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Root chemistry of Douglas-fir seedlings grown under different nitrogen and potassium regimes Terry M. Shaw, James A. Moore, and John D. Marshall
Abstract: Root chemistry and biomass allocation of Douglas-fir (Pseudotsuga menziesii var. glauca (Bessn.) Franco) seedlings under optimal and deficient levels of nitrogen (N) and potassium (K) were studied. Seedlings receiving high-N treatments were significantly larger and allocated more dry matter to their stems and less to their roots than those receiving the low-N treatments. The K treatments did not significantly affect total seedling biomass or root/shoot ratios. Root tip starch concentrations were significantly higher and root tip sugar concentrations were lower in plants receiving the low-N treatments. Seedlings receiving the high-N, low-K treatment had significantly lower concentrations of phenolics and tannins and lower ratios of these compounds to sugars in the root tips than seedlings receiving the high-K treatments. Samples taken from two locations on the root system show that concentrations of phenolics, tannins, sugars, and starches were substantially higher in the root collar than in the root tips. Because of lower within tissue variation, we recommend sampling at root tips to better detect treatment differences. This study shows that N levels affect starch concentrations in the roots, while K levels affect root phenolic and tannin concentrations. Possible relationships between low root phenolic and tannin concentrations and lessened resistance of Douglas-fir to root disease are discussed. Résumé : La chimie des racines et l’allocation de la biomasse ont été étudiées chez des semis de Douglas taxifolié (Pseudotsuga menziesii var. glauca (Bessn.) Franco) soumis à des niveaux faible ou optimal d’azote (N) et de potassium (K). Les semis soumis au niveau élevé de N avaient une dimension significativement plus forte et allouaient plus de matière sèche vers la tige que vers les racines comparativement aux semis soumis au niveau faible de N. Les traitements avec K n’ont pas significativement affecté la biomasse totale des semis ni le ratio racines/tige. La concentration en amidon était significativement plus élevée et la concentration en sucre plus faible dans l’apex racinaire des plants soumis au faible niveau de N. Les plants soumis aux niveaux élevé de N et faible de K avaient des concentrations significativement plus faibles de composés phénoliques et de tannins et des ratios plus faibles de ces composés par rapport aux sucres, dans l’apex racinaire, que les semis soumis au niveau élevé de K. Des échantillons prélevés à deux endroits du système racinaire montrent que les concentrations de composés phénoliques, de tannins, de sucres et d’amidon étaient substantiellement plus élevées au collet qu’à l’apex racinaire. Étant donné la plus faible variation à l’intérieur de ces tissus, nous recommandons d’échantillonner à l’apex racinaire pour mieux détecter les différences entre les traitements. Cette étude montre que le niveau de N affecte la concentration d’amidon dans les racines tandis que le niveau de K affecte les concentrations de composés phénoliques et de tannins dans les racines. On discute de la relation qui pourrait exister entre de faibles concentrations de composés phénoliques et de tannins dans les racines et une moins grande résistance du Douglas taxifolié aux maladies des racines. [Traduit par la rédaction]
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Concentrations of storage and secondary compounds in plant tissue depend to a considerable extent on the environment in which plants grow (Waring et al. 1985; Huber and Arny 1985). In particular, mineral nutrition influences concentrations of secondary carbon compounds such as sugars, starches, phenolics, and tannins (Bryant et al. 1983; Entry et al. 1991a; Moore et al. 1994). The concentrations of these compounds and the balance among them help to determine the resistance of plants to herbivores and pathogens (Wargo Received February 11, 1998. Accepted July 31, 1998. T.M. Shaw, J.A. Moore,1 and J.D. Marshall. Department of Forest Resources, University of Idaho, Moscow, ID 83844, U.S.A. 1
Author to whom all correspondence should be addressed. e-mail:
[email protected]
Can. J. For. Res. 28: 1566–1573 (1998)
1972; Garraway 1975; Ostrofsky and Shigo 1984; Larsson et al. 1986; Mwangi et al.1990; Dudt and Shure 1994). Therefore, the levels of available nutrients such as N and K may influence the ability of plants to resist disease. Douglas-fir (Pseudotsuga menziesii var. glauca (Bessn.) Franco) was selected for this experiment because it is commonly managed in the forests of the Inland Northwest and has been the subject of many previous fertilization field experiments (Heath and Chappell 1989; Shafii et al. 1989, 1990; Moore et al. 1991). Based on foliar analysis and growth response results from these experiments, nitrogen and potassium are commonly deficient (Mika and Moore 1991; Moore et al. 1994). Thus, N and K were chosen for experimentation in this greenhouse study. One control over the ability of a tree to resist stress is the availability of carbohydrate reserves (Waring and Schlesinger 1985). However, competition for photosynthate may reduce the levels of carbohydrate reserves available for mobilization in various tissues. Resource availability may also influence © 1998 NRC Canada
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Table 1. Nutrient treatments under which Douglas-fir seedlings were grown: low nitrogen and low potassium (nk), low nitrogen and high potassium (nK), high nitrogen and low potassium (Nk), and high nitrogen and high potassium (NK). Years
Treatment
Nitrogen
Potassium
1990 and 1992
nk nK Nk NK nk nK Nk NK
10 10 100 100 25 25 100 100
10 100 10 100 10 100 10 100
1991
Note: Values are percentages of solution concentrations developed by Ingestad and Lund (1979).
production of secondary compounds like phenols and tannins (Mooney 1972; Bazzaz et al. 1987). Changes in plant tissue composition result from competition for photosynthate among compounds. These changes can be predicted from the carbon–nutrient balance (CNB) hypothesis (Bryant et al. 1983), which suggests that carbonrich compounds such as phenolics and tannins are produced in relatively greater amounts when photosynthate is more available and mineral nutrients are less available. The CNB hypothesis is an extension of the earlier growth–differentiation balance hypothesis (Loomis 1932; Herms and Mattson 1992), which suggests that growth and differentiation compete for photosynthate. Both hypotheses predict increases in the concentration of carbon-rich compounds when nutrients are less available. We therefore examined correlations between plant biomass, phenolics, tannins, sugars, and starch concentrations, as well as ratios of compounds occurring in the roots. To better estimate and understand the relationship between plant nutrition, root chemistry, and plant susceptibility to disease, the effects of sampling location on root chemistry must also be understood. Wargo et al. (1972) found that glucose and fructose concentrations in sugar maple (Acer saccharum Marsh.) were higher in the outermost root wood than the inner root bark. In another study with sugar maple, Parker and Houston (1971) found that levels of sugars were higher in root bark than in root collar bark. Likewise, levels of storage and defensive compounds have been shown to vary considerably along the gradient of stem and root bark (Kelsey and Harmon 1989). The principal objective of this study was to determine the effects of varied N and K regimes on the chemical composition of Douglas-fir roots. A second objective was to evaluate two sampling locations by comparing variations in their chemical composition.
Treatments One hundred and four 1-year-old, containerized Douglas-fir (Pseudotsuga menziesii var. glauca (Bessn.) Franco) seedlings were planted in 2.9-L plastic containers filled with medium-grade silica sand. Seedlings were grown at the University of Idaho Forest Nursery in a shadehouse covered by a clear, corrugated fiberglass
roof from June to December for three growing seasons (1990, 1991, and 1992). To obtain wider representation of the Douglas-fir gene pool, seedlings from two northern Idaho Douglas-fir seed sources collected from different locations and elevations were distributed equally by treatment and block. Seedlings were randomly assigned to four different N and K treatments within two blocks. The solution used to supply the Douglas-fir seedlings with nutrients was adapted from Ingestad and Lund (1979) and is considered nutritionally optimal. A stock solution was formulated with levels for the macronutrients as follows: N, 100; K, 65; P, 13.8; Ca, 7; Mg, 8.5; and S, 15 mg/L. The micronutrient concentrations were as follows: Fe, 700; Mn, 400; B, 200; Cu, 30; Zn, 30; Cl, 30; Mo, 7; and Na, 3 ppm. The nutrient stock solution was modified to meet the treatment regimes shown in Table 1. Nutrient solutions were given to plants by adding them to well water through an injection system at a ratio of 1:100. The average pH of the solution for all treatments was 7.1. Seedlings were irrigated as needed, and 500 mL of nutrient solution was applied every 4 days throughout the growing season. Irrigation was reduced in late September of each year to allow the onset of dormancy. Periodic foliar sampling was used to adjust treatments so that the N and K foliar concentrations were similar to treatment ranges observed from Douglas-fir field fertilization studies. Mika and Moore (1991) observed Douglas-fir foliar N concentrations as low as 1.0% on untreated field sites and as high as 1.87% on N-treated sites. In the same study, foliar K concentrations ranged between 0.6% on untreated sites and 0.9% on K-fertilized sites. These results from field experiments, along with the results from van den Driessche (1979) and Webster and Dobkowski (1983), helped determine the target N and K concentrations in our experiment. The target concentrations in our study were as follows for N: high-N treatments, 1.9%; low-N treatments, 1.0%; and for K: high-K treatments, 0.8%; low-K treatments, 0.6%. We felt that, even if the target concentrations were not completely achieved, an acceptable range of nutrient levels would be produced by the experiment. Each treatment consisted of a 3-year nutrient regime periodically adjusted to attain the target nutrient concentrations. Early during the second growing season it became apparent that seedlings grown in the low-N treatments required more N based on very low (
