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CROP ECOLOGY, MANAGEMENT & QUALITY

Effect of Row Width and Nitrogen on Cotton Morphology and Canopy Microclimate

North Florida Research and Education Center, University of Florida, Quincy, FL 32351-5677

^* Corresponding author (marois{at}mail.ifas.ufl.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Canopy structure is a function of cultural practices, genetics, and environment; specifically, practices affecting plant row width and soil fertility can change canopy structure. Canopy structure affects canopy microclimate. Canopy microclimate determines the rate of fruit development, pest pressure, and crop response to ambient conditions. The rank growth associated with excess N in cotton (Gossypium hirsutum L.) is associated with delay in fruit development and increased boll rot. This study was conducted to determine the effect of ultra narrow row (UNR) planting on canopy microclimate in cotton. Canopy microclimate in UNR cotton (18- to 25-cm row width) and conventional (76- to 91-cm row width) plantings with low N (16.8 kg ha^–1 at planting only) or high N (16.8 kg ha^–1 at planting and 202 kg ha^–1 3 wk after bloom) fertilizer was quantified by monitoring the plant canopy temperature, relative humidity (RH), and vapor pressure deficit (VPD) every 15 min during the growing season. The study was conducted at Quincy, FL, on a Dothan sandy loam soil (fine loamy siliceous, thermic Plinthic Kandiudult) in 2000 and 2001. Plant height and nodes per plant were determined 60, 90, and 120 d after planting (DAP). A split-plot design was used with row width as the main effect and N rate as subplot. During the hours from 0700 to 1900, temperature was 1 to 2°C higher and RH 3 to 7% lower in the conventional vs. the UNR plantings. In general, N and row width effects did not have as great an impact on canopy microclimate as did plant height. In 2001, when correlations were significant, as plant height increased to 1 m, canopy temperature during the hours of 0700 to 1900 on average decreased from 33 to 25°C, RH increased from 55 to 85%, and VPD decreased from 0.16 kPa to less than 0.08 kPa. In 2002, plants grew much faster and correlations of microclimate and plant height were not observed. Plant height under typical production conditions had more of an effect on canopy microclimate than did plant density or N treatment.

Abbreviations: DAP, days after planting • DOY, day of year • N, nitrogen • RH, percent relative humidity • UNR, ultra narrow row • VPD, vapor pressure deficit

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
CANOPY MICROCLIMATE is a result of interactions among prevailing climatic conditions and canopy density and structure. Practices that alter the plant canopy can have a significant impact on the canopy microclimate, resulting in changes in pest pressure, irrigation requirements, and ultimately yield. By manipulating crop canopies through shoot positioning and leaf removal, English et al. (1989) showed that the changes in relative humidity and wind speed within the canopy were associated with a reduction in disease caused by Botrytis cinerea Pers. Pierce et al. (2001) found that increased survival of boll weevil in cotton planted in narrow 17-cm row widths was associated with higher relative humidity and cooler temperature within the plant canopies as compared to conventional 96-cm row widths. Factors that affect the rate of plant growth can also affect canopy microclimate. Pettigrew and Jones (2001) reported that the faster emergence and plant development in conventional tillage compared to no-till cotton resulted in plants intercepting 28% more sunlight during prebloom and 17% more during midbloom.

The impact of canopy microclimate on cotton fiber quality can be significant. Experiments conducted in growth chambers showed that light and temperature interactions affect cotton fiber, with the most uniform fibers produced under high light and low temperature conditions (Roussopoulos et al., 1998). Jost and Cothren (2000) reported that fiber length tended to be reduced in 19-cm row width compared with wider row widths. Early canopy closure (Heitholt et al., 1992) and earlier fruit set (Buxton et al., 1979) associated with ultra narrow row (UNR) cotton may negatively affect fiber quality; however, several studies have shown that fiber quality is not significantly affected (Heitholt et al., 1993; Gerik et al.,1998; Jost and Cothren, 2001). However, the perception of inferior fiber quality has resulted in discounts paid to the growers producing UNR cotton and is the primary reason UNR cotton is decreasing in the southeastern USA (D. Wright, personal communication).

A major factor affecting canopy variables is plant density. Recent planting technology has allowed widespread use of UNR planting strategies for cotton. In UNR cotton, plant row spacings of 17 to 25 cm between rows is used rather than conventional 76 to 95 cm. With 200 000 to 320 000 plants ha^–1 in UNR cotton, as compared to 50 000 to 150 000 plants ha^–1 in conventional plantings, fertilizer and water requirements can be affected. Increased plant density in cotton can reduce plant height and the main stem node number (Kittock et al., 1986; Bednarz et al., 2000), shorten the length of sympodial branches (Kerby et al., 1990), and decrease the number of nodes per monopodial branch (Buxton et al., 1977).

Ultra narrow row planting of cotton may change many aspects of management of the crop as well as physical growth parameters of the plants. A significant challenge associated with UNR cotton is its harvest. Excessive growth of UNR cotton may interfere with stripper harvesters, resulting in an inefficient harvest as well as increased trash in the fiber (Vories et al., 2001). Use of less N and growth regulators can address this problem. There are advantages to harvesting cotton with a stripper, including a more efficient harvest and reduced cost of equipment. If a stripper is to be used, it is important to produce small plants to keep bark from getting into the lint. Applications of a growth regulator, such as mepiquat chloride, can be useful if applied early (beginning at pinhead to matchhead square). Timing of application is important since the fruiting window of UNR cotton is much less (3–4 wk) than with conventional cotton (7–8 wk) (Jost and Cothren, 2000). The goal is to keep the top five internodes 5 cm or less in length.

Another critical factor is fertilizer N. In general, with higher plant densities, higher rates of N may be needed than in conventional plantings; however, with UNR less growth from each plant is required to make a given yield. Therefore, UNR cotton often does not require N rates different from conventional cotton, and applications of 67 to 112 kg ha^–1 are sufficient to produce maximum yield. As with conventional plantings, high N rates can actually reduce yield by encouraging unnecessary growth and extending vegetative periods of the cotton crop (Bell et al., 2003).

The decreased row width in UNR cotton often results in more rapid canopy development, which significantly reduces weed competition. Whereas conventional cotton production systems may take 60 to 75 d to provide a complete canopy, UNR cotton often takes only 30 d. Ultra narrow row cotton cannot be cultivated to control weeds; however, herbicide resistant cotton varieties can provide effective weed control strategies with over-the-top applications of herbicides (Culpepper and York, 2000). Other weed control strategies include a complete burndown of the preceding winter crop and broadcast applications of residual or soil applied herbicides before planting.

In addition to the potential impact on weeds mentioned above, the increased plant canopy may also affect plant pathogens, since many plant diseases are associated with dense canopies (Chellemi and Britton, 1992; Melzer and Boland, 1994; English et al., 1989). In general, dense canopies can increase RH and reduce temperature and air movement. These interact to affect the evaporative potential of the canopy, which ultimately affects the duration of free water available for use by plant pathogens for spore germination and the establishment of penetration structures before infection (English et al., 1990). Boll rot of cotton is often associated with dense canopies because of rank growth resulting from high levels of applied N (Roncadori et al., 1975) or ineffective application of growth regulators (Snow et al., 1981).

Temperature may be affected by plant density as a direct result of increased shading or an indirect result of decreased air movement (Whiting and Lang, 2001). Insect pests and pathogens are directly affected by temperature. Insect degree-day models predict life stages of many insect pests, and are used to determine optimum timing of pesticide applications (Wilson and Barnett, 1983). Furthermore, models have been developed for some crops integrating periods of wetness or RH with temperature to provide valuable information on the activities of plant pathogens (Broome et al., 1995).

The objective of this study was to determine the impact of UNR and conventional cotton plantings produced with high and low N rates on plant growth and canopy microclimate. Treatments included plantings in 17.8- and 91.4-cm row widths with and without additional N added at 202 kg ha^–1 at 3 wk after bloom.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Crop Management
The experiment was conducted at the University of Florida's North Florida Research and Education Center at Quincy, FL, in 2000 and 2001. Delta Pine 458 B/RR cotton seed [containing genes with Bt resistance to insects and RoundUp Ready (Monsanto) resistance to glyphosate, N-(phosphonomethyl)glycine] was planted in a split plot design. The main plot treatment was row width (conventional, 91.4 cm, or UNR, 17.8 cm) and the subplot was rate of N (0 or 202 kg ha^–1 at third week of bloom). Each block was replicated twice and was 18.3 m long and 13.4 m wide. The main plots were 18.3 m long, conventional plots were 3.65 m wide (4 rows) and UNR plots were 3.05 m wide (13 rows). Subplots were the width of the main plots and 9.15 m long. Cotton was planted in the conventional planting system with a vacuum planter in 91.4-cm rows and 12 seeds m^–1 of row or in the UNR plots with a no-till drill in 17.8-cm rows and 12 seeds m^–1.

The study was conducted on a Dothan sandy loam (fine loamy siliceous, thermic Plinthic Kandiudult). In 2000, it contained 34 mg kg^–1 P, 124 mg kg^–1 K and 232 mg kg^–1 Ca and a pH of 6.5 before fertilization. On 10 May (DOY 130) 2000, the entire experiment was fertilized with 3-9-18 (N-P-K) at 560 kg ha^–1. Ammonium nitrate was applied at 202 kg ha^–1 to N plots 25 July (DOY 206) (3 wk after bloom). The study was repeated in 2001 on the same soil type containing 49 mg kg^–1 P, 122 mg kg^–1 K and 113 mg kg^–1 Ca and a soil pH of 5.6 before fertilization. On 13 June (DOY 164) 2001, the entire experiment was fertilized with 3-9-18 (N-P-K) at 560 kg ha^–1. Ammonium nitrate was applied at 202 kg N ha^–1 to N plots on 31 August (DOY 243)(3 wk after bloom). The study was managed according to standard extension recommendations except in regard to the application of N. Overhead irrigation was applied as needed, two times in 2000 (0.63 cm on 26 May and 1.27 cm on 18 August) and no times in 2001.

Data Collection
Plant growth parameters were taken 60, 90, and 120 DAP. Plant height, nodes per plant, and height to node ratio number were obtained for each plot by measuring 10 randomly selected plants 60, 90, and 120 DAP in two adjacent 3.3-m-long rows. Plant height, number of nodes, and height to node ratio were determined for each plant and averaged for the plot. Yield was determined by harvesting 8 m of two adjacent rows within each plot with a spindle cotton picker.

Temperature and RH were monitored every 15 min during the season with CR10 Microloggers equipped with Type HKP45C temperature and RH sensors enclosed in 41002 Radiation Shield (Campbell Scientific, Logan, UT). Two sensors were placed randomly in one of the middle rows within a plant row in each sub plot 60 cm above the ground and at least 3 m apart. Temperature and RH data were down loaded weekly and analyzed after averaging the temperature and RH from 0700 to 1900 h each day. Data were collected for the entire day, but analysis indicated that night temperatures did not differ significantly enough during the season to be correlated with the growth parameters recorded. Days with rain or irrigation events were eliminated from the averages because they were statistically different from the typical day. The temperature and RH were then averaged over the week immediately proceeding the dates that the growth data were obtained. This allowed correlation of growth data parameters with average daytime canopy microclimate.

Vapor pressure deficit (VPD) integrates temperature and RH and is an indicator of how much more water a given amount of air is able to hold at a given temperature. It has been used to quantify the atmospheric humidity in relation to pathogen behaviors (Delp, 1954; Lacy and Pontius, 1983; Alderman et al., 1987). Prevailing VPD under greenhouse conditions has been correlated with plant susceptibility to the fungal pathogen Botrytis cinerea (Marois et al., 1988). English et al. (1990) used VPD to quantify the impact of leaf removal on the evaporative potential of the plant canopy. In their study, vapor pressure and wind speed were correlated by path analysis with evaporative potential, the ability of the plant canopy to evaporate free water. They found that in plant canopies, wind speed had a greater impact on evaporative potential than did VPD. In the studies reported here, VPD was used as an integrator of temperature and RH in the microclimate of the cotton canopy.

Vapor pressure deficit was calculated from the temperature and RH readings by the empirical equation

[1]

Where VPD = vapor pressure deficit (kPa), T = temperature (°C); and RH = % relative humidity (Snyder and Shaw, 1984).

Statistics
Data were analyzed with SAS (1989) by general linear models procedures for a split-plot design. The impact of row widths and N rates on plant height, number of nodes, ratio of plant height to number of nodes, average temperature, RH, and VPD were subjected to analysis of variance to determine if there were significant effects. Linear regression procedures were used to compare the plant growth parameters of height, number of nodes, and the ratio of plant height to number of nodes to the microclimate variables, temperature, RH, and VPD.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Analysis of variance was conducted for the data set including both years (Table 1). Year, DAP, N, and row width had a significant effect on plant height. Year, DAP, and row width affected the number of nodes per plant, but only DAP affected the ratio of plant height to number of nodes. The ratio of plant height to number of nodes, temperature, RH, and VPD was affected by year, DAP, and row width, but not by N. In general, plants grew much faster and taller in 2001 than 2000, probably in part because of the later DOY planting date (DOY 130 for year 2000 and DOY 164 for year 2001). All possible interactions were significant with regards to plant height, but no interactions had a significant effect on the number of nodes per plant or the ratio of plant height to number of nodes. Canopy temperature and RH during the daylight hours was significantly affected by year, row width, and the year x DAP interaction only. The canopy VPD was affected by year, DAP, row width, and year by DAP interaction. Cotton lint yield was significantly affected by year, N and the year x N interaction. The effects of nitrogen fertilizer and row width on temperature and RH were similar both years; however, the effect of treatments changed during the cropping season (Table 2). In general, RH and temperature were not affected by nitrogen treatment, but were affected by row width. During daylight hours, temperature was lower and RH greater in UNR plots as compared with the conventional plots (Fig. 1 and 2) . Even when RH and temperature were affected by row width, the effects were more marked earlier in the cropping season. By the end of the season, there were no consistent effects. Throughout the experiments, temperature and RH were not influenced by the treatments at night (Fig. 1 and 2).

View larger version (27K): [in this window] [in a new window]   Fig. 1. Effect of a rain event at 1200 h on temperature (A) and relative humidity (B) in a cotton canopy in conventional (_ _ _ _) and ultra narrow row (_____) cotton. Normalized data plotted against the mean of the conventional and ultra narrow row cotton canopy (C) indicate the differences between the conventional temperature (_____) and ultra narrow row temperature (_ _ _ _) and conventional relative humidity (___ _ __) and ultra narrow row relative humidity (___ ___). If the line is above the 0, the value is greater than the mean; if it is below the value, it is lower than the mean.

View larger version (26K): [in this window] [in a new window]   Fig. 2. Typical temperature (A) and relative humidity (B) during a day without rain or irrigation in conventional (_ _ _ _) and ultra narrow row (_____) cotton. Normalized data plotted against the mean of the conventional and ultra narrow row cotton canopy (C) indicate the differences between the conventional temperature (_____) and ultra narrow row temperature (_ _ _ _) and conventional relative humidity (___ _ __) and ultra narrow row relative humidity (___ ___). If the line is above the 0, the value is greater than the mean; if it is below, the value is less than the mean.

Plant Growth Parameters
Plant height and number of nodes were less in 2000 than in 2001 (Tables 3 and 4). This could be due to the drought conditions in 2000, even though the plants were irrigated as needed, or due to the earlier planting date in 2000 (DOY 130 in 2000 vs. 164 in 2001). In general, row width did not affect height, number of nodes, or the height-to-node ratio at 60 DAP. In 2000, at 90 DAP (DOY 220), nodes per plant and the height to node ratio were greater in the UNR plantings that did not receive additional N (Table 3). At 120 DAP (DOY 280), nodes per plant were less in the UNR plantings, and the height to node ratio was greater in the UNR plantings. In 2001 at 90 DAP (DOY 254) plant height and nodes per plant were less in the UNR plantings across N application (Table 3). The height to node ratio was not affected since both plant height and number of nodes were reduced in the UNR cotton. The same was true at 120 DAP (DOY 284), except that the number of nodes in the N treated plots was not affected by plant density. Jost and Cothren (2001) also reported annual variation in the impact of UNR on growth parameters. They found that height and node counts were reduced in UNR cotton in only one year of a 2-yr study.

Interaction of Plant Growth and Canopy Microclimate
The microclimate for the week immediately preceding plant growth sampling was analyzed in combination with plant growth data. An average temperature, RH, and VPD during the daytime were obtained by averaging each 15 min reading collected over the hours from 0700 to 1900. These data were then analyzed for treatment effect for each week. Days with rain or irrigation were not used because they had very different data that were affected more by the prevailing short-term weather conditions (Fig. 1 and 2). In 2000, temperature and VPD were not affected by treatment. When additional N was applied, RH was greater in the UNR plantings at 90 DAP (DOY 220) and 120 DAP (DOY 250) (Table 4). In 2001, with taller plants during the entire season, neither row width nor N application affected the canopy microclimate during the week preceding the dates when plant growth data were collected (Table 4).

Although specific treatment effects did not often indicate an interaction between plant growth and canopy microclimate, there were several highly significant correlations among plant growth parameters and canopy microclimate when the data for each year were pooled across treatments and plant growth sampling dates, especially in 2000 when the plants were shorter than in 2001 (Table 5). In 2000, plant canopy temperature decreased, RH increased, and VPD decreased as plant height and number of nodes increased (Table 6, Fig. 3) . These correlations were not as consistent and even reversed in 2001 (Table 5) because of the greater plant height during the 2001 season, where the plant height 60 DAP for 2001 was already equivalent to the average plant height at 120 DAP in 2000, and plants continued to grow taller during the season (Fig. 3).

View this table: [in this window] [in a new window]   Table 5. Correlation of plant growth parameters and daytime canopy microclimate.

View larger version (23K): [in this window] [in a new window]   Fig. 3. Temperature (A), relative humidity (B), and vapor pressure deficit (C) vs. plant height across all treatments and years. In year 2000 Temperature = 44.4 – Height x 19.8, r = 0.70, p < 0.016 Relative Humidity = –17.18 + Height x 120, r = 0.90, p < 0.001 and Vapor Pressure Deficit = 0.406 – Height x 0.41, r = 0.90, p < 0.001. In year 2001 Temperature = 44.7 – Height x 18.5, r = 0.58, p < 0.063 Relative Humidity = 112 – Height x 37.6, r = 0.58, p < 0.063 and Vapor Pressure Deficit = –0.114 + Height x 0.22, r = 0.57, p < 0.068.

Year and N significantly affected lint yield (Table 7). Overall, lint yield averaged 669 kg ha^–1 in 2000 and 822 kg ha^–1 in 2001. This was due in part to the severe drought conditions experienced in 2000. The application of 202 kg ha^–1 of N 3 wk after bloom reduced the overall lint yield from 829 to 661 kg ha^–1. Row width did not affect yield. Vories et al. (2001) reported that in 2 out of 3 yr seed cotton yields were higher in UNR cotton, but lint yield was higher only one year because of lower gin turnout. High N rates can reduce yield (Bell et al., 2003), but results are affected by cultivar used. Meredith et al. (1997) found that more recent cultivars responded to higher N rates differently than older cultivars. Planting UNR cotton may affect many other aspects of the plant community. Karban et al. (1989) found that induced resistance to spider mites (Tetranychus urticae Koch) and Verticillium wilt (caused by Verticillium dahlia Kleb.) was reduced when cotton plants were grown in crowded conditions. Cotton growth was shown to be affected by higher planting density in UNR cotton, but the response was dependent upon soil type (Jost and Cothren, 2001). Plant density within a row can also affect cotton growth. Bednarz et al. (2000) found that yield was stable across a wide range of seeding rates (3.5–25.1 m^–2 on constant 91-cm row widths). They reported that lower plant densities resulted in plants with more mainstem nodes and monopodial branches, as well as greater fruit retention. This resulted in a greater number of fruit per plant in the lower density plantings. They also found that the size of bolls was inversely correlated with the increase in plant density. Other cultural practices such as no-till planting may affect blooming period (Pettigrew and Jones, 2001) and prebloom crop growth rate (Kennedy and Hutchison, 2001). The interactions of cultural practices and plant growth are complex, and the resulting impact on canopy microclimate can be significant.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
This research was supported by the Florida Agricultural Experiment Station, and approved for publication as Journal Series No. R-09103. A grant from Cotton Incorporated provided a portion of the cost of the research.

Received for publication October 2, 2002.

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