«（落葉樹の根圏動態に対する高CO2とO3及び高窒素負荷の影響に関する研究） Wang Xiaona 王 晓娜 Division of ...»
The changes of ECM community were visible under different treatments. When individual types of ECM were compared, the proportion of type D increased and the proportion of type C decreased in elevated CO2 as compared to the control (Fig. 3.4a, b). Under O3 exposure, type B did not colonize F1, type D colonized at a significantly higher degree, and type A colonized at a lower degree in comparison with the control (Fig. 3.4c). Under mixed fumigation with elevated CO2 and O3, the proportion of type C increased relative to the control, and it became the dominant species (Fig. 3.4d).
3.3.4 Growth of seedlings and element concentrations Ozone markedly reduced seedling growth after one growing season (Table 3.3). The height and stem diameter of seedlings were not significantly changed by elevated CO2 compared with the control in 2011 and 2012, but they were significantly reduced by O3 at the end of the 2011 growing season. Elevated CO2 + O3 did not have any effects on the growth of height and stem diameter during the two-year treatments, and both the diameter and height were unaffected by treatments in 2012. Elevated CO2 increased the biomass of root, stem, and total aboveground biomass, and O3 reduced the stem and root biomass. The elevated CO2 + O3 did not differ from the control, and there was no interaction between elevated CO2 and O3 for all biomass parameters
(Table 3.4). The root/shoot (needle + branch + stem) ratio (R/S) was also unaffected under all gas treatments.
No clear differences were found among gas regimes for the concentrations of N, K, Ca, and Mg in needles (Table 3.5). The concentrations of P, Al, Fe, and Mo were significantly reduced by O3, and those of P and Mn were clearly increased by elevated CO2. There was an interactive effect of elevated CO2 and O3 in the P concentration.
Under the fumigation of elevated CO2 + O3, the P concentration in needles greatly increased compared to the control. In roots, K and Mg were increased by O3, and there was no significant effect of the four treatments on other elements.
3.3.5 Gas exchange rate In the elevated CO2 treatment, Agrowth was significantly enhanced in 2011 and 2012 (Fig. 3.5). The values of Agrowth at elevated O3 and control did not differ significantly, while they were increased at elevated CO2 and CO2 + O3 fumigation in both 2011 and
2012. No significant influence on Gs was exerted by any individual gas treatment.
However, under elevated CO2 + O3 treatment, the Gs value was similar to the control value, but with a tendency to be lower in 2011 (P 0.1). Ozone tended to increase Gs in the second year (Fig. 3.5).
3.4 DISCUSSION Overall, the composition of the ECM community changed greatly after the treatments.
Inocybe lacera (G) and Thelephoraceae spp. (H) colonized F1 before the treatments,
but Suillus laricinus (C) and other ECM species replaced them during the two-year fumigated growth period. Despite the efficiency of the symbionts, the shift of ECM species in all treatments may partly be attributed to ECM succession. Nara et al.
(2003b, 2006a) found a common succession sequence of symbiotic ECMs, and for larch, Suillus spp. appeared in late succession on well-weathered lava flows. The same pattern was observed in Japanese larch in a mature forest (Yamakawa 2012).
In general, carbon allocation to belowground parts increased with elevated CO2, as reported in Japanese larch (Choi et al. 2005) and other tree species (Jackson et al.
2009; Nowak et al. 2004). Carbon allocation to belowground parts stimulates symbiosis involving ECM (Lukac et al. 2003), thus a significantly increased total ECM colonization of F1 was observed under elevated CO2. However, the diversity did not follow the same colonization rate pattern under elevated CO2, and this demonstrates that the ECM composition did not change with an increased total colonization rate.
On the contrary, the vital support of host photosynthates for ECM survival was reduced under O3 due to limited carbon allocation to belowground parts (e.g., Grantz and Farrar 2000), and this resulted in a colonization rate decrease. Lower ECM diversity was also found under O3, which indicated a shift (or even a reduction) of ECM abundance, I discussed this point below.
The proportion of Suillus grevillei (D) increased sharply, and was slightly greater than that of S. laricinus (C) in the elevated O3 regime as compared to the control. This likely occurred because S. grevillei colonizes larch seedlings faster than S. laricinus, and larch seedlings have higher shoot biomass with S. grevillei colonization than with S. laricinus (Qu et al. 2003b). Even with reduced ECM colonization, the proportion of most represented species such as Suillus spp. increased under elevated O3 within the
As previously mentioned, lower ECM diversity caused by O3 indicated a shift in the ECM abundance. Particularly, temporary partners could not have a symbiotic relationship with F1, such as the species of Peziza spp. (B) in my case. Moreover, Laccaria cf. laccata (F) and Tomentella spp. (A) individuals were present in lower proportions under elevated O3, which suggests that the assistance and function of these species to the host F1 was weak, or that their symbiotic activity was lower than other ECM types (Suillus spp.). A study of long-term exposure of aspen-birch to elevated CO2 and O3 supports my results (Edwards and Zak 2011).
The study revealed that Laccaria spp. and Tomentella spp. declined along with decreased cello-bio-hydrolase activity in an elevated O3 regime. Both the ECM colonization rate and diversity were reduced by elevated CO2 + O3 compared to the control, but ECM diversity was significantly higher than under the single O3 treatment.
Specifically, Suillus spp. colonized the same proportion of the roots as in the control.
Therefore, I conclude that under the combined treatment, elevated CO2 counteracted the reduction of diversity induced by O3.
The F1 growth (stem diameter and height) and ECM abundance did not accelerate significantly under elevated CO2. However, the increased stem and root biomass proved that F1 benefited from elevated CO2, and the same result was also observed in seedlings of Japanese larch (Yazaki et al. 2004). Ozone decreased the stem diameter and height at the end of the first growing season, which is similar to the result of Noormets et al. (2001a) in aspen. For instance, they found that the growth of two aspen clones was reduced by O3 (especially the stem diameter). Moreover, the inhibition of stem diameter by O3 was also found in potted F1 plants (Koike et al.
As symbiotic partners, ECMs surely enhance the capability for nutrient uptake of the host plant (Buscot et al. 2000). Katanic et al. (2014) found that shift of ECM community into types aided the distant nutrient acquisition when plants were treated by ethylenediurea (EDU) under O3 treatment. In the present study, O3 did not inhibit the growth of F1 during the second growing season. The shift of ECM community mentioned previously led to higher nutrient uptake, which resulted in avoidance of the O3 harmful effects on F1 growth. Therefore, I examined the element composition of aboveground and belowground parts of F1 plants. With higher ECM colonization rate under elevated CO2, I found an increased concentration of P and Mn in needles. This increase might be derived from an efficient uptake due to higher ECM colonization.
Phosphorus is an essential macro-element for ATP and NADPH, which are related to light reactions in photosynthesis (e.g., Reich et al. 2009). Manganese is also important for photosynthesis as a co-factor for photosynthetic oxygen evolution (Henriques 2003; Raven 1990). Higher ECM colonization stimulated the uptake of P and Mn, and thus enhanced carbon assimilation and the growth of F1 (Cairney 2011).
The concentrations of K and Mg in F1 roots were also increased by O3. This change may help maintain a stable concentration of Fe in roots, since K is vital for maintaining the root iron balance (e.g., Kraemer 2004). In addition, Fe is also essential for ECM formation (e.g., Van Hees et al. 2006). As a result, I postulate that F1 selected the most beneficial ECM partners under elevated O3, thus the structure of ECM community changed. This is one reason why the structure of the ECM community was altered with Peziza spp. absence under elevated O3 compared with control and elevated CO2. Although the reduced concentrations of Fe and Mo by O3 in needles may inhibit the growth of F1 seedlings (Raven et al. 1990), macro-element concentrations of N and K did not change significantly.
Furthermore, concentrations of Ca and Mg were unaffected in the needles under elevated O3. These two elements are important in that Ca is usually correlated with the activity of various enzymes that regulate photosynthesis, and Mg is essential for chlorophyll function (e.g., Liang et al. 2009). Therefore, this might be one reason supporting my hypothesis (slight inhibition of photosynthesis). On the other hand, under external stress, the internal allocation of nutrients is liable to be altered by various ECM species (Weigt et al. 2011). The stable concentrations of Mg and Ca in needles may be due to positive effects of ECM and the changes in species abundance, which may have indirectly led to the weak inhibition of photosynthesis by O3 treatment. Because colonizing mycorrhizal species are regulated by their host plants according to the efficiency of symbionts, especially when carbon allocation to shoots or fine roots changes under external stress, lower carbon-requiring and effective species are selected (Hoeksema and Kummel 2003).
Further, Suillus spp. were reported to reduce the transfer of large quantities of metals towards the plant-fungus interface without hampering normal nutrient uptake to the host plant (Colpaert et al. 2011). In my case, the O3-reduced concentration of Al in needles was potentially caused by the shift in ECM abundance to protect F1 from metal toxicity. This might be beneficial to the maintenance of photosynthetic activity under elevated O3.
The net photosynthetic rate (Agrowth) at elevated CO2 and elevated CO2 + O3 was markedly higher than in the control. Usually, plant growth and root systems increase under elevated CO2, and this is accompanied by higher efficiency in plant water and nitrogen use (e.g., Koike et al. 2010; Norby and Zak, 2011; Qu et al. 2004). As a result, the stem and root biomasses were increased by elevated CO2. I did not find any significant effects on the growth and biomass of F1 with elevated CO2 + O3, or any
interactive effects of these two factors. The Agrowth of seedlings in elevated CO2 + O3 was higher than that of seedlings in elevated O3 alone, indicating the positive effects of elevated CO2. The potential impact of O3 on the growth of F1 seedlings may be ameliorated by elevated CO2 associated with abundant P uptake via ECM, which has been reported for beech in central Europe (Matyssek and Sandermann, 2003; Weigt et al. 2012).
Moreover, this might explain why the structure of ECM abundance under elevated CO2 + O3 was similar to that of the control. The Gs was not influenced by any treatment, but it was increased by O3 in the second growing season. As it has been reported, the decrease in the hydraulic conductance of plant stems (Saliendra et al.
1995; Sperry and Pockman 1993), roots (Kavanagh et al. 1999; Meinzer and Grantz 1990), and even leaves (Salleo et al. 2000) may lead to stomatal closure. However, the water uptake of host plant was efficient with the ECM assistance, as it is supported by higher stomatal conductance occurred in the high mycorrhization (Auge et al. 2004;
Ebel et al. 1997).
In the present results, although higher Gs increased the risk of high O3 damage, the growth of F1 was found to be unaffected. Moreover, Agrowth was not reduced by O3 in the second year when compared with the control. Thus, the increased stomatal conductance might be a driving factor for nutrient uptake through specific ECM species under O3 stress. This trail requires further research.
3.5 SUMMARY Elevated CO2 increased the ECM colonization rate but not ECM type diversity. The
higher Agrowth of F1 under elevated CO2 stimulated the biomass of belowground and stem, which increased the ECM colonization rate. Elevated O3 negatively affected the ECM colonization rate and species abundance, with a great shift in the latter. The growth of F1 was restricted, and the biomass was reduced by O3. The altered concentrations of individual elements in needles and roots interactively affected the shift of ECM abundance under elevated O3. This might contribute to the defense capability of F1 seedlings against O3 by selecting those ECM species capable of flourishing under enhanced O3. However, this point needs further study with specific ECM species. Elevated CO2 + O3 reduced the colonization rate and diversity but increased Agrowth with parallel unaffected growth and biomass, which suggests that elevated CO2 diminished the influence of O3 on photosynthetic ability. Therefore, it seems that a symbiotic partnership between host F1 seedlings and ECM specialists such as Suillus spp. may be essential for the survival of F1 seedlings. The present results provide information on ECM symbiosis with F1 seedlings under elevated CO2 and/or O3 conditions, which are useful for further field inoculations of F1 with optimal ECM species to enhance survival under changing environments.
Figure 3.2 Ectomycorrhiza (ECM) colonization rate and diversity of hybrid larch F1 under different treatments at the end of the experimental period.
The ECM diversity (H') was calculated as Shannon’s diversity index, and each value is the average of four chamber replications. A vertical bar indicates the standard error. Different character symbols denote the degree of significance between the four treatments (P 0.05).
ANOVA: **P 0.01, *** P 0.001, “ns” means not significant.
Figure 3.3 Abundance of colonized ECM species in response to four fumigation treatments at the end of the experimental period.