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Floral bud removal from specific fruiting positions in cotton: yield and fiber quality.

Under normal field conditions, more lint is harvested from cotton bolls on proximal fruiting sites (Fig. 1) on sympodial branches than from distal sites (Constable, 1991; Jenkins et al., 1990; Kerby et al., 1987). Typically, lint from FP1 (first fruiting position) is longer and more mature than lint from FP2 (Heitholt and Schmidt, 1994) or FP3 (Kerby et al., 1993). Although cotton bolls located on the distal sites (FP2 and greater) only produce a small portion of the yield, their contribution of lower quality fiber is proportionally greater than other positions and may be detrimental to the overall fiber quality. Because the cotton industry wishes to reduce the contribution of lower quality fiber associated with distal fruiting sites, physiologists and breeders have proposed that a plant type fulfilling this objective would produce primarily FP1 fruit, few FP2 fruit, and no FP3 fruits. There are at least two approaches to achieving plants with primarily FP1 fruit. First is the use of genotypes that exhibit this kind of reproductive growth (e.g., "columnar" genotypes). Secondly, and only for research purposes, one can selectively remove fruiting forms so that boll production at FP1 or FP2 predominates. An advantage of this latter approach is that yield potential of plants restricted to only one fruit per main stem node could be evaluated without genotype presenting a confounding effect.

[Figure 1 ILLUSTRATION OMITTED]

Numerous studies have quantified the effects of fruiting form removal on cotton yield or fiber properties when the FP site of removal has not been specified. Fruiting-form removal treatments that lower yield generally increase the ratio of vegetative growth to reproductive growth (Kletter and Wallach, 1982; Kennedy et al, 1986; Jones et al., 1996b; Pettigrew et al., 1992; Ungar et al., 1987; Sadras, 1995 and references therein). In two reports by Jones et al. (1996a, b), the effects of selected times of boll removal on yield, fiber quality, and vegetative growth were examined. In the first study, Jones et al. (1996a) found that late flower removal (4th week of flowering and later) decreased yield but increased boll size and micronaire approximately 10%. In the second study (Jones et al., 1996b), flower removal increased the ratio of vegetative to reproductive biomass. This increased ratio provided existing bolls with a greater potential assimilate supply per boll (Jones et al., 1996a). A greater assimilate supply per boll could allow bolls to develop longer and thicker fibers unless the removal delays maturity and causes boll development to occur in unfavorable environmental conditions.

To my knowledge, there are only two published reports of the effects of fruit removal on boll growth and fiber properties when the FP of removal was specified (Pettigrew, 1994, 1995). In one of those reports, a one-time removal of all FP2 fruiting forms from selected plants increased boll size, seed size, boll retention, and seed per boll of tagged FP1 bolls (Pettigrew, 1994). In the companion report, this treatment also increased fiber strength, length, micronaire, maturity, and wall thickness of lint from FP1 bolls (Pettigrew, 1995). Although these two studies reported the effects of selective FP removal on boll characteristics and fiber quality of selected bolls, the question still remains as to whether a similar treatment would affect yield, yield components, and fiber quality of the whole plant or plant community. It is logical to reason that extensive floral bud removal will reduce yield. However, a study where FP2 fruit are prevented from developing needs to be performed so that its effects can be quantified. Therefore, the primary objective of this research was to quantify the effects of floral bud removal at selected FP sites on yield, yield components, and fiber properties of cotton. A secondary objective was to compare yield components and fiber properties among FP1, FP2, and FP3 sites.

MATERIALS AND METHODS

Cotton (cv. Deltapine 5415) was grown in the field on a Beulah fine sandy loam (coarse-loamy, mixed, thermic Typic Dystrochrepts) near Stoneville, MS, in 1993 and 1994. Planting dates were 28 April 1993 and 6 May 1994. Weather data for the two growing seasons are provided in Fig. 2. At the first true leaf stage, plants were thinned to 8 plants [m.sup.-2]. plots consisted of three rows, 5.2 m in length, with a 1.0-m row spacing. Treatments were imposed on only the middle (bordered) row and treatments began when bracts of early squares (floral buds) reached approximately 2 cm in width. Treatments began at 63 d after planting (DAP) which was at 583 heat units in 1993 and 46 DAP (450 heat units) in 1994 (Fig. 2). Square removal treatments ceased at late bloom. The last days of removal were 105 DAP (1160 heat units) in 1993 and 89 DAP (955 heat units) in 1994. Squares were removed twice weekly. Squares from the entire plant and from all plants in the treated row were considered for removal. In 1993, the three treatments (Fig. 1) were (i) check (no removal), (ii) P1 [FP2 and greater (i.e., FP3, FP4, etc.) squares removed], and (iii) P2 [FP1 and FP3 (and greater) squares removed]. In 1994, an additional treatment, PIP2 was included where only FP3 and greater squares were removed. If not stated otherwise, the use of FP3 in the remainder of this paper refers to FP3 sites and greater.

[Figure 2 ILLUSTRATION OMITTED]

Most squares were removed at the youngest stage possible with the goal of minimizing damage to the plants and to limit removal of dry matter from the plants. However, a small number of squares of slightly greater age were removed during the week preceding their anthesis. Squares produced on monopodia ("vegetative" branches) were determined to be either FP1, FP2, etc. and treated accordingly. As discussed in the Results section, monopodial bolls did not contribute greatly to the final boll number. Despite efforts to remove all FP2 squares in the P1 treatment and all of the FP1 squares in the P2 treatment, several squares went undetected or began development after the treatment ceased. Regardless of FP, undetected fruiting forms that reached anthesis were not removed. The number of squares removed was not recorded.

Handling plants may affect their growth (Jaffe and Forbes, 1993). During the search for selected squares check plants were not touched and plants in the P1, P2, and P1P2 treatments were handled so that touching was minimized as much as possible. Plants in all four treatments were touched during the twice weekly flower counts (described later) in 1994. Touching the plants for flower counts was not as severe as it was for the square search.

At harvest, mature (open) bolls from 4.6 [m.sup.2] were separated into either FP1, FP2, or FP3 sites and counted. The few bolls from monopodia were determined to be FP1, FP2, or FP3 and combined with those groups. Harvest dates were 131, 148, and 163 DAP in 1993 and 117, 124, 129, 136, 143, and 153 DAP in 1994. Seed cotton for each harvest was weighed separately and the date (DAP) that 65% of the seed cotton was on open bolls was calculated for each plot (Heitholt, 1993). In both years, seed cotton from FP1, FP2, and FP3 was ginned (a 10-saw plot gin) separately but each ginning included seed cotton from all harvests for each FP. The harvests were combined because the objective was to compare treatments (not harvests) and because there were often few or no bolls at an FP for a given harvest.

Yield and yield components were determined for each of the three positions separately, as well as for the whole plot. Boll size was lint mass divided by the number of bolls. For seed size (mass per seed), 100 nondelinted (fuzzy) seed from each ginned sample were weighed. Fiber properties for each of the three positions and a calculated plot composite (weighted averages or calculated blend values) were determined (Heitholt et al., 1993). Fiber analysis was performed by Starlab (Knoxville, TN) as described by Heitholt et al. (1993).

Petals of blooms of most cotton genotypes are concealed the day prior to anthesis, are white and conspicuous on the day of anthesis, and are pink the day after anthesis. Therefore, the number of flowers reaching anthesis on a given day can be determined by counting only those flowers whose petals are white. In 1994, open blooms at anthesis (white flowers) were counted twice weekly from beginning bloom (58 DAP) until the end of the bloom period (118 DAP). Seasonal flower production was determined as previously described (Heitholt, 1993). At maturity, plant height (both years), the number of main stem nodes (1994 only), and the number of branch bolls (1994 only) were determined.

Experiment Design and Statistical Analysis

A randomized complete block design with eight replications was used in both years. For boll distribution and yield, the primary objective was to compare treatments within each FP. Therefore, a separate analysis of variance (ANOVA) for e9ch FP (rather than a combined ANOVA across FP sites) was performed for each year and for data combined across years. A separate ANOVA for each FP helped to avoid heterogeneity of error.

For the yield components and fiber properties, a combined ANOVA included data from both years and each of the three FP sites. Treatments were considered main plots and FP sites were considered subplots. The LSD for treatments within FP was calculated with a combination of error a and residual error (Steel and Torrie, 1980). The LSD for FP within treatments was calculated with the residual error. For the composite values from all variables, a combined ANOVA across years was performed but did not include data from the P1P2 treatment in 1994. Sources of variation for this combined analysis were year, replicate (year), treatment, and year x treatment.

RESULTS AND DISCUSSION

Yield and Boll Distribution

An ANOVA for the boll distribution at each position indicated that the mean squares for year x treatment interactions were 11- to 40-fold lower than treatment effects (Table 1). The year x treatment interaction for total yield was not significant. Therefore, summaries of data averaged across years as well as for each year are included. When compared with the check, the P1 treatment forced a greater percentage of bolls (82% vs. 68%) and significantly more lint (7%) to be produced at FP1 sites (Table 2). However, the number of FP1 bolls on the P1 treatment was similar to that of the check. The failure for the P1 treatment to increase the number of FP1 bolls in this study was unexpected and contrasts with the reports of others that showed FP2 fruiting form abscission (Kerby and Buxton, 1981) or removal (Pettigrew, 1994) increased retention of FP1 bolls. The lack of increase in the number of FP1 bolls may have been partly due to the production of 10.7 non-FP1 bolls [m.sup.-2] (1.3 per plant) on the P1 treatment. However, the main objective of the P1 treatment, to decrease the contribution of FP2 and FP3 fruit, was achieved. The P1 treatment provided a physiological situation that favored increased assimilation to FP1 bolls as indicated by greater lint production at FP1. The percentages of bolls harvested from FP2 (42%) and FP3 (20%) were greater on the P2 treatment than the other treatments. Boll numbers at FP2 and FP3 for the P2 treatment were greater than the check. Averaged across years, lint yield (all positions) averaged 11% higher in the check than in the P1 treatment and 28% higher than the P2 treatment. The P1 treatment outyielded the P2 treatment by 20%.

Table 1. Analysis of variance of the percentage of bons at selected fruiting positions, FP, (% of total) and total lint yield (kg [ha.sup.-1]) of Deltapine 5415 cotton. Data for both years were included.
                                Fruiting position

                            FP1([dagger])

Source             df     MS   P [is greater than] F

Year                1    449         0.01
Replicate (year)   14     36         0.02
Treatment           2   8164         0.01
Year x treatment    2    202         0.01
Error              28     14           --

                         FP2([double dagger])

Source               MS   P [is greater than] F

Year                469         0.01
Replicate (year)      9         0.51
Treatment          4075         0.01
Year x treatment    363         0.01
Error                10           --

                            FP3([sections])

Source                MS   P [is greater than] F

Year                   1          0.88
Replicate (year)      24          0.01
Treatment            781          0.01
Year x treatment      38          0.02
Error                 28            --

                                 Total yield

Source                    MS     P [is greater than] F

Year                   586 977          0.01
Replicate (year)        11 040          0.40
Treatment              404 036          0.01
Year x treatment         4 596          0.63
Error                   10 073            --


([dagger]) FP1 represents the most proximal position on the sympodial branch (Fig. 1).

([double dagger]) FP2 represents the second most proximal fruiting position (Fig. 1).

([sections]) FP3 represents the third and greater most proximal fruiting positions (Fig. 1).

Table 2. Effect of selective square removal on the distribution of bolls, number of bolls and yield of Deltapine 5415 cotton in 1993 and 1994.
                              Boll distribution

Treatment       FP1([dagger])   FP2([double   FP3([sections])
                                dagger])

                                 % of total

1993

Check              65.7           24.3           10.1
P1                 82.2           10.1            7.7
P2                 31.0           50.3           18.7
LSD (0.05)          4.0            3.6            3.0

1994

Check              69.8           23.0            7.2
P1                 82.9            9.5            7.7
P2                 44.7           33.2           22.1
P1P2               69.2           23.3            7.3
LSD (0.05)          4.1            3.3            3.0

1993 and 1994

Check              67.6           23.7            8.6
P1                 82.4            9.9            7.7
P2                 38.0           41.7           20.2
LSD (0.05)          2.7            1.8            2.2

                                 Bolls

Treatment           FP1        FP2       FP3        Total

                             no. [m.sup.-2]

1993

Check               52.7       19.4       8.1        80.2
P1                  53.9        6.7       5.1        65.7
P2                  16.4       26.8      10.0        53.3
LSD (0.05)           3.9        3.2       2.1         6.2

1994

Check               47.4       14.8       4.6        64.3
P1                  46.1        5.2       4.3        55.6
P2                  20.0       14.8       9.9        44.8
P1P2                44.1       14.9       4.7        63.7
LSD (0.05)           5.6        2.2       1.7         6.9

1993 and 1994

Check               50.0       17.1       6.3        73.5
P1                  50.0        6.0       4.7        60.7
P2                  18.2       20.8      10.0        49.0
LSD (0.05)           3.3        1.8       1.4         4.4

                             Lint yield

Treatment           FP1        FP2       FP3        Total

                              kg [ha.sup.-1]

1993

Check               800        290       124         1210
P1                  898        108        86         1090
P2                  249        457       164          870
LSD (0.05)           74         55        40          118

1994

Check               702        209        64          975
P1                  713         74        62          849
P2                  303        230       155          688
P1P2                664        222        65          951
LSD (0.05)           91         36        28          110

1993 and 1994

Check               751        249        94         1090
P1                  805         91        74          971
P2                  276        344       159          779
LSD (0.05)           52         31        24           73


([dagger]) FP1 represent the most proximal fruiting position on the sympodial (Fig. 1).

([double dagger]) FP2 represent the second most proximal fruiting position (Fig. 1).

([sections]) FP3 represents the third and greater most proximal fruiting positions (Fig. 1).

In 1994, boll distribution and yield of the PIP2 treatment was similar to that of check. The similar response of the P1P2 treatment to that of the check indicated that FP3 squares were probably not prevalent or few were retained in 1994. This also suggested that the physical handling of the plants described earlier probably did not have an appreciable effect on plant growth at least for the P1P2 treatment.

Because the P1 treatment reduced yield, the results also indicated that permitting FP2 fruit to develop is probably necessary to attain maximum yield. Likewise, the lower yield of the P2 treatment indicated that FP2 sites were unable to compensate for lost FP1 squares. The yield of the P2 treatment was predicted to be greater than obtained here, because others have shown that boll set at FP2 increased when FP1 fruiting forms abscised as squares (Kerby and Buxton, 1981) or were manually excised (Pettigrew, 1994).

In general, mature boll distribution (Table 2) was similar to that reported in the literature with the exception that the contribution of FP3 fruit in the check (9%) was slightly greater than reported by Jenkins et al. (1990). A discussion of the differences in boll distribution and lint yield among FP sites in untreated plants has already been provided in the literature (Constable, 1991; Jenkins et al. 1990; Kerby et al., 1987) and is not repeated here.

Yield Components

As was shown for boll distribution and yield, an ANOVA of boll size indicated that the year x treatment interaction was not significant, whereas the treatment effects were significant (Table 3). Therefore, yield component data were averaged across years. Although composite boll size was not significantly affected by treatment, boll size at FP1 on the P1 treatment was 7% greater than the check (Table 4). This 7% greater boll size explained the 7% greater lint yield at FP1 for the P1 treatment. Boll size at FP2 on the P2 treatment was 12% greater than the check. Boll size at FP3 for both the P1 and P2 treatment was 6 to 9% greater than the check. Increased boll size in response to fruiting form removal is similar to responses reported by others (Pettigrew, 1994; Jones et al., 1996a).

Table 3. Analysis of variance of boll size (g [boll.sup.-1]) and fiber maturity (%) of Deltapine 5415 cotton. Data for 1993 and 1994 were included. The analysis included data from all fruiting positions (FP) and treatments.
                                        Boll size

Source                           df    MS     P [is greater than] F

Year                              1   0.447        0.01
Rep (year)                       14   0.024        0.20
Treatment                         2   0.163        0.01
Year x treatment                  2   0.036        0.13
Rep x treatment (year) error a   28   0.017        0.02
FP                                2   0.002        0.69
Treatment x FP                    4   0.047        0.01
Year x FP                         2   0.032        0.01
Year x treatment x FP             4   0.007        0.29
Residual (error b)               84   0.005         --

                                    Fiber maturity

Source                            MS    P [is greater than] F

Year                             405            0.01
Rep (year)                        36            0.18
Treatment                        259            0.01
Year x treatment                  13            0.60
Rep x treatment (year) error a    24            0.01
FP                                23            0.14
Treatment x FP                    15            0.26
Year x FP                          4            0.69
Year x treatment x FP            13             0.34
Residual (error b)               11              --


Table 4. Effect of selective square removal on the yield components of Deltapine 5415 cotton, averaged across 1993 and 1994.
                                    Boll size

                   Fruiting site

Treatment     FP1         FP2        FP3            LSD   Composite
             ([dagger])   ([double   ([sections])         ([para
                           dagger])                         graph])

                                g lint [boll.sup.-1]

Check         1.50         1.45        1.47         0.05    1.52
P1            1.61         1.51        1.57         0.05    1.59
P2            1.52         1.63        1.60         0.05    1.58
LSD (0.05)    0.07         0.07        0.07         --      ns

                               Seed size

                              mg [seed.sup.-1]

Check         79.5         79.7        75.7          2.8    79.1
P1            84.3         82.5        80.3          2.8    83.9
P2            80.2         83.1        80.5          2.8    81.6
LSD (0.05)     2.8          2.8         2.8         --       2.2

                              Lint per seed

                                  mg

Check         49.8         51.8        52.1          1.9    50.4
P1            53.3         52.3        54.0         ns      52.9
P2            51.3         53.8        53.4          1.9    53.2
LSd (0.05)     2.1         ns          ns           --       1.7

                                    Lint percentage

                   Fruiting size

Treatment       FP1      FP2      FP3      LSD     Composite

                                    %

Check           38.5     39.4     40.7     0.6      38.9
P1              38.7     38.9     40.2     0.6      38.8
P2              39.0     39.3     39.9     0.6      39.3
LSD (0.05)      ns       ns        0.7     --        0.4

                               Seed per boll

                                   no.

Check           30.1     28.1      28.1    1.1      30.2
P1              30.2     28.9      29.0    1.1      30.0
P2              29.6     30.2      29.8    ns       29.9
LSD (0.05)      ns        1.3       1.3    --        ns

                           Seed cotton per seed

                                  mg

Check          129      131       128      ns      129
P1             138      135       134      4       137
P2             131      137       134      4       134
LSd (0.05)       5        5         5      --        4


([dagger]) FP1 represents the most proximal fruiting position on the sympodial branch (Fig. 1).

([double dagger]) FP2 represents the second most proximal fruiting position (Fig. 1).

([sections]) FP3 represents the third and greater most proximal fruiting positions (Fig. 1).

([paragraph]) The composite is the calculated weighted average (or blend) which represents the value that would have been obtained had FP1, FP2, and FP3 been harvested and ginned together.

Except for the FP1 sites in the P2 treatment, both the P1 and P2 treatment increased seed size at each FP and in the composite (Table 4). The P2 treatment significantly increased lint percentage in the composite. At FP2, the P2 treatment produced more seed per boll than the check. The composite seed per boll was unaffected by treatments. Apparently, although not consistently observed here, an increase in assimilate supply per boll has the potential to increase the number of seed per boll that reach maturity. Pettigrew (1994) showed that FP2 fruit removal increased seed per boll at FP1 from 27 to 30 in one year of a 2-yr study. The composite lint per seed and seed cotton per seed were greater in the P1 and P2 treatments than the check. The nonsignificant 4% increase in composite boll size due to treatments was more closely associated with the 5% increase in composite lint per seed than with seed per boll (which was unaffected).

The greater boll size, seed size, and lint per seed in the P1 and P2 treatments may have been associated with a greater ratio of vegetative to boll dry matter. Although aboveground vegetative biomass was not determined in this study, others have reported that aboveground vegetative biomass can either increase (Kennedy et al., 1986; Jones et al., 1996b) or remain unchanged (Jones et al., 1996b) as a result of fruiting form removal. Assuming the treatments did not change aboveground vegetative dry matter, the ratio of vegetative to reproductive biomass, and therefore the potential assimilate supply per boll, was probably greater in the P1 and P2 treatments than the check. Therefore, the increase in boll size and seed size was expected for the P1 and P2 treatments.

Differences in yield components were also found among FP sites (Table 4). For the check, seed size and seed per boll were greater at FP1 than FP3. The greater number of seed per boll for FP1 compared to FP2 agrees with a previous report by Kerby and Ruppenicker (1989). For the P2 treatment, boll size and seed size were greater at FP2 than FP1 suggesting that many FP1 fruit on the P2 treatment may have developed late in the season under a less advantageous environment (data not shown). This claim is supported by the observation that the percentage of all FP1 bolls that came from the final harvests (FP1 bolls at final harvest/total number of FP1 bolls) was significantly greater in the P2 treatment (data not shown). In 1993, a significantly greater percentage of all FP1 bolls were collected on the third (i.e., final) harvest for the P2 treatment (21%) compared with 17% for the P1 treatment and 13% for the check. In 1994, a significant difference was found again with 39% of all FP1 bolls on the P2 treatment being collected on the final two (5th and 6th) harvests, compared with 23% for the P1 treatment and 21% for the check. This indicates that many of the FP1 bolls harvested from the P2 treatment probably developed after the square removal treatments ceased and when environmental conditions were likely to be less favorable than for bolls developing earlier.

Flower Production and Earliness

In 1994, total flower production (Fig. 3A) in the check (154 [m.sup.-2]) was significantly ([LSD.sub.0.05] = 15) greater than the P1 (119 [m.sup.-2]) and P2 treatments (105 [m-.sup.2]). Flower production on the P1P2 treatment (144 [m.sup.-2]) was not significantly different from the check (data not shown). The greater total flower production by the check, as compared with the P1 and P2 treatments (Fig. 3A), was due to a greater flower production from 75 to 101 DAP (Fig. 3B). Both yield and boll numbers of the four treatments were correlated with seasonal flower production (r = 0.96*).

[Figures 3A-3B ILLUSTRATION OMITTED]

Boll retention (number of bolls on the whole plant divided by the number of whole plant seasonal flowers) averaged 44% in 1994 (only year it was measured) and was unaffected by the square removal treatments (data not shown). Boll retention was 42, 47, 43, and 44% for the check, P1, P2, and P1P2 treatments, respectively. The fact that boll retention was unaffected by the treatments is not surprising given the strong correlation between boll numbers and seasonal flower production.

Averaged across years, the check reached 65% open (mature) bolls 1 d ([LSD.sub.0.05] = 1; P = 0.05) earlier (138 DAP) than the P1 treatment (139 DAP) and 7 d earlier than the P2 treatment (145 DAP). Delayed maturity of the P2 treatment further indicates that many of its bolls developed later, and possibly under different environmental conditions than the other treatments.

Main Stem Growth, Monopodial Bolls, and Plant Density

In 1993, plant height was significantly ([LSD.sub.0.05] = 7) greater in the P2 treatment (125 cm) than in the P1 treatment (110 cm) or the. check (114 cm). In 1994, neither plant height (average of 119 cm) nor the number of main stem nodes (average of 23) differed significantly among treatments.

Because the fiber quality of lint from monopodial bolls is generally lower than sympodial FP1 and FP2 bolls (Kerby and Ruppenicker, 1989), it is important to indicate whether monopodial boll production was greater in the P1 treatment or P2 treatment than the check. Monopodial boll production was not measured in 1993. However, in 1994, the number of bolls harvested from monopodial (vegetative) branches averaged 1.9 [m.sup.-2] and did not differ among treatments (data not shown). Therefore, the lower fiber quality often suspected from monopodial bolls probably did little to dilute FP1 fiber properties.

Plant density can also affect boll distribution. Increasing plant density from 3 to 15 plants [m.sup.-2] increased the percentage of bolls that were found at FP1 from 40 to 80% (Kerby et al., 1987). Therefore, results different from those reported in this paper may be expected to occur in studies using plant densities other than the 8 plants [m.sup.-2] used in this study.

Fiber Properties

In contrast to yield, but in concert with boll size, square removal increased the values for several fiber properties (Table 5). For most FP sites and the composite, fiber properties related to cell wall thickening (micronaire, maturity, and wall thickness) were significantly greater in the P1 and P2 treatments than the check. As was the case for boll size, the increases in micronaire, maturity, and wall thickness in the P1 and P2 treatments were somewhat expected because these treatments reduced boll load and provided a greater potential assimilate supply per boll than the check treatment. Increases in micronaire and wall thickness as a result of partial fruit removal support previous reports that showed similar fruit removal treatments increased micronaire (Pettigrew, 1995; Jones et al., 1996a). The greater seed cotton per seed combined with greater micronaire found in the P1 treatment compared to the check (Tables 4 and 5) also support the findings of Kerby et al. (1993) that micronaire was positively associated with seed cotton per seed. Fiber strength at FP2 was greater in the P2 treatment than the check but the composite was unaffected. Treatments did not consistently affect the 2.5% span length or elongation.

Table 5. Effect of selective square removal on selected fiber properties at FP1, FP2, and FP3 fruiting sites of Deltapine 5415 cotton. Values are averaged across 1993 and 1994.
                               2.5% span length

                  Fruiting site

Treatment     FP1         FP2        FP3            LSD   Composite
             ([dagger])   ([double   ([sections])         ([para
                          dagger])                         graph

                                    mm

Check          28.0         27.8        27.5        0.3     27.9
P1             28.2         27.8        27.6        0.3     28.1
P2             27.8         27.9        27.7        ns      27.8
LSD (0.05)      0.3         ns          ns          --      ns

                                 Micronaire

Check          4.56         4.61        4.57        ns      4.57
P1             4.85         4.77        4.86        ns      4.84
P2             4.70         4.97        4.81        0.11    4.84
LSD (0.05)     0.15         0.15        0.15        --      0.13

                               Well thickness

                                 [micro] m

Check          2.77         2.83        2.79        ns      2.78
P1             2.96         2.93        3.02        0.08    2.96
P2             2.91         3.02        2.97        0.08    2.97
LSD (0.05)     0.10         0.10        0.10        --      0.09

                              Strength

                  Fruiting site

Treatment       FP1         FP2         FP3      LSD      Composite

                                  kN m [kg.sup.-1]

Check          198         196         197        ns       197
P1             203         196         196        5        202
P2             198         203         200        5        200
LSD (0.05)      ns           6          ns        --        ns

                                  Maturity

                                      %

Check             83.4        84.1       84.1     ns          83.6
P1                87.5        86.8       89.8     ns          87.5
P2                86.8        88.5       87.9     ns          87.6
LSD (0.05)         2.8         2.8        2.8     --           2.3

                                 Elongation

                                       %

Check              8.47        8.30       8.09     0.26        8.40
P1                 8.20        8.03       8.28     ns          8.23
P2                 8.41        7.95       8.16     0.26        8.15
LSD (0.05)       ns            0.32      ns        --         ns


([dagger]) FP1 represents the most proximal fruiting position on the sympodial branch (Fig. 1).

([double dagger]) FP2 represents the second most proximal fruiting position (Fig. 1).

([sections]) represent the third and greater most proximal fruiting positions (Fig. 1).

([paragraph]) The composite is the calculated weighted average (or blend) which represents the value that would been obtained had FP1, FP2, and FP3 been harvested and ginned together.

Differences in fiber properties among FP sites were also observed (Table 5). In the P1 treatment, fiber length and strength were greater at FP1 than either FP2 or FP3. For the check, fiber length and elongation were greater for FP1 than FP3. In most cases fiber properties of FP1 lint from the P1 treatment were similar to FP2 lint from the P2 treatment. The greater micronaire and wall thickness at FP2 compared to FP1 in the P2 treatment was unexpected. However, this may be explained by the production of FP1 bolls whose squares developed after the FP1 square removal treatments ceased (discussed earlier). A failure to detect increased fiber maturity at FP1 compared to FP2 in the current study partly agrees with data from Pettigrew (1995) that showed little difference in fiber properties between FP1 and FP2 lint when only one boll was allowed to occupy a sympodial fruiting branch.

SUMMARY

Preventing more than one boll from developing per fruiting branch (by selective FP1 or FP2 square removal) increased fiber properties related to cell wall thickening but reduced yield in both years of a 2-yr study. The fact that the P1 treatment reduced yield indicated that allowing FP2 fruit to develop is probably necessary (at least at this plant density) for the plant to produce its greatest yield. Similarly, because the P1 treatment outyielded the P2 treatment, the results indicated that FP2 sites were unsuccessful in compensating for removed FP1 squares. The gain in fiber micronaire and maturity attained by either reducing FP2 boll numbers or by reducing their proportion relative to FP1 boll numbers did not appear advantageous on a seasonal basis because the improvements; in fiber properties were small, of no economic importance, and probably of minor biological importance.

ACKNOWLEDGMENTS

The author thanks Joseph Graves and Don Richardson for technical assistance and Ellen Keene for help with the drawing.

Abbreviations: ANOVA, analysis of variance; DAP, days after planting; FP, fruiting position; FP1, the most proximal fruiting position on a sympodia (fruiting branch); FP2, the second most proximal fruiting position on the sympodia; FP3, the third most proximal and greater fruiting position on the sympodia.

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J. J. Heitholt(*)

Cotton Physiology and Genetics, USDA-ARS, P.O. Box 345, Stoneville, MS 38776. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the product or service, and the use of the name by USDA implies no approval of the product or service to the exclusion of others that may also be suitable. Received 10 May 1996. (*) Corresponding author (E-mail: jheithol@ag.gov).
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Date:May 1, 1997
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