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COTTON

Canopy Light Environment and Yield of Narrow-Row Cotton as Affected by Canopy Architecture

^a Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP), Apartado Postal 247, Torreón, Coahuila 27000, Mexico
^b Dep. of Agron. and Hortic., New Mexico State Univ., Las Cruces, NM 88003

* Corresponding author (retad{at}cirnoc.inifap.conacyt.mx)

Received for publication January 14, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Alterations of plant architecture in narrow-row cotton (Gossypium hirsutum L.) using management and genetic strategies to improve light penetration into the canopy may increase crop yields. The objective of this study was to quantify how plant architecture changes affect light penetration into the canopy, yield, and yield components of narrow-row cotton. The study was conducted on a Glendale clay loam soil (fine montmorillonitic, thermic Typic Torrert). Two field experiments were established on 0.76-m rows in 1995 and 1996. Treatments consisted of the following plant architecture modifications: pruning leaves throughout the canopy, mechanical topping, trimming of branches, and temporarily opening the canopy during boll production. Photosynthetic photon flux density (PPFD) interception and PPFD penetration into the canopy were measured when the canopy was fully developed. Seed-cotton yield and yield components by plots, fruiting positions, and strata by four main-stem node groups were obtained. Early canopy modifications simulating plant characteristics such as reduced plant height, short branches, and modified leaf shape increased light availability at the medium and upper part of the canopy. Modified canopy treatments grown at 97 000 plants ha^-1 reached high PPFD interception (90–97%), with leaf area index from 3.7 to 5.2. Treatments to increase light distribution in the canopy while maintaining a high PPFD interception increased seed-cotton yield by 34% due to a 26% increase in number of bolls per square meter. A canopy light environment improved during the first 3 wk after canopy closure (86–107 d after sowing) increased number of bolls per square meter by 33%.

Abbreviations: DAS, days after sowing • LAI, leaf area index • PPFD, photosynthetic photon flux density

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
NARROW-ROW cotton culture has demonstrated the potential for increasing the yield over conventional production. Yield advantage of narrow rows is due to an increase in light interception early in the season and increased boll production (Heitholt et al., 1992). Because development of cotton bolls depends primarily on adjacent leaves (Kerby et al., 1980), the early closure of rows and excessive vegetative growth above developing fruiting branches causes less penetration of sunlight into the canopy and, consequently, an increase in the abscission of fruiting forms (Guinn, 1974) and a decrease in fiber quality (Kerby and Ruppenicker, 1992; Kerby et al., 1993), presumably due to the lack of assimilates.

Alterations of canopy architecture that allow more light penetration into the lower depths of the canopy may be a way to increase the yield of cotton through greater boll production. Guinn (1985) found that thinning at different times during the season caused increases in flowering per plant and boll retention, probably due to a higher irradiance in the lower portion of the canopy. Pettigrew (1994) increased the availability of light into the canopy by using open canopies and reflectors and found a 17 and 6% increase in lint yields, respectively, due to a higher production of bolls per square meter.

Genotypes with characteristics such as mutant leaf types, reduced plant height, and short branches have been evaluated for increasing light penetration into the canopy. The use of genotypes such as okra-leaf or sub-okra-leaf has allowed competitive or higher yields compared with normal leaf types (Meredith, 1984; Wells et al., 1986). Kerby et al. (1980) indicated that efficient use of assimilates in cotton grown in narrow rows could be encouraged by selecting plants with normal-type leaves near developing bolls and erect mutant-type leaves at the top of the canopy. Kerby and Buxton (1981) suggest that cultivars with short sympodial branches and reduced leaf area in the upper canopy could improve efficiency of assimilate utilization in narrow-row cotton. Also, Kerby and Ruppenicker (1992) propose that the use of columnar phenotypes allows for enhanced development of bolls located in the lower part of the canopy due to an improved light penetration into the canopy. In fact, the yield advantage of modern cultivars has been associated with an earlier boll set to coincide with peak assimilate production capacity and a greater harvest index (Wells and Meredith, 1984). However, modern cultivars still show a source-to-sink problem due to the small leaf area of subtending leaves and a deteriorating light environment in the lower part of the canopy before bolls are sufficiently developed (Kerby et al., 1993).

The objective of this study was to quantify how plant architecture changes affect the canopy light environment, yield, and yield components of narrow-row cotton.

	MATERIALS AND METHODS

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This study was conducted at the New Mexico State University Leyendecker Plant Science Research Center (32°12' N, 106°44' W; elevation 1168 m above sea level). The soil was a Glendale clay loam (fine montmorillonitic, thermic Typic Torrert). Land preparation was consistent with cultural practices used in this locality for cotton production and consisted of chiseling, plowing, disking, and bedding in an east-to-west direction. In both years, 18–46–0 fertilizer was applied at 224 kg ha^-1 by broadcasting before preparation of the beds. Before the first irrigation, urea fertilizer (46–00–00) was applied at 200 kg ha^-1. Herbicides trifluralin (,, -trifluoro-2, 6-dinitro-N, N-dipropyl-p-toluidine) and prometryn [2,4-bis(isopropyl-amino-6-(methylthio)-S-triazine] at 1.0 and 1.75 kg a.i. ha^-1, respectively, were incorporated into the soil in March during seedbed preparation. Cultivation and hand hoeing were used to control emerging weeds as needed.

Field experiments were established on 15 May 1995 and 8 May 1996. In both years, a presowing irrigation of about 230 mm was applied, and a locally adapted full-season cultivar, Acala 1517 91, was sown by machine on rows 0.76 m apart and at 20 seed m^-1 row. Acala 1517 91 was selected for these studies because it is a high-yielding cultivar with normal leaves and long vegetative branches. Plant spacing within rows of 20.4 cm (65 000 plants ha^-1) in 1995 and 13.6 cm (97 000 plants ha^-1) in 1996 were achieved by hand thinning at 35 d after sowing (DAS). Each experimental plot consisted of six 3-m-long rows. In 1995, five irrigations were applied at 43, 68, 84, 102, and 117 DAS. In 1996, four irrigations were applied at 49, 82, 98, and 113 DAS. In both years, the irrigations were applied when the leaf water potential, measured with a pressure chamber (Scholander et al., 1965), reached the critical value of 1.9 MPa (Grimes and Yamada, 1982). Measurements were made at midday (1200 to 1400 h MDT) on two leaves (third fully expanded leaf from the apex) randomly selected from the central rows of each plot onto two replications per day. Insects were controlled by insecticide applications during the growing season.

Treatments consisted of the following plant architecture modifications: removing part of the leaves, mechanical topping, temporarily opening of canopy, and trimming of branches (Tables 1 and 2). For the distribution of treatments in the field, a randomized complete block design with four replications was used. In 1995, Treatments 3, 4, and 6, where the canopy was modified above main-stem node 16, were applied when the leaves above node 16 reached at least a third of their final size (90 DAS). The removal of leaves in these treatments was repeated during the growing season at least once per week. In Treatment 2, where all branches were trimmed, the treatment was applied after the first fruiting branch produced more than two nodes (65 DAS). Later, the branches were trimmed at least once per week. In 1996, the canopy modifications were applied earlier than in 1995. In Treatments 2, 3, and 7, where all branches were trimmed, the treatment application began at 66 DAS. After this date, the branches were trimmed at least once a week. In Treatments 4, 5 and 7, the removal of a third of the blade in each leaf produced on the main stem and vegetative branches began at 54 DAS. In Treatment 6 (opened canopy), when plants were bent away, Rows 2 and 5 were used for measurements of light penetration into the canopy. At harvest, the yield and yield components were also determined in Rows 2 and 5.

In 1996, measurements of PPFD intercepted by the canopy were taken when the canopy was fully developed (102 DAS). Two readings per plot were taken between 1200 and 1500 h solar time using a LI-COR LI-191SA line quantum sensor (LI-COR, Lincoln, NE) above and below the canopy, parallel to the earth's surface. The line quantum sensor was held at a 40° angle from perpendicular to the row so that each end of the sensor was aligned with the center of each row and the center of the sensor was at the midpoint between the two rows. The PPFD intercepted by the canopy (µmol m^-2 s^-1) was determined using the following equation:

To quantify the leaf area, in 1996, all leaves of five randomly selected plants per plot were harvested when the canopy was fully developed (102 DAS). Leaf area was determined with a LI-COR LI-3000 leaf area meter and a LI-3050A conveyor belt accessory (LI-COR, Lincoln, NE.). Leaf area index (LAI) was determined as the ratio between the total leaf area of the sample and the ground area occupied by the sample.

To evaluate the effect of canopy modification on yield and yield components, selected measurements were taken at maturity. The retention and distribution of fruits into the canopy were determined with plant mapping in 10 randomly selected plants per plot. The bolls produced in 10 randomly selected plants per plot were harvested and weighed individually. From these data, the seed-cotton yield, number of bolls, and average boll weight by plot, fruiting position, and strata (0–8, 9–12, 13–16, and 17–20 main-stem nodes) were obtained.

Because canopy modification treatments were not the same in the 2 yr of the study, the analysis of all variables measured is presented by year. Data from each year were subjected to analysis of variance. The analysis of data was done by plot and by strata and fruiting positions. In the analysis by plot, an analysis of variance for a randomized complete block design with four replications was used. In the analysis by strata and fruiting positions, a split-plot design with main plots in a randomized complete block design with four replications was used where the main plots were assigned to canopy treatments and the subplots to the strata and fruiting positions. Mean separation for LAI, light interception, seed-cotton yield, and yield components by plot were performed using single-degree comparisons at the 0.05 level of significance. For light penetration into the canopy, seed-cotton yield, and yield components by fruiting position and strata, a protected LSD test at = 0.05 was used (SAS Inst., 1985).

	RESULTS

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Leaf Area Index and Light Interception
Some of the plant architecture changes applied in 1996 modified the leaf area size and light interception capacity of cotton (Table 3). The modification of shape and area of all vegetative and main-stem leaves and the limitation of the growth above node 18 by mechanical topping in Treatment 5 reduced the leaf area by 29%. On the other hand, the canopy alteration by treatment application significantly reduced the light interception capacity of the cotton only in Treatment 2 [shortened branches (two nodes)] compared with that of the control. However, the light interception in all modified canopy treatments was high (90–97%). The results suggest that the arrangement of leaves in the canopy was more important than the LAI size for light interception capacity. This behavior can be observed in the similar light interception capacity in Treatment 5 (modified leaves and plants decapitated) and the control, in spite of a larger LAI (29%) in the control (Table 3).

View larger version (37K): [in this window] [in a new window]   Fig. 1. Photosynthetic photon flux density (PPFD) availability at three canopy positions resulted from modified plant architecture treatments of narrow-row cotton in 1995. * = significance at the 0.05 probability level (LSD = 13.63) for each control–treatment comparison. Average PPFD above the canopy of all treatments = 1738 µmol m-2 s-1.

View larger version (41K): [in this window] [in a new window]   Fig. 2. Photosynthetic photon flux density (PPFD) availability at three canopy positions resulted from modified plant architecture treatments of narrow-row cotton in 1996. * = significance at the 0.05 probability level (LSD = 7.37) for each control–treatment comparison. Average PPFD above the canopy of all treatments = 1738 µmol m-2 s-1.

Response of Yield and Yield Components to Leaf Shape Modifications
Response of yield and yield components to leaf shape modifications depended on the light penetration deep into the canopy. In 1995, partial upper leaves pruned (Treatment 6), and modified upper leaves (Treatment 4) did not affect the yield and yield components (Table 4) due to the limited response to the light penetration into the canopy (Fig. 1). However, a different response was observed in Treatments 4 (modified leaves) and 7 (shortened branches and modified leaves), which were evaluated in 1996 at higher plant population. In Treatment 4, where all vegetative and main-stem leaves were modified, a greater light availability at main-stem nodes 12 and 16 (Fig. 2) promoted a higher seed-cotton yield (30%) than the control (Table 5). On the other hand, the combination of modified leaf and short-branch characteristics in Treatment 7 also improved the light availability at main-stem nodes 12 and 16 (Fig. 2) and increased the seed-cotton yield by 43% compared with the control. A greater production of bolls per square meter in Treatments 4 (25%) and 7 (36%) was the principal factor for higher yields (Table 5). The yield response in Treatment 4 was observed only in stratum 9 to 12 (Table 6) and in the first fruiting position (Table 7). In Treatment 7, the seed-cotton yield was increased in strata 9 to 12 and 17 to 20 (Table 6) and in the three fruiting positions per branch (Table 7).

Response of Yield and Yield Components to Artificial Open Canopies
When the light environment in the canopy was improved by artificial open canopies after row closure, seed-cotton yield was increased as a response to a greater production of bolls per square meter. In Treatment 6 (opened canopy), where row closure was delayed for 3 wk, the higher light availability at main-stem nodes 8, 12, and 16 (Fig. 2) increased the seed-cotton yield by 40% due to a greater number of bolls per square meter (Table 5). Seed-cotton yield response occurred only in strata 9 to 12 and 13 to 16 main-stem nodes (Table 6). On the other hand, the yield was increased in fruiting positions 1 and 3 compared with the control (Table 7) while in fruiting position 2, only boll size was increased (data not shown). The results showed that the light competition during 3 wk after row closure reduced, in a significant way, the number and weight of fruits growing at the lower and middle part of the canopy.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Plant characteristics such as reduced plant height, short branches, and modified leaf shape increased light penetration into the canopy. However, certain characteristics were more efficient in modifying light distribution into the canopy. Kerby et al. (1980) proposed that cotton plants with a combination of normal-type leaves near developing bolls and erect mutant-type leaves at the top of the canopy could be more efficient for increasing production and use of assimilates in narrow-row cotton. In our study, plants with modified leaf shape at the upper part of the canopy and normal leaves at the medium and lower part of the canopy increased light availability only at node 16 (Fig. 1). This behavior suggests the necessity of an earlier and greater canopy modification using other plant characteristics such as short branches (Kerby and Buxton, 1981; Kerby and Ruppenicker, 1992) and modified leaf shape (Kerby et al., 1980; Wells et al., 1986; Peng and Krieg, 1991). The results showed that modified leaf shape allowed a deeper light penetration into the canopy than plants with short branches. Furthermore, the combination of leaf shape, reduced plant height, and short branches gave a greater light penetration through the canopy than the control (Fig. 2). In fact, plants with reduced plant height, short branches, and modified leaves grown at 97 000 plants ha^-1 reached high values of light interception (90–97%), with LAI ranging from 3.7 to 5.2 (Table 3). This behavior, in combination with a greater light penetration into the canopy, allowed altered plant architecture treatments a better light distribution in the canopy than the control. However, plant architecture has to be modified early during plant development to obtain these results.

The increased light penetration into the canopies of modified plant architecture treatments was related to changes in seed-cotton yield and yield components (Table 8). This relationship depended on the management conditions. In 1995, the relationships among light penetration, yield, and yield components were negative; that is, the alterations of plant architecture to increase light penetration into the canopy reduced or did not affect the yield of cotton. The principal factor that may have adversely affected the yield under these conditions was the medium plant density utilized (65 000 plants ha^-1), which reduced the number of fruiting positions per area of ground in plants with compact plant architecture. In 1996, when the cotton was planted at higher plant density than 1995 (97 000 plants ha^-1), the relationships among light penetration, yield, and yield components were positive and significant. These relations mean that the greater number of fruiting positions per square meter in modified canopy treatments (Table 5) compensated for the more compacted plant structure compared with the control. The significant relationship among the variables studied at node 12 suggests that the main response to an improved light environment occurred in the medium part of the canopy (Table 8). This behavior can be explained because a high proportion of fruits are produced in the middle part of the canopy and because of the low light conditions in the canopy after row closure. The yield increases due to treatments in 1996 appear to be due to an overcrowding phenomenon in the control. The 34% yield reduction in the control from 1996 to 1995 was related to a lower boll production and a smaller average boll size (Tables 4 and 5). Canopy alteration via leaf and branch removal alleviated the overcrowding of full-season cultivar utilized.

Evidence exists that a potential increase in seed-cotton yields is possible through a better light distribution in the canopy. However, alteration of plant architecture to improve the light environment in the canopy has been associated with a reduction in light interception. Genotypes with characteristics such as short branches (Kerby and Ruppenicker, 1992) and mutant plant types (Kerby et al., 1980; Wells et al., 1986; Peng and Krieg, 1991) have been evaluated for increasing light penetration into the canopy, but the reduced leaf area in genotypes such as super okra leaf has limited canopy photosynthesis and yields (Kerby et al., 1980; Wells et al., 1986). A cultural approach to take advantage of genotypes with these characteristics is to increase their light interception capacity through the use of high plant populations (Heitholt, 1994) and narrow rows (Heitholt et al., 1992). In our study, the utilization of these factors and the improvement of canopy light environment through plant architecture modifications increased the seed-cotton yield by 34% due mainly to a greater production of bolls per square meter by 26% (Table 5). Similar response has been reported by Pettigrew (1994).

There is a potential to increase seed-cotton yields while maintaining a high capacity to intercept light yet reduce the disadvantage of an early shading of leaves nearest the developing bolls in narrow rows through plant architecture modification. Cultivars of cotton with reduced plant height, short branches, modified leaves, and combinations of these characteristics grown at high plant densities and in a narrow-row system could be a good alternative to increase the yield of cotton.

	ACKNOWLEDGMENTS

 
The authors thank Dr. Marta Remmenga for helpful suggestions in statistical analysis of the experimental data.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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