Crop Science 43:879-885 (2003)
© 2003 Crop Science Society of America
CROP PHYSIOLOGY & METABOLISM
Glyphosate and Water-Stress Effects on Fruiting and Carbohydrates in Glyphosate-Resistant Cotton
Wendy A. Plinea,
Randy Wellsb,
Gary Littleb,
Keith L. Edmistenb and
John W. Wilcut*,b
a Syngenta, Jealott's Hill International Research Centre, Bracknell, Berkshire, RG42 6ET, U.K
b Dep. of Crop Science, North Carolina State Univ., Raleigh, NC 27695-7620
* Corresponding author (john_wilcut{at}ncsu.edu)
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ABSTRACT
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Water stress and glyphosate treatments to glyphosate-resistant (GR) cotton (Gossypium hirsutum L.) can cause abscission of young bolls although the interaction of these factors is not well defined. Studies were conducted to quantify the effects of water stress and glyphosate treatments on fruit retention, fruit placement, and carbohydrate partitioning in GR and conventional cotton varieties grown in a phytotron environment. Glyphosate-resistant plants treated with glyphosate at the four-leaf stage, postemergence (POST), and at the eight-leaf stage, POST-directed (PDIR), had fewer first-position bolls after 0 and 1 d of water stress than nontreated GR and conventional plants but did not differ after 2 and 3 d of water stress. Glyphosate-treated GR plants reached first bloom 3 to 4 d later than nontreated plants. Five-day-old bolls from plants of one genotype, SG 125RR, treated with glyphosate had lower fructose content than bolls from nontreated plants. Subtending leaf carbohydrates and boll sucrose, glucose, and starch content did not differ after glyphosate treatments. Increasing water stress caused reductions in subtending leaf glucose, sucrose, and starch content, as well as reductions in boll starch and sucrose content. Reductions in boll starch and sucrose content in response to water stress may indicate the potential for abscission. Water stress and glyphosate treatments to GR cotton do not alter carbohydrate profiles in boll or leaf tissues in a like manner. Differences in carbohydrate profiles of young bolls and leaves from glyphosate-treated and water-stressed cotton plants suggest that water stress and glyphosate treatments may promote fruit abscission in different manners.
Abbreviations: EPSPS, 5-enolpyruvylshikimiate 3-phosphatesynthase • GR, glyphosate-resistant • IAA, indole-3-acetic acid • POST, postemergence foliar application • PDIR, postemergence-directed application
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INTRODUCTION
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THE NUMBER OF cotton fruiting structures (bolls) is positively correlated with the amount of lint produced (Wells and Meredith, 1984; Heitholt, 1993). Therefore, factors that reduce boll retention often directly reduce lint yield. Environmental, insect, as well as chemical factors, may induce boll abscission (Guinn, 1982). Guinn (1998) proposes that the plant hormones ethylene and indole-3-acetic acid (IAA) promote and inhibit, respectively, the development of the abscission zone where the boll peduncle attaches to the fruiting branch, resulting in either boll abscission or retention. Factors that result in increased ethylene production or decreased IAA production promote abscission. Carbohydrate deficits caused by shade, nutrient deficiency, temperature stress, or high boll load often result in boll abscission (Guinn, 1998). One of the most potent promoters of boll abscission is water stress (Guinn and Mauney, 1984). Boll retention decreases when water potential decreases below -1.9 MPa, and when the plant has a high boll load (Guinn and Mauney, 1984).
Young bolls seem to be more susceptible to water stress-induced abscission than mature bolls (Guinn and Mauney, 1984). There are at least two potential reasons for water stress-induced shedding of young bolls. The first explanation is that the xylem vascular tissue does not function in young bolls which depend instead on phloem transport to obtain water (van Iersel et al., 1994; van Iersel and Oosterhuis, 1995). Second, young bolls may not yet be strong enough sinks to attract the limited amount of photosynthate produced during water stress. Therefore, young bolls produce ethylene and abscisic acid in response to the deficit and abscise (Guinn, 1982). Gersani et al. (1980) describes a strong sink as one that is able to produce IAA, gibberellins, and cytokinins, all of which promote fruit retention. Levels of abscisic acid and ethylene increase in weak sink tissues, promoting abscission by activating enzymes in the abscission zone. Heitholt and Schmidt (1994) attempted to associate retention differences between first, second, and third position sympodial bolls with carbohydrate levels in excised receptacles and ovaries taken 5 d preanthesis to 2 d postanthesis. They concluded that assimilate levels alone did not appear to explain sufficiently the variations in boll retention among different fruiting positions. Because water stress causes severe reductions in photosynthate production (Ackerson and Hebert, 1981), differences in carbohydrate content and sink strength of young nonstressed and water-stressed bolls may be more evident and correspond with water-stress-induced decreases in fruit retention.
Because boll abscission, in its simplest form, is caused by any factor that promotes the development of an abscission zone, several forms of plant stress, whether caused by environmental factors, plant pests, nutrient stress, or other abiotic factors, could all interact to promote abscission (Guinn, 1982). It remains unclear, however, whether all forms of plant stress induce abscission in a similar manner. Herbicide treatments may be considered a stress to cotton plants that are not sufficiently tolerant to a particular herbicide. Glyphosate resistance has been conferred on cotton by the incorporation of a GR CP4-EPSPS (5-enolpyruvylshikimiate 3-phosphatesynthase) gene cloned from Agrobacterium sp. strain CP4 into its genome (Barry et al., 1992; Padgette et al., 1995; Nida et al., 1996). Cotton varieties resistant to glyphosate have been commercially available since 1997. Since that time, decreased boll retention in glyphosate-treated GR cotton has been reported by both researchers and growers (Ferreira et al., 1998; Jones and Snipes, 1999; Yasuor et al., 2000). Previous research has demonstrated translocation and accumulation of 14C-glyphosate in reproductive structures, as well as reductions in pollen viability and abnormal floral anatomy in glyphosate-treated GR cotton (Pline et al., 2001, 2002a,c; Yasuor et al., 2000). The reductions in pollen viability and shorter stamens caused by glyphosate applications result in less pollen deposition on the stigma tissue (Pline et al., 2002c) and correspondingly fewer seeds per boll (Pline et al., 2002a,b). Increases in young boll abscission due to glyphosate treatments may arise from poor pollination, which reduces production of the retention-promoting hormones IAA, cytokinin, and gibberellins by the nonfertilized ovules (Bhardwaj et al., 1975). A second possible explanation for glyphosate-induced boll shed is inhibition by glyphosate of aromatic amino acid biosynthesis in reproductive tissues to the point where the tissues themselves are killed, resulting in boll shed. Pline et al. (2001) showed that the level of CP4-EPSPS in male reproductive organs is significantly lower than that in vegetative leaf tissue. In either case, poor pollination or starvation for aromatic amino acids resulting in tissue death, the levels of carbohydrates in leaf or young boll tissue may be altered in response to glyphosate treatments.
Therefore, the objectives of this research were to quantify the effects of water stress and glyphosate treatments and their potential interactions on fruit placement and retention in conventional and GR cotton varieties and to associate differences in fruit retention with differences in carbohydrate content in young bolls and subtending leaves.
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MATERIALS AND METHODS
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Plant Culture and Water-Stress Treatments
Plants were grown in a climate-controlled greenhouse at the Southeastern Plant Environmental Laboratories with a 26/22°C day/night temperature regime. Delta and Pine Land cultivars DP 50, DP 90, DP 5415 RR, and SureGrow cultivar SG 125 RR seeds were planted in 25-cm pots containing a gravel-metro mix combination soil (Downs and Thomas, 1991). DP 50 and DP 90 are parental lines for DP 5415 RR. Plants were thinned to one per pot and were watered with a standard Phytotron nutrient mixture twice daily (Downs and Thomas, 1991). Applications of 1.12 kg a.i. ha-1 glyphosate (Roundup Ultra, Monsanto, St. Louis, MO) to some GR cotton plants were made using a backpack sprayer delivering 140 L ha-1, at the four-leaf stage (foliar application) and at the eight-leaf stage (PDIR application to stem) according to the Roundup Ultra supplemental label for GR cotton (Anonymous, 1999). Conventional and other GR plants remained nontreated. To assess the impact of transport and handling of plants on fruit retention, an independent study was conducted where plants handled and sprayed with a blank at the four-leaf and eight-leaf stage were compared with plants that were not transported. No significant differences in fruit retention or placement were evident when plants were plant-mapped at cutout (data not shown). The growth regulator, mepiquat-chloride (N,N-dimethylpiperidinium chloride) was applied to upper leaves at the rate of 0.84 kg a.i. ha-1 at the 10-leaf stage to control vegetative growth according to North Carolina Cooperative Extension Service guidelines (Edmisten, 2001).
The first day of flowering for each plant, and the date of anthesis for each flower were recorded. One week after first bloom, water-stress treatments were initiated by withholding water for either 3, 2, 1, or 0 d. Between 0900 and 1200 h, all plants were sampled by replication on the same day following respective water-stress treatments. Water potential of the main-stem leaf on the node with a 4- to 6-d old boll were taken using a pressure bomb. Water potential readings lower than -2.5 MPa were not read due to safety concerns with the pressure chamber. Plants undergoing 1 d of water stress showed few visible signs of stress. Two days of water stress caused leaves to wilt but not to desiccate. Plants undergoing 3 d of water stress were severely water-stressed with many leaves beginning to desiccate by the day of sampling. After water-stress sampling, plants were rehydrated and allowed to recover for 1 wk, at which time each plant was mapped and the number and location of bolls, squares, incompletely abscised fruit, and aborted positions were recorded.
Carbohydrate Analysis
For carbohydrate analysis, at the time of water-stress sampling, a 4- to 6-d postanthesis, first-position boll, and its subtending leaf were removed from each plant. Ten 0.42-cm2–diameter disks weighing 
100 mg were taken from leaf tissue at the base of each leaf and transferred immediately to microphage tubes containing 1 mL of ice-cold ethanol. Leaf samples were kept at -20°C until analysis. The 4- to 6-d postanthesis bolls were collected, frozen, and then freeze-dried. Freeze-dried bolls were then ground in a coffee grinder, and a 100-mg subsample from each boll was analyzed. Nonstructural carbohydrates (glucose, fructose, sucrose, and starch) were extracted and analyzed according to the methods of Hendrix (1993). In brief, tissue was boiled in ethanol to extract ethanol-soluble carbohydrates. Starch was converted to glucose by 
-amylase and amyloglucosidase and was assayed using a glucose kit (glucose diagnostic kit 115-A). All enzymes and the glucose kit were purchased from the Sigma Chemical Company, St. Louis, MO. Ethanol-soluble glucose, fructose, and sucrose were assayed by measuring the release of glucose after treatment with phosphoglucose isomerase or invertase, also using the glucose kits.
Experimental Design and Statistical Analysis
Experiments were conducted in a randomized complete block design in a two-factor factorial arrangement with four replications of treatments. The main effects were genotype/herbicide treatments and level of water stress. Cotton genotypes included two conventional genotypes, two GR genotypes, and two glyphosate-treated GR genotypes. The levels of water stress were 3, 2, 1, or 0 d of water stress. The entire experiment was conducted two times and data from both runs were combined due to nonsignificant run x treatment interactions. Data from plant mapping, water potential, and carbohydrate content were subjected to ANOVA using the SAS version 8, general linear model procedure (SAS Institute, 1999), to test for significance of main effects and interactions. Pairwise statistical comparisons were used to test for differences between glyphosate and nonglyphosate treated GR and conventional varieties. Fisher's protected LSD test at = 0.05 was used to separate means from main effects.
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RESULTS AND DISCUSSION
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Glyphosate and Water-Stress Effects on Fruiting Patterns
Because both glyphosate and water-stress treatments may cause GR or conventional cotton plants to shed fruit, the potential for interactions between these two factors was investigated. Phytotron-grown GR and conventional cotton plants that underwent water stress for 1, 2, or 3 d reached leaf water potentials of -1.8, -2.4, and <-2.5 MPa, respectively (Table 1). Water stress significantly affected fruit retention on all plants. Cotton plants undergoing moderate and severe water stress (2 and 3 d) had 2.8 fewer bolls per plant on Nodes 1 through 7 and 10.4 to 12.6 fewer total fruiting structures (bolls + squares) than did nonstressed plants (Table 1).
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Table 1. Number of bolls on lower portion of plant (Nodes 1–7), total fruit at 2 wk after first bloom, and water potential as effected by 0, 1, 2, or 3 d of water stress. Data pooled over cultivar because water stress x genotype-glyphosate treatment interaction was not significant.
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SG 125RR cotton plants treated with glyphosate at the four-leaf (POST) and eight-leaf (PDIR) stages reached first bloom 3.9 d later than nontreated SG 125RR plants (Table 2). Treated plants did not exhibit any visible phytotoxic symptoms as a result of herbicide treatment, which is in agreement with Jones and Snipes (1999). This delay in flowering suggests that glyphosate applications to some GR cotton varieties may slow some aspects of development. In glyphosate-sensitive species, growth is suspended and chloroplast swelling is observed within 16 to 20 h after treatment (Mollenhauer et al., 1987). A delay in growth of reproductive tissues, which have reduced glyphosate tolerance, may exist in GR plants which have been treated with glyphosate (Pline et al., 2002c).
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Table 2. Days to first bloom, first-position bolls, and total bolls for conventional (DP 50 and DP 90) and glyphosate-resistant (DP 5415RR and SG 125RR) cotton as affected by glyphosate treatment.
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The interaction between the main effects of cotton genotype-glyphosate treatment and level of water stress was significant for first-position bolls, total bolls, total squares, aborted positions, and attached dead bolls/squares (Tables 2 and 3). Minor (1 d) water stress did not reduce first-position or total bolls in any cotton genotype (Table 2). Guinn and Mauney (1984) reported that boll retention was high when leaf water potentials were between -1.4 and -1.9 MPa, which agrees with the high fruit retention at leaf water potentials of -1.3 to -1.8 MPa in the current study. However, plants undergoing moderate to severe (2 and 3 d) water stress had 1.7 to 5.6 fewer first-position bolls than did water-nonstressed plants (Table 2). This decrease in fruit retention is also in agreement with Guinn and Mauney (1984), who reported decreases in boll retention in plants with leaf water potentials below -1.9 MPa. Guinn and Mauney (1984) describe a sophisticated balance between water stress and boll load in determining fruit retention. Plants with a higher boll load are generally more susceptible to water stress and, therefore, shed a greater amount of young fruit than do plants with a lower boll load. This is clear in the current study where the genotype with the highest boll load (DP 90) lost 12.4 bolls under severe water stress, while the genotype with the lowest boll load (DP 5415RR-Treated) lost only 4.3 bolls (Table 2).
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Table 3. Effect of water stress and glyphosate treatments on retained squares, aborted positions, and attached dead bolls or squares per plant in conventional (DP 50 and DP 90) or glyphosate-resistant (DP 5415RR and SG 125RR) cotton varieties at 2 wk after first bloom. Plants were subjected to 0, 1, 2, or 3 d of water stress.
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Glyphosate treatments also affected fruit retention in water-stressed and water-nonstressed plants. DP 5415RR and SG 125RR plants treated with glyphosate and undergoing 1 d water stress had fewer first-position bolls than glyphosate-nontreated DP 5415RR plants, but did not differ at 2 and 3 d of water stress (Table 2). Glyphosate-treated DP 5415RR plants with 0 d water stress also had fewer total bolls than nontreated DP 5415RR with 0 d water stress but did not differ at 1, 2, or 3 d water stress (Table 2). In contrast, glyphosate-treated SG 125RR had fewer total bolls at 3 d water stress than nontreated SG 125RR did, but did not differ at 0, 1, or 2 d of water stress (Table 2). These data suggest that reductions in boll retention following glyphosate treatments may occur under water-nonstressed or minor water-stress conditions, but are overshadowed at moderate to severe water stress conditions by water-stress-induced boll shed. Therefore, water stress appears to exert a stronger influence on boll retention than do glyphosate treatments. The combination of glyphosate and moderate to severe water stress did not act additively to decrease boll retention. The effects of glyphosate on boll retention are more pronounced on first-position bolls than on total bolls on the plant. This observation is in agreement with previous reports that loss of first-position fruit due to glyphosate applications is greater than loss of second- or third-position fruit (Jones and Snipes, 1999).
Reproductive squares are the future fruit of the cotton plant. Therefore, substantial loss of squares due to environmental conditions, insect injury, or chemical treatment damage can reduce numbers of fruit on the plant later in the season. When bolls have been lost due to stress, the formation of additional squares is necessary for yield compensation. Moderate and severe (2- and 3-d) water stress caused significant losses of squares across all varieties compared with water-nonstressed plants (Table 3). Glyphosate treatments of DP 5415RR did not influence retention of squares at moderate (2-d) water stress; however, glyphosate-treated SG 125RR cotton retained significantly more squares than did nontreated SG 125RR after 2 d of water stress. The increased number of squares on glyphosate-treated SG 125RR plants may be a reflection of the 3.9-d delay in maturity in glyphosate-treated plants (Table 2). Generally, mild water stress (1 d) did not reduce the number of squares on plants with the exception of DP 50, where 1 d of water stress caused a loss of 8.8 squares per plant, compared with water-nonstressed DP 50 (Table 3).
Glyphosate treatment of GR cotton did not increase the number of total positions aborted except under severely (3-d) water-stressed conditions in one genotype, SG 125RR (Table 3). The number of aborted positions generally increased with increasing level of water stress. Severe (3-d) water stress resulted in more aborted positions in all genotypes (except DP 90) than in water-nonstressed plants. Among all genotypes, but particularly in DP 90, fruit from plants which had undergone moderate and severe water stress (2- and 3-d) was often killed but failed to abscise completely from the plant, leaving the dead bolls or squares remaining attached to the plant (Table 3). Incomplete abscission of fruit in DP 90 has been observed in field situations and seems to be a genotype-related trait (Legé and Kerby, 2001). Moderate (2-d) water stress increased the number of incompletely abscised fruit in all genotypes except for those treated with glyphosate (Table 3). In order for proper abscission to occur, the middle lamella in the abscission zone swells considerably, pushing the leaf or organ away from the branch to which it is attached (Addicott, 1981). If water is severely limited, as is the case under severe water stress, sufficient water to drive the swelling of the middle lamella may not be available, thus preventing the procession of the abscission process. Citrus (Citrus reshni hort. ex Tanaka) leaves injured by water stress do not to abscise promptly, remaining attached to the plant until water relations improve, at which time they are abscised (Addicott, 1981; Tudela and Primo-Millo, 1992). A similar phenomenon occurs in loblolly pine (Pinus taeda L.), where needles of severely water-stressed plants fail to shed due to premature cell death in the abscission zone (Heikkenen et al., 1986). It seems plausible, therefore, that water stress may inhibit the proper maturation of the abscission zone, preventing abscission of affected cotton fruit. Glyphosate treatments did not affect the occurrence of incomplete fruit abscission, regardless of the level of water stress (Table 3).
Glyphosate and Water-Stress Effects on Leaf and Boll Carbohydrates
Because both glyphosate treatments and water stress can promote abscission of young bolls (Table 2), studies were conducted to determine whether changes in carbohydrate profiles in leaves and bolls were altered in response to glyphosate or water stress. Shifts in carbohydrate partitioning may signal that the fruiting structure will be shed or retained. Interactions between water stress and genotype-glyphosate treatments were not significant for leaf glucose, fructose, or starch, or for boll glucose, fructose, and sucrose.
Glyphosate treatments of GR cotton did not significantly affect the concentrations of glucose, fructose, or starch in subtending leaves or glucose and sucrose levels in 5-d old bolls (Table 4). Although no differences were seen with DP 5415RR, the fructose content in bolls from glyphosate-treated SG 125RR was less than that of SG 125RR, but no other carbohydrate was significantly effected by glyphosate treatment (Table 4). The fructose content in nontreated SG 125RR plants was significantly higher than in all other genotypes whether conventional or GR (Table 4). The lack of effect on the carbohydrate content in leaves and bolls of glyphosate-treated GR cotton suggests that boll shed following glyphosate treatments is not caused by changes in carbohydrate profiles.
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Table 4. Nonstructural carbohydrate content in first-position boll and subtending leaf tissue from conventional(DP 50 and DP 90) and glyphosate-treated and nontreated GR (DP 5415RR and SG 125RR) cotton pooled over water-stress treatments.
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Increasing levels of water stress generally reduced the glucose and starch content of leaf tissue and the fructose content in bolls (Table 5). Water stress did not affect the levels of fructose in leaf tissue or glucose and sucrose in bolls. The interactions between genotype-glyphosate treatments and water stress were only significant for the sucrose content in leaves and starch content in bolls. Glyphosate-treated SG 125RR had reduced sucrose with 0 d of water stress, but sucrose content increased with 1 d of water stress vs. nontreated SG 125RR (Table 6). Severe water stress (2 and 3 d of water stress) reduced the sucrose concentration in DP 90, SG 125RR, and glyphosate-treated SG 125RR (Table 6). Glyphosate treatments of GR cotton did not affect the starch content in bolls. Water stress (2 and 3 d) reduced the starch content in DP 50, DP 90, DP 5415RR, and SG 125RR bolls. However, starch levels were not affected in glyphosate-treated DP 5415RR and SG 125RR bolls (Table 6). Again, this effect may occur because of reduced maturity or boll load, which could have alleviated the effects of water stress (Table 2). Overall, increasing water stress generally seemed to cause greater reductions in leaf carbohydrate profiles than those of reproductive bolls.
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Table 5. Glucose, fructose, and starch content of subtending leaf and glucose, fructose, and sucrose content of first-position boll as effected by days of water stress (0, 1, 2, or 3 d).
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Table 6. Sucrose content of subtending leaf and starch content of first-position boll as effected by days of water stress (0, 1, 2, or 3 d) in conventional (DP 50 and DP 90) and glyphosate-treated or nontreated glyphosate-resistant (DP 5415RR and SG 125RR) cotton.
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Studies of osmoregulation in cotton following water stress have indicated that the levels of several carbohydrates, in particular, sucrose and starch, rise significantly in stress-adapted plants (Ackerson, 1981; Timpa et al., 1986). Ackerson (1981) implicated starch as the key component in osmotic adjustment following several cycles of water stress and subsequent rehydration. Increases in sucrose accumulation in leaves of cotton plants undergoing repeated midday water stress were observed by both Ackerson (1981) and Timpa et al. (1986). In contrast, our data suggest a general decrease in leaf carbohydrate contents with increasing levels of water stress. These apparent differences are likely due to differences in water-stress treatments in water stress and osmoregulation studies (one-time severe water stress vs. repeated cycles of water stress). The plants used in the current study were never previously water-stressed and were not rehydrated following water stress until after samples for carbohydrate analysis were collected. Therefore, carbohydrate contents of leaves sampled as they were undergoing water stress were significantly reduced compared with nonstressed plants. Ackerson and Hebert (1981) reported declines in photosynthesis in water-stress-adapted plants compared with control plants, and Ackerson et al. (1977) reported that water stress substantially reduced photosynthesis in both vegetative and reproductive tissue in cotton. Reductions in photosynthesis due to water stress would seem to explain reductions in carbohydrate content of leaves occurring in the current study.
In conclusion, both water stress and glyphosate applications caused fruit loss in cotton, but water stress exerted a larger influence on fruiting. Glyphosate applications to GR cotton delayed maturity or reduced boll load, effects which tended to make glyphosate-treated plants less susceptible to water stress than more mature, nontreated plants. Levels of soluble carbohydrates in leaves and young bolls were generally not affected by glyphosate treatments, suggesting that altered carbohydrate balance may not be the major cause of boll abscission following glyphosate treatments. Certain carbohydrates were affected by water-stress treatments. Leaves from water-stressed plants had reduced levels of glucose and starch in their leaves, and reduced levels of fructose and starch in their bolls. Increases in starch content have previously been implicated in osmoregulation of cotton plants to water stress (Ackerson and Hebert, 1981). It seems likely that other factors, such as hormonal regulation of the abscission zone, may play a larger role than the levels of nonstructural carbohydrates in leaf and fruit tissues in determining whether fruiting structures on glyphosate-treated and water-stressed plants are retained or abscised. Future studies measuring hormone levels in bolls of glyphosate-treated and water-stressed plants would aid in determining if both of these factors induce abscission in similar manners or, if because of their mechanistic differences, abscission is induced differently by the two factors.
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ACKNOWLEDGMENTS
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The authors wish to thank Cotton Incorporated and the North Carolina Cotton Growers for funding of this research.
Received for publication November 8, 2001.
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ABSTRACT
INTRODUCTION
