizal dependency

(%)

WW

DS

WW

DS

WW

DS

WW

DS

G. mosseae 0.87¡Ó0.27a 0.42¡Ó0.03b 0.65¡Ó0.24a 0.47¡Ó0.06a 1.45¡Ó0.51a 0.90¡Ó0.06a 309

265

G. versiforme 0.81¡Ó0.54ab 0.25¡Ó0.07c 0.50¡Ó0.20abc 0.30¡Ó0.11bc 1.33¡Ó0.74ab 0.54¡Ó0.17b 283

159

G. geosporum 0.83¡Ó0.35ab 0.57¡Ó0.19a 0.53¡Ó0.18ab 0.39¡Ó0.11ab 1.39¡Ó0.52a 0.96¡Ó0.28a 296

282

G. diaphanum 0.41¡Ó0.15bc 0.25¡Ó0.04c 0.28¡Ó0.10cd 0.20¡Ó0.06cd 0.69¡Ó0.25bc 0.45¡Ó0.06b 147

132

G. etunicatum 0.36¡Ó0.10c 0.23¡Ó0.05c 0.35¡Ó0.14bcd 0.26¡Ó0.12c 0.72¡Ó0.23bc 0.48¡Ó0.16b 153

141

Non-AMF 0.30¡Ó0.03c 0.22¡Ó0.02c 0.17¡Ó0.01d 0.13¡Ó0.02d 0.47¡Ó0.04c 0.34¡Ó0.01b 100

100

Mean¡ÓSD (n=6) followed by the same letter within a column shows non-significant difference at p<0.05 level.

Table 1. Effects of five Glomus species inoculation and drought stress on AM colonization, entry points, vesicules and arbuscules in

Citrus tangerine.

AMF status AM colonization (%) Entry points (no. cm

-1

root) Vesicules (no. cm

-1

root) Arbuscules (no. cm

-1

root)

WW

DS

WW

DS

WW

DS

WW

DS

G. mosseae 64.09¡Ó7.86a 34.60¡Ó3.61b 7.4¡Ó2.4a 4.2¡Ó1.1a 4.6¡Ó1.5a 2.0¡Ó0.7a 8.7¡Ó2.4a 3.1¡Ó0.6b

G. versiforme 47.80¡Ó5.16b 44.37¡Ó4.33a 4.3¡Ó1.3bc 4.6¡Ó1.1a 1.9¡Ó0.7b 1.7¡Ó0.7abc 4.6¡Ó0.7b 4.4¡Ó0.6a

G. geosporum 63.43¡Ó11.91a 29.91¡Ó3.49b 3.7¡Ó0.3bc 1.3¡Ó0.4bc 3.0¡Ó0.1ab 1.8¡Ó1.0ab 3.0¡Ó0.1bc 1.4¡Ó0.4c

G. diaphanum 25.76¡Ó0.92c 16.69¡Ó1.34c 6.3¡Ó2.1ab 2.2¡Ó0.4b 2.4¡Ó1.6b 0.8¡Ó0.3cd 5.5¡Ó3.2b 1.4¡Ó0.4c

G. etunicatum 27.07¡Ó4.55c 17.60¡Ó4.92c 3.6¡Ó0.3cb 1.4¡Ó0.4b 1.7¡Ó0.9bc 0.9¡Ó0.1bcd 2.5¡Ó1.2bc 1.1¡Ó0.4cd

Non-AMF

0¡Ó0d

0¡Ó0d

0¡Ó0d

0¡Ó0c

0¡Ó0c 0¡Ó0d

0¡Ó0c

0¡Ó0d

Mean¡ÓSD (n=3) followed by the same letter within a column shows non-significant difference at p<0.05 level.

150

Botanical Studies, Vol. 48, 2007

> G. versiforme > G. etunicatum > G. diaphanum. Under

DS conditions the ranking of Glomus species for the

mycorrhizal dependency was as follows: G. geosporum

> G. mosseae > G. versiforme > G . etunicatum > G .

diaphanum.

Plant height, leaf number per plant, stem diameter and

RWC were stimulated by AM colonization (Table 3).

Glomus geosporum inoculated seedlings had the highest

values and G. etunicatum inoculated seedlings the lowest

values.

The five Glomus species used in this experiment all

significantly increased the soluble sugar and NSC levels

in leaves of both WW and DS seedlings (Table 4). The

highest soluble sugar and NSC content were observed

in seedlings colonized by G . geosporum regardless of

water status. The AMF colonization also stimulated

starch accumulation in leaves of seedlings. Glomus

geosporum seedlings had the highest starch contents, but

G. etunicatum seedlings had the lowest starch contents.

The soluble protein, SOD, G-POD and CAT activities

were increased by the five Glomus species colonization

regardless of water status (Table 5). The soluble protein

concentration was significant higher in G. mosseae,

G . versiforme and G . diaphanum seedlings than in

the non-inoculated control seedlings under WW and

DS conditions. Except for G . etunicatum colonized

treatment, other Glomus species colonized treatments

significantly increased the SOD activities of leaves under

WW conditions, whereas the beneficial increments of

AMF were found only in G. mosseae and G. diaphanum

inoculated treatments under DS conditions. Except

for G. versiforme, other Glomus species colonization

did not affect the G-POD activities of leaves under

WW conditions. However, under DS conditions, other

Glomus species except for G . diaphanum significantly

increased the G-POD activities of leaves. Although AMF

colonization did not affect the CAT activities of leaves

under WW conditions, G. mosseae and G . versiforme

colonization notably increased the CAT activities of leaves

under DS conditions. Glomus mosseae seedlings showed

the highest soluble protein concentration and CAT activity

in leaves, G. diaphanum seedlings showed the highest

SOD activity, and G. versiforme seedlings showed the

highest G-POD activity.

Glomus mosseae and G. geosporum inoculations

significantly increased leaf E under two water regimes

compared with control treatment (Table 6). Comparison

of five Glomus species efficiency showed that G. mosseae

under WW conditions and G . versiforme under DS

conditions significantly increased Pn compared with non-

inoculated control treatment. Glomus mosseae-seedlings

showed the highest gs and G . etunicatum seedlings the

lowest value. The AM colonization usually decreased

Lt regardless of water treatments, and G. diaphanum

seedlings had the lowest Lt under WW and DS conditions.

Table 3. Effects of five Glomus species inoculation and drought stress on plant height, leaf number per plant, stem diameter and

relative water content in Citrus tangerine.

AMF status

Plant height (cm) Leaf number per plant Stem diameter (cm)

Relative water content (%)

WW

DS

WW

DS

WW

DS

WW

DS

G. mosseae 18.90¡Ó7.00ab 12.33¡Ó1.91b 22.3¡Ó3.3a 15.7¡Ó4.2ab 0.314¡Ó0.032ab 0.263¡Ó0.028a 94.54¡Ó3.10ab 92.08¡Ó2.72ab

G. versiforme 15.65¡Ó3.01bc 9.78¡Ó1.38bc 19.0¡Ó3.2a 13.8¡Ó2.9abc 0.269¡Ó0.038bc 0.217¡Ó0.023bc 94.84¡Ó4.27ab 94.25¡Ó3.01a

G. geosporum 21.40¡Ó4.27a 17.43¡Ó4.57a 22.2¡Ó3.9a 17.7¡Ó4.5a 0.323¡Ó0.040a 0.268¡Ó0.034a 96.04¡Ó1.97a 94.75¡Ó3.02a

G. diaphanum 17.25¡Ó5.49ab 9.62¡Ó2.06bc 20.8¡Ó7.4a 12.0¡Ó3.0bcd 0.315¡Ó0.067ab 0.231¡Ó0.031ab 92.74¡Ó1.47ab 90.26¡Ó3.13ab

G. etunicatum 11.27¡Ó2.42cd 8.22¡Ó1.46c 13.7¡Ó2.9b 10.7¡Ó2.5cd 0.250¡Ó0.042cd 0.205¡Ó0.046bc 90.97¡Ó0.32bc 88.18¡Ó4.19b

Non-AMF 8.70¡Ó1.88d 7.28¡Ó0.89c 10.8¡Ó1.7b 9.8¡Ó2.4d 0.201¡Ó0.028d 0.189¡Ó0.026c 88.25¡Ó1.86c 86.52¡Ó2.44b

Mean¡ÓSD followed by the same letter within a column shows non-significant difference at p<0.05 level. N=6 for plant height, leaf

number per plant and stem diameter; n=3 for relative water content.

Table 4. Effects of five Glomus species inoculation and drought stress on soluble sugar, starch, and total non-structural

carbohydrates (NSC) of leaf in Citrus tangerine.

AMF status

Soluble sugar (% fwt.)

Starch (% fwt.)

NSC (% fwt.)

WW

DS

WW

DS

WW

DS

G. mosseae

9.65¡Ó0.32a 10.30¡Ó0.17a 8.44¡Ó1.33b 8.60¡Ó0.40bc 18.09¡Ó1.08b 18.90¡Ó0.37b

G. versiforme 10.01¡Ó1.53a 10.25¡Ó1.37a 7.95¡Ó1.24b 8.97¡Ó0.23bc 17.96¡Ó2.30b 19.22¡Ó1.17b

G. geosporum 10.13¡Ó0.70a 11.18¡Ó0.85a 13.50¡Ó0.46a 12.69¡Ó0.96a 23.62¡Ó1.02a 23.87¡Ó0.28a

G. diaphanum 9.85¡Ó0.64a 10.20¡Ó0.34a 9.26¡Ó0.56b 9.80¡Ó0.07b 19.11¡Ó0.89b 20.00¡Ó0.28b

G. etunicatum 9.75¡Ó0.69a 9.97¡Ó1.42a 7.79¡Ó1.40b 8.16¡Ó1.66cd 17.54¡Ó1.02b 18.13¡Ó1.91b

Non-AMF

7.49¡Ó0.49b 7.95¡Ó1.11b 5.60¡Ó0.79c 6.79¡Ó0.55d 13.10¡Ó1.07c 14.74¡Ó1.56c

Mean¡ÓSD (n=3) followed by the same letter within a column shows non-significant difference at p<0.05 level.

WU et al. ¡X Mycorrhizal effects on

Citrus tangerine

151

152

Botanical Studies, Vol. 48, 2007

DISCUSSION

Different Glomus species inoculated on C. tangerine

showed different mycorrhizal development (Table 1).

Glomus species apparently responded differently to

soil conditions and host plant affected mycorrhizal

development. Moreover, plant species of low mycorrhizal

dependency tended to limit mycorrhizal colonization

(Graham et al., 1991). Under WW conditions, AM

colonization was significantly positive correlated

(r=0.9664, p<0.01) with corresponding mycorrhizal

dependency, whereas there was no correlated (r=0.3891,

p>0.05) between AM colonization and corresponding

mycorrhizal dependency under DS conditions. Thus,

variations of the correlation in this experiment were due

to different soil water status, which affected the responses

of AMF. The correlation between AM colonization and

mycorrhizal dependency in low P soil could represent

a functional response of the fungus to host carbon

availability (Graham et al., 1991).

In general, AM fungal efficiency is measured in

terms of growth status of host plant under different

environmental conditions (Ruiz-Lozano et al., 1995). In

this study, we observed that G. mosseae and G. geosporum

colonization resulted in higher plant biomass (shoot dry

wt., root dry wt. and plant dry wt.) and plant growth

responses (plant height, leaf number per plant, and stem

diameter) than other three Glomus species regardless of

water status (Tables 2, 3). Moreover, for mycorrhizal

dependence, C. tangerine highly depended on G. mosseae

and G. geosporum under two water regimes. Thus, the two

more efficient AMF used in this study were G. mosseae

and G. geosporum under WW and DS conditions. Our

study showed that AM seedlings had the higher levels of

soluble starch, soluble sugar and NSC in leaves than non-

AM seedlings regardless of water status (Table 4). These

results are supported by higher rates of Pn perhaps in

response to great carbon demand by its allocation to roots.

The result was in accord with the finding of Nemec and

Guy (1982), who reported that G. macrocarpus inoculation

increased the levels of total soluble sugars, starch and NSC

in leaves of Cleopatra mandarin.

It is well documented that AM symbiosis causes the

increases of antioxidant enzymes activities of host plants.

In an early study, SOD activity was higher in the roots of

mycorrhizal Lactuca sativa plants than those of non-AM

plants (Ruiz-Lozano et al., 1996). Lambais et al. (2003)

reported that, under low P soil conditions, CAT activity

was induced in Phaseolus vulgaris roots colonized by

G. clarum. Inoculation with G. claroideum increased the

activities of SOD and CAT in Rhamnus lycioides. We

also observed that inoculation with five Glomus species

usually increased the SOD, G-POD and CAT activities

in leaves of C. tangerine (Table 5). Moreover, Glomus

species had different responses to SOD, G-POD and CAT.

For example, G . diaphanum colonization significantly

increased SOD activity, G . versiforme colonization

G-POD activity, and G . mosseae colonization CAT

activity. It is known that SOD converts superoxide into

hydrogen peroxide, which is then eliminated by CAT.

When mycorrhizal plants had higher antioxidant enzymes

activities, cellular and photo-oxidative damages would

be decreased by AMF colonization. Thus, mycorrhizal

plants may have important ecological implications for

adaptation to adverse environmental conditions. Nodules

from stressed mycorrhizal Anthyllis cytisoides exhibited

higher antioxidant enzyme activities, which were

related to the removal of ROS (Goicoechea et al., 2005).

Although external hyphae, stomatal regulation and indirect

P nutrition can enhance water uptake by mycorrhizal

roots (Nelsen and Safir, 1982; Faber et al., 1991; Ruiz-

Lozano and Azcon, 1995; Goicoechea et al., 1997), it

seems that AM symbiosis may lead to a lower drought-

induced oxidative stress in mycorrhizal plants due to

higher antioxidant enzymes. Enzymatic polymorphism

in different strains of AMF might be related to their

efficiency in root colonization and their influence on

plant growth (Garcia-Garrido et al., 2000). In addition,

our results indicated that the level of AM colonization

was negative correlated with SOD activities (r=-0.6579,

p<0.05).

Glomus mosseae and G. versiforme had no significant

effect on gs of Vigna unguiculata under WW and DS

conditions (Diallo et al., 2001). Mycorrhizal infection

improved gs in Rosmarinus officinalis plants under DS

conditions (Sanchez-Blanco et al., 2004). The g s of

citrus taxa was usually not changed by AM colonization

(Auge, 2001). Our results found that C . tangerine

inoculated with G. mosseae, G . diaphanum and G.

geosporum had significant higher gs than non-inoculated

seedlings regardless of water status (Table 6), whereas

the inoculation of G. etunicatum used in this study did

not affect gs (Table 6). Mycorrhizal effects on g

s

were

attributable to mycorrhizal roots and mycorrhizal soils,

as well as to mycorrhizal infection of roots alone (Auge

et al., 2004). Ruiz-Lozano et al. (1995) reported that

the effect on gs was related to growth promotion. In

addition, we observed in this study that five Glomus

species significantly decreased Lt of mycorrhizal seedlings

compared with non-inoculated control treatment (Table 6).

This was likely due to the greater evaporative cooling of

higher E in mycorrhizal seedlings (Wu and Xia, 2006).

In conclusion, all the data suggested that five Glomus

species used in the present study had an ability to improve

the water relations of C. tangerine and higher antioxidant

enzmyes activities of leaves. The more efficient fungus

in C. tangerine was G. mosseae or G. geosporum and

the least was G . etunicatum under both WW and DS

conditions.

Acknowledgements. We are grateful to Y.-S. Wang

for providing the AM fungal inocula. This work was

supported by Science and Technology Exploitation Special

Item (2003EP090018; 2004EP090019) for Three-Gorge

migrant, Ministry of Science and Technology Department

of the People¡¦s Republic of China.

WU et al. ¡X Mycorrhizal effects on

Citrus tangerine

153

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Botanical Studies, Vol. 48, 2007