Agronomy Journal 94:876-882 (2002)
© 2002 American Society of Agronomy
COTTON
Stem and Root Carbohydrate Dynamics of Two Cotton Cultivars Bred Fifty Years Apart
Randy Wells*
Dep. of Crop Sci., North Carolina State Univ., Box 7620, Raleigh, NC 27695-7620
* Corresponding author (randy_wells{at}ncsu.edu)
Received for publication May 22, 2001.
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ABSTRACT
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Little is known concerning carbohydrate pools within the nonleaf and nonreproductive portions of the cotton (Gossypium hirsutum L.) plant. A 2-yr field study was conducted to ascertain both the concentration and total content of soluble carbohydrate and starch in upper stems, lower stems, vegetative branches, and roots of two cultivars released approximately 50 yr apart [Deltapine (DPL) 14 and 5690] from the same breeding program. In addition, yield, canopy photosynthesis, nodes above white flower, and white flowers per square meter were measured in the second year. Cultivar main effect was significant for soluble carbohydrate, total carbohydrate (soluble carbohydrate + starch), root dry weight, and total stem and root dry weight per plant. These differences are reflected by the generally greater dry weight and larger soluble carbohydrate concentration of DPL 5690 at 143 d after planting in 1994. Deltapine 5690 also exhibited larger late-season, integrated canopy photosynthesis in 1994, attributing to its larger soluble carbohydrate content. There was an initial increase in total stem and root carbohydrate per plant in each year followed by a decrease. Timing of the maxima and minima differed between years and between cultivars in 1994. Both the upper/lower stem carbohydrate ratios and the upper/lower stem starch ratios indicate declining upper-stem carbohydrate as flowering approached a hiatus and a shift in carbohydrate content towards the lower half of the stem. The data indicate that carbohydrate concentration and content per plant vary throughout the season; however, the cultivars exhibited little alteration in carbohydrate trends due to breeding efforts.
Abbreviations: DAP, days after planting • DPL, Deltapine
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INTRODUCTION
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BOTH LEAF AND CANOPY PHOTOSYNTHESIS have been characterized as out of phase with reproductive development in cotton. Photosynthesis reaches a maximum before completion of reproductive maturation at the branch (Constable and Rawson, 1980; Wullschleger and Oosterhuis, 1990) and plant level (Constable and Rawson, 1980; Wells et al., 1986). Constable and Rawson (1980) found in a greenhouse experiment that 13 to 28% of total C required for complete boll maturation would come from previously assimilated sources. Wullschleger and Oosterhuis (1990) found that bolls at main stem node 8 needed more than 60% of the required C to be imported from other sources. Further, Wells et al. (1986) found that a large portion of plant boll maturation occurred after canopy photosynthesis had declined appreciably. These observations indicate that stored carbohydrate plays an important role in supplying the needs of boll development.
Storage carbohydrates in the cotton plant are starches, amylose, and amylopectin (Chang, 1980). There have been reports of starch reserves in cotton, starting with Mason and Maskell (1928a)(1928b), who studied the factors governing the transport of carbohydrate in the plant. Another study examined starch and carbohydrate levels in response to ozone and water stress (Miller et al., 1989). De Souza and Da Silva (1987) found a greater concentration of starch in the roots of perennial tree cotton than in roots of the more annual-type cotton, the latter being the phenotype that is commonly grown around the world. The greater starch content of the perennial cotton appeared to be associated with greater root/shoot dry weight ratios and drought tolerance, which imparted a survival mechanism well suited to the cyclic availability of water in northeastern Brazil. Subsequent selections for high and low root starch resulted in 219 and 105 g glucose equivalents kg-1 root dry weight, respectively. This contrast in starch storage capacity resulted in large variations in plant growth and boll development.
Breeding efforts, both public and commercial, have been primarily aimed toward increasing fiber yield. A pertinent question is whether two annual-type cultivars from a common breeding program have altered carbohydrate concentrations and content per plant. The present study was conducted to determine if two such cultivars, released approximately 50 yr apart from the same breeding program, have similar carbohydrate levels in root and stem during reproductive growth.
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MATERIALS AND METHODS
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Cultural Practices
A field study was conducted in 1993 and 1994 on a Dothan sandy loam (fine-loamy, siliceous, thermic Plinthic Paleudults) at the Central Crops Research Station in Clayton, NC. Seed of Deltapine (DPL) 14 (released 1941) and 5690 (released 1990) were planted on 4 May 1993 and 28 Apr. 1994 in four 7.6-m rows spaced 1.0 m apart. Six seeds were planted in hills spaced 1.0 m apart within the rows. Hills were thinned to three plants after plants reached the first true-leaf stage. There were four replications. Six weeks before planting, 14, 14, and 160 kg ha-1 N, P, K, respectively, were broadcast on the plots. At approximately 50 d after planting (DAP), 27, 13, and 54 kg ha-1 N, P, and K, respectively, were sidedressed. Each cotton crop followed soybean [Glycine max (L.) Merr.] in the yearly rotational scheme.
Aldicarb [2-methyl-2-(methylthio) propionaldehyde O-(methylcarbomoyl) oxime] was applied at 1.0 kg a.i. ha-1 in the furrow during planting. Later applications of pesticides were made as needed and consisted of either 70 g a.i. ha-1 esfenvalerate [(s)-cyano-(3-phenoxyphenyl)-methyl-(s)-4chloro-
-(1-methylethyl) benzene acetate] or 30 g a.i. ha-1 cyhalothrin [-cyano-3-phenoxybenzyl 3-(2-chloro-3,3,3-trifluoro-prop-1-enyl)-2,2-dimethylcyclopropanecarboxylate]. Overhead sprinkler irrigation (2.5 cm) was applied at 48, 110, and 118 DAP in 1993.
Plant Sampling
Three plants (one hill) were sampled on six dates starting on 72 DAP and ending on 149 DAP in 1993. In 1994, the five sampling dates selected ranged from 73 to 183 DAP. The top 30 cm of the root systems were dug using a spade, with care given to recover the maximal amount of root. Successive plant samplings were from completely bordered hills and made from 1000 to 1300 h. Three plants were separated in the field and pooled into three distinct strata based on the height of the plant: upper one-half, lower one-half, and roots. Each stem section was separated into main stem and reproductive branches. Leaves, petioles, and reproductive structures were not included in the samples due to the transient nature of their carbohydrate levels during a diurnal cycle. Roots were separated into taproot and lateral roots. Upon separation of each sample, the different plant segments from the three plants were pooled and frozen in liquid N. They were transported to the laboratory on dry ice and remained frozen until they were lyophilized. Upon lyophilization, the sample weights were recorded. Samples were ground to pass through a 40-mesh screen for carbohydrate analyses.
Carbohydrate Analysis
Starch and carbohydrate analyses were made using the methods of Hendrix (1993). One hundred milligrams of ground tissue was extracted with 20 mL of hot 0.8 L L-1 ethanol in a series of three washings. The sample was centrifuged (3000 x g), and the supernatant was frozen after adding 20 mg of activated charcoal.
Starch Analysis
The pellet was reacted with 1 mL of 0.2 M KOH during 1 h in a 100°C water bath. After cooling, 0.2 mL of 1 M acetic acid was added to adjust the reaction mixture pH to near neutrality. Three hundred and sixty units of -amylase from Bacillus licheniformis (Sigma A-3403, EC 3.2.1.1) was added and reacted for 30 min at 85°C. Acetic acid was added to lower the pH below 5.0, and 122 units of amyloglucosidase from Aspergillus niger (Sigma A-3042, EC 3.2.1.3) was added and reacted for 1 h at 55°C. The incubation time for the amyloglucosidase reaction was reported by Hendrix (1993) to be sufficient for starch degradation but was short enough to avoid ß-glucan interference (Denison et al., 1990). Before use, the amyloglucosidase was dialyzed against 50 mM of sodium acetate buffer (pH 4.5) until no glucose was detected in the preparation. The reaction was stopped in a boiling-water bath and clarified in a microcentrifuge. Samples (20 µL) were pipetted into a 96-well microplate and analyzed for glucose using 100 µL of Sigma 115A glucose detection reagent in low light.
Soluble Carbohydrate Analysis
Twenty microliters of the ethanol supernatant was placed in each well of the microplate and dried at 35°C for 20 min. The glucose detection reagent (100 µL) was added to each well. The glucose detection reagent contains glucose-6-phosphate dehydrogenase (EC 1.1.1.49), hexokinase (EC 2.7.1.1), indonitrotetrazolium violet, ATP, NADP+, and buffer. Glucose is converted to glucose-6-phosphate by the hexokinase, and the fructose is converted to fructose-6-phosphate. The glucose detection reagent only reacts with D-glucose-6-phosphate, and only the glucose contained in the soluble fraction is initially detected. Phosphoglucose isomerase (EC 5.3.1.9) was then added to convert fructose-6-phosphate to glucose-6-phosphate. The plate was read again, and the increase in absorbance at 490 nm was attributed to the fructose in the original soluble sample. A final addition of invertase (EC 3.2.1.26) converted the sucrose in the wells to readily converted glucose and fructose. The final increase in the absorbance at 490 nm was proportional to the concentration of sucrose. All measurements were quantified against a glucose standard curve created from a serial dilution and a blank created from a well with only dried 80% ethanol. Two analyses were performed on each sample. Preparation of all enzyme solutions were as in Hendrix (1993).
The quantities of soluble carbohydrates and starch per plant (nonleaf) were calculated by multiplying the dry weight of each plant organ within a plant–strata combination by the concentration of the hexoses, sucrose, and starch concentration in each. Glucose, fructose, and sucrose have been pooled together for this report and are referred to henceforth as soluble carbohydrate.
Canopy Photosynthesis and Other Measurements
In 1994, canopy photosynthesis was measured in a closed, infrared gas analyzer system using a chamber made of angle aluminum covered with mylar (Wells, 1991). The chamber body was 1.0 m high by 1.0 m wide by 1.25 m long. The chamber lid was 0.4 m high by 1.0 m wide by 1.25 m long and was placed on the lower section during measurement. All contact points had weather stripping to avoid air exchange with the atmosphere. The chamber body was placed in a soil base installed for the season. The base was essentially a trough that had the same dimensions as the chamber base. When filled with water, an airtight seal was created. The rate of photosynthesis was measured over a 30-s to 1-min period using a LI-COR Model 6200 infrared gas analyzer. Measurements were made when the photosynthetic photon flux density (PPFD) was above 1200 µmol m-2 s-1. Seven measurements were made from 81 to 144 DAP. Integrated area under the canopy photosynthesis curve was calculated using the trapezoidal rule for area of the trapezoid created by the measured rate of each pair of measurement dates, area = [(rate on the first date + rate on the second date)/2 x the number of days between the dates].
The number of white flowers were recorded on one row per plot on a weekly basis in 1994, starting at 72 DAP and ending 128 DAP. Nodes above white flowers were determined on five plants per plot at the same time. The top node was considered the node that had a leaf at least 3 cm2 in area.
Yield
The plants were defoliated at 144 and 168 DAP in 1993 and 1994, respectively. In 1993, defoliation was accomplished with a mixture of 0.2 kg a.i. ha-1 tribufos (S,S,S-tributyl phosphorotrithioate) and 0.2 kg a.i. ha-1 ethephon [(2-chloroethyl) phosphonic acid]. The same mixture was used in 1994 with the exception that 0.28 kg a.i. ha-1 tribufos was included. Following leaf drop in 1994, the plants within the canopy photosynthetic chamber bases were harvested for estimates of the fiber yield and number of bolls per plant. The seed cotton was hand-harvested and the number of bolls recorded. Seed cotton was subsequently ginned and the weight of fiber determined.
Statistical Analyses
Analyses of variance were performed using the General Linear Model procedure (SAS Inst., 1995). The design was a split-split plot, with year as the whole plot, cultivar as the split plot, and sampling date as the split-split plot. Year was tested using replication(year) as the error. Cultivar and cultivar x year were tested using replication(year) x cultivar as the error term. Because dates of sampling were variable between years, the following dates were paired from the respective years for an analysis of year effects: 72 and 72, 100 and 87, 114 and 114, and 149 and 143 DAP for 1993 and 1994, respectively.
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RESULTS
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Heat units accumulated more quickly in 1993 than in 1994 (Fig. 1A) . The crop in 1993 also reached maturity at an earlier date than in 1994, which is evident in the different defoliation dates. The crop was defoliated at 144 and 168 DAP in 1993 in 1994, respectively. Irrigation was applied on three dates in 1993, and none was applied in 1994 (Fig. 1B).
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Fig. 1. Accumulated (A) weekly heat units and (B) weekly rainfall and irrigation totals during 1993 and 1994. Irrigation in 1993 is indicated by a plus sign (+).
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The mean squares for total stem and root dry weight per plant, carbohydrate concentration, starch concentration, and stem and root carbohydrate content per plant are shown in Table 1. Cultivar main effect was significant (P 
0.1) for root, stem, and total stem and root dry weight. In addition, root soluble carbohydrate concentration, stem and root soluble carbohydrate content per plant, and total stem and root carbohydrate content per plant were significantly different between cultivars. Cultivar x year interaction was significant for root soluble carbohydrate concentration, stem and root soluble carbohydrate content per plant, and total stem and root carbohydrate content per plant. Date of sampling and the date x year interaction were consistently significant for most variables. Year was significant for dry weight of the root and total stem and root dry weight and for stem and root soluble carbohydrate and total stem and root carbohydrate per plant.
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Table 1. Analysis-of-variance mean squares for dry weight, soluble carbohydrate concentration, and starch concentration in various plant organs and for soluble carbohydrate, starch content, and total stem and root carbohydrate per plant. The following dates were paired for analysis of year effects: 72 and 73, 100 and 87, 114 and 114, and 149 and 143 d after planting for 1993 and 1994, respectively.
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Root and stem dry weights were larger in 1994 than in 1993 (Fig. 2A and 2B) . When averaged across cultivar and harvest date, the root, stem, and total stem and root dry weights were 75, 24, and 48% greater in 1994 than in 1993. Deltapine 5690 had a significantly larger stem, root, and total stem and root dry weight than DPL 14 as determined by the cultivar main effect (Table 1). Most of this effect was due to differences in 1994; when averaged over all dates, DPL 5690 had 14% greater structural mass.
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Fig. 2. Dry weight of Deltapine (DPL) 5690 and DPL 14 for the (A) root, (B) shoot, and (C) total stem and root in 1993 and 1994. Error bars indicate least significant difference between cultivars (P = 0.1) determined within year and date of measurement. No significant cultivar differences at P = 0.05 were found.
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The maximal dry weights were observed in 1993 at approximately 79 DAP while in 1994, the maximum values of dry weight were observed at 114 DAP (Fig. 2). Similarly, the accumulated heat units at 80 DAP were approximately 800 in 1993, a level not seen in 1994 until after 100 DAP (Fig. 1A). The slower accumulation of heat units in 1994 was combined with a long frost-free period late in the season, thus allowing considerable late-season plant development, thereby contributing to the larger stem and root dry weight observed in 1994 compared with 1993.
The concentration of soluble carbohydrates was characterized in the aboveground stem portions by a declining trend following an initially maximal level at the first date in each year (Fig. 3) . The root soluble carbohydrate concentration was also greatest on the first sampling date; however, a later increase was seen about 140 DAP in each year (Fig. 3C). Cultivar main effect was not significant for soluble carbohydrate concentration (Table 1). The greatest initial values were found in the upper stem, with approximately 195 and 150 mg glucose equivalents g-1 dry weight in 1993 and 1994, respectively (Fig. 3A). By the last sampling date, stems exhibited about 20 mg glucose equivalents g-1 dry weight, regardless of year. When analyzed within year and date of sampling, significant differences in soluble carbohydrates were found between the cultivars in the lower and upper stem at 100 DAP in 1993 and at 143 DAP in 1994. At 143 DAP in 1994, DPL 14 possessed 24 and 48% lower soluble carbohydrate concentration than DPL 5690 in the upper and lower stem portions, respectively.
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Fig. 3. Soluble carbohydrate concentration of Deltapine (DPL) 5690 and DPL 14 for the (A) upper stem, (B) lower stem, and (C) root in 1993 and 1994. Error bars indicate least significant difference between cultivars (P = 0.1) determined within year and date of measurement. No significant cultivar differences at P = 0.05 were found.
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Starch concentrations were not significantly different between cultivars, regardless of the plant organ examined (Fig. 4)
. The trends due to year were not significant either despite being quite different between years (Table 1; Fig. 4). Roots had a much greater concentration of starch than either the lower or upper stem. Overall seasonal averages of starch concentration in the root, lower stem, and upper stem were 63, 18, and 12 g kg-1 for 1993 and 58, 18, and 10 g kg-1 for 1994.
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Fig. 4. Starch concentration of Deltapine (DPL) 5690 and DPL 14 for the (A) upper stem, (B) lower stem, and (C) root in 1993 and 1994. No significant cultivar differences between cultivars were found in either year.
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Stem and root content per plant of soluble carbohydrate, starch, and total carbohydrate are shown in Fig. 5
. Cultivar main effect was significantly different for soluble carbohydrate per plant (Table 1). The largest difference between the cultivars was observed at 143 DAP in 1994 (Fig. 5). Soluble carbohydrate content of DPL 5690 at 143 DAP was nearly twice that found for DPL 14 (Fig. 5A). Starch content, on the other hand, was not significantly different due to cultivar, despite having seasonal patterns that were very similar to soluble carbohydrate content (Fig. 5B). Total stem and root carbohydrate content per plant showed maximal levels at 100 DAP in 1993 and 143 DAP in 1994 (Fig. 5C). Cultivar differences were observed in 1994 when DPL 5690 had nearly 80% more total carbohydrate content than DPL 14.
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Fig. 5. Stem and root (A) soluble carbohydrate, (B) starch, and (C) total carbohydrate of Deltapine (DPL) 5690 and DPL 14 in 1993 and 1994. Error bars indicate least significant difference between cultivars (P = 0.1) determined within year and date of measurement. No significant cultivar differences at P = 0.05 were found.
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The upper/lower stem soluble carbohydrate concentration ratio declined from a high value of 1.6 at 79 DAP to a low of 0.75 at 148 DAP in 1993 (Fig. 6A)
. A similar pattern was observed in 1994, with a maximum ratio of nearly 1.9 at 73 DAP and a minimum of 0.4 at 143 DAP. Similarly, the ratio of total carbohydrate content in the upper and lower stem declined in each year (Fig. 6B). The minimum in 1993 (0.22) and 1994 (0.13) was reached at 100 and 143 DAP in 1993 and 1994, respectively. No significant cultivar effect was found for the upper/lower stem ratio of either soluble carbohydrate concentration or total carbohydrate content.
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Fig. 6. Upper/lower ratio averaged over cultivar for (A) stem and root soluble carbohydrate and (B) stem and root total carbohydrate in 1993 and 1994. Error bars indicate least significant difference between years (P = 0.1) determined within date of measurement. No significant cultivar differences at P = 0.05 were found.
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In 1994, cultivar canopy photosynthetic rates were not significantly different on any date of measurement (Fig. 7A)
. The integrated area under the canopy photosynthetic curve was significantly larger (38%) for DPL 5690 than DPL 14 during the period from 116 to 144 DAP. The nodes above white flower, on the other hand, exhibited a significant main effect due to cultivar, indicating slightly later maturity for DPL 14 (Fig. 7B). When analyzed within date of sampling, significant differences (P = 0.05) were found at 79, 94, 101, and 107 DAP. The number of white flowers were not significantly different due to cultivar (Fig. 7C). Fiber yield per plant was 40 and 37 g plant-1 for DPL 5690 and DPL 14, respectively, in 1994. These differences were not significant.
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Fig. 7. (A) Canopy photosynthesis (Pn), (B) nodes above white flower, and (C) white flowers per plant of Deltapine (DPL) 5690 and DPL 14 in 1994. Error bars indicate least significant difference between cultivar (P = 0.05) determined within date of measurement.
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DISCUSSION
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Few differences in stem and root carbohydrate content per plant or concentration were evident in the two cultivars examined in this study. The data indicate that seasonal carbohydrate levels and patterns have not been greatly altered in two cultivars produced 50 yr apart by the same commercial breeding program. Generally, the concentration of carbohydrate in the various plant organs was not significantly different due to cultivar. However, stem and root total and soluble carbohydrate content per plant was larger in DPL 5690 than DPL 14 at P = 0.1 in 1994 (Fig. 5). There are several reasons for this difference. The first is the greater dry weight per plant of DPL 5690 compared with DPL 14 in 1994 (Fig. 2C). At 143 DAP, DPL 5690 had a plant dry weight that was 20% larger than that of DPL 14. Secondly, DPL 5690 had four and two times more soluble carbohydrate in the upper and lower stem, respectively, than DPL 14 at 143 DAP (Fig. 3A and 3B). Contributing to this greater relative soluble carbohydrate content was the 38% greater integrated canopy photosynthesis exhibited by DPL 5690 than DPL 14 for the period after 116 DAP (Fig. 7A). This greater canopy photosynthetic rate may merely be due to the larger plants of DPL 5690, as indicated by dry weight. Larger plant dry weight would indicate larger leaf area and greater capacity to intercept light. Spaced plant hills would not reach canopy closure, and light interception capacity would continue to be a limiting factor concerning canopy photosynthetic rates (Wells, 1991).
Maximum concentrations of root starch were 160 g kg-1 found at 100 DAP in 1993 and 80 g kg-1 found at 87 DAP in 1994 (Fig. 4). The 1994 concentration, while seemingly less than observed in 1993, was maintained for a longer period. Approximately 20% more integrated area under the root starch concentration curve was evident in 1994 compared with 1993. Both cultivars reflected the changing trend between years. De Souza and Da Silva (1987) found maximal root starch concentrations of approximately 200 g kg-1 at 90 d after emergence in a more annual-type cotton genotype, similar to the upland genotypes presently grown around the world. The more perennial tree line of De Souza and Da Silva (1987) possessed maximal starch concentrations of 350 g kg-1, a level that is apparently important for survival in drought-prone regions of South America.
The upper/lower stem ratios of soluble carbohydrate concentration and total carbohydrate content declined as growth continued in each year (Fig. 6). The relationship indicates a relative declining presence of carbohydrate and starch in the upper stem concurrent with boll development in the lower-stem region of the plant. The first flowers were observed at about 73 DAP in 1994 (Fig. 7C). Benedict et al. (1976) reported that 14C photosynthate continued to be imported into bolls until 45 d postanthesis. Therefore, the bolls developing from the first flowers at 73 DAP in 1994 would be approaching completion of import needs on or about 118 DAP. Subsequent flowers would produce bolls of younger age and with a considerable need for carbohydrates. The lower carbohydrate presence in the upper stem is consistent with the hypothesis posed by Guinn (1987) that growth, flowering, and boll retention decline when the supply exceeds the demand for carbohydrates. Because the meristematic regions are at the top of the plant, low carbohydrate levels at that stratum in the plant would have inhibitory consequences on further reproductive growth.
Although the lines studied herein are more annual in nature than the perennial tree line studied by De Souza and Da Silva (1987), they are still perennial in nature. This fact may contribute to the large amount of total stem and root carbohydrate after chemical defoliation (Fig. 5C), 144 and 168 DAP in 1993 and 1994, respectively (Fig 2A). At that time, the plants contained approximately 1.5 and 3.0 g glucose equivalents per plant, respectively. This observation is reasonable in light of the multiple reproductive cycles characteristic of perennial species. Perennial species must have reserves to initiate renewed vegetative growth following the previous growth cycle or a period of stress. The importance of this carbohydrate pool is certainly implicated in the regrowth of plants when conditions for growth are favorable (i.e., high temperature, adequate moisture, and excessive N) following defoliation with current management practices.
In summary, the cultivars observed in this study did not differ greatly in carbohydrate concentration (g kg-1) or stem and root content (g plant-1). In 1994, carbohydrate content did reflect a larger plant mass, greater stem soluble carbohydrate concentrations, and greater integrated late-season canopy photosynthesis of DPL 5690 compared with DPL 14. While this greater canopy photosynthesis may merely be explained by the greater mass per unit ground area of DPL 5690, different photosynthetic capacities per unit leaf mass or area may have been changed indirectly through breeding selection for greater fiber yield. The ratios of carbohydrate concentration and content between the upper and lower stem sections indicate a lowered carbohydrate presence in upper-stem portions as reproductive development proceeds. Further, the carbohydrate pool is not completely exhausted during reproductive development and is apparently withheld for a new growth cycle that will not occur under annual production systems.
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ACKNOWLEDGMENTS
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The author expresses gratitude to John B. Graeber and Gary A. Little for their hard work and dedication in completing this study.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
