Raising exotic tropical tree seedlings particularly for the plantations is still challenging and knowledge on the interaction of arbuscular mycorrhizal fungi (AMF) with the nursery substrate appears to be an important step for AMF-dependent tree species like mahogany. The knowledge on the AMF specificity for a beneficial interaction is necessary for the early establishment and growth of mahogany seedlings. However, concerning the use of native or non-native AMF as inoculum it is argued that besides the higher costs involved, the application of commercially available AMF inoculum is not favourable when compared to the local inoculum production and application for reforestation. The main arguments are that when inoculated seedlings with non-native fungi are planted into the field, exotic fungi often have difficulty in establishing within the pre-existing fungal community, and if they do establish, they may become fast spreading invaders out-competing the local fungal community as reviewed by Schwartz et al. (2006). A study has shown that local fungi were found to be finally more effective in colonization (Allen et al. 2005). In the Southern part of India, studies have shown that the predominant genus of AMF are Rhizophagus, Funelliformis (Harikumar and Potty 1999; Gopal et al. 2005). This was the reason behind the selection of this native AMF species in this experiment. Three native species of AMF (Funneliformis mosseae, Rhizopahagus intraradices, and Rhizopahagus proliferus) were selected at different levels (10, 25, and 50 g inoculum per seedling). The data shows that exotic mahogany seedlings are a mycorrhizal dependent species as the results show that inoculated seedlings performed better than the uninoculated seedlings. Besides this, however, it is revealed that they preferred the mycorrhizae as shown by the difference in their response to the difference in mycorrhizal species and quantity of the inoculum used in the experiment (Table 1–4). Many other studies also indicated clear colonization of vesicles and arbuscules in the secondary roots of seedling and mahogany trees in natural areas (Herrera and Ferrer 1980), in young plantations (Noldt and Bauch 2001). It is reported that about twenty-three AMF species belonging to genera Acaulospora, Glomus, Gigaspora, and Rhizophagus were found to have a symbiosis with mahogany trees growing in its natural habitat (Rodríguez-Morelos et al. 2014). Mahogany plants grown in the presence of AMF showed a general increase in plant growth parameters like plant height, stem girth, leaf area, and total dry weight as against those grown in soils uninoculated with AMF. The seedlings inoculated with Funneliformis mosseae responded better for all the growth characters of the seedlings. One of the reasons for this may be since besides higher photosynthetic rate the plant water status of the seedlings treated with Funneliformis mosseae was at a higher level as exemplified with high values for the relative water content and lower leaf temperature (Table 2).
Higher water potential of AMF inoculated plants would help to increase photosynthetic assimilation and in turn meet the carbon demands of the AMF as reported by Pooter et al. (1990). It is also documented that the symbiosis with AMF also helps the plants through improvement in absorption of soil water in addition to the nutrient absorption and enhancing the conductivity of roots and regulating the concentration of cellular osmolytes (Augé 2004; Aroca et al. 2008). Thus, it can be concluded that physiologically these seedlings were sound. A similar result was observed for inoculated mahogany seedlings under stressed conditions in the nursery (Lakshmy et al. 2020). Studies on Tectona grandis (Ajeesh et al. 2017), Azadirachta indica (Sumana and Bagyaraj 1996), Acacia mangium (Ghosh and Verma 2006), Anacardium occidentale ( Ananthakrishnan et al. 2004), and Santalum album (Binu et al. 2015) also confirmed our results.
There are many reports on the host preference among AMF (Rajan et al. 2000; Wu et al. 2011; Binu et al. 2015). Several authors have also stressed the need for selecting efficient AMF strains that can be used for inoculating different mycotrophic plants (Jeffries and Rhodes 1987; Bagyaraj and Varma 1995). There are many reports, where indigenous AMF were found to be ineffective or less effective (Bagyaraj et al. 1989; Reena and Bagyaraj 1990) when compared with exotics. For example, unsuitable AMF did not affect the growth of sandal seedlings (Binu et al. 2015). So, it becomes inevitable to select the best performing mycorrhizae for the establishment of exotic like mahogany. The present investigation indicated that the photosynthetic rate of the seedlings inoculated with both the species of Rhizophagus were higher when compared to the Funneliformis mosseae inoculated seedlings, but this was not transformed to the increased growth of the plants and thus the quality of seedlings was not good compared to those treated with Funneliformis mosseae. This might be due to the utilization of the photosynthate for the maintenance respiration and also for higher root respiration. Higher root respiration resulting in the loss of the photosynthate was observed for Plantago major for AMF inoculated seedlings (Baas and Kuiper 1989). There may be another possibility that the carbon flux out of the plant in the plant-AMF continuum was more for this particular mycorrhizal association resulting in the higher energetic cost of the symbiosis that can be high enough to cause growth decrease in mycorrhizal plants (Johnson et al. 1997; Zangaro et al. 2015). A positive dose response relation was not observed for the growth characters of the mahogany seedlings as it is usually observed with other species(Ajeesh et al. 2017)). However, a positive relation was observed for some of the physiological characteristics of the seedlings such as chlorophyll content, leaf temperature, water potential, and relative water content, and for the parameters like total spore count and the colonization percentage. This positive linear relation is usually attributed to better colonization of the rhizosphere by the introduced microorganisms and increased plant protection (Bull 1991; Raaijmakers 1995).
Some detrimental effects on root growth were also observed with high inoculation doses (Kapulnik et al. 1985). The critical level of spores for the mycorrhizal inoculation to cause an increase in the plant height, dry matter yield, root length, and per cent root infection in Prosopis cineraria seedling was found to be 400 germinable spores per polybag (1 kg soil) (Hatchell 1985; Verma et al. 2009). In Tecomella undulata the best dose of AMF for better seedling performance was found to be 100 g rhizosphere soil (500 germinable spores; Srivastava et al. 2004).
Further, seedlings raised in the presence of F. mosseae showed a greater biovolume index, quality index, and mycorrhizal efficiency index compared to all other treatments (Table 4) and this increase was to an extent of 146.00 per cent, 10 per cent, and 58 per cent, respectively, over those uninoculated seedlings. Such high values of biovolume index and quality index indicate a sturdier stem and a proportionate top dry weight compared to the seedling dry weight, qualities which are desirable among nursery seedlings (Hatchell 1985). The positive growth effect seen in the nursery can be significantly used to improve the tree survival and field establishment of mahogany seedlings as demonstrated earlier for other species like Acacia koa (Habte et al. 2001), Eucalyptus tereticornis (Karthikeyan and Krishnakumar 2012),Ceratonia siliqua (Manaut et al. 2015).
Inoculating with AMF improved the physiology of seedlings (Table 2). The high percentage of root colonization in AM fungal treated plants is directly correlated with better nutrient uptake, increased total chlorophyll content, an increase in the rate of photosynthesis and transpiration (Reid et al. 1983; Nylund and Unestam 1987; Rajasekharan and Nagarajan 2005). The difference in photosynthetic rate could probably be due to excessive starch accumulation in leaves of seedlings inoculated with AMF. This observation was further strengthened by the present study as mycorrhizal fungi used in this study significantly improved the chlorophyll content and photosynthesis of soil compared to the uninoculated treatment (Table 2).
The interaction effect was more pronounced for the combination R intraradices at 10 g. For this combination, the plant height and collar diameter were higher when compared to all other combinations. The total photosynthetic area expressed as the leaf area was significantly higher in plants with this combination. This increased leaf area and growth of the seedlings colonized by this specific combination of the mycorrhizae and doses have probably resulted in significantly higher biomass observed compared to other treatments. The enhancement in growth and physiology is also related to the higher Mycorrhizal efficiency index (Table 4). The growth of mahogany seedlings was significantly high for the parameters leaf area ratio and leaf weight ratio. Further, seedlings raised in the presence of R. intraradices at 10 g inoculum showed a greater bio volume index, quality index, and Mycorrhizal index.