New Forests 19: 27–49, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
Stock-type patterns of phosphorus uptake, retranslocation, net photosynthesis and morphological development in interior spruce seedlings RAYMUND S. FOLK and STEVEN C. GROSSNICKLE Forest Biotechnology Centre, BCRI Vancouver, British Columbia, Canada, V6S 2L2 Received 20 March 1998; accepted 4 May 1999 Key words: Picea glauca (Moench) Voss × Picea engelmannii Parry, phosphorus, phosphorus-use efficiency, retranslocation, specific absorption rate, spring-stock, stock-type, summer-stock Abstract. One-year-old interior spruce (Picea glauca (Moench) Voss × Picea engelmannii Parry) spring-stock and summer-stock were grown under two phosphorus (P) fertility regimes, with (+P) or without (−P), followed by a simulated winter, and a second growing period under an adequate fertility regime in a controlled environment room. The two stock-types differed in their response to low P availability. For spring-stock, morphological development, phosphorus-use efficiency (PUE) and P specific absorption rate (SAR) were similar between −P and +P seedlings. For summer-stock, −P seedlings compared to +P seedlings had lower (p ≤ 0.05) morphological development, but greater PUE and SAR. For both stock-types, P content increased in +P seedlings, remained low in −P seedlings, and P concentration decreased in nursery-needles (i.e., formed in the nursery) of −P seedlings. The difference in stock-type response to low phosphorus availability (−P) was attributed to internal supply of P and it’s retranslocation. Assimilation (A) of CO2 in nursery-needles was similar between −P and +P seedlings for both stock-types. For spring-stock, +P seedlings had greater A in new-needles (i.e., needles formed during the trial) than −P seedlings. It was recommended that the spring-stock be selected over summer-stock for sites low in P availability.
Introduction Phosphorus is an important element for seedling establishment. However, uptake in newly planted conifer seedlings on reforestation sites is often low due to poor root system uptake capability and low levels of phosphorus availability in forest soils (van den Driessche 1991). Soils supporting coniferous forests and plantations have a much lower phosphorus status than most soils used for agricultural production (Ballard 1980). Low levels of plant available phosphorus have been reported for forest soils in British Columbia, Canada (Radwan and Shumway 1983), Japan (Kawana 1969), Australia (Raupach
28 1967; Attiwill 1983), New Zealand (Ballard 1978), the southern United States (Allen 1987), South Africa (Donald et al. 1987), and Europe (Leikola and Rikala 1974; McIntosh 1981). When soil nutrient supply is sufficiently low, retranslocation of nutrients from previously produced foliage to points of new growth, is enhanced (Florence and Chuong 1974; Ballard 1980; van den Driessche 1985, 1991). On reforestation sites, up to 75% of the phosphorus requirements of new tissue is achieved through internal transfer of this nutrient (Meyer and Tukey 1965; Krueger 1967; Mutoh 1972; Ballard 1980; van den Driessche 1985, 1991). Consequently, storage and retranslocation of phosphorous is important in newly planted conifer seedlings (Miller et al. 1979; Waring and Franklin 1979; Miller 1984; Nambiar and Fife 1987; van den Driessche 1985, 1991), and in established conifer plantations (Florence and Chuong 1974; Malkonen 1974; Turner and Singer 1976; Madgwick et al. 1977). In particular, growth of newly planted conifer seedlings is largely supported by phosphorus reserves in needles produced in the previous year (Krueger 1967; Mutoh 1972; Turner and Singer 1976; van den Driessche 1991). The capability of retranslocated phosphorus to meet the growth requirements of newly planted seedlings is determined by the relative sink strength associated with the developing tissues (Williams 1955), as well as the amount of phosphorus available in previously formed tissues (i.e., needles formed in the nursery) (Florence and Chuong 1974; Ballard 1980; van den Driessche 1991). Phosphorus uptake during periods of growth cessation result in greater levels of available P for future retranslocation to points of new growth (Benzian et al., 1974; van den Driessche 1985). Thus, the fertility regime in the nursery, particularly between bud-set and planting, can determine the effectiveness of P retranslocation. Several studies have shown that nutrient uptake after bud-set affects subsequent field performance in the following year (Anderson and Gessel 1966; Benzian et al. 1974; Mullin and Bowdery 1977; van den Driessche 1985; Margolis and Waring 1986; van den Driessche 1991). In addition, phosphorus deficiency has also been attributed to reduced photosynthetic rates, needle conductance, and growth in young conifer seedlings (Bradbury and Malcolm 1977; Black 1988; Conroy et al. 1988, 1990). In British Columbia, one-year-old interior spruce seedlings are planted either in the spring (spring-stock), or in early summer (summer-stock). The spring-stock is produced in forest nurseries during the growing season before planting. Under this regime, seedlings set a terminal bud during late July or early August, are lifted in November, and frozen-stored (−2 ◦ C) until planting in the following spring. Between terminal bud set and lifting, these seedlings continue to receive an adequate supply of all nutrients, and build up nutrient
29 reserves that will be available for retranslocation and growth after planting. In contrast, the summer-stock is produced in the same year it is planted. Shortly after terminal bud-set, seedlings are shipped to reforestation sites, and receive a much lower supply of nutrients after bud-set than seedlings produced as a spring-stock. Consequently, the summer-stock may have a lower supply of phosphorus for retranslocation and new growth after planting, than the springstock. This implies that the two stock-types may have different performance potentials on sites low in soil P availability. This study determined P uptake, retranslocation, morphological development and photosynthetic patterns of spring-stock and summer-stock interior spruce during periods when the two stock-types are typically planted.
Materials and methods Plant material One-year-old interior spruce (Picea glauca (Moench) Voss × Picea engelmannii Parry) (seed-lot #4061, Lat. 51◦ 190 N, Long. 119◦ 570 W, Kamloops Forest Region, British Columbia, Canada) seedlings were grown in 410B styroblock containersr in a commercial nursery (Lat. 49◦ 120 N, Long. 122◦ 220 W), and produced as spring-stock (i.e., spring/summer grown, falllifted, frozen-stored and spring planted) and summer-stock (winter/spring grown, summer lifted and planted). Operationally, both stock-types are characterized by their shoot development after planting. Spring-stock breaks bud and shoot elongation occurs shortly after planting. In contrast, summerstock sets a terminal bud just prior to lifting in the nursery and progresses through terminal bud dormancy after planting. Shoot elongation does not occur in summer-stock until the following spring when spring-stock commences its second post-planting shoot elongation period. In this study, spring-stock was grown in the nursery between March and August 1993, set bud in mid August, were lifted and packaged in late November, 1993, and frozen stored (−2 ◦ C). Actively growing summerstock were removed from the nursery in July, 1994 and received a short-day treatment (8 h photoperiod for 1 week at 22 ◦ C air temperature, 50% relative humidity) at BC Research Inc. to induce bud-set. No effort was made to inoculate seedlings with mycorrhizae, though naturally occurring mycorrhizae were visually evident on some seedlings of both stock-types. One hundred seedlings were randomly selected from each stock-type population to determine height and diameter. For each experiment, only seedlings within ±0.5 standard deviations of the mean height and mean diameter of the sample were used to ensure morphological uniformity within
30 the experimental populations. Total number of seedlings in the spring-stock experiment were n = 414, and in the summer-stock experiment were n = 300. Less seedlings were used for the summer-stock study because fewer measurements were required (i.e., none required for new shoot parts during the first growing period). No effort was made to select or avoid mycorrhizal seedlings since it’s effect was not considered to be important: the peat media had very low levels of P so that the major supply of P for seedling uptake was controlled though liquid fertilizer, as described below. Growing environment Both spring-stock and summer-stock experiments consisted of an initial growing period, followed by a simulated winter, and a second growing period. For the first growing period, spring-stock was removed from frozen-storage in early March, 1994, and grown in a controlled environment room (22 ◦ C air temperature, 50% relative humidity, 300 µmol m−2 s−1 photosynthetically active radiation with metal halide, 1000W) until bud-set in late May, 1994. Summer-stock were grown (growth did not consist of shoot elongation) in the above described controlled environment room between July and September 1994. Photoperiod was set at 20 h for spring-stock to ensure full growing season shoot elongation, and at 14 h for summer-stock to simulate average day-length for the period of July to September in the Kamloop’s Forest Region, where seed was originally collected. Two nutrient regimes were used during the first growing period; optimum liquid nutrient regime containing all macro and micro nutrients with (+P) or without (−P) phosphorus. The peat-based medium was analyzed for available − levels of all macro nutrients (NH+ 4 , NO3 , P, K, Ca, Mg, and S) at the beginning of the study. Nutrient solutions were developed that supplemented the container medium nutrient supply and achieved target levels for coniferous seedlings (Landis et al. 1989) (Table 1). Seedlings were fertilized weekly with a constant volume (175 mL container−1 ) and nutrient rate, allowing for determination of the total amount of applied P, and ensuring the amount of P applied was equal between the two studies. Seedlings were monitored for nutrient content. In October, 1994, sub-samples of both stock-types (n = 21) were simultaneously put through a simulated winter (5 ◦ C air and soil temperatures, 8 hour photoperiod at 100 µmol m−2 s−1 ). Seedlings were maintained under these conditions for five weeks in order to advance terminal bud dormancy from rest to a quiescence (Grossnickle et al. 1994). Both stock-types were then grown together for a second growing period of approximately eight weeks, under the same conditions used for the first growing period of spring-stock, except both −P and +P seedlings were fertilized with the +P nutrient regime,
31 Table 1. Target, initial peat levels, and application rates (mg/L fertilizer) of soil substrate macronutrients for rapid growth of 1 + 0 spring-stock and summer-stock interior spruce seedlings grown in 1.2L pots with peat.
Target Initial peat levels
NH+ 4
NO− 3
P
K
Ca
Mg
S
75 58
75 3
60 3
150 109
80 1265
40 253
60 12
+P (NH4 )2 SO4 KH2 PO4 Ca(NO3 )2 .4H2 O Mg(NO3 )2 .6H2 O Total added Total
42
48 57
69
60 35 42 100
95 98
86 30 57 60
69 178
86 1351
30 283
48 60
−P (NH4 )2 SO4 KNO3 Ca(NO3 )2 .4H2 O Mg(NO3 )2 .6H2 O Total added Total
42
48 25 35 35
42 100
95 98
69 50 30 0 3
69 178
50 1315
30 283
48 60
but at a reduced dosage (70 ppm N). After shoot elongation and bud-set (i.e., after eight weeks), both stock-types were measured for new-shoot length (i.e., leader length) and root collar diameter. Morphological assessment At four, eight and 14 weeks for the spring-stock study, and at four, eight and 12 weeks for the summer-stock study, new-shoot length (spring-stock seedlings only), root collar diameter, shoot dry weight, and new-root dry weight were determined on 21 seedlings. New-shoot length was defined as the length of terminal leader elongation, and new-root dry weight was defined as the weight of roots extending out of the original container plug media. At 12 or 14 weeks, for spring- and summer-stock, respectively, terminal bud primordia (Templeton et al. 1993) and number of shoot buds were also determined.
32 Assimilation of CO2 Assimilation of CO2 (A) was recorded with a LI-6200 (Li-Cor Inc.) gas exchange system and a 1/4-L chamber (LI-6200-13) on randomly selected seedlings (n = 9) from each nutrient regime. Measurements were taken biweekly, two to four hours after photoperiod commencement, at 600 µmol m−2 s−1 of photosynthetically active radiation (i.e., samples were moved closer to light source), 24 ◦ C leaf temperature, and 50% relative humidity. Afterwards, sample branches were removed, projected surface area (LI3000, Li-Cor Inc.) and dry weight of the needles were determined, and gas exchange measurements were re calculated according to each samples total surface area. For spring-stock, gas exchange measurements were measured on nursery-needles (i.e., needles produced in the nursery prior to the study) from two to 14 weeks, and on new-needles (i.e., needles produced during the study) of the same seedlings from six to 14 weeks after planting. For summerstock, no new shoot growth occurred during the first growing period and gas exchange measurements were measured only on nursery-needles. Needle Phosphorus Both nursery-needles and new-needles for spring-stock, and nursery-needles from summer-stock, were collected from seedlings used for A measurements (biweekly). Needle samples were oven dried (65 ◦ C) for 48 hours, ground in a Wiley Mill, and analyzed for P with the colorimetric method using molybdenum blue (Allen 1989). Total shoot P was calculated by the product of P concentration and shoot dry weight, determined at four, eight and 12 or 14 weeks. Phosphorus-use efficiency (PUE), defined as dry matter production per unit of nutrient uptake, was based on total shoot P content and shoot dry weight (Chapin and van Cleve 1989; Munson and Timmer 1990): PUE =
shoot dry weight (mg) total-shoot P content (mg)
(1)
The effectiveness of roots in acquiring P is indicated by the specific absorption rate (SAR), defined as the amount of nutrient absorbed per unit root mass (Gray and Schlesinger 1983). It was calculated by total shoot P content and new-root dry weight (Munson and Timmer 1990): SAR =
shoot P content (mg) new-root dry weight (mg)
(2)
Munson and Timmer (1990) used total root dry weight for SAR calculation. For this study, new-root dry weight was considered to better reflect effective
33 root P absorption capacity than total root dry weight, which would include woody plug-roots formed in the nursery. The calculation of SAR was expressed on a per unit shoot dry weight basis (e.g. SAR/shoot dry weight) to account for changes in P content related to shoot size. This allowed P absorption by new-roots to be assessed in the context of shoot requirements (sink effect), and allowed for better qualitative comparisons of SAR differences between the two stock-types which differed in shoot dry weight. Thus, the rate of P absorption is expressed per unit of shoot P dry weight instead of per unit of time. Experimental design Both the spring-stock and summer-stock studies were conducted as separate experiments. Both experiments used a randomized block (three) design, where blocking was determined by seedling distance from each light source in the controlled environment room. Morphological and physiological samples were selected evenly from among the three blocks. Analysis of variance was conducted separately for each experiment and measurement period, and included blocking, fertility regime (+P or −P), and error terms. No interactions were found between fertility regime and block, thus, fertility regime differences were assessed with F-tests of p levels at each measurement period. Means and standard errors were calculated for each fertility regime and measurement period. Shoot dry weight, root dry weight, and P concentration means for each block were used to calculate three values (i.e., 3 blocks) of P content, PUE and SAR for each P regime and stock-type. When new shoot growth commenced for spring-stock seedlings during the first growing period, the P concentration value used to calculate total P content was derived from mixed homogenized samples of both new- and nursery-needles. The springand summer-stock studies were analyzed separately, and only qualitative comparisons were made between the two stock-type studies.
Results Morphological development during the first growing period Differences in morphological development between +P and −P regimes differed according to stock-type. For the spring-stock, shoot dry weight (Figure 1A), new-shoot length (Figure 1A insert), diameter (Figure 1B), and new-root dry weight (Figure 1C) increased in a similar manner between +P and −P treated seedlings during the 14-week trial. In contrast, +P summerstock had greater shoot dry weight (Figure 1D) and diameter (Figure 1E) at
34
Figure 1. Morphological development of interior spruce seedlings during the first growing period, under a balanced nutrient regime with (+P) and without (−P) a phosphorus component. The parameters (mean ±SE) measured were shoot dry weight (A), new-shoot length (A insert), diameter (B), and new-root dry weight (C) of a one-year-old spring-stock, and shoot dry weight (D), diameter (E), and new-root dry weight (F) of a one-year-old summer-stock. One, two or three asterisks indicate that +P and −P means within a stock-type are different (F-test) at the ∝ = 0.10, 0.05, or 0.01 level of probability, respectively.
eight and 12 weeks, and greater new-root dry weight (Figure 1F) at 12 weeks, compared to −P summer-stock. Stock-type response to low P availability also differed for bud development at the end of the first growing period. Shoot growth had ceased by week ten, and terminal buds with brown bud scales were visible by week 14 in both +P and −P spring-stock. At the end of the first growing period, the number of needle primordia in the terminal bud (Figure 2A), and the
35
Figure 2. Number of terminal bud needle primordia (A) and shoot buds (B) at the end of the first growing period (mean ±SE) for interior spruce spring-stock and summer-stock, grown under a balanced nutrient regime with (+P) and without (−P) a phosphorus component. One, two or three asterisks indicate that +P and −P means within a stock-type are different (F-test) at the ∝ = 0.10, 0.05, or 0.01 level of probability, respectively.
total number of buds per shoot (Figure 2B) were similar between +P and −P spring-stock. For summer-stock, +P seedlings had greater number of buds and terminal bud primordia than −P seedlings. Terminal bud primordia in −P summer-stock were at levels similar to both the +P and −P spring-stock, but spring-stock had approximately twice the number of buds than summerstock. Greater number of buds in the spring-stock was due to their greater shoot size (i.e., greater height, and number of branches, not reported). At the end of the first growing period, shoot elongation had occurred twice (i.e., once in the nursery and once during the trial) for spring-stock, but only once for summer-stock (i.e., only in the nursery). Needle P content and concentration Both spring-stock and summer-stock had greater needle P concentration when grown under the +P regime, compared to those grown under the −P regime. Differences in nursery-needle P concentration between +P and −P seedlings were detected as early as two weeks after planting, and concen-
36 tration progressively decreased in −P seedlings of both stock-types (Figure 3A & 3C). However, mean P concentration differences between +P and −P were greater throughout the first growing period with summer-stock than with spring-stock. Differences in new-needle P concentration also occurred between +P and −P spring-stock, but these differences were not as great as those recorded for nursery-needles during the same measurement periods (Figure 3B). Total P content progressively increased in +P seedlings, but net uptake was negligible in −P seedlings for both stock-types (Figure 4A & 4B). Consequently, differences in total P content between −P and +P seedlings of both stock-types occurred as early as week two. Phosphorus-use efficiency Phosphorus-use efficiency (PUE) was relatively constant in +P seedlings of both stock-types, but increased in −P seedlings (Figure 5A & 5B). Generally, seedlings maintained under a −P regime had greater PUE than seedlings under the +P regime. Greater PUE was observed in −P seedlings only at week 14 for spring-stock, but at all measurement periods for summer-stock. The greater PUE for −P seedlings indicated greater shoot dry mass production per unit content of shoot P. Spring-stock generally had lower levels of PUE than summer-stock under low P availability. The highest PUE level recorded for −P spring-stock (1380 mg SDW mg−1 P at 14 weeks) was lower than the levels recorded for summerstock at eight and 12 weeks after planting. Specific absorption rate Response of P specific absorption rate (SAR) to low P availability varied according to stock-type. Both +P and −P spring-stock had similar SAR (i.e., p > 0.05), but summer-stock had greater (i.e., p < 0.05,) SAR in +P seedlings at two and eight weeks after planting, compared to −P seedlings (Figure 6A & 6B). The mean SAR levels for −P summer-stock were close to both +P and −P spring-stock at similar periods after planting (e.g., ∼ =40 at 2 weeks, ∼ =20 at 4 weeks, and ∼ =5 to 10 at 8 weeks). In contrast, mean SAR was twice as high in +P summer-stock during the same periods. Thus, +P summer-stock were absorbing more P per unit of new-root dry weight, for each unit of shoot dry weight, than the other three experimental populations.
37
Figure 3. Needle phosphorus (P) concentration (mean ±SE) in interior spruce seedlings grown under a balanced nutrient regime with (+P) and without (−P) a phosphorus component: spring-stock nursery-needles (i.e., needles formed in the nursery) (A), spring-stock new-needles (i.e., needles formed during the first growing period) (B), and summer-stock nursery-needles. One, two or three asterisks indicate that +P and −P means within a stock-type are different (F-test) at the ∝ = 0.10, 0.05, or 0.01 level of probability, respectively.
38
Figure 4. Total shoot phosphorus (P) content (mean ±SE) for interior spruce seedlings grown under a balanced nutrient regime with (+P) and without (−P) a phosphorus component: for a spring-stock (A) and summer-stock (B). One, two or three asterisks indicate that +P and −P means within a stock-type are different (F-test) at the ∝ = 0.10, 0.05, or 0.01 level of probability, respectively.
Assimilation of CO2 The effect of low P availability on net assimilation of CO2 (A) appeared to be dependent on whether needles were formed before or during exposure to P deficient conditions. Throughout the first growing period, nursery-needle A was similar (i.e., p > 0.05) between +P and −P seedlings of spring- and summer-stock (Figure 7A & 7C). However, for the spring-stock, the +P seedlings had greater new-needle A than −P seedlings (Figure 7B) from week six through week 14.
39
Figure 5. Phosphorus-use efficiency (PUE) (mean ±SE) for interior spruce seedlings grown under a balanced nutrient regime with (+P) and without (−P) a phosphorus component: spring-stock (A) and summer-stock (B). One, two or three asterisks indicate that +P and −P means within a stock-type are different (F-test) at the ∝ = 0.10, 0.05, or 0.01 level of probability, respectively.
Morphological development during the second growing period The effect of P availability on seedling shoot development at eight weeks after exposure to a simulated winter environment differed according to stock-type. Shoot growth after this chilling period was the second shoot extension for spring-stock and the first for summer-stock after removal from the nursery. New-shoot length (i.e., leader length) (Figure 8A) and diameter (Figure 8B) were not different between +P and −P spring-stock. In contrast, +P summerstock had greater new-shoot length and diameter than −P summer-stock.
40
Figure 6. Specific absorption rate (SAR) (mean ±SE) of phosphorus (P) for interior spruce seedlings grown under a balanced nutrient regime with (+P) and without (−P) a phosphorus component: for spring-stock (A) and summer-stock (B). SAR is calculated as P uptake (i.e., P content in the shoot), per P root absorption capacity (i.e., new-root oven dry weight), per P sink capacity (i.e., shoot oven dry weight). One, two or three asterisks indicate that +P and −P means within a stock-type are different (F-test) at the ∝ = 0.10, 0.05, or 0.01 level of probability, respectively.
Discussion Morphological response to high and low P availability Response of one-year-old interior spruce seedlings to soil P availability after planting varied according to stock-type. Over two growing periods, morphological development did not differ between spring-stock exposed to either adequate (+P) or low levels (−P) of phosphorus availability. In contrast, −P summer-stock were lower in all measured morphological attributes during
41
Figure 7. Assimilation of CO2 (A) (mean ±SE) of interior spruce seedlings grown under a balanced nutrient regime with (+P) and without (−P) a phosphorus component. Measurements were taken on nursery-needles (i.e., needles formed in the nursery) (A) and new-needles (i.e., needles formed during the first growing period) (B) of a spring-stock, and on nursery-needles of a summer-stock. One, two or three asterisks indicate that +P and −P means within a stock-type are different (F-test) at the ∝ = 0.10, 0.05, or 0.01 level of probability, respectively.
42
Figure 8. New-shoot length (A) and diameter (B) at the end of the second growing period (mean ±SE) for interior spruce spring-stock and summer-stock. Seedlings were grown under the +P fertility regime during this period. The symbols +P and −P refer to the fertility regime of the first growing period. One, two or three asterisks indicate that +P and −P means within a stock-type are different (F-test) at the ∝ = 0.10, 0.05, or 0.01 level of probability, respectively.
the first growing period, compared to +P summer-stock. Consequently, −P summer-stock had reduced bud development during the first growing period, and lower levels of morphological development during the second growing period. Other studies with spruce species have found reduced total biomass accumulation (Armson and Sadreika 1979; Proe and Millard 1995), and reduced bud development (Chandler and Dale 1990) if P levels were deficient. The one-year-old spring-stock appeared to have a greater ability to buffer the effect of low P availability on morphological development over two growing periods, compared to the one-year-old summer-stock. Phosphorus retranslocation and uptake Differences in the morphological response of the two stock-types to P availability after planting was probably related to P retranslocation patterns. Production of new tissue and biomass is supplied with P from two processes; one is the uptake from external sources (soil), and the other is from redistri-
43 bution through withdrawal from older organs (retranslocation). Phosphorus retranslocation was not measured directly in this study, but the potential of nutrient retranslocation can be inferred by changes in both old and new needle P content and concentration during the first growing period (Turner 1977; Munson and Timmer 1990). The critical period of P accumulation for storage and later retranslocation occurs primarily after bud-set (van den Driessche 1991). By virtue of the typical period of planting for each stock-type, the spring-stock went through this period of P accumulation in the nursery under adequate fertilization and before exposure to the low P conditions after planting. In contrast, the summer-stock went through this stage of P accumulation after planting when exposed to two levels of P availability. Thus, the two stock-types were exposed to the P deficient conditions at different periods of seedling phenology and stages of P uptake. The P content of the roots was not determined, but constant or low shoot P content inferred negligible net P uptake in the shoots of both stocktypes grown in the −P regime. For spring-stock seedlings in the −P regime, low net P uptake in shoots, constant P concentration (0.08 and 0.10%) in newly expanding needles, decreasing P concentration in nursery-needles, and increasing shoot dry weight, all suggested that P was being translocated to new-needles from nursery-needles or other seedling components during shoot growth. For −P summer-stock, negligible net P uptake in shoots and decreasing P concentration in nursery-needles indicated a dilution of P within the nursery-needles as diameter and shoot dry weight increased. This dilution suggested that P retranslocation may have been occurring within the nursery-needles of −P summer-stock. Other studies have shown that nutrient retranslocation can occur within conifer needles formed in the current growing season (Fife and Nambiar 1982). The ability of retranslocation to meet the nutrient requirements of current tissue is partially controlled by the ability of current uptake to meet these requirements (Florence and Chuong 1974; Ballard 1980; Millard and Proe 1993; Proe and Millard 1995). Retranslocation tends to be enhanced under nutrient deficient conditions (van den Driessche 1991). Thus, plants are capable of tolerating nutrient stress with a buffer of internally stored nutrients (Bradshaw 1965). Despite poor mobility within soils, once P has been extracted from the soil it tends to be recirculated within the plant (Ballard 1980; Salisbury and Ross 1992). In this study, most of the P available for retranslocation was probably obtained while both stock-types were being grown in the nursery under adequate fertilization, although this amount was much lower for summer-stock.
44 Phosphorus-use efficiency Phosphorus-use efficiency (PUE) differed by stock-type, and differences were attributed to apparent P retranslocation. Under conditions of decreased nutrient availability, nutrient-use efficiency will increase (Miller et al. 1976; Vitousek 1982; Birk and Vitousek 1986; Gray and Schlesinger 1983), but both the levels of nutrient available from soil (uptake) and from older tissue (retranslocation) must be low before this will occur (Crawford et al. 1991). Similar PUE between +P and −P spring-stock during the first eight weeks indicated that retranslocation was capable of meeting the P requirements of newly forming needles in −P seedlings to the same degree as the combination of both retranslocated and/or root-absorbed P was in +P seedlings. However, PUE increased after week eight in −P spring-stock when levels of P available for retranslocation were low in nursery-needles (i.e., at lowest recorded P concentration). In contrast, summer-stock had only current-year produced tissue from which to obtain retranslocated P, and thus −P seedlings had greater PUE than +P seedlings throughout the first growing period. Very high PUE levels (≥1400) in −P summer-stock were associated with reductions in shoot dry weight, diameter, and new-root dry weight, suggesting that PUE ≥1400 is a critical level for one-year-old interior spruce. Specific absorption rate of phosphorus The response of specific absorption rate (SAR) to low P availability during the first growing period differed by stock-type. Patterns of SAR reflected differences in the amount of P available for retranslocation in each stocktype. Generally, SAR increases as nutrients become more available in the soil (Barrow 1977; Chapin 1980; Gray and Schlesinger 1983), and decreases with time as plant roots exhaust supplies in their immediate vicinity (Khasawneh 1975). Nutrient retranslocation is a more efficient process in trees than nutrient uptake from the soil (Nambiar 1985), and changes in SAR can be modified by internal supply (Barrow 1977). This appeared to be the case for spring-stock, where +P and −P seedlings had similar SAR. Negligible net P uptake in the shoots of −P seedlings but similar SAR between −P and +P seedlings, reflected a preference for retranslocated P over root absorbed P. Nambiar and Fife (1987) reported that P retranslocation in radiata pine (Pinus radiata D Donn) was independent of nutrient supply, and was probably controlled more by growth rate than soil nutrient availability. In contrast, +P summer-stock had mean SAR levels that were higher than all other tested populations at similar measurement periods, indicating greater uptake of P per unit of root absorbing capacity and shoot sink capacity. Greater SAR in
45 +P summer-stock is the result of a lower supply of retranslocated P for shoot growth. Assimilation of CO2 Low P availability did not have an effect on net assimilation of CO2 (A) in nursery-needles of either stock-type, but it did result in reduced A in new-needles of −P spring-stock. The response of A to low P may have been affected by the timing of needle development with respect to P deficiency. Other studies have shown that carbon assimilation rate in conifers is sensitive to P nutrition (Black 1988; Conroy et al. 1988, 1990), but these studies conducted their photosynthetic measurements on needles that had formed during a low P available condition. In this study, nursery-needles had completed most of their development before exposure to the −P regime. In contrast, new-needles in spring-stock were formed during the period of exposure to P deficiency, and −P seedlings had lower new-needle A than +P seedlings for almost all measurement periods. This indicated that the photosynthetic system in developing needles is more sensitive to P deficiency than in developed or mature needles of interior spruce. A longer period (e.g., two growing periods) of P deficiency may have eventually resulted in reductions of A in nursery-needles. Natr (1972) found that P deficiency resulted in a decrease in A, a change in photosynthate distribution, and a decrease of assimilate transport out of fully formed leaves only after a long-term and pronounced P deficiency. Miller et al. (1979) indicated that nutrients are held within plant tissues at two levels of availability for retranslocation: 1) readily available nutrients from recent uptake and temporary seasonal storage, and 2) less readily available nutrients that can only be used at the expense of primary physiological processes. If high amounts of P occur in the first level, P in the second level will be available only under substantial nutrient stress (van den Driessche 1991). Thus, mature leaves or needles may require a greater period of deficiency before P associated with specific photosynthetic processes is limited. In contrast, a P deficiency during needle or photosynthetic system development will most likely result in an immediate reduction in A. Operational considerations The controlled testing environment of this study is not wholly representative of reforestation site conditions. Seedlings were grown under low light conditions, and the effect of native reforestation mycorrhizae on P uptake was not assessed. Despite this, the trial was adequate to assess the relative
46 performance of stock-types (Folk and Grossnickle 1997). Field studies should be conducted to confirm these differences. Two-year-old seedlings were not included in this study. The performance of one-year-old summer-stock may not be representative of the performance of two-year-old summer-stock under low P conditions. Two-year old seedlings would have an internal supply of P from needles produced in the previous year, and would be exposed to an adequate fertility regime after budset during the first year in the nursery. However, nursery-needles produced in the year of planting will still not have the benefit of a nutrient loading period between bud-set and planting. Studies should be conducted to compare the performance of one- and two-year-old summer-stock. In a similar manner, the two stock-types differed significantly in shoot dry weight at the beginning of the first growing period (i.e., spring stock was 3 times greater than summer-stock). The effect of this difference on the relative performance and internal supply of P for each stock type should be determined.
Conclusions This controlled environment study has shown that stock-type selection may play a critical role in determining the success of reforestation efforts on sites low in soil P availability. Seedlings require a good supply of internal P to buffer the effects of low soil P. A one-year-old spring-stock is a better choice than a one-year-old summer-stock for sites having low soil P, as these seedlings remain in the nursery after bud-set and receive an adequate supply of fertilizer before lifting in late fall or early winter for frozen-storage. Summerstock is planted shortly after bud-set, and the lack of an adequate period of nutrient loading after bud-set results in seedlings that are more susceptible to low soil P availability. Thus, certain stock-types can result in reduced growth during the first and second growing seasons if improperly matched with site fertility conditions.
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