Use of Plant Growth Regulators as a Management Tool in Cotton

Use of Plant Growth Regulators as a Management Tool in Cotton Philip Jost 1, Jared Whitaker, Steve M. Brown 1, and Craig Bednarz Department of Crop and Soil Sciences College of Agriculture and Environmental Sciences 1 Cooperative Extension Service Introduction Cotton is a subtropical, perennial plant with an indeterminate growth habit. Vegetative and reproductive development occurs simultaneously. While vegetative growth is necessary to support reproductive growth, excessive vegetative growth can be detrimental. Under excessive vegetative growth, fruit abortion may increase, crop maturity may be delayed, and harvest reduced. Growth habits of cotton varieties are inconsistent, with many characterized by their tendency for aggressive vegetative growth. The growth habit of these varieties combined with high availability of nutrients, timely rainfall or irrigation, and delayed fruit retention can encourage excessive vegetative growth. Cotton plants produce several natural growth regulators or plant hormones, which modify plant growth and divert energy allocation within the plant. Producers often apply growth regulators to the foliage in an effort to maintain a balance of vegetative and reproductive growth. Plant growth regulators decrease cotton vegetative growth by modifying the production of plant hormones such as gibberellins, auxins and cytokinins. The most commonly used growth regulator is mepiquat chloride, which decreases vegetative growth by reducing gibberellic acid formation. Understanding the causes of excessive vegetative growth and the mode of action of Mepiquatbased growth regulators is essential for satisfactory vegetative growth management. Problems with Excessive Vegetation Growth No optimal plant size exists for cotton; however, excessive vegetative growth can lead to several production problems. Fruit abortion, delayed maturity, boll rot, and harvest difficulties often coincide with excessive vegetative growth. Fruit Abortion The cotton plant develops in an ordered fashion. Fruit initiates at the bottom of the plant and then proceeds up the plant and out to more distal fruiting positions as the season progresses (Ritchie et al., 2004). Excessive vegetative growth causes shading in the lower canopy and may lead to the abscission of early fruit (Oosterhuis, 2001) (Figure. 1). Numerous environmental and management factors can also contribute significantly to fruit shed, such as low light, excessive nitrogen fertility, and root pruning from cultivation. Figure 1. Excessive shading can cause fruit shed or abscission A) Cotton plant with excessive fruit shed B) Cotton plant with normal fruit shed Delayed Maturity and Yield Loss of early fruit may be compensated for when favorable conditions exist to extend the growing season. This compensatory growth results in plants setting more fruit at higher main-stem nodes and more distal fruiting

positions than would occur normally. Compensatory growth generally results in delayed crop maturity (Silvertooth et al., 1999). Boll size is strongly correlated with boll location on the plant (Figure 2, page 2 ). Bolls located on main-stem nodes 10-16 at the first or second fruiting position tend to produce the majority of crop yield (Silvertooth et al., 1999). If the cotton plant compensates for early season fruit loss by setting bolls later in the season, yield may still be compromised if water, nutrient, or temperature stress is encountered. Under these conditions, fruit produced outside this zone tend to be smaller (Jones and Wells, 1998). Compensatory growth may result in similar boll numbers but yield may still be reduced due to smaller boll size. Figure 2. Boll size is correlated to position on fruiting branch. First and second position bolls tend to occur more frequently and weigh more than third position bolls (Bednarz et al., 2005). Boll Size (g/boll) 5 4 3 2 1 1st Position 2nd Position 3rd Position 5 10 15 20 25 Main Stem Node Shading of the Lower Canopy Excessive shading may also decrease the micronaire of bolls that are retained in the lower portion of the plant (Eaton and Ergle, 1954). If shedding does not occur boll rot is often more prevalent with excessive vegetation due to a more humid microenvironment in the lower portion of the canopy (Percival et al., 1999). Excessive vegetative growth inhibits penetration of insecticides to the lower portion of the canopy and can also reduce harvest of the crop. A smaller plant with a more compact fruiting zone is harvested more easily than a taller plant with excessive vegetative growth (Stewart, 2005). Hormones and Their Growth Regulating Roles in Cotton Plants The three types of growth hormones affected by foliar-applied products are gibberellins, cytokinins, and auxins (Addicot, 1970; Addicot and Lynch, 1955). These hormones are integral for cotton growth and each has a unique role in the regulation of vegetative and reproductive growth. Gibberellins, or gibberellic acid, promote cell division and expansion (Taiz and Zeiger, 1998). This hormone is most closely related to vegetative growth. Mepiquat chloride, a foliar-applied growth regulator, reduces the concentration of gibberellic acid in the plant (Hake et al., 1991). When the concentration of this acid is reduced, a more compact plant cell, and subsequently a more compact plant develop. Cytokinins promote cell division and elongation, but are also involved with the transport, accumulation, and retention of carbohydrates (Taiz and Zeiger, 1998). While this hormone may contribute to the regulation of vegetative growth, it may also play a significant role in fruit retention and boll fill (Letham, 1967). Auxin increases cell wall plasticity and is considered a growth promoting hormone. Auxin also delays sensecence (Taiz and Zeiger, 1998). This hormone is similar to gibberellic acid in its regulation of vegetative growth. The Basics of Mepiquat Chloride/Pentaborate Mode of Action Limiting vegetative growth in cotton has been and continues to be researched extensively. In the late-1960s and early-1970s, the chemical 2-chlroethyltrimethylammonium chloride (also known as CCC, Cycocel, and chlormequat) was used in India where excessive vegetative growth was a problem (Guinn, 1986). Currently, mepiquat (N, N-dimethyl piperidinium) is used by many cotton producers to manage vegetative growth. Mepiquat inhibits a key enzyme in the production of gibberellic acid (Rademacher, 2000). The effect of mepiquat is a function of plant size and rate of product applied, and only affects new growth. Mepiquat has no effect on portions of the plant that have ceased growing. This means that a given rate of product applied at first-square to a relatively small plant has a much greater effect than that same rate applied to a larger plant in the bloom stage.

Morphological Effects The morphological response of a cotton plant treated with mepiquat chloride is the suppression of vegetative growth through a reduction in the total number of mainstem nodes, and internode lengths (Figure 3, Reddy et al., 1992). A reduction in leaf area is also ob-served with the use of mepiquat chloride. Since mepi-quat chloride does not stop the growth of the cotton plant, the reduction in leaf size is also generally corre-lated with an increase in leaf thickness (Walter et al., 1980). Mepiquat chloride-treated plants are typically shorter, more compact, and possess a characteristically darker green color than untreated plants (Stewart, 2005) (Figure 4). The use of mepiquat chloride may also cause a shift in boll location creating a more concentrated fruiting zone. In a study by Kerby et al. (1986), both the mepiquat chloride treated and untreated cotton had similar numbers of bolls, yet the mepiquat chloride treated cotton had a greater percentage of those bolls on lower main stem nodes (Figure 5). It is important to realize that even without a mepiquat chloride treatment; boll retention on lower nodes can limit vegetative growth. High early fruit retention provides the plant with a reproductive sink limiting the resources available for vegetative growth (Jost et al., 2005). Figure 4. Mepiquat treated plants (right) are typically shorter, more compact, and darker green than untreated plants (left). Figure 5. The effect of mepiquat on number of harvestable bolls per square meter on all sympodial branch fruiting positions at each main stem node (Kerby et al. 1986). Mepiquat generally causes a greater percentage of the total bolls to lower nodes on the plant. 12 Figure 3. Plant height (A) and total number of main-stem nodes (B) are reduced by mepiquat applications (Bednarz, unpublished data, 2005). Harvestable Bolls m -2 10 8 6 4 2 Mepiquat Control 0 4 6 8 10 12 14 16 18 20 Main Stem Node Benefits of Mepiquat Chloride Treatments The benefits gained with a mepiquat chloride application are directly related to the morphological response exhibited by the treated plants. The size of a cotton plant can influence harvest efficiency. Taller and ranker plants significantly reduce the speed in which the crop can be harvested. Bolls set over a more compact zone on the plant increases harvest efficiency by allowing plant material to travel more easily through the picker head (Stewart, 2005). Reducing crop height is the most consistent response from a mepiquat chloride treatment (Walter et al., 1980).

The leaf area index (LAI) of a mepiquat chloride treated crop can be decreased by 5 to 10 percent compared to a non-treated crop (Hake et al., 1991). This lower LAI can improve pesticide distribution into the lower canopy (Stewart, 2005). Lower LAI also allows more light to penetrate the lower canopy for boll development. Most photosynthetic energy for boll development originates from adjacent or subtending leaves. Boll development in the lower portions of the plant on interior fruiting positions in shaded conditions may result in reduced maturity, quality, and potentially reduced yield (Kerby, 1985). In extreme cases, shading of the lower canopy may cause fruit abortion. Despite many benefits of a mepiquat chloride application, yield effects with mepiquat chloride applications have always been inconsistent (Biles and Cothren, 2000). Studies have shown yields may be increased, decreased, or unaffected treatment. Yield effects are generally small when the cotton plant has high early fruit retention. Positive yield effects are more likely to occur when early fruit retention is reduced and vegetative growth is excessive (Cook and Kennedy, 2000). Increases in yield are usually 100 lbs lint/a, or less (Table 1). Yield reductions are more likely to occur with the use of mepiquat chloride at excessive rates in the presence of early-season environmental or management stress. Under these conditions the growth regulating effects of mepiquat act as an additional stress on the plant. Mepiquat chloride should be viewed as a management tool, not a yield enhancer. Table 1. Effect of mepiquat chloride treatment on yield vs. untreated check in large plot trials, Evans Co., Ga., 2002 and 2003. Treatment Rate* Yield 2002 2003 oz/a lbs lint/a Mepiquat pentaborate 24 30 2962 a Mepiquat chloride 24 30 2923 a Untreated - - 2674 b Pr>f 0.0009 C.V. 4.2 *In 2002 3 applications of 8 oz/a of each product were made when cotton had 9 main-stem nodes. Subsequent applications of 8 oz/a were made at 2 week intervals. In 2003, 4 applications were made; 8 oz/a @ 9 main-stem nodes, 4 oz/a 2 weeks after initial (WAI) application, 8 oz/a 4 WAI application, and 10 oz/a 6 WAI application. Types of Mepiquat Products Mepiquat Chloride Mepiquat chloride is the primary growth regulator used in cotton production. It is most commonly formulated as a 4.2 percent concentration of nitrogen, N- dimethyl piperidinium (or mepiquat) chloride salt. Mepiquat chloride is generally absorbed by the plant in 4 to 8 hours (Micro Flo, 2006). The addition of a surfacetant may enhance uptake but is generally unnecessary (Hake et al., 1991). Mepiquat Chloride has been labeled for the use in cotton since the 1980 s (Stewart, 2005), and is sold under many trade names such as Pix, Mepex, Mepichlor, and Mepiquat Chloride. Mepiquat Pentaborate Mepiquat pentaborate has been recently developed and is sold under the brand name Pentia by BASF Corporation. Pentia contains a 9.6 percent concentration of mepiquat as a pentaborate salt. While Pentia appears to have more active ingredient than the original mepiquat chloride formulations, this increase is due to the weight of the pentaborate salt, which is heavier than the chloride salt of mepiquat chloride. There is no differrence in the amount of actual mepiquat between the two products. BASF has indicated the uptake rate of the pentaborate formula is greater than mepiquat chloride and is advertised as a more rain-fast product (BASF Corporation, 2004). Mepiquat Chloride Premixes As the use of Mepiquat chloride gained acceptance in the cotton industry, attempts have been made to improve the effectiveness and yield response to this product. Several premixes have been developed combining Mepiquat chloride with reproductive growth enhancers or other growth regulating hormones. The performance of each of these formulations is discussed in a later section. Pix Plus contains a 4.2 percent concentration of Mepiquat chloride with the bacterium Bacillus cereus. This bacterium was touted to be a reproductive growth enhancer. Mepex Ginout also includes a Mepiquat chloride with kinetin, a synthetic cytokinin hormone (McCarty et al., 2002) The most recently released mepiquat product is Stance, which consists of an 8.4 percent formulation of Mepiquat chloride, but also includes an additional active ingredient cyclanilide. Cyclanilide is an auxin transport and synthesis inhibitor (Pedersen et al., 1997). Stance consists of two products, each potentially regulating a different hormone involved in vegetative growth control.

Strategies for Management of Vegetative Growth Consider many factors when assessing the occurrence and potential for excessive vegetative growth including, growth stage, growth rate, fruit retention, availability of irrigation, field fertility status, field growth history, and previous history of applications of mepiquat chloride. No two fields or situations are identical and there is no universally accepted method of growth management. Not every field of cotton needs the same amount of mepiquat at the same time. There are several characteristics of the cotton plant which indicate the onset of excessive vegetative growth and should be considered when making mepiquat applications. Early-Season Fruit Retention Regardless of the type of mepiquat product utilized or timing of application, the most effective vegetative growth management factor in cotton is high early-season fruit retention. The retention of early fruit reduces the amount of carbohydrates available for vegetative growth later in the season. Prior to any applications of mepiquat products, efforts should focus on maintaining an environment that promotes early fruit set and retention. Growth Stage The growth stage of the cotton plant does much to determine the need for vegetative growth regulation. Prior to the development of the first square, or approximately 7 to 9 main-stem nodes, a cotton plant is strictly vegetative. Restriction of growth at this time has not proven necessary or effective (Jost, unpublished data). Once cotton has reached 5 nodes above white flower (NAWF) future vegetative growth is of little concern (see discussion on Late-Season Mepiquat Chloride Applications). Vegetative growth management can be limited to the time between the appearance of the first square and cut-out (5 NAWF), approximately 6 to 8 weeks during the growing season. Plant Monitoring The height to node ratio (HNR) should be monitored to evaluate vegetative growth. The HNR is calculated by dividing the plant height by the number of main stem nodes, providing an average internode length. A cotton plant is considered to be vegetative when the average internode length is greater than a specific value at a specific developmental stage (Table 2). This measurement alone can be deceiving however, in that all growth is included. Mepiquat chloride has no effect on growth that has already occurred; it will only affect growing plant cells. Table 2. Height to Node Ratios for normal, stressed, and vegetative plants (Jost et al., 2005). Crop Stage Normal Stressed Vegetative HNR (inches/node) Seedling Cotton 0.5-0.75 - - Early Squaring 0.75-1.2 0.7 >1.3 Large Square-1st bloom 1.2-1.7 <1.2 >1.9 Early Bloom 1.7-2.0 <1.6 >2.5 Early Bloom + 2 weeks 2.0-2.2 <1.8 >2.5 The internode lengths of the uppermost five nodes on the cotton plant should also be monitored (Figure 6, page 6). Any internode located below this region has ceased expanding. The uppermost 5 nodes is the region of the plant actively growing which can be managed by applications of mepiquat chloride. Measurements of these nodes are incorporated into the decision flow chart presented in the Plant-Based Rate Recommendations section of this bulletin. Several tools have been developed to monitor the growth of cotton plants in this region. They provide guidelines for mepiquat chloride treatment. The Pix Stick, developed by BASF, provides a quick check of average internode length of the uppermost 5 internodes, and then gives a recommended Mepiquat chloride application rate (Figure 7, page 6). The Slide Stick, developed by Delta and Pine Land Company, bases Mepiquat chloride application rates on plant height, main stem node number, and internode length between the fourth and fifth main stem nodes (Figure 8, page 6). The additional crop parameters incorporated into the Slide Stick are intended to allow more precise recommendations. Many of these tools are based on the compiled works conducted by Dr. Juan Landivar (e.g., Landivar et. al., 1992).

Figure 6. The top 5 internodes of the plant are critical to examine when determining whether applications of mepiquat should be made. This portion of the plant is actively growing. Internodes below this point have ceased expanding. Figure 7. The PIX STIK from BASF. This tool is used to measure the length of the top 5 nodes of the plant. These node lengths determine the rate of mepiquat application. Figure 8. The Delta and Pine Land Slide Stick. This tool incorporated individual internode lengths, plant height, and total nodes into rate recommendation. Field History Field history is also an important part of predicting and managing excessive vegetative growth. If the field has a previous history of rank growth, more attention should be paid to the use of mepiquat products for vegetative management. Conversely, fields prone to drought stress may require less or no use of mepiquat chloride. Soil Fertility While nitrogen (N) management is key to producing a cotton crop, low rates of nitrogen will lead to less vegetative growth and can also reduce lint yield (Radin and Mauney, 1986). High rates of nitrogen can lead to rank growth (Silvertooth and Norton, 1998), and potentially boll rot and delayed maturity. Cotton grown under high nitrogen fertility situations will require more mepiquat chloride to attain adequate vegetative growth control. The key is to target nitrogen rates so that yield is maximized while minimizing excessive vegetative growth. Ideal nitrogen rates are provided for various growing conditions in the University of Georgia Cotton Production Guide which is updated annually. The practice of using excessive nitrogen rates and managing growth with excessive mepiquat chloride applications is not recommended.

Cotton Variety Not all varieties of cotton exhibit the same growth habits, which further confounds the vegetative growth management issue. For example, some cotton varieties have been bred for increased vigor that, while beneficial under cool early-season conditions, may be linked to excessive vegetative growth. Some full-season varieties tend to be more prone to excessive vegetative growth than shorter season varieties. Understanding the growth habits of a particular variety can be very useful for successful vegetative growth management. Documentation shows not all varieties respond to mepiquat the same (Stewart, 2005). Some of the new FiberMax offerings appear to be very responsive to mepiquat and may require a lower application rate and fewer overall treatments (Jost, unpublished data). Other varieties such as DP 555 BG/RR and ST 5599 BR require a very timely early season application in order to attain satisfactory vegetative growth control. Both seed companies and University of Georgia extension personnel evaluate the response of varieties to mepiquat applications. Especially with new varieties, it is important to pay particular attention to university and company-generated information in regards to mepiquat usage until some experience can be gained. Rates and Timing of Mepiquat Chloride Applications Opinions about mepiquat chloride application rates, timing, and products differ greatly. General programs can consist of a few high rate, multiple low rate, or plant growth based applications. All inclusive recommendations for mepiquat chloride applications are nearly impossible to make as excessive vegetative growth is influenced by many factors. The following recommendations are based on any product containing mepiquat except for Reign, for which insufficient research exists. This product is currently being examined as a one-rate-fits-all-situations material. If successful, this would surely simplify the regulation of excessive vegetative growth. The use rates of all materials containing a 4.2 percent concentration of Mepiquat chloride or a 9.6 percent concentration of mepiquat pentaborate are identical. Current research and grower experience indicates no difference in performance among these products regardless of additives or salt formulation (Table 3). Table 3. Comparison of height control and yield from three formulations of mepiquat vs. untreated check in small plot trials, Midville, Ga., 2003 and 2004.* Product Height at Mid-bloom Inches/plant Yield Seed cotton/a Mepiquat chloride 38.4 a 2991 a Mepiquat chloride + kinetin** 38.1 a 3090 a Mepiquat pentaborate 35.7 a 2917 a Untreated 50.7 b 2983 a Pr>f 0.0001 0.8113 C.V. 9.1 12.0 **8 oz/a of each product was applied to cotton variety DP 555 BR at match-head square followed by subsequent applications 2 and 4 weeks later. **Mepiquat chloride + kinetin provided by Mepex Plus in 2003 and Mepex Ginout in 2004. Calendar or Growth-Stage Based Programs Irrigated Cotton On irrigated cotton, two growth regulator programs are recommended. One approach is to begin low rate applications of 4 oz/a during the second week of squaring. Applications should continue every 14 days for 3 or 4 applications (Jost et al., 2005). This approach was more applicable when producers made multiple trips across the field for insect control and mepiquat products could be added into the tank. With the adoption of Bt technology, producers may not make over the top applications of insecticides until late into the bloom period. Some varieties may require higher rates. A more applicable approach is to treat with a higher rate (8 to 12 oz/a) at first bloom and subsequent higher rate treatments 2 to 3 weeks later if good growing conditions persist (Jost et al., 2005). For many cotton varieties, waiting to make initial applications until first bloom is an acceptable program. However, with DP 555 BG/RR and ST 5599 BR in particular, waiting until first bloom to make the initial mepiquat application is often too late. Various research trials and grower experience has indicated that these varieties need to be treated during the squaring period (Jost, unpublished data).

Non-Irrigated Cotton Applying mepiquat chloride to non-irrigated cotton is a more delicate task and plant monitoring is vital. While relatively uncommon, it is possible to apply too much mepiquat chloride if the treatment is followed by extremely dry conditions. If this occurs, cotton may be stunted and not recover even more favorable conditions return. Non-irrigated cotton can safely be treated with 8 oz/a at first bloom. Subsequent applications are then dependent upon adequate rainfall and growth, with rates and timings refined by monitoring overall plant growth (Jost et al., 2005). With ample early season rainfall many varieties should be treated prior to first bloom. Plant Based Rate Recommendations Calendar or growth-stage based programs have weaknesses. The most successful programs are those that consider the growth of plants on a field by field basis. Figures 9 through 11 (page 9) provide flow charts by which a range of mepiquat chloride usage rates can be identified based on stage of crop development. These charts incorporate the HNR, top 5 internode lengths, and previous mepiquat chloride applications into the recommendations. Some adjustments may be necessary. Late-Season Mepiquat Chloride Applications Applications of mepiquat chloride are not recommended once cotton has reached 5 nodes above white flower. The theory behind such applications is that they would halt any further vegetative growth and force the plant to channel all growth into filling out top bolls. Research to date in Georgia and many parts of the United States has not shown any consistent benefit to such applications (Tables 4 and 5). Table 4. Effect of varying rates of mepiquat chloride applied at cutout (5 NAWF) vs. untreated check on yield in small plot trials, Midville, Ga., 2003. Table 5. Effect of varying rates of mepiquat chloride applied at cutout (5 NAWF) vs. untreated check on yield in small plot trials, Midville, Ga., 2004. Treatment at cut-out* Rate Yield oz/a Seed cotton/a Mepiquat chloride 8 3955 a Mepiquat chloride 16 3951 a Mepiquat chloride 32 3513 b Untreated - 3775 ab Pr>f 0.0409 C.V. 5.3 *8 oz/a of mepiquat chloride + kinetin as Mepex Ginout was applied to all plots of cotton variety DP 555 BR at match-head square with subsequent applications at 2 and 4 weeks.. Summary Excessive vegetative growth can be detrimental to the production of a cotton crop. Vegetative growth can be managed by insuring early-season fruit retention and the use of mepiquat chloride when necessary. A cotton crop treated with mepiquat chloride will consistently exhibit shorter internodes, increased leaf thickness, a more compact fruiting zone, and potentially earlier maturity. These changes can enhance harvesting, improve pesticide penetration to lower portions of the plant canopy, and reduce the occurrence of boll rot. Yield enhancement is not consistently observed. Mepiquat chloride usage should be viewed as a management tool. Research to date indicates that there is no difference in performance among mepiquat products. While calendar and growth-stage based programs of mepiquat applications are easy, in-season plant monitoring provides the most reliable information when considering mepiquat chloride applications. Treatment at cut-out* Rate Yield oz/a Seed cotton/a Mepiquat chloride + kinetin 16 1986 a Mepiquat chloride + kinetin 32 1934 a Untreated - 1995 a Pr>f 0.9511 C.V. 14.7 *8 oz/a of mepiquat chloride was applied to all plots of cotton variety DP 555 BR at match-head square with subsequent applications at 2 and 4 weeks.

Figure 9. Flow chart for mepiquat applications for a crop 10 to 14 days after first square. Figure 10. Flow chart for mepiquat applications for a crop in the early-bloom stage. Figure 11. Flow chart for mepiquat applications for a crop 2 weeks or later past the early-bloom stage.

References Addicott, F.T. 1970. Plant Hormones in the Control of Abscission. Biol. Rev. Cambridge Phil. Soc. 45:485-524. Addicott, F.T., and R.S. Lynch. 1955. Physiology of Abscission. Ann. Rev. Plant Physiol. 6:211-238. BASF Corporation. 2004. Pentia Plant Regulator. [Online]. C&P Press Inc., Available at http://www.greenbook.net/search/quicksearch/ (verified 10 January 2006.) Bednarz, C.W., R.L. Nichols, W.S. Anthony, and W.D. Shurley. 2005. Yield, quality, and profitability of cotton produced at varying plant densities. Agron J. 97:235-240. Biles, S. P., and J.T. Cothren. 2001. Flowering and Yield Response of Cotton to Application of Mepiquat Chloride and PGR-IV. Crop Sci. 41:1834-1837. Cook, D. R., and C. W. Kennedy. 2000. Early Flower Bud Loss and Mepiquat Chloride Effects on Cotton Yield and Distribution. Crop Sci. 40:1678-1684. Eaton, F.M., and D.R. Ergle. 1954. Effects of Shade and Partial Defoliation on Carbohydrate Levels and the Growth, Fruiting, and Fiber Properties of Cotton Plants. Plant Physiol. 29:39-49. Guinn, G. 1986. Hormonal Relations During Reproduction. in Cotton Physiology. J.R. Mauney and J.M. Stewart eds. The Cotton Foundation, Memphis, TN. Hake, K., T. Kerby, W. McCarty, D. O Neal, and J. Supak. May 1991. Physiology of PIX. Physiology Today Volume 2, Number 6. Jones, M.A., and R. Wells. 1998. Fiber Yield and Quality of Cotton Grown at Two Divergent Population Densities. Crop Science. 38:1190-1195. Jost, P., S. M. Brown, S. Culpepper, G. Harris, B. Kermerait, P. Roberts, D. Shurley, and J. Williams. 2005. 2005 Georgia Cotton Production Guide p 37-39. Landivar, J.A., S. Zypman, D.J. Lawlor, J.Vasek, and C. Crenshaw. 1992. The Use of and Estimated Plant Pix Concentration for the Determination of Timing and Rate of Application. Proc. Beltwide Cotton Conf. 1047-1049. Letham, D.S. 1967. Chemistry and Physiology of Kenetin- Like Compounds. Ann. Rev. Plant Physiol. 18:349-364. Kerby, T.A. 1985. Cotton Response to Mepiquat Chloride. Agron. J. 77:515-518. Kerby, T.A., K. Hake, and M. Keely. 1986. Cotton Fruiting Modification with Mepiquat Chloride. Agron. J. 78:907-912. McCarty, J.C., P.A. Hardin, and R.W. Hayes. 2002. Minimal Effects of Foliar Applications of Gibberellic Acid and Carbohydrates on the Yield of Cotton Lint. [Online]: http://msucares.com/pubs/reasearchreports/r22-5.htm. Micro Flo Company. 2006. Mepichlor 4.2% Liquid. [Online]. C&P Press Inc., Available at http://www.greenbook.net/search/quicksearch/ (verified 10 January 2006.) Oosterhuis, D. 2001. Physiology and Nutrition of High Yielding Cotton in the USA. Informacoes Agronomicas. No. 95. Sept. 2001. Pedersen, M.K., J.D. Burton, H.D. Coble, J.R. Collins, and D. Fritz. 1997., Efficacy of Finish and its Mechanism of Action. Proc. Beltwide Cotton Prod. Res. Conf. 93-94. Percival, A.E., J.F. Wendel, and J.M. Stewart. 1999. Taxonomy and Germplasm Resources. in Cotton: Origin, History, Technology, and Production. W.C. Smith and J.T. Cothren eds. John Wiley and Sons, Inc., New York, NY. Rademacher, W. 2000. Growth Retardants: Effects on Gibberellin Biosynthesis and Other Metabolic Pathways. Ann. Rev. of Plant Physiol. and Plant Mol. Biol. 51:501-531. Radin, J.W. and J.R. Mauney. 1986. The Nitrogen Stress Syndrome. in J.M. Brown ed. Cotton Physiology. The Cotton Foundation, Memphis, TN. Reddy, V.R., A. Trent, and B. Acock. 1992. Mepiquat Chloride and Irrigation versus Cotton Growth and Development. Agron. J. 84:930-933. Ritchie, G.L., C.W. Bednarz, P.H. Jost, and S.M. Brown. 2004. Cotton Growth and Development. University of Georgia Cooperative Extension Service Bulletin 1253. http://pubs.caes.uga.edu/caespubs/pubcd/b1252.htm Silvertooth, J.C., and E.R. Norton. 1998 Evaluation of a Feedback Approach to Nitrogen and Pix Applications, 1997. Cotton, A College of Agriculture Report. Series P- 112, Univ. of AZ, Tuscon, AZ p.469-475. Silvertooth, J.C., K.L. Edmisten, and W.H. McCarty. 1999. Production Practices. in Cotton: Origin, History, Technology, and Production. W.C. Smith and J.T. Cothren eds. John Wiley and Sons, Inc., New York, NY. Stewart, S. 2005. Suggested Guidelines for Plant Growth Regulator Use on Louisiana Cotton. Louisiana Cooperative Extension Service Publication Number 2918. Taiz, L., and E. Zeiger. 1998. Plant Physiology 2 nd edition. Sinauer Associates, Inc., Publishers. Sunderland, MA. Walter, H., H.W. Gausman, F.R. Rittig, L.M. Namken, D.E. Escobar, and R.R. Rodgriquiez. 1980. Effect of Mepiquat Chloride on Cotton Plant Leaf Area and Canopy Structure and Dry Weights of its Components. Proc. Beltwide Prod. Res. Conf., pp. 32-35.

The University of Georgia and Ft. Valley State University, the U.S. Department of Agriculture and counties of the state cooperating. Cooperative Extension, the University of Georgia College of Agricultural and Environmental Sciences, offers educational programs, assistance and materials to all people without regard to race, color, national origin, age, gender or disability. An Equal Opportunity Employer/Affirmative Action Organization Committed to a Diverse Work Force Bulletin 1305 Date April, 2006 Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College of Agricultural and Environmental Sciences and the U.S. Department of Agriculture cooperating. J. Scott Angle, Dean and Director