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a Département de Phytologie, Université Laval, Sainte-Foy, QC, Canada G1K 7P4
b Agriculture and Agri-Food Canada, Sainte-Foy, QC, Canada G1V 2J3
* Corresponding author (castonguayy{at}em.agr.ca)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: DW, dry weight • GDD, growing degree days • HPLC, high performance liquid chromatography • MCW, methanol chloroform water • RFO, raffinose family oligosaccharide • TNC, total non structural carbohydrates
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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It has long been documented that defoliation reduces TNC concentrations in taproots of alfalfa (Graber et al., 1927) and that low concentrations of carbohydrates are associated with poor winter survival and reduced spring regrowth (Reynolds, 1971). However, mid-December TNC concentrations were correlated poorly to spring yields in a study in Virginia (Edminsten et al., 1988), suggesting that high TNC concentrations in taproots prior to the onset of winter acclimation might not be a prerequisite for alfalfa persistence and vigor of spring regrowth. Starch, a major nonstructural polysaccharide that accumulates in alfalfa taproots, is greatly reduced after a defoliation or when growth resumes in spring (Habben and Volenec, 1991; Kim et al., 1993). Defoliation also leads to a decrease in root concentrations of reducing sugars (Barta, 1988). Even though Habben and Volenec (1991) established a positive relationship between starch mobilization and initial vigor of alfalfa regrowth, Boyce and Volenec (1992) subsequently observed that alfalfa genotypes selected for low starch concentration in taproots consistently showed greater regrowth than genotypes selected for their capacity to maintain high starch concentration. Ourry et al. (1994) also noted that starch concentration in alfalfa roots was poorly related to shoot regrowth, with plants having low starch and high root N contents showing higher shoot dry matter yield than plants having high starch and low N content.
Fall accumulation of soluble sugars (mainly sucrose) in taproots is considered a determinant factor for cold tolerance and winter survival of alfalfa (MacKenzie et al., 1988). During late cold acclimation, Castonguay et al. (1995) noted that the fall accumulation in crowns of alfalfa of raffinose and stachyose, members of the raffinose family of oligosaccharides (RFO), was more closely related to the differences in alfalfa freezing tolerance than sucrose accumulation.
The timing of a fall harvest on C reserves and its impact on spring regrowth of alfalfa remains unclear. Additionally, the effect of fall harvest management on cryoprotective sugars including sucrose and RFO has not been investigated. The majority of results on C reserves in overwintering alfalfa are expressed on a concentration basis, and the total amount of carbohydrates available in roots for winter survival and spring regrowth has seldom been considered. A study of carbohydrate pools could provide valuable information on their role in the regrowth of alfalfa harvested in the fall. We investigated the effect of the timing of a fall harvest on quantitative changes, expressed on concentration and total amount bases, of specific C reserves in roots of alfalfa. The relationship between alfalfa regrowth potential and these specific C reserves was also studied.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Transfer to Outdoor Conditions
Immediately after the second harvest (7 Aug. 1997), pots were transferred to an experimental field near Québec City (46° 49' 15'' N; 71° 12' 00'' W; altitude 45 m) and buried in the soil. Plants initiated their regrowth under natural irradiance and temperatures. Plants remained outdoor from 7 Aug. 1997 to 10 Nov. 1997, and were allowed to acclimate under the natural hardening conditions that prevail in late summer and early fall in Québec. Fall harvest treatments were applied during this period as described below.
Fall Harvesting Treatments
Four harvest treatments were applied in the fall: no additional harvest (two harvest system; second harvest on 7 Aug. 1997), or a third harvest taken either at 400, 500, or 600 growing degree days (GDD) cumulated after the second harvest; the third harvest occurred on 8 Sept. 1997, 17 Sept. 1997, or 7 Oct. 1997, respectively. The GDD were calculated by subtracting 5°C from the average of daily maximum and minimum air temperatures (Bélanger et al., 1992). Harvesting height was about 4 cm, and there was little leaf area remaining.
Transfer to Unheated Greenhouse
Pots were dug on 10 Nov. 1997 and transferred to an unheated greenhouse for overwintering. Plants completed their acclimation to low temperatures in the unheated greenhouse, and remained there throughout the winter. Plants were not defoliated. The unheated greenhouse was continuously ventilated during daytime to maintain the inside temperature similar to that of the outside. When the air temperature inside remained permanently below freezing, plants were covered with a layer of insulating fiberglass wool to simulate snow cover. Air temperatures inside the unheated greenhouse and soil temperatures in pots were monitored and recorded from mid-October 1997 to mid-March 1998 (Fig. 1)
, as described in Castonguay et al. (1995). Data of air temperatures from mid-October to mid-November 1997 and of soil temperatures from mid-February to mid-March 1998 are missing. The first killing frost (<-5°C) occurred on 24 Oct. 1997. Soil temperatures were near freezing throughout the winter and remained at subzero levels (Fig. 1). Accordingly, no plant damage or mortality was noted during the overwintering period.
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Alfalfa Regrowth Potential
At each sampling during the overwintering period (10 Nov. 1997, 12 Jan. 1998, and 11 Mar. 1998), four pots of each harvest treatment were transferred to an environmentally controlled chamber, and plants were allowed to regrow under the initial establishment conditions described above. Shoots were harvested after a 3-wk regrowth period on 2 Dec. 1997, 4 Feb. 1998, and 2 Apr. 1998, respectively, and oven dried at 55°C until constant weight for dry weight determination.
Tissue Extractions
Soluble sugars were extracted as described previously by Castonguay et al. (1995). Tissues were ground on ice in 8 mL of MCW (12:5:3, v/v/v) with a Polytron homogenizer (Brinkmann Instruments Canada Ltd, Rexdale, ON, Canada). Seven milliliters of MCW were used to rinse the homogenizer and then combined with the first 8 mL of homogenate. Tubes were centrifuged for 10 min at 1000 x g and the supernatant was collected. For phase separation, 4 mL of water and 1 mL of chloroform were added to the 15 mL of extract. After shaking, the tubes were centrifuged for 10 min at 1000 x g and the aqueous phase was collected. A 1-mL subsample was evaporated to dryness on a rotary evaporator, solubilized in ethylene-diaminetetraacetic acid (EDTA; Na+, Ca2+; 50 mg L-1), and then kept frozen at -40°C until its analysis by HPLC. The nonsoluble residues left after extraction were washed twice with 10 mL of methanol and used for starch determination.
Carbohydrate Analyses
Mono-, di-, tri-, and tetrasaccharides were separated and quantified by HPLC (Waters, Milford, MA). The analytical system used Waters Millenium32 Software, and consisted of a Model 510 pump, a Model 717plus Autosampler, and Model 410 Differential Refractometer. A Waters Sugar-Pak I column (6.5 x 300 mm) was eluted isocratically at 85°C with EDTA (Na+, Ca2+; 50 mg L-1) at a flow rate of 0.5 mL min-1. Peak identity and sugar quantity were determined by comparison to standards.
Starch was quantitated as glucose equivalents with the p-hydroxybenzoic acid hydrazide method of Blakeney and Mutton (1980) after gelatinization at 100°C and digestion for 90 min with amyloglucosidase (Sigma A7255, Sigma Chemical Co., St. Louis, MO). Starch amounts were determined spectrophotometrically by reference to a glucose standard curve.
Results from carbohydrate analyses were expressed on a concentration basis (mg g-1 DW) or on a total amount basis (mg plant-1) by multiplying for each pot the concentrations by the root dry weight (DW) per plant.
Data Analysis
The experimental unit was a pot containing eight plants. The experiment was conducted by means of a completely randomized design with four replicates. Analyses of variance were done on data from the harvest dates corresponding to the harvest treatments, and on data from each sampling date during the overwintering period (Table 1). A priori contrasts were used for comparison of harvest, and cultivar x harvest means (Table 1). Standard errors of the mean (SEM) were calculated for each sampling date. Correlations between shoot regrowth and root DW, concentrations and amounts of carbohydrates at the onset of regrowth were calculated (Table 2). Statistical significance was postulated at P < 0.05. Statistical analyses were performed by SAS statistical procedures (Statistical Analysis System Institute, Inc., 1996).
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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II. Concentrations of Carbohydrates
Soluble Sugar Concentrations
Raffinose was present on all harvest dates from 7 Aug. to 7 Oct. 1997 (Fig. 3A and B
; Harvests) but stachyose was almost completely absent until the beginning of October (Harvest at 600 GDD) in both cultivars (Fig. 3C and D; Harvests). Raffinose accumulated to greater concentrations in AC Caribou than in WL 225 on all harvest and sampling dates (Fig. 3A and B; Table 1). In addition, the increase in raffinose concentration with an additional harvest taken at 400, 500, or 600 GDD was greater for AC Caribou than for WL 225 in November and January (Fig. 3A and B; cultivars x harvests contrasts, Table 1). In contrast, cultivars did not differ in stachyose concentrations on all harvest and sampling dates, except in March where stachyose dropped to a lower concentration in WL 225 than in AC Caribou (Fig. 3C and D; Table 1). Concentrations of raffinose and stachyose were higher in roots of plants harvested three times as compared with plants harvested only twice (Fig. 3 A–D; Table 1). In January and March, raffinose concentrations decreased as the previous fall harvest was delayed, whereas stachyose concentrations showed an opposite response.
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Starch and Total Nonstructural Carbohydrate Concentrations
As fall harvest was delayed from 400 (8 Sept. 1997) to 600 GDD (7 Oct. 1997), there was a progressive accumulation of starch, primarily in AC Caribou (Fig. 4A and B
; Harvests). Starch concentrations declined progressively from mid fall (10 Nov. 1997) to early spring (11 Mar. 1998) in both cultivars. This decline in starch concentration was more pronounced in plants harvested in the fall at 500 or 600 GDD for AC Caribou, and at 500 GDD for WL 225 (Fig. 4A and B). In contrast to soluble sugars, a fall harvest generally reduced starch concentrations at each sampling date, with the exception of November, when starch concentration in AC Caribou harvested at 500 or 600 GDD was higher than in plants harvested only twice (Fig. 4A; cultivars x (2 harvests vs. 500) and cultivars x (2 harvests vs. 600), Table 1).
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In fall, there was no significant correlation between shoot regrowth and root carbohydrate concentrations at the onset of the regrowth period, except in WL 225 for which root sucrose concentration was correlated negatively to shoot regrowth (Table 2). In March, the only significant positive correlation was observed in AC Caribou for starch concentration (Table 2). Correlations between root carbohydrate concentrations assessed on 12 Jan. 1998 and shoot regrowth subsequently measured on 4 Feb. 1998 were not significant (data not shown).
III. Pools of Carbon Reserves
Soluble Sugar Pools
Carbohydrate reserves were also expressed on the basis of their total accumulation in roots (Fig. 5 and 6)
. In both cultivars, raffinose markedly increased in roots from mid-summer (7 Aug. 1997) to mid-winter (12 Jan. 1998), and tended to decline thereafter in spring (Fig. 5A and B). The raffinose amounts were higher in roots of AC Caribou than in those of WL 225 on all harvest and sampling dates (Fig. 5A and B; Table 1). In November, a third harvest taken at 400 or 500 GDD reduced raffinose amounts. However, harvesting at 600 GDD caused a decrease in raffinose amounts in WL 225 but not in AC Caribou as compared with plants harvested twice [Fig. 5A and B; cultivars x (2 harvests vs. 600), Table 1]. Stachyose amounts started to increase later in fall than raffinose to reach similar values in both cultivars, and declined thereafter in March (Fig. 5C and D; Table 1). In November and January, fall harvests taken at 400 and 500 GDD strongly reduced the amount of stachyose per plant in both cultivars as compared with plants harvested twice (Fig. 5C and D; Table 1). Harvesting at 600 GDD had no effect on stachyose amounts as compared with plants defoliated only twice, except for AC Caribou in November [Fig. 5C and D; cultivars x (2 harvests vs. 600), Table 1]. Total sucrose amounts in roots of alfalfa increased during fall hardening and decreased in the spring. Sucrose amounts were generally reduced by fall harvest treatments. Fall harvesting at 400 or 500 GDD had the most adverse effects on sucrose amounts on all sampling dates (Fig. 5E and F; Table 1). However, in November, harvesting at 600 GDD caused a greater decrease in sucrose amounts in WL 225 than in AC Caribou [Fig. 5E and F; cultivars x (2 harvests vs. 600), Table 1]. In January, pools of sucrose were lower in roots of plants harvested at 600 GDD as compared with plants harvested twice.
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In November, the two cultivars differed in their correlations between shoot regrowth and root carbohydrate amounts at the onset of the regrowth period. In WL 225, root DW and all carbohydrate components (raffinose, stachyose, sucrose, starch, and TNC) were correlated highly to shoot regrowth (Table 2). However, no significant correlation was observed in AC Caribou for root DW or any other component of the carbohydrate pool (Table 2). In spring, the raffinose and stachyose pools of both cultivars were not correlated significantly to shoot regrowth, but starch and TNC pools were positively related to shoot regrowth (Table 2). Differences between cultivars were observed for the relationship between shoot regrowth, and sucrose pool and root DW; significant correlations were found in WL 225 but not in AC Caribou (Table 2). Correlations assessed between root carbohydrate pools on 12 Jan. 1998 and shoot regrowth subsequently measured on 4 Feb. 1998 were not significant (data not shown).
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Shoot Regrowth and Root DW
Our observation that shoot regrowth of alfalfa can be reduced by an additional harvest in the fall is in accordance with earlier observations by Sheaffer and Marten (1990) who showed that fall harvest management affects alfalfa yield in the following year. Increasing the length of the regrowth interval between the second summer harvest and the fall harvest mitigated the negative impact on shoot regrowth. Previous field studies also concluded that a reduction in the regrowth interval between the last two harvests reduced alfalfa yield in the following year (Edminsten et al., 1988; Brink and Marten, 1989; Sheaffer and Marten, 1990; Bélanger et al., 1992). Using the accumulation of GDD since the last summer harvest for the timing of the fall harvest, Bélanger et al. (1992) concluded that an accumulation of 500 GDD was sufficient to achieve maximum DM yield. In the current study, a fall harvest taken at 600 GDD was required to achieve a regrowth that did not differ from that of plants harvested only twice in the summer. Our results confirm the field observations of Bélanger et al. (1999) in Atlantic Canada where first harvest yield and total seasonal yield of the subsequent year increased with the lengthening of the interval between the second summer harvest and a third harvest in the fall.
In our system, a root biomass of approximately 60 g m-2 (0.9 g plant -1) at the time of the plant transfer to the field conditions (Second harvest, 7 August) was comparable to values of 50 to 100 g m-2 measured at the same period in eastern Canada (4–9 August) with field-grown plants in their seeding year (Bélanger and Richards, 2000). Our values of 120 g m-2 (2 g plant -1) for root biomass in mid-September (500 GDD harvest date) were also close to those observed by Bélanger and Richards (2000) during the same period (130–170 g m-2). Our results on root biomass changes from the time of the second harvest to mid-September are also in agreement with those of Khaiti and Lemaire (1992) in France, and Bélanger and Richards (2000) in eastern Canada, who reported no or very limited increase in root biomass in the first 20 to 30 d following defoliation (essentially August) followed by a large increase in September. In our study, alfalfa root DW increased markedly in October and early November with the decline in temperature and photoperiod. In spite of several reports on the accumulation of carbohydrate reserves in October and November in alfalfa roots (Li et al., 1996, Cunningham et al., 2001), there are no reports in the literature on the concomitant alfalfa root growth during that period, especially in the seeding year. Our data indicate that TNC accumulation accounted for 40 to 50% of the root DW increase in the fall. The remaining portion of the root biomass increase could be attributable to an increase in structural DM and the accumulation of other organic components such as N or lipid reserves. Therefore, in our study, the transfer of pot-grown plants to outdoor conditions on 7 August and the subsequent exposure to field conditions led to root growth and TNC accumulation that are typical of field grown plants.
The root growth until mid-November of plants harvested twice was greater than that of plants harvested in the fall, even when the fall harvest was delayed until 600 GDD. Plants harvested for a third time in the fall had a root DW that was 30 to 60% lower than plants harvested only twice during the summer. Root DW varied with the interval between the second and a third harvest, the shorter regrowth intervals of 400 and 500 GDD having the most adverse effects. Avice et al. (1997a) noted that shorter regrowth intervals between harvests taken at the beginning of the growing season led to a reduction of root DW. Smaller root DW in plants harvested early in the fall (400 GDD) can be explained in part by the mobilization of existing C reserves to sustain initial shoot regrowth (Habben and Volenec, 1991), along with preferential allocation of newly assimilated C to the shoot to restore the imbalance created between sources and sinks (Frankow-Lindberg, 1997). A late fall harvest at 600 GDD had a lesser impact on root DW, as the period of accumulation of root organic reserves was increased and little regrowth occurred before the first killing frost.
The cultivars differed in their relationship between root DW and shoot regrowth. In fall and spring, root DW was related to subsequent shoot regrowth in WL 225 but not in AC Caribou, suggesting that cold adaptation of cultivars could affect the initiation of regrowth upon exposure to growing conditions. Even though the two cultivars are rated with similar levels of fall dormancy (Certified Alfalfa Seed Council, 1994; Michaud, personal communication, 1996), our results indicate differences in fall dormancy under our experimental conditions, with AC Caribou being more dormant than WL 225. Indeed, WL 225 presented superior regrowth in December than did AC Caribou. Furthermore, a greater accumulation of carbohydrates in roots of AC Caribou than in WL 225 in the fall agrees with Cunningham et al. (1998) who reported superior root soluble sugar accumulation in November and December in populations selected for higher fall dormancy.
Carbohydrate Concentrations
One of our objectives was to characterize the changes occurring during winter in soluble sugar and starch concentrations in alfalfa following different fall harvest treatments. As previously documented, concentrations of soluble sugars increased markedly during fall hardening (McKenzie et al., 1988; Li et al., 1996). Sucrose and the galactose-containing oligosaccharides, raffinose and stachyose, were the most abundant soluble sugars that accumulated in roots of cold acclimated alfalfa, in accordance with measurements made in crowns by Castonguay et al. (1995). Contrary to our expectations of a reduction in soluble sugars with a fall harvest, root concentrations of sucrose, stachyose, and raffinose in plants harvested in the fall were increased relative to plants harvested twice in the summer. The concentrations of these soluble sugars have been related to the acquisition of freezing tolerance in forage legumes (Volenec et al., 1991; Svenning et al., 1997; Castonguay et al. 1995, 1998) and therefore, it seems unlikely that fall harvesting impairs soluble sugar-based cryoprotection.
Differential accumulation of soluble sugars in taproots of alfalfa cultivars of contrasting hardiness was previously reported (Cunningham et al., 1998). In our study, changes in sucrose concentrations during cold acclimation and in response to fall harvest treatments were similar in both cultivars. However, raffinose accumulated to higher concentrations in roots of the hardier cultivar AC Caribou than in WL 225. Castonguay et al. (1995)(1998) reported that both stachyose and raffinose accumulated to higher levels in cultivars of superior winterhardiness, and related these oligosaccharides to the acquisition of cold tolerance. In contrast with raffinose accumulation, root concentrations of stachyose did not differ between cultivars.
Few studies have characterized the evolution of carbohydrate concentrations from early fall to spring in relationship with fall harvest management. It has been shown previously that TNC concentrations in the fall are maximized in a two-harvest system, and that the reductions in TNC concentration are minimized when a third harvest is delayed in the fall (Gervais and Bilodeau, 1985; Gervais and Girard, 1987). Edminsten et al. (1988) reported that TNC concentrations were higher if the regrowth interval between the last two harvests lasted for a minimum of 50 d. In contrast with those results, we observed that delaying fall harvest did not lead to higher TNC concentrations. Starch concentrations throughout the overwintering period were generally reduced by fall harvests while TNC concentrations were only slightly affected. The stability of TNC concentrations, irrespective of fall harvest treatments, was the result of a balance between a decrease in starch and a concomitant increase in soluble sugar concentrations. Our results agree with those of Sheaffer and Marten (1990) who observed that root TNC concentrations in November are not affected by fall harvests. Edminsten et al. (1988) also reported that in spite of differences in TNC concentrations prior to the onset of winter, there were no differences in TNC concentrations in mid-March before spring regrowth.
Root TNC and starch concentrations in fall and winter were not correlated to shoot regrowth. This agrees with Sheaffer and Marten (1990) who reported that root TNC concentrations in fall were not reliable predictors of alfalfa yields in the spring. Edminsten et al. (1988) also observed that December TNC concentrations were not related to spring yields. It is not known whether a carbohydrate concentration threshold in alfalfa taproots is required to sustain regrowth, but there are indications that modifications in the carbohydrate status will affect the initial rate of regrowth (Morvan-Bertrand et al., 1999; Skinner et al., 1999). The lack of consistent relationships between carbohydrate concentrations and shoot regrowth observed in our study does not preclude the importance of C reserves for regrowth as they make a contribution as a source of energy to sustain metabolic activity during heterotrophic periods, when respiration exceeds photosynthesis (Avice et al., 1997a,b).
Carbohydrate Pools
There is little information available on total amounts of carbohydrate reserves that accumulate in roots of alfalfa along with their fluctuations throughout winter as most of the published data have been expressed on a concentration basis. In contrast to our observations based on concentrations, a fall harvest strikingly depleted total amounts of starch and TNC in roots, with the fall harvest taken at 400 GDD having the greater effect (e.g., 200 mg TNC plant-1 in 400 GDD vs. 800 mg TNC plant-1 in 600 GDD in AC Caribou in November). Harvesting alfalfa early in the fall at 400 or 500 GDD interrupted root growth and the accumulation of carbohydrates. In contrast, plants harvested at 600 GDD accumulated higher amounts of carbohydrates in the fall before growth cessation. The reduction of carbohydrate amounts in response to a fall harvest was not likely due to their mobilization for regrowth, since fall regrowth after the third harvest did not differ among harvest treatments, with the exception for WL 225 harvested at 400 GDD which showed superior growth (data not shown).
The TNC amounts decreased throughout winter, especially in plants harvested twice, and this reduction followed a trend similar to that observed for root DW (e.g., 50% in AC Caribou, and 25% in WL 225 harvested twice). However, the minimum amounts of carbohydrates reached at the end of the winter were sufficient to maintain stable and consistent regrowth in early spring. Larger accumulation of root carbohydrates during the fall may provide some benefits to the plants harvested twice as compared with those harvested a third time in the fall, such as superior cold tolerance, or greater resistance to pathogens.
The reduction in TNC amounts represented about 35% of the decrease of root DW during winter. Hence losses of other root cell constituents occurred. Farrar and Jones (2000) indicated that in addition to CO2, carbon losses may occur through rhizodeposition, which includes components such as mucilage, lactic acid, sugars, amino acids, and other organic components, as well as sloughed cells. It is not known how pools of constituents other than TNC are affected during overwintering in alfalfa subjected to different fall harvest managements.
Although all types of carbohydrates were correlated to shoot regrowth in fall in WL 225, no such relationship was observed in AC Caribou. Such differences in the relationship between the availability of C reserves and shoot regrowth between the two cultivars could be related to different fall acclimation response. Skinner et al. (1999) reported that there was no relationship between regrowth and the total TNC that accumulated in roots or that were remobilized. However, in spring, TNC amounts were related positively to the regrowth in both cultivars, indicating that total C reserves could be a determinant factor of alfalfa regrowth as plants come out of dormancy.
Our study showed that carbohydrate amounts were more affected by fall harvest and were better correlated to shoot regrowth than carbohydrate concentrations, underlining the importance of total availability of C reserves for shoot regrowth. Marked variations in the relationship between regrowth and total carbohydrates between cultivars and sampling dates clearly indicate that additional factors need to be considered for a more comprehensive understanding. Several reports have recently indicated that N reserves contribute to winter survival and shoot regrowth of alfalfa (Volenec et al., 1996; Avice et al., 1996, 1997b). Analyses of amino acids and soluble proteins accumulation currently underway will lead to a better understanding of the biochemical bases of the effects of fall harvest management on alfalfa regrowth.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Received for publication April 17, 2001.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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