Introduction

Arbuscular mycorrhizal fungi (AMF), an important group of soil inhabiting microorganisms, can establish symbiotic associations with roots of approximately 80 % of land plants, based upon the reciprocal exchange of nutrients (Grace and others 2009). As obligate biotrophs, arbuscular mycorrhizal (AM) symbiosis needs to acquire 4–20 % additional carbohydrates from the host plant for the process of symbiosis remain metabolic active. In this regard, hexoses originated from sucrose cleavage are predominantly taken up by the AM fungus within the root and converted into trehalose and glycogen, the typical fungal carbohydrates (Bago and others 2003). Studies in the past have shown that root AMF colonization could induce significant changes in sucrose-cleaving enzymes (Schubert and others 2003; Wu and others 2013), involved in releasing hexoses to either the colonized root cells to cope with its relatively higher metabolic activity or to the fungus itself (Schubert and others 2003; Ferrol and Pérez-Tienda 2009). These sucrose-cleaving enzyme transcripts such as acid invertase and sucrose synthase (breakdown direction) act on the arbuscule-enriched root cortical cells of the host (Blee and Anderson 2002). Therefore, increased carbon (C) allocation to AM roots is generally associated with a stimulation of activities of sucrose-cleaving enzymes in AM roots (Wright and others 1998).

On the other hand, AMF functioning is affected by substrate phosphorus (P) levels, with positive effects at low soil P levels and negative effects at high soil P levels (Srivastava and others 2002; Schmidt and others 2010; Beltrano and others 2013). For example, at high P supply, inoculation with Glomus intraradices strongly depressed plant growth of Volkamer lemon (Citrus volkameriana), due to the greater C expenditure by AM roots than non-AM roots (Peng and others 1993). Until now, there was no evidence to evaluate whether AMF-mediated sucrose metabolism of the host is affected by substrate P levels.

It has been shown that AMF colonization can impact root development, with consequential effects on the anatomy, physiology, morphology of roots (Gutjahr and others 2009). Plant root morphology by and large determines root–soil contact areas, which is so important for increased P uptake (Gohoonia and Nielsen 2004; Wu and others 2012). In red tangerine, AMF inoculation improved root morphological traits by regulating endogenous polyamine metabolism (Wu and others 2012). However, it is still inconclusive as to whether AMF-induced root modification is dependent on substrate P level.

Trifoliate orange [Poncirus trifoliata (L.) Raf.], a close Citrus plant, possesses a shallow root system with few or no root hairs, thereby, being strongly dependent on AMs to absorb nutrients from the soil (Zobel and others 2006). In this background information, studies were undertaken to evaluate the effects of an AM fungus, Funneliformis mosseae, on growth, root morphology, and sucrose metabolism of trifoliate orange seedlings in response to different substrate P levels.

Materials and Methods

Experimental Setup

The experiment was conducted in 32 randomized factorial design with inoculation with or without F. mosseae (AMF and non-AMF) and three substrate P levels (0 (P0), 3 (P3), and 30 (P30) mM KH2PO4). Each treatment was replicated four times. The experiment was set up under glass-house conditions (photosynthetic photon flux density is 768 μmol/m2/s, day/night temperature 28 °C/21 °C, and relative air humidity 85 %) at the College of Horticulture and Gardening, Yangtze University, Jingzhou, China.

Seeds of trifoliate orange were sown in autoclaved sand after surface-sterilized with 70 % ethanol for 15 min, and grown in a controlled growth chamber at 28 °C/20 °C (day/night), 745 μmol/m2/s photosynthetic photon flux density, and 80 % relative humidity. After 3 weeks, 5-leaf-old seedlings with uniform size (nucellar seedlings) were transplanted into plastic pots (20 cm length × 15 cm width × 18 cm height) filled with acid washed sand (<4 mm size), collected from the riverside of the Yangtze River near the Jingzhou city.

The strain of F. mosseae (Nicol. & Gerd.) Schüßler & Walker used for the experiment was isolated from the rhizosphere of Incarvillea younghusbandii in Dangxiong, Tibet, China. The inoculum including AMF-infected roots, spores (~23 spores/g), hyphae, and sand were derived from a pot culture with the identified fungal spores and Trifolium repens as the host for 16 weeks. A 60 g (corresponding to ~1380 spores) sample of AMF inoculum was placed in the rhizosphere of trifoliate orange seedlings in each pot at the time of transplanting. The non-AMF treated pots were also supplied with 60 g of autoclaved (121 °C, 0.11 Mpa, 2 h) AMF inoculum plus 2 mL filtrate (25 μm) of inoculum to keep other microbial communities except F. mosseae.

The plants were acclimatized for 2 weeks of AMF inoculation daily with 100 mL distilled water per pot, and then three P levels (0, 3, and 30 mM KH2PO4) were imposed daily through 100 mL standard Hoagland solution per pot until the harvest. The three P levels (0, 3, and 30 mM KH2PO4) in standard Hoagland solution used here were referred to as low, medium, and high P levels for trifoliate orange growth, according to a number of studies (Jifon and others 2002; Guo and others 2003).

Observations and Analysis

Growth attributing parameters such as plant height, stem diameter, and leaf number per plant were recorded before harvest. Whole seedlings were harvested by uprooting and measured the shoot and root fresh weights. Subsequently, the root systems were washed with tap water to remove soil particles, placed in Regent’s water-proof trays without root overlap, and scanned by an Epson Perfection V700 Photo (Seiko Epson Corp, Nagano, Japan). The acquired images of roots were analyzed using the software WinRHIZO 2007b (Regent Instruments Incorporated, Quebec, Canada) to estimate total length, surface area and volume. The number of different order lateral roots was also manually counted.

Root segments 1.0 cm long were stained using the protocol as described by Phillips and Hayman (1970), and the root mycorrhiza was observed under a microscope for the presence of mycorrhizal mycelium. Root AM colonization was finally expressed as the percentage of colonized root lengths against observed total root lengths.

Fructose, glucose and sucrose in leaves and roots were extracted with 4 mL 80 % ethanol after 50 mg ground (0.5 mm sieve) dry samples were incubated for 40 min at 80 °C, and centrifugated at 2500×g for 5 min. The centrifuged residues were extracted again as per the above procedure, and the two supernatants were combined for analysis of carbohydrate concentration. The concentration of sucrose, glucose, and fructose was colorimetrically determined according to the procedure outlined by Zhang and Zai (2004). Sucrose was assayed with a mixture of 0.15 mL supernatant and 0.15 mL 2 mol/L NaOH at 100 °C for 5 min, which were then mixed together with 2.1 mL 30 % HCl and 0.6 mL 0.1 % resorcinol at 80 °C for 10 min and followed by measurement of absorbance of assay at 480 nm using sucrose as one standard. Glucose concentration was determined by mixing 0.5 mL of supernatant with 1.0 mL of prepared solution (1 mg/mL o-dianisidine dihydrochloride, 1 mg/mL horseradish peroxidase, and 1 U/mL glucose oxidase) at 30 °C for 5 min, then added 2 mL 10 mol/L H2SO4 solution to terminate the reaction, and followed by measurement of absorbance at 460 nm using one glucose standard. Fructose was assayed using a mixture of 0.8 mL 0.1 % resorcinol, 0.4 mL H2O, and 0.4 mL supernatant at 80 °C for 10 min. The absorbance of the mixed solution was determined at 480 nm using fructose as one standard solution.

A 0.2 g fresh tissue sample was homogenized in 4 mL 100 mM Hepes–NaOH buffer (pH 7.5), containing 20 mM EDTA, 1 mM NaF, 1 mM benzamidine, 20 mM cysteine and 1 % polyvinyl pyrrolidone, and centrifuged at 10,000×g for 30 min. The supernatants were dialyzed with 100 mM Hepes–NaOH buffer (pH 7.5) in a 21 mm dialysis bag for 12 h at 4 °C. The activity of acid invertase (AI), neutral invertase (NI), and sucrose synthase (SS, synthesis direction) in the supernatants, was assayed using the method described by Wu and others (2013). Meanwhile, an assay of SS (synthesis direction) activity is based on Tris–HCl buffer containing fructose and uridine diphosphate glucose, which can synthesize sucrose by SS (synthesis direction), and the enzyme reaction is ended by addition of NaOH.

Statistical Analysis

Data generated were statistically analyzed using the SAS software (v 8.1). Variance (ANOVA) was used to compare the significant difference with t tests at P < 0.05. The Pearson’s correlation coefficients between different variables were performed using the Proc Corr’s procedure of SAS.

Results

Root Mycorrhizal Colonization and Plant Growth

The varying magnitude of colonization of trifoliate orange seedlings was observed by F. mosseae in response to different P levels. Root AM colonization was rated as P0 > P3 > P30 (Fig. 1). Compared with the P3 treatment, P0 treatment significantly decreased plant height, stem diameter, leaf number, and shoot and root fresh weight, irrespective of AMF or non-AMF seedlings (Fig. 2a–e). On the other hand, P30 treatment did not significantly alter plant height and stem diameter, but significantly increased root fresh weight and decreased leaf number and shoot fresh weight, regardless of AMF and non-AMF seedlings. In addition, AMF inoculation significantly increased the different growth traits such as plant height, stem diameter, leaf number, and shoot and root fresh weight, irrespective of substrate P levels, except for a non-significant difference in plant height between the AMF and non-AMF seedlings exposed at the P30 level (Fig. 2a–e).

Fig. 1
figure 1

Root colonization of trifoliate orange seedlings by F. mosseae under 0, 3, and 30 mM P level (defined as P0, P3, and P30). Data (mean ± SD, n = 4) followed by different letters above the bars among treatments indicate significant differences at the 5 % level

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Fig. 2
figure 2

Effect of F. mosseae on plant growth performance of trifoliate orange (P. trifoliata) seedlings grown under 0, 3, and 30 mM P level (defined as P0, P3, and P30). Data (mean ± SD, n = 4) are significantly different (P < 0.05) if followed by different letters above the bars between P levels for the same F. mosseae treatment (a–c) or between F. mosseae treatments for the same P level (x, y). Black columns and white columns indicate the seedlings inoculated with AMF and non-AMF, respectively

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Root Morphology

Root total length was significantly higher under P3 and P30 than under P0 treatment, irrespective of AM or non-AM status (Fig. 3a). Other root traits such as root surface area and volume significantly increased with increasing levels of substrate P (Fig. 3b, c). AMF colonization significantly increased the root total length, surface area, and volume, irrespective of substrate P level, except that root surface area and root volume were not significantly different between the AMF and non-AMF seedling grown under P3 and P30, respectively (Fig. 3a–c). Mycorrhizal seedlings showed significantly higher numbers of first-, second-, and third-order lateral roots than under non-mycorrhizal seedlings, regardless of substrate P level, although no significant difference in the number of first-order and second-order lateral roots was observed between AMF and non-AMF seedlings under P30 and P3, respectively (Fig. 3d–f).

Fig. 3
figure 3

Effects of F. mosseae on root morphological traits of trifoliate orange (P. trifoliata) seedlings grown under 0, 3, and 30 mM P level (defined as P0, P3, and P30). Data (mean ± SD, n = 4) are significantly different (P < 0.05) if followed by different letters above the bars between P levels for the same F. mosseae treatment (a–c) or between F. mosseae treatments for the same P level (x, y). Black columns and white columns indicate the seedlings inoculated with AMF and non-AMF, respectively

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Carbohydrate Concentrations

Different P treatments in the order of their increasing efficiency of leaf sucrose were ranked as P3 > P0 ≈ P30 and P30 > P0 > P3 under non-mycorrhization and mycorrhization, respectively (Fig. 4a). The effect of different P treatments in relation to root sucrose concentration was classified as P30 ≥ P3 ≥ P0 and P0 > P3 ≈ P30 under non-mycorrhization and mycorrhization, respectively (Fig. 4b). The concentration of glucose in both leaves and roots followed a different pattern of response accruing through mycorrhization versus non-mycorrhization. Significantly higher glucose concentration in leaves was ranked as P0 ≈ P3 > P30 under non-mycorrhization and as P0 ≥ P3 ≥ P30 under mycorrhization (Fig. 4c), whereas significantly higher glucose concentrations in root was ranked as P0 > P3 > P30 under both mycorrhization and non-mycorrhization (Fig. 4d). The concentration of fructose in response to different P levels followed almost the same pattern as sucrose, irrespective of AMF treatments, with significantly highest fructose concentration in leaves observed at P30 (Fig. 4e). However, in roots, the pattern of fructose accumulation in response to substrate P levels was ranked as P0 > P3 > P30 under mycorrhization and P0 ≈ P30 > P3 under non-mycorrhization, respectively (Fig. 4f).

Fig. 4
figure 4

Effects of F. mosseae on sucrose, glucose, and fructose concentrations in leaves and roots of trifoliate orange (P. trifoliata) seedlings grown under 0, 3, and 30 mM P level (defined as P0, P3, and P30). Data (mean ± SD, n = 4) are significantly different (P < 0.05) if followed by different letters above the bars between P levels for the same F. mosseae treatment (a–c) or between F. mosseae treatments for the same P level (x, y). Black columns and white columns indicate the seedlings inoculated with AMF and non-AMF, respectively

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Compared with non-AMF seedlings, AMF seedlings recorded 33.8 and 58.5 % higher leaf sucrose concentrations under P0 and P30, respectively (Fig. 4a). However, AMF inoculation produced no variation in sucrose concentration in roots with P3 treatment, but root sucrose significantly increased by 18.0 % and decreased by 23.1 % under P0 and P30 treatment, respectively (Fig. 4b).

AMF inoculation induced no change in leaf glucose concentration under P0 and P3 but significantly increased leaf glucose by 24.8 % under P30 (Fig. 4c). In roots, AMF seedlings observed 27.9, 12.7, and 21.3 % higher glucose concentrations with P0, P3, and P30, respectively, as compared with non-AMF seedlings (Fig. 4d).

Leaf fructose concentration was 10.2, 14.3, and 17.3 % significantly higher in AMF seedlings than in non-AMF seedlings under P0, P3, and P30 treatments, respectively (Fig. 4e). On the other hand, root fructose concentration was 73.1 and 73.4 % significantly higher in AMF seedling than in non-AMF seedlings under P0 and P3, respectively. However, root sucrose concentration was 18.1 % significantly lower under mycorrhization than under non-mycorrhization with P30 (Fig. 4f).

Sucrose Relevant Enzyme Activities

Compared with non-AM seedlings, AMF seedlings accounted for 21.7 and 5.3 % higher leaf AI activity under P0 and P3 but 25.3 % significantly lower leaf AI activity under P30 (Fig. 5a). In roots, AMF colonization showed no change in AI activity under P3, but significantly increased AI activity by 27.0 % under P30 and decreased by 25.7 % under P0 (Fig. 5b). In addition, the significantly higher rank of leaf AI activity was P3 > P0 ≈ P30 under mycorrhization and P30 > P3 > P0 under non-mycorrhization. The effect of different P treatments in relation to root AI activity was classified as P3 > P30 > P0 and P3 > P0 > P30 under mycorrhization and non-mycorrhization, respectively.

Fig. 5
figure 5

Effects of F. mosseae on acid invertase, neutral invertase, and sucrose synthase (synthesis direction) activities in leaves and roots of trifoliate orange (P. trifoliata) seedlings grown under 0, 3, and 30 mM P level (defined as P0, P3, and P30). Data (mean ± SD, n = 4) are significantly different (P < 0.05) if followed by different letters above the bars between P levels for the same F. mosseae treatment (a–c) or between F. mosseae treatments for the same P level (x, y). Black columns and white columns indicate the seedlings inoculated with AMF and non-AMF, respectively

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Mycorrhizal seedlings had 45.2 and 26.9 % significantly higher leaf NI activity under P0 and P30 (Fig. 5c). Although root NI activity was 56.2 and 48.0 % significantly higher in AMF than non-AMF seedlings with P0 and P3 treatments, respectively (Fig. 5d). No significant difference in root NI activity was observed between AMF and non-AMF seedlings under P30. Under P3, AMF colonization significantly decreased leaf NI activity by 35.7 %. Under mycorrhization, the significantly higher rank of leaf and root AI activity was P30 > P0 > P3 and P3 > P0 > P30, respectively. Under non-mycorrhization, leaf and root NI activity significantly increased with the increase of substrate P level.

Compared with non-AMF treatment, AMF treatment showed three types of responses on leaf SS activity: no change under P0 treatment, a significant increase under P30, and a significant decrease under P3 (Fig. 5e). Significantly higher leaf SS activity ranked as P30 > P0 > P3 under mycorrhization and as P3 ≈ P30 > P0 under non-mycorrhization. In roots, AMF seedlings recorded 25.4, 39.5, and 33.4 % lower SS activity than non-AMF seedlings under P0, P3, and P30, respectively (Fig. 5f). Root SS activity was significantly higher under P0 than under either P3 or P30, irrespective of AM or non-AM seedlings.

Correlationship Studies

Correlationship revealed that root AMF colonization was significantly positively correlated with all the three fractions of carbohydrates (glucose, sucrose, and fructose) concentrations of roots (Fig. 6). Correlation of root AMF colonization with root glucose was strongly higher than with root fructose. Leaf SS activity was significantly positively correlated with leaf sucrose concentration under both AMF or non-AMF conditions (Fig. 7). Interestingly, the correlation between leaf SS activity and leaf sucrose was higher under mycorrhization than under non-mycorrhization, suggesting that AMF inoculation aided in transforming the host plant, to metabolically more active.

Fig. 6
figure 6

Linear regression between root AM colonization and root sucrose, glucose, and fructose concentrations in F. mosseae-colonized trifoliate orange (P. trifoliata) seedlings grown under 0, 3, and 30 mM P level (n = 12)

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Fig. 7
figure 7

Linear regression between leaf sucrose synthase (synthesis direction) activity and leaf sucrose concentration in F. mosseae-colonized trifoliate orange (P. trifoliata) seedlings grown under 0, 3, and 30 mM P level (n = 12)

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Discussion

Our study showed that root mycorrhizal colonization of trifoliate orange by F. mosseae decreased with increasing substrate P levels. Earlier results by Beltrano and others (2013) showed a significant reduction in root AMF colonization with increasing P levels from 10 to 40 mg/kg soil in pepper plants inoculated with Glomus intraradices. A significant positive correlation of root AMF colonization with fructose, glucose, and sucrose concentrations of roots suggested the direct involvement of AMF in changes in root carbohydrate levels, irrespective of substrate P level. In addition, the response of root colonization to substrate P level is attributable to spore behavior, because external hyphae of spores were often mediated by substrate P level (Trindade and others 2006).

In the present study, the trifoliate orange seedlings between P3 and P30 conditions seemed to be little different in growth performance, indicating that high P is not required to accelerate the plant growth process, because the roots of trifoliate orange are relatively shallow and possess fewer and short root hairs, sometimes even no root hairs (Zhang and others 2013). We also observed that mycorrhizal seedlings maintained a significantly higher magnitude of growth, irrespective of substrate P level, suggesting that AMF-induced enhancement in plant growth is independent of the substrate P level, or this strain of F. mosseae is an efficient AM fungus for growth of trifoliate orange (Wu and others 2006), though the substrate P level strongly affected root mycorrhizal colonization in the present study. The present study further revealed that AMF inoculation improved plant growth performance, more distinctly under P0 than under P30. The AMF effect on plant growth responses resulted from the enhancement in uptake of native soil P by extraradical mycorrhizal mycelium (Grimoldi and others 2005). Possibly, AMF plants possessed a higher P-use-efficiency at a low P concentration than at a high P concentration (Ning and Cumming 2001).

Phosphorus application has been shown to increase root morphology in many plants (Li and others 2012). In our work, P0 treatment generally inhibited root morphological characters, but P30 treatment failed to produce the same magnitude of response (except a higher surface area and volume and lower third-order lateral root number), in comparison to P3 treatment, suggesting that trifoliate orange was more sensitive at low P than at high P. On the other hand, such changes of root morphology in response to P availability are an important factor in P-use-efficiency (Zobel and others 2006). However, AMF trifoliate orange showed higher root morphological traits than non-AMF seedlings, irrespective of substrate P level. Our previous studies in trifoliate orange inoculated with AMF species such as F. mosseae, G. versiforme, and Paraglomus occultum showed similar kinds of responses on root morphological characters (Wu and others 2011). However, the substrate P level produced no significant effect of AMF on root morphology of trifoliate orange, because increase in root total length did not account for the effect of substrate P level on arbuscules, an exchanged interface of nutrients between AMFs and the host plant (Braunberger and others 1991). It is well documented that AMF plants contained a modified endogenous balance of growth regulators (Berta and others 1993; Wu and others 2012). These dynamic changes in hormone balance especially auxin, which has not been determined in this study, could trigger the desired increase in cell size and cell elongation of roots, thereby, inducing a favorable change in root morphology (López-Bucio and others 2002; Padilla and Encina 2005).

AMF must obtain hexose from sucrose cleavage, primarily glucose, and transform it into trehalose and glycogen, typical fungal carbohydrates (Bago and others 2003). Our studies indicated that leaf sucrose concentration was significantly higher under mycorrhization than under non-mycorrhization with P0 and P30 treatments, without any significant response at P3. There observations suggested that AMF plants provided a greater sucrose source in leaves to load into the phloem for long-distance transport to the target sink organs such as AMs, which is dependent on the supply substrate P level. Doidy and others (2012) cloned three sucrose transporters from Glomus intraradices-colonized Medicago truncatula, which modulated C fluxes from the photosynthetic leaf source towards the AM symbiont. As a result, plants have been shown to direct 4–20 % of photoassimilates towards AM roots for sustaining growth and respiration requirements of intraradical as well as extraradical mycelia (Smith and Read 2008).

Sucrose synthase (SS) has the synthesis direction and breakdown direction, and can collectively catalyze the reversible reaction of sucrose into hexoses (Verma and others 2013). The present study indicated that the leaf sucrose concentration was more strongly positively correlated with leaf SS (synthesis direction) activity in AMF seedlings than in non-AMF seedlings, confirming that AMs would induce higher SS (synthesis direction) activity to synthesize sucrose in leaves as a source. There is certainly evidence that the net photosynthesis rate of the host plant is enhanced to meet the increased carbohydrate requirement for AM (Ferrol and Pérez-Tienda 2009). It is well known that under physiological conditions, SS is commonly considered to be involved in sucrose breakdown and glucose channeling to cellulose synthase, rather than sucrose synthesis alone (Amor and others 1995; Baier and others 2010). Schubert and others (2003) observed no significant change in root SS activity of soybean plants inoculated with F. mosseae at an earlier stage (<35 days), but after a later growth stage (>35 days) root SS activity was significantly higher under the mycorrhization with F. mosseae than with non-AMF. Baier and others (2010) further cloned a symbiosis-induced M. truncatula SS gene MtSucS1, which directly or indirectly helped in sustaining the normal maturation of arbuscules and shelflife of these arbuscules in efficient functioning of AMs. In this context, further studies are needed to consider SS (breakdown direction) activity and expression of related SS genes in different mycorrhizal structures, beside SS (synthesis direction) activity.

Plants deliver sucrose to AM roots, where sucrose is cleaved into glucose and fructose for the utilization of AMF (Smith and Read 2008). In the present study, mycorrhization induced higher glucose and fructose concentrations in roots, irrespective of substrate P level, except a lower root fructose concentration in AMF than in non-AMF seedlings with P30 treatment. It seems that more sucrose from source leaves in AMF plants was cleaved into hexoses for AMF development. A higher correlation of root AMF colonization with root glucose than with root fructose further suggested that AMs would preferably utilize glucose to transform it into typical fungal carbohydrates. This accounted for significantly lower root SS activity in AMF seedlings as compared with non-AMF seedlings. In fact, AMF inoculation down-regulated SS gene expression in roots of a AMF–plant system (Baier and others 2010).

When the plant delivers sucrose to AMF roots, sucrose cleavage by plant enzymes maintains a pool of hexoses available for transfer (Smith and Read 2008). Meanwhile, plant invertases primarily catalyze the irreversible hydrolysis of sucrose into glucose and fructose (Moscatello and others 2011). In our study, root AI activity remained unchanged between AMF and non-AMF seedlings under P3, but higher in AMF than non-AMF seedlings under P30 and lower in AMF than in non-AMF seedlings under P0. AMF inoculation showed no change in root NI activity under P30 but indicated significantly higher root NI activity under both P0 and P3 conditions. These results suggest that AMF inoculation had a varied change on root AI and NI activity in relation to different substrate P levels. Higher root AI activity as observed in our study in AMF seedlings than non-AMF control under P30 condition, was attributable to the strong expression of the acid invertase gene in arbuscule-infected roots under high P conditions (Blee and Anderson 2002). However, the AMF inoculation showed no change in root AI activity under P3 condition. Schubert and others (2003) earlier reported no significant difference in root AI activity between F. mosseae-colonized and non-AMF soybean. On the other hand, root AI activity was significantly lower in inoculated seedlings than uninoculated seedlings under P0 condition, suggesting that low P altered AMF functioning on root AI activity. Based on this effect on root AI activity, AMF-induced root AI changes were observed dependent on substrate P level. Wu and others (2013) earlier reported divergent effects of F. mosseae on AI and NI activity between leaves and roots in three citrus genotypes. These studies further concluded that AMF-changed sucrose-cleaving enzyme activities are dependent on enzyme types, tissue types and substrate P levels. Such variation in sucrose-cleaving enzymes caused by mycorrhization was accountable to apoplast pH value, which partly controlled these sucrose-cleaving enzymatic activities (Schaeffer and others 1995). It seems that plant invertase has an important contribution in expanding the C sink capacity for AMs and subsequently the mycorrhizal plants.

Conclusion

In this work, AMF inoculation improved the growth performance and root morphological traits of trifoliate orange. These structural modifications were independent of substrate P level. However, AMF-induced sucrose cleavage was dependent on P levels, plant tissue type, and sucrose-cleaving enzyme types. In a better mycorrhizal root, more sucrose would be diverted into hexose primarily as glucose, thus, resulting in an increased allocation of glucose and decreased allocation of sucrose to roots. The functional characterization of such regulatory mechanisms would pave the way in providing a better understanding about the physiology as well as biochemistry involved in AMF functioning.