Base cation stimulation of mycorrhization and photosynthesis of sugar maple on acid soils are coupled by foliar nutrient dynamics

Authors

  • Samuel B. St Clair,

    1. Intercollegiate Graduate Program in Ecological and Molecular Plant Physiology, The Pennsylvania State University, 102 Tyson Building, University Park, PA 16802, USA
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  • Jonathan P. Lynch

    Corresponding author
    1. Intercollegiate Graduate Program in Ecological and Molecular Plant Physiology, The Pennsylvania State University, 102 Tyson Building, University Park, PA 16802, USA
    2. Department of Horticulture, The Pennsylvania State University, 221 Tyson Building, University Park, PA 16802, USA
      Author for correspondence: Jonathan Lynch Tel: +1 814 8632256 Fax: +1 814 8636139 Email: jpl4@psu.edu
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Author for correspondence: Jonathan Lynch Tel: +1 814 8632256 Fax: +1 814 8636139 Email: jpl4@psu.edu

Summary

  • • The nutritional benefits that mycorrhizal associations provide to plants may be constrained by acidic soil conditions resulting in decreased photosynthetic function.
  • • Sugar maple (Acer saccharum) and red maple (Acer rubrum) seedlings were grown on a native acidic (pH 4.1) soil both unamended and amended with base cations (pH 6.2). In a second study a fungicide treatment was included. Foliar nutrition, mycorrhizal colonization, photosynthesis and their relationships were assessed.
  • • On the native soil, red maple maintained higher levels of mycorrhizal colonization and photosynthesis than sugar maple but showed little response to base cation amendments. Mycorrhizal colonization and photosynthesis of sugar maple increased significantly in response to base cation amendments. Correlations were observed among mycorrhizal colonization, foliar nutrition and photosynthesis. The fungicide treatment indicated that 50% of the base cation-induced increase in sugar maple photosynthesis was mycorrhiza related.
  • • The results suggest that base cation stimulation of mycorrhization and photosynthesis of sugar maple on acid soils are coupled by foliar nutrient dynamics. Red maple exhibits much less sensitivity to these same edaphic conditions.

Introduction

The eastern forest of North America, including both the mixed deciduous forest and transitional forest, is a principal terrestrial biome, encompassing c. 65 million ha. Much of this forest is in the north-east USA where anthropogenic influences are significant. Over the last few decades, symptoms of health decline in sensitive tree species have been observed (Tomlinson & Tomlinson, 1990). The decline of sugar maple (Acer saccharum) has been particularly well documented (Bernier & Brazeau, 1988a; Horsley et al., 2000; Drohan et al., 2002). Symptoms have included reduction in growth, canopy deterioration, leaf chlorosis, tree mortality and low rates of seedling regeneration (Bernier & Brazeau, 1988c; McWilliams et al., 1996; Horsley et al., 2000; Duchesne et al., 2002).

Soil acidification and the disruption of nutrient cycles appears to be an important factor that weakens sugar maple resistance to both abiotic and biotic stresses and predisposes them to health decline symptoms (Horsley et al., 2000; Driscoll et al., 2001; Duchesne et al., 2002). Forest ecosystems with acidic soils are often limited by low base cation availability (Huttl & Schaaf, 1997; McLaughlin & Wimmer, 1999). Low availability of both Ca and Mg has been correlated with health decline symptoms in sugar maple (Bernier & Brazeau, 1988b; Horsley et al., 2000; Duchesne et al., 2002). Calcium and magnesium amendments on acidic, nonglaciated forest soils improved crown vigor and growth in sugar maple (Long et al., 1997; Moore et al., 2000) but not in other tree species examined (Long et al., 1997; Kobe et al., 2002), suggesting that sugar maple may be particularly sensitive to base cation deficiencies.

Unlike sugar maple, the range and dominance of red maple (Acer rubrum) has expanded significantly in North American forests over the last 100 yr (Abrams, 1998). Surveys of the Allegheny Plateau revealed that mortality and health decline symptoms which have been correlated with edaphic stresses in this region (Horsley et al., 2000) were three times greater in sugar maple than red maple (McWilliams et al., 1996). It has been proposed that greater nutrient acquisition and nutrient use efficiency has contributed to the expansion and success of red maple on acidic, nutrient-poor soils (Abrams, 1998; St Clair, 2004). Interspecific variation in tolerance to edaphic stresses may be an important factor influencing the differential success of these two species on acidic, nutrient-poor soils.

Sugar maple and red maple form arbuscular mycorrhizal (AM) associations (Schultz, 1982; Brundrett et al., 1990), which can significantly improve their foliar nutrient status (Schultz, 1982). Arbuscular mycorrhizal associations can increase nutrient acquisition (Tinker et al., 1992; Clark & Zeto, 2000) and ameliorate Al (Koslowsky & Boerner, 1989; Clark & Zeto, 2000) and Mn toxicity (Arines et al., 1989; Bethlenfalvay & Franson, 1989; Clark & Zeto, 2000), which are significant constraints to the health of sensitive tree species on acidic forest soils (Cronan & Grigal, 1995; Horsley et al., 2000). In the literature there are conflicting reports on the influence of soil pH and base cation saturation on mycorrhizal colonization of sugar maple roots (Cook et al., 1992; Ouimet et al., 1995, 1996; Coughlan et al., 2000) and the influence that this relationship has on tree health (Spitko et al., 1978; Ouimet et al., 1995). Coughlan et al. (2000) showed that raising the pH of acidic forest soils increased both the growth and mycorrhizal colonization of sugar maple seedlings, although there was no indication whether there was a relationship between these two responses. Much less is known about these relationships in red maple.

The important benefits that mycorrhiza provide plants on acidic soils (Clark & Zeto, 2000) can increase photosynthesis, resulting in greater photosynthate availability for each symbiont (Marschner, 1995; Smith & Read, 1997). However, the benefits of this relationship may be limited if the association itself is sensitive to acidic soil conditions and excessive soil solution activities of Al and Mn, which can inhibit sporulation, hyphal growth and the AM–root interactions (McGee, 1987; Porter et al., 1987; Danielson & Visser, 1989; Bartolomeesteban & Schenck, 1994; Zahka et al., 1995). The objective of this study was to examine the relationships among mycorrhiza colonization, foliar nutrition and photosynthesis in sugar maple and red maple seedlings growing on acidic forest soils amended with base cations. We hypothesized that base cation amendments of the native acidic soils would increase mycorrhizal colonization of sugar maple and red maple seedlings, resulting in improved foliar nutrient balance and higher rates of photosynthesis.

Materials and Methods

Plant material and growth conditions

Second-year bare-root seedlings of sugar maple (A. saccharum Marsh.) (Penn Nursery, Spring Mills, PA, USA) and red maple (A. rubrum L.) (Musser Forest Nursery, Indiana, PA, USA) grown from seed were used as plant material. All seedlings were cultivated in 11.37-l plastic tree pots (15 cm wide, 41 cm deep) (Steuwe and Sons, Corvallis, OR, USA) filled with forest soil collected from the SSF #1 site on the Susquehannock State Forest in Potter County, PA, USA, described by Kolb and McCormick (1993) in their study of sugar maple decline in Pennsylvania. Soil was collected May 3, 2002 at four equally spaced locations along a 100 m transect at the top of the ridge using a shovel with a long tapered blade (40 cm long, 13 cm wide). Soil columns the full-length of the blade were removed. The upper 10 cm of soil was an Oa/A horizon and the lower 30 cm was B horizon soil. The two horizons were separated and each stored in 37-l plastic containers for transport to Penn Sate University. The Oa/A and B soils were each placed on separate plastic tarps and thoroughly homogenized by hand using a shovel. After mixing, five 100-g soil samples from each horizon were placed in plastic soil sample bags for physical and chemical analysis.

The first experiment was a completely randomized 3 × 2 factorial design replicated five times. Factor combinations included three different soil conditions (soil, soil + Ca and soil + Ca + Mg) and two species (A. saccharum and A. rubrum). The control was the soil in its natural chemical condition. The soil + Ca treatment had laboratory grade CaO mixed into each of the two horizon soils at a ratio of 890 mg kg−1 of undried soil for the B-horizon soil and 1150 mg kg−1 of undried soil for the Oa/A horizon soil. This was calculated as the soil : Ca ratio which brought the soil pH to 6.2 in a 1 : 1 soil–water mixture. The soil profile for the soil and soil + Ca treatments were then reconstructed. During soil profile reconstruction, the top of the seedling roots were placed just below the surface of the Oa/A horizon soil and extended down into the B-horizon as observed in the root systems of natural seedlings at the SSF#1 site. The soil + Ca + Mg treatment was created by placing 20 g of coarsely ground limestone (53.5% CaCO3, 42% MgCO3) on the soil surface of seedlings that had been prepared according to the soil + Ca treatment explained above. The experiment was initiated May 11, 2002 and continued until September 5, 2002. Seedlings were irrigated two or three times a week, as needed, using an automated drip system, which delivered approx. 500 ml of deionized water.

A second experiment was conducted the following year from June 7 to September 3, 2003. In this experiment soil was collected from the same site and processed in the same way as already described. Second-year sugar maple seedlings obtained from the same source (Penn Nursery) were used as treatment units. This experiment was a completely randomized 2 × 2 factorial design replicated five times. Factor combinations included the soil and soil + Ca treatments already described with or without fungicide applications to the soil. The fungicide treatments were applied according to the methods of Ganade and Brown (1997). The fungicide Rovral (iprodione; Bayer Cropscience, Research Triangle Park, NC, USA), was applied at a rate of 2.0 g m−2 of soil surface in 500 ml of deionized water at 5-wk intervals throughout the study to reduce AM colonization of roots. The seedlings were irrigated two or three times a week using an automated drip system. However, instead of using deionized water, as in the first study, we used a solution that was formulated to reflect the chemical properties of rainwater collected at the Kane Experimental Forest on the Allegheny National Forest during the summers of 1999–2002. The solution pH was 4.2 and contained (in µmol l−1): 32 SO4, 26 NO3, 19 NH4, 3.3 Ca, 0.83 Mg, 0.41 K, 0.70 Na and 0.29 Cl.

Both experiments were conducted in the same climate-controlled glasshouse at Penn State University (40°49′ N, 77°49′ W). A HOBO data logger (MicroDAQ; Warner, NH, USA) was used to monitor temperature and relative humidity. Mean daytime temperature was 28 ± 5°C and relative humidity averaged 62 ± 6%. Photosynthetic photon fluence rate (PPFR) varied throughout the day, with a maximum of 1500 µmol photons m−2 s−1.

Leaf gas exchange and chlorophyll fluorescence

Gas exchange and chlorophyll fluorescence measurements were made using the Li-Cor 6400 gas exchange system with the 6400–40 leaf chamber fluorometer (Li-Cor, Lincoln, NE, USA) on the last day of the experiment. Light intensity of 1500 µmol photons m−2 s−1 was the targeted leaf chamber irradiance generated by the 6400-40 LED light source under which light-adapted gas exchange and fluorescence measurements were taken. Measurements were made at ambient temperature and humidity. All measurements were taken on a pair of fully expanded mature leaves. Measurements were initiated by sealing the 2 cm2 leaf chamber proximally to the major leaf sinus avoiding the primary veins on either side. When dF/dt (the rate of change in the fluorescence signal) approached zero and CO2 concentrations in the leaf chamber reached a steady state (60–90 s), Ft (steady-state yield of fluorescence in the light) was measured followed by a 0.8 s saturating light flash to induce maximal fluorescence in the light-adapted state (F′m). These two coefficients were used to calculate ΦPSII = (F′m − Ft)/F′m (Genty et al., 1989). Electron transport rate (ETR) through photosystem II was determined according to the equation, ETR = ΦPSII × PFDa × 0.5 (Maxwell & Johnson, 2000). Gas exchange data was logged concurrently with fluorescence data and were calculated according to Caemmerer & Farquhar, 1981). Photosynthetic measurements were taken between 10 : 00 hours and 15 : 00 hours. Diurnal gas exchange measurements taken on red maple and sugar maple seedlings indicated that CO2 exchange rates (CER) values were stable during these hours. The measurement sequence was randomized so that treatment effects were not biased by the time of day.

Foliar element analysis

Upon completion of the photosynthetic measurements, leaves were harvested for elemental analysis. Leaf tissue was dried for 72 h at 65°C. Dried leaf tissue was pulverized by placing the sample in a 20-ml plastic scintillation vial with a glass bead followed by vigorous shaking in a mixer/mill (Spex Industries, Metuchen, NJ, USA). Samples were ashed at 550°C and then dissolved in solution for analysis of P, K, Ca, Mg, Mn, Fe, Cu, Al and Zn using inductively coupled plasma spectroscopy (ICP) (Dahlquist & Knoll, 1978). Before ashing a subsample of the pulverized tissue was set aside for the determination of total nitrogen using the combustion method (Campbell, 1991).

Arbuscular mycorrhizal colonization

At the end of the experiment, seedling roots were carefully removed from the soil and rinsed with water. Feeder roots were collected from upper, mid and lower portions of the root system. Roots were boiled in 10% KOH (w : v) for 15 min followed by rinsing with deionized water. The roots were then placed in 5% bleach for 2 min followed by a second rinsing with deionized water. The roots were then placed in 5% HCl for 1 min followed by staining in a Trypan blue solution consisting of 950 ml deionized water, 50 ml acetic acid, 1 l glycerol, and 0.2 g of Trypan blue. Total AM colonization per cent was quantified using the grid intersect method (Giovannetti & Mosse, 1980) using a dissecting microscope (Olympus SZ40; Olympus, Tokyo, Japan).

Soil methods and analysis

Soil pH was determined in a 1 : 1 soil: water (w : v) mixture using a pH meter. Soil N was determined according to (Bremner & Sparks, 1996). The Mehlich-3 protocol was used to extract available P, K, Ca, Mg, Mn, Fe, Cu, Zn (Wolf & Beegle, 1995). Available Al was extracted with 0.01 m SrCl2 (Lyon & Sharpe, 1999). Element concentrations were determined using ICP (Dahlquist & Knoll, 1978). For soils collected in 2002, soil organic matter was assessed by loss on ignition (Schulte, 1995) and soil particle size was determined using the hydrometer method (Gee & Bauder, 1986).

Statistics

anova and Fisher's Protected Least Significant Difference were used to test the significance of treatments on dependent variables. An ancova model with CER and ETR as dependent variables, fungicide treatment as the independent variable and foliar elements as regressors was used to examine the interactive effects of the fungicide treatment and foliar nutrition on photosynthesis. Simple linear regression was used to examine relationships between dependent and independent variables. Statistical significance in this study was defined as α ≤ 0.05. Statistical analysis was performed using statview statistical software (SAS institute, Cary, NC, USA).

Results

Soil

The soil used in both experiments was classified as a nonglaciated, Typic Fragiochrept, Mardin channery silt loam (31% sand, 45% silt, 24% clay). The Oa/A- and B-horizons had 7% and 3% organic fractions, respectively. With the exception of Al in both experiments and P in the 2003 experiment, element concentrations were higher in the AO horizon than in the B horizon soil (Table 1). Except for some variation in N and Mn the chemical properties of the soil collected in 2002 and 2003 were similar (Table 1).

Table 1.  Extractable element concentrations of A and B horizon soil collected at SSF#1 (Kolb & McCormick, 1993) in early May of 2002 and 2003
 20022003
AO-HorizonB-HorizonAO-HorizonB-Horizon
  1. Soil element concentrations extracted (µg g−1) using Mehlich 3.

pH   4.1   4.4   3.9   4.3
N3120120076001300
P  49  38  33  44
K  61  27  82  30
Ca 209 137 331 116
Mg  33  21  52  19
Mn 384  92 144  45
Fe 293  86 321 214
Cu   1.53   1.10   1.35   1.05
Zn   3.76   1.9   3.66   1.54
Al 131 165  98 210

Leaf nutrition

In the 2002 experiment, foliar Ca in both species was significantly increased by base cation amendments, while concentrations of foliar Mg were significantly increased by the soil + Ca + Mg treatment (Table 2). In both species, foliar Mn, Zn and Al concentrations were significantly decreased by both base cation treatments (Table 2). For sugar maple, Fe was significantly decreased by both base cation treatments and for red maple the soil + Ca + Mg significantly decreased Fe concentrations (Table 2). For red maple, both base cation amendments significantly lowered foliar N and K concentrations while foliar P was significantly increased by the soil + Ca + Mg treatment (Table 2).

Table 2.  Mean foliar element concentrations of sugar maple (Acer saccharum) and red maple (Acer rubrum) seedlings grown on an acidic, nonglaciated forest soil (soil) collected at SSF#1 (Kolb & McCormick, 1993) amended with base cations (soil + Ca or soil + Ca + Mg) during the summer of 2002
 NPKCaMgMnFeCuZnAl
  1. Foliar element concentrations are µg g−1 d. wt.

  2. Data are means ± 1 SE (n = 5). Different superscript letters in each column category indicate statistically significant differences among the treatments at the α≤ 0.05 level.

Sugar maple
Soil17720 ± 4171840 ± 3054160 ± 42012080 ± 491a 840 ± 67a6277 ± 298a 104 ± 13a5.0 ± 0.9526.0 ± 2.9a61.8 ± 8.2a
Soil + Ca17720 ± 20371820 ± 1934380 ± 378 16560 ± 1896b1100 ± 202a2766 ± 788b56.6 ± 14b4.8 ± 0.74 17.2 ± 2.2b19.2 ± 1.4b
Soil + Ca + Mg16300 ± 5612400 ± 2163480 ± 213 16240 ± 1113b2600 ± 291b 954 ± 166c49.6 ± 7b3.8 ± 0.66 14.0 ± 1.7b14.2 ± 0.74b
Red maple
Soil14220 ± 660a1040 ± 51a4160 ± 222a10360 ± 1037a1100 ± 84a4802 ± 745a39.4 ± 4.5a3.8 ± 0.6657.4 ± 7.1a28.4 ± 4.2a
Soil + Ca11740 ± 850b1240 ± 233a3240 ± 191b14620 ± 1669ab1380 ± 208a1139 ± 158b28.6 ± 3.0ab6.4 ± 2.028.4 ± 5.5b 8.6 ± 0.82b
Soil + Ca + Mg11425 ± 526b1900 ± 280b3125 ± 298b16125 ± 1492b4225 ± 394b 674 ± 126b25.7 ± 3.6b6.3 ± 1.024.0 ± 5.0b 7.0 ± 1.2b

Foliar analysis of the 2003 experiment indicated that the Ca amendment significantly increased foliar concentrations of Ca and significantly decreased foliar concentrations of Zn and Al. Calcium was the only element that was significantly affected by the fungicide treatment, with fungicide-treated seedlings accumulating less foliar Ca than control seedlings (Table 3).

Table 3.  Mean foliar element concentrations of sugar maple (Acer saccharum) seedlings grown on an acidic, nonglaciated forest soil (soil) collected at SSF#1 (Kolb & McCormick, 1993) as influenced by CaO and fungicide treatments in the 2003 experiment
 NPKCaMgMnFeCuZnAl
  1. Foliar element concentrations are µg g−1 d. wt.

  2. Data are means ± 1 SE (n = 5). Different superscript letters in each column category indicate statistically significant differences among the treatments at the α≤ 0.05 level.

Soil18240 ± 8102960 ± 3904860 ± 29510840 ± 684a1209 ± 1236532 ± 385 89 ± 127.6 ± 1.644 ± 4.0a108 ± 6.5a
Soil +19980 ± 7901950 ± 2884933 ± 287 8316 ± 818b1116 ± 1625246 ± 725144 ± 347.0 ± 1.646 ± 4.8a 95 ± 9.6a
fungicide
Soil + Ca19580 ± 15803060 ± 6664520 ± 22020480 ± 1586c1140 ± 815072 ± 788 83 ± 255.2 ± 0.9721 ± 1.1b 33 ± 3.4b
Soil + Ca +18700 ± 9603057 ± 5574385 ± 29017471 ± 1125c1300 ± 974583 ± 632121 ± 295.0 ± 0.8419 ± 0.85b 31 ± 1.6b
fungicide

Arbuscular mycorrhizal root colonization

The AM colonization of sugar maple roots on unamended soil was < 10% in both experiments (Figs 1 and 2). In the 2002 experiment red maple had significantly higher AM colonization in unamended soil than sugar maple (P < 0.01). For red maple only the soil + Ca + Mg treatment significantly increased AM colonization (Fig. 1). Both base cation treatments significantly increased AM colonization in sugar maple seedlings (Fig. 1).

Figure 1.

Influence of base cation amendments (filled columns, soil; tinted columns, soil + Ca; open columns, soil + Ca + Mg) on CO2 exchange rate, electron transport rate and mycorrhization of sugar maple (Acer saccharum) and red maple (A. rubrum) seedlings in the 2002 experiment (n = 5; error bars, 1 SE). Different letters indicate statistically significant differences within each species at the α ≤ 0.05 level. AM, arbuscular mycorrhiza.

Figure 2.

Influence of Ca and fungicide treatments (filled columns –fungicide; open columns, +fungicide) on mycorrhiza colonization, CO2 exchange rates, electron transport rates and relative growth rates of sugar maple (Acer saccharum) seedlings in the 2003 experiment (n = 5; error bars, 1 SE). Different letters indicate statistically significant differences among the treatment conditions at the α ≤ 0.05 level. AM, arbuscular mycorrhiza.

In the 2003 study, the soil + Ca-treated seedlings had higher mean mycorrhizal colonization levels than seedlings grown in the native soil but the difference was not statistically significant (P = 0.09). The fungicide treatment significantly decreased AM colonization in seedlings growing in calcium-amended soil (P = 0.03).

Photosynthesis

In the 2002 experiment, red maple had a significantly higher CER in unamended soil than sugar maple (P < 0.01). CO2 exchange rates and ETR of red maple seedlings were not responsive to base cation amendments (Fig. 1). By contrast, CER in sugar maples seedlings increased significantly in response to the soil + Ca + Mg treatment, while the soil + Ca treatment significantly increased ETR (Fig. 1).

In the anova models for the 2003 experiment, the Ca amendment significantly increased CER (P = 0.01) and ETR (P < 0.01). The fungicide treatment significantly decreased CER and ETR in Ca-amended seedlings but not in the control seedlings (Fig. 2), resulting in a significant Ca × fungicide interaction for both CER (P = 0.04) and ETR (P = 0.03). The ancova model indicated that there was a significant interactive effect between foliar Ca and the fungicide treatment on ETR (P = 0.05). The influence of the fungicide treatment differentially affected CER and ETR, as their correlation was weaker in seedlings treated with fungicide (R2 = 0.21, P = 0.02) compared with control seedlings (R2 = 0.44, P < 0.01).

Relationships among mycorrhization, leaf nutrition and photosynthesis

In the 2002 experiment, mycorrhization was positively correlated with CER and ETR in sugar maple but not red maple seedlings (Fig. 3). Mycorrhization of red maple roots was significantly correlated with foliar concentrations of Mg, Mn and Al (Table 4). However, no significant linear relationships were identified between foliar element concentrations and photosynthesis in red maple seedlings (Table 4). Mycorrhizal colonization of sugar maple seedlings was positively correlated with P, Ca, and Mg but stronger negative relationships were found for Mn, Fe and Al (Table 4). Foliar concentrations of P, Mg, Mn and Al were significantly correlated with CER (Table 4). By contrast, ETR was not significantly correlated with foliar element concentrations (Table 4).

Figure 3.

Bivariate plots indicating the relationship between mycorrhiza (AM) colonization and CO2 exchange rates (a) and electron transport rates (b) of sugar maple (Acer saccharum; closed circles, solid line; (a) R2 = 0.49**, (b) R2 = 0.37*) and red maple (A. rubrum; open symbols, dashed line; (a) R2 = 0.06, (b) R2 = 0.01) seedlings in the 2002 experiment (significance designated as: *P = 0.05, **P = 0.01).

Table 4.  Coefficients of determination indicating the relationship between foliar nutrition with arbuscular mycorrhiza (AM) root colonization, CO2 exchange rate (CER) and electron transport rate (ETR) of sugar maple (Acer saccharum) and red maple (Acer rubrum) seedlings in the 2002 experiment
 AM colonizationCERETR
  1. n = 14. Significance designated as: *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001.

Sugar maple
N< 0.010.030.25
P0.33*+0.39*0.05
K0.06< 0.010.16
Ca+0.31*0.060.05
Mg+0.32*+0.34*0.02
Mn−0.58***−0.38*0.09
Fe−0.55**0.110.08
Al−0.70***−0.42*0.15
Red maple
N0.10< 0.010.08
P0.150.03< 0.01
K0.12< 0.010.09
Ca0.200.08< 0.01
Mg+0.36*0.03< 0.01
Mn−0.34*0.06< 0.01
Fe0.240.130.01
Al−0.30*0.090.02

In the 2003 experiment, marginally significant positive linear relationship was found between mycorrhization of nonfungicide sugar maple roots and CER (R2 = 0.42, P = 0.06) and ETR (R2 = 0.34, P = 0.09). For nonfungicide seedlings only foliar Al concentrations were significantly correlated with mycorrhization. Carbon dioxide exchange rates were negatively correlated with foliar Mn and Al concentrations (Table 5), while foliar Ca and Al were strongly correlated with ETR (Table 5). Relationships among mycorrhization, foliar nutrition and photosynthesis were not significantly correlated among fungicide-treated seedlings.

Table 5.  Coefficients of determination showing the relationship between foliar nutrition and arbuscular mycorrhiza (AM) root colonization, CO2 exchange rate (CER) and electron transport rate (ETR) in sugar maple seedlings, as influenced by fungicide treatments in the 2003 experiment
 AM colonizationCERETR
–Fungicide+Fungicide–Fungicide+Fungicide–Fungicide+Fungicide
  1. n = 10. Significance designated as: *P ≤ 0.05, **P ≤ 0.01.

N   0.04   0.040.110.060.030.19
P   0.02   0.080.140.03< 0.010.08
K   0.20   0.14< 0.010.060.02< 0.01
Ca   0.28< 0.010.15< 0.01+0.63**< 0.01
Mg   0.08   0.100.020.180.150.06
Mn   0.24   0.04−0.47*< 0.010.180.08
Fe   0.04   0.090.07< 0.01< 0.01< 0.01
Al−0.53*   0.08−0.56*< 0.01−0.68**0.19

Discussion

In this study, we examined relationships between mycorrhizal colonization, leaf nutrition and photosynthesis of sugar maple and red maple seedlings on acidic forest soil amended with base cations. The results show that red maple seedlings maintained higher levels of root mycorrhizal colonization and CER than sugar maple on the native soil (Fig. 1). The base cation treatments confirmed that the two species differed in their sensitivity to soil acidity and/or Ca and/or Mg availability as mycorrhizal colonization, CER and ETR of sugar maple seedlings were significantly more responsive to the base cation amendments than red maple. Although photosynthesis is a good indicator of physiological sensitivity to environmental factors, it is not always a precise predictor of growth or survivorship patterns that could drive changes in the structure and composition of forest ecosystems. However, photosynthetic results from this study are consistent with other studies showing that growth responses of sugar maple seedlings are sensitive to acidic soil conditions while red maple seedlings are not (St Clair, 2004). Growth and health status of sugar maple growing on acid soils are also strongly responsive to base cation amendments (Long et al., 1997; Moore et al., 2000).

Mycorrhizal colonization of sugar maple roots in the native soil (pH 4.2) compared with base cation-amended soils (pH 6.2) in this study were in the same range as a study that showed a similar increase in mycorrhizal colonization of sugar maple roots in response to an increase in soil pH from 4.0 to 6.0 (Coughlan et al., 2000). The increase in mycorrhizal colonization of sugar maple roots in response to base cation amendments in this study is consistent with results that have shown significant correlations between mycorrhization of sugar maple roots and leaf and root concentrations of Ca and Mg (Ouimet et al., 1995).

Calcium was the principal element being manipulated in these studies (Mg in soil + Ca + Mg treatment of 2002 study being the exception). With the exception of ETR in the second experiment, foliar Ca concentrations were not strongly correlated with photosynthesis measurements in either study (Table 4). Therefore, the effects of Ca amendments on photosynthesis are more likely indirect through the influence of Ca on soil pH and its interactions with other elements. Calcium can increase foliar Mg, K and P concentrations, possibly by the displacement of cations from soil exchange sites or by increasing fine root growth (Long et al., 1997; Kobe et al., 2002). Calcium also has strong interactions with Al and Mn, decreasing their toxic effects by buffering against acid soil conditions and competitively interfering with their uptake in the root zone (Rengel, 1992; El-Jaoual & Cox, 1998). The results show that photosynthetic measurements in both experiments were negatively correlated with foliar Al and Mn concentrations, indicating that Ca amendments may have increased photosynthesis through amelioration of Mn and Al toxicity. Sugar maple seedlings growing in the native soil in both studies had foliar Mn and Al concentrations that were significantly higher than values found in healthy foliage of mature sugar maple trees (Kolb & McCormick, 1993). Foliar Al and Mn at these concentrations have been shown to decrease growth (Thornton et al., 1986; St Clair, 2004) and photosynthesis (St Clair, 2004) in sugar maple seedlings.

Photosynthesis was positively correlated with mycorrhizal colonization of sugar maple roots in both experiments (Fig. 3). This relationship could be the result of (1) base cation amendments independently increasing both mycorrhizal colonization and photosynthesis, (2) increased photosynthesis in response to base cation amendments resulting in more photosynthate to support AM associations, or (3) base cation amendments stimulating mycorrhizal colonization, which then had positive effects on photosynthesis.

The 2003 experiment indicates that at least 50% of the base cation-induced increase in photosynthesis was related to mycorrhizal colonization, as indicated by the reductions in CER and ETR by fungicide in seedlings amended with Ca (Fig. 2). This indicates that the corresponding increases in mycorrhization and photosynthesis are not independent. The significant interaction between base cation amendments and fungicide treatments indicates that the fungicide effect resulted from interference with mycorrhizal colonization (Fig. 2) rather than phytotoxicity. The fungicide strongly suppressed mycorrhization in seedlings amended with Ca but not seedlings growing on the native soil, which may partly be the result of the small contrast potential caused by the inherently low mycorrhization rates in the native soil.

As outlined in the hypothesis, regression analysis in both experiments suggests that base cation enhancement of mycorrhizal colonization may have increased CER by improving foliar nutrient balance. In the 2002 study, mycorrhizal colonization was significantly correlated with foliar concentrations of P, Mg, Mn and Al, all of which were significantly correlated with CER. In the 2003 study, improvement of mycorrhization by the Ca amendments may have increased photosynthesis by altering foliar nutrient balance. Reductions in mycorrhizal associations in the Ca-amended, fungicide-treated seedlings did not, however, result in increased uptake of Al and Mn (Table 3). Therefore, it does not appear that the decrease in photosynthesis of these seedlings can be explained by higher foliar Mn or Al concentrations that occur in the absence of mycorrhiza. It appears that this response is more likely explained by the observed decrease in foliar Ca caused by the fungicide treatment (Table 3). Foliar Ca concentrations were strongly correlated with ETR in nonfungicide seedlings but were poorly correlated with CER. Because of the indirect role of Ca in many cellular processes it is difficult to pinpoint why PSII activity was more responsive to foliar Ca than CER. The responsiveness of ETR to foliar Ca may mean little in terms of the performance of the plant if it is not strongly coupled to CER and carbohydrate dynamics.

It is possible that enhancement of mycorrhizal colonization by the Ca amendment could stimulate photosynthesis independently of foliar nutrition by increasing sink strength (Dosskey et al., 1990). However, it is unlikely that sugar maple seedlings could support such a carbon sink with higher rates of photosynthesis on this acidic, nutrient-depleted soil without receiving a nutritional benefit from the mycorrhizal association and/or the base cation amendments.

Results from this study indicate that base cation amendments stimulate mycorrhization, which is correlated with foliar nutrient status and photosynthesis. The fungicide treatment demonstrated that base cation stimulation of mycorrhization and photosynthesis are not independent. Two distinct conceptual models could explain the observed relationships in this study. First, as we hypothesized, base cation amelioration of acidic soil conditions could lead to the stimulation of mycorrhizal associations, resulting in improved foliar nutrient status and higher rates of photosynthesis. Alternatively, base cation amendments may directly improve foliar nutrient imbalances, resulting in increased amounts of photosynthate to support greater mycorrhizal symbiosis. Both mechanisms may operate concurrently and synergistically, whereby incremental improvements in one process (photosynthesis or mycorrhizal-mediated nutrient acquisition) promote incremental improvements in the other.

Acknowledgements

The authors thank Dr Roger Koide for making available laboratory equipment for the quantification of AM colonization, helpful discussion on several aspects of the study and for reviewing the manuscript. We appreciate the technical assistance of Ylva Besmer in quantifying AM colonization of maple roots and for reviewing the manuscript. Thanks to Dr Bill Sharpe for site information and suggestions related to soil treatment conditions. Thanks to Ron Walter and Penn Nursery for donation of sugar maple seedlings. This research was supported by USDA (NRI) grant #2002-35100-12055 to J.P.L. and J.C. Carlson.

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