Potassium Effects on Partitioning, Yield, and Earliness of Contrasting ...

Published online June 26, 2007

Potassium Effects on Partitioning, Yield, and Earliness of Contrasting Cotton Cultivars Jenny Clement-Bailey* and C. Owen Gwathmey Saleem and Buxton (1976) determined that nonstructural carbohydrates accumulated in stem tissue before reproductive development, but were depleted as fruit weight accumulated. As boll filling was completed, stem carbohydrates increased again. Since a more determinate cultivar has less vegetative growth in late season, it may place a higher demand on reserve carbohydrates to support boll filling than does a more indeterminate growth habit. Pace et al. (1999) found that, in early reproductive growth, a short-season cultivar distributed more dry matter and photosynthate to fruit than did a long-season cultivar. Bange and Milroy (2004) also found that two early maturing genotypes partitioned a higher proportion of total dry matter to reproductive organs than did later, more indeterminate genotypes. Cultivar differences in K response were found by Halevy (1976) and by Cassman et al. (1989), working with Acala cottons. These researchers did not describe the growth habit or maturity of the cultivars in their studies, but these traits can be inferred from their data. Halevy (1976) showed similar dry matter accumulation in two cultivars, but it appears that the earlier, more determinate cultivar exhibited more K deficiency symptoms with limited K than did the later, more indeterminate one. The determinate cultivar also had a higher proportion of dry weight in reproductive organs. Cassman et al. (1989) found two Acala cultivars differed in K-use efficiency, defined as higher yield under limited K supply. They also reported that the more K-efficient cultivar produced more leaf and stem biomass during the season than the less efficient cultivar. The two cultivars partitioned similar proportions of biomass to reproductive parts with adequate K, but not with limited K. In the 1990s, attention focused on K deficiencies that were being observed in early maturing or “fast fruiting” cultivars. Tupper et al. (1996) reported differences in yield response to K in eight genotypes, noting that earlier maturing cultivars required higher K rates to maximize yields. In contrast, Pettigrew et al. (1996), who also studied K deficiency of eight genotypes varying in maturity, showed that genotype 3 K interaction for lint yield was due to the lack of response in just one variety (HS-26) with poor local adaptation, not by maturity differences. Genotypic maturity did not appear to be associated with extent of yield reduction due to K, and their study offered no support for the hypothesis that “fast fruiting,” early maturing varieties are more susceptible to K deficiency. Though K interactions may be due to particular genotypic characteristics or environmental adaptations (Pettigrew et al., 1996), it is also important to consider

Reproduced from Agronomy Journal. Published by American Society of Agronomy. All copyrights reserved.

ABSTRACT Potassium nutrition requirements may differ in cotton (Gossypium hirsutum L.) cultivars that contrast in maturity and growth habit. Our objectives were to determine the effects of K fertility on carbohydrate and biomass partitioning, earliness, and lint yields of two contrasting cultivars. Fertilizer rates of 56 and 112 kg K ha21 yr21, representing 13 and 23 recommended K rates, were applied to long-term K fertility plots on a no-tilled Loring silt loam (thermic Oxyaquic Fragiudalf) in Jackson, TN. Plant samples were harvested at early bloom and cutout, to evaluate carbohydrate and dry matter partitioning during boll filling. Earliness was determined as the percentage of total yield at the first of two harvests. The more determinate cultivar, ‘Paymaster PM1218 BG/RR’, had higher aboveground dry weight, main stem starch concentrations and percentage first harvest, representing earlier maturity than the more indeterminate ‘Deltapine DP555 BG/RR’. The 23 K rate delayed maturity relative to the 13 rate, reflected in biomass partitioning and lint yield distribution. Lint yields were lower at the 13 than at the 23 K in the more determinate cultivar, but K did not affect the indeterminate yields. The two cultivars had equivalent lint yields at the 23 K rate, but the indeterminate cultivar produced more vegetative biomass, further delaying its maturity. Results suggest that an earlier, more determinate cultivar may require more K fertilization for optimal yield response with no tillage.

P

is an essential nutrient for reproductive development in cotton, partly due to its role in transport of carbohydrates to developing bolls. Studies on K deficiency have shown reductions in translocation of photoassimilates to bolls (Ashley and Goodson, 1972), resulting in decreased lint yields (Bennett et al., 1965; Cassman et al., 1989; Pettigrew, 1999). However, since the classical study of Bennett et al. (1965), few studies have been published on effects of higher than recommended rates of K fertilization on lint production. Improvements in cultivars and production technology over the past 40 yr have elevated yield potential, along with K requirements. Cultivar differences in growth habit and maturity may affect their K response, due to K effects on carbohydrate transport (Ashley and Goodson, 1972). After first bloom, earlier and more determinate cultivars generally partition a greater proportion of photosynthate to fruit growth than to new vegetation, compared with more indeterminate types (Bange and Milroy, 2004). This shift tends to lower the photosynthetic source/sink ratio in late season, possibly increasing the role of reserve carbohydrates in boll filling (Constable and Rawson, 1980). OTASSIUM

Plant Sciences Dep., Univ. of Tennessee, 605 Airways Blvd., Jackson, TN 38301. This research was sponsored in part by PPI/FAR Project TN-19F. Received 19 Oct. 2006. *Corresponding author (jclemen3@ tamu.edu). Published in Agron. J. 99:1130–1136 (2007). Cotton doi:10.2134/agronj2006.0288 ª American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: BG, Bollgard; DAP, days after planting; DP, Deltapine; PM, Paymaster; RR, Roundup Ready.

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CLEMENT-BAILEY & GWATHMEY: POTASSIUM EFFECTS ON COTTON CULTIVARS

the tillage system used in planting. Howard et al. (1997) compared ‘DP50’ response to K fertilization in conventional and no-tilled soils with high extractable K. They obtained significant yield response to broadcast K fertilizer in more site-years with no tillage than with conventional tillage, but a higher K rate was required to elicit a yield response with no tillage. They speculated that K uptake efficiency may have been impaired by the lack of incorporation with no tillage. Varco (2000) also speculated that cotton uptake of K fertilizer applied to no-tilled ground may be less efficient than when fertilizer is incorporated, due to surface stratification. He studied a no-tilled high-cation exchange capacity silty clay loam in which the majority of exchangeable K was stratified in the top 5 cm of soil. He suggested that higher rates of K fertilization may be required to optimize fertilizer use efficiency with no tillage relative to conventional tillage practices. Despite these observations, most K fertilizer recommendations for cotton production do not differentiate between conventional and no-tilled soils (e.g., Savoy and Joines, 2001). Our objectives were to investigate effects of K fertility on carbohydrate and biomass partitioning, and on lint yield of two contemporary cultivars with contrasting earliness and growth habits. In light of recent trends of increasing K fertilization rates of cotton in the midSouth and Southeast U.S. cotton belt (Snyder et al., 2005), we compared nominally adequate (13 recommended) and above-adequate (23 recommended) K fertilization applied with no tillage. We hypothesized that additional K would (i) promote remobilization of stored carbohydrates during boll filling, especially in the more determinate cultivar; (ii) increase partitioning to reproductive structures relative to vegetative growth, especially in the more determinate cultivar; (iii) increase lint yields more in the determinate than the indeterminate cultivar; and (iv) delay maturity more in the indeterminate cultivar. MATERIALS AND METHODS Field Study A 3-yr field experiment was conducted in long-term K fertility plots on a no-tilled Loring silt loam at the West Tennessee Education and Research Center, Jackson, TN. The cultivars, Paymaster PM1218BG/RR, (PM1218) and Deltapine DP555BG/RR, (DP555) were evaluated under two levels of K fertilization (56 and 112 kg K ha21 yr21) (Gwathmey, 2005). These two cultivars were selected to represent relatively determinate and indeterminate growth habits, respectively. They are higher yielding than older cultivars (Camberato and Jones, 2005) and had similar lint yields in state variety trials (Gwathmey et al., 2003). Characteristics of the two cultivars were described by Albers and Williams (1999) and Lege´ and Leske (2003) when commercially released. Plots containing cultivars and K treatments were arranged in a randomized complete block design with six replications. In the winter prior, soil samples were collected from all plots at 0- to 15-cm depth to determine residual (carryover) K fertility using Mehlich I extraction (Hanlon, 2001) performed by University of Tennessee Soil and Forage Test Laboratory, Nashville, TN. Potassium chloride fertilizer treatments were applied to designated long-term fertility plots in March or April each year, at

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the same rates as had been applied annually for three or more years before the present study. The plot area had been in continuous no-tillage cotton for 10 yr before the present 3-yr study, and uniformly managed according to Agricultural Extension Service (2001) guidelines except for K fertilization. Cultivars were planted in 9.15-m rows in 4-row plots with a JohnDeere MaxEmerge no-till planter equipped with Almaco cone units on 30 Apr. 2003, 5 May 2004, and 5 May 2005. The two middle rows of each plot were hand-thinned for uniformity by the third true leaf stage, and were used for data collection and sampling. Final populations ranged by year from 7.5 to 12 plants m22. Agricultural Extension Service (2001) guidelines for growing Bollgard (Bt)/Roundup Ready (glyphosate resistant) cotton with no tillage were used in crop management. No supplemental irrigation was applied in 2003. In 2004, 1.1 cm of irrigation was applied by sprinkler boom at 104 days after planting (DAP). In 2005, a total of 5.7 cm of irrigation was sprinkler applied between 18 and 102 DAP. Plants were sampled at early bloom, when boll set began; and at cutout, the cessation of flowering. These samples were taken to determine carbohydrate and biomass partitioning during boll filling. The two sampling dates were treated as subplots in the experimental design. Plants were sampled around solar noon on 89 and 118 DAP in 2003, at 69 and 112 DAP in 2004, and 74 and 109 DAP in 2005. The sampling dates were selected to bracket the period of active boll filling. Variations in first bloom sampling dates were due to differences in first bloom dates between years. Eight consecutive plants from a row were cut at the soil level and a 2-cm sample of stem tissue below the cotyledonary node was cut and immediately placed on dry ice. The samples were then lyophilized and ground through a 20-mesh screen with a Wiley-type mill. The ground tissue was stored with desiccant until carbohydrate analysis could be performed. The remaining aboveground plant was dissected into vegetative and reproductive parts and dried at 60jC for 72 h before weighing for biomass partitioning analysis. Plots were mechanically harvested twice each year for yield, and earliness was measured as the percentage of total yield picked at first harvest. Seedcotton samples from each plot were weighed and ginned as described by Gwathmey et al. (2003) to calculate lint yields.

Carbohydrate Analysis Glucose, fructose, sucrose, and starch concentrations were each determined colorimetrically by methods developed by Hendrix (1993). One hundred mg of ground stem tissue from each plant sample was subjected to a series of three hot ethanolic washes to extract the soluble sugars. The ethanolic extract was then decanted and brought to a volume of 10 mL, and a 1.5-mL aliquot was purified with 20 mg of activated charcoal, and centrifuged. Two 20-mL subsamples were placed in separate wells of a microplate and dried. The sugars were resolubilized with 20 mL of water. Glucose assay reagent (glucose kit GAHK-20, Sigma-Aldrich, St. Louis, MO), phosphoglucose isomerase (Sigma P-9544), and invertase (Sigma I-4504) were sequentially added to each well, and absorbances were read on a microplate reader (Multiskan MCC/340, Thermo Labsystems, Helsinki, Finland) at 340 nm, 15 mn after each enzyme was added. Increases in absorbance were attributed to glucose, fructose, and sucrose, respectively. Starch analysis was performed on the pellet remaining after soluble sugar extraction. It was treated with 1 mL of 0.1 M KOH and placed in a hot water bath to gelatinize the starch. Once cooled, pH was lowered and a-amylase (Sigma A-3403) was added and placed in an 80jC bath for 30 min. Amyloglucosidase (Sigma A-3042) was then added and placed in a

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AGRONOMY JOURNAL, VOL. 99, JULY–AUGUST 2007

55jC bath for 1 h. To stop all reactions, the samples were placed in boiling water for 4 min. Aliquots were taken, centrifuged, and placed in wells for analysis. Glucose assay reagent was added and absorbance read at 340 nm as described above. The absorbances were compared to standard glucose curves to determine the glucose concentrations.


Statistical Analysis Data were analyzed using the Mixed Procedure in SAS 9.0 and means were separated using pairwise Tukey comparisons at P 5 0.05. Years, treatments, and sampling dates were fixed effects with replications being treated as random effects. The year effects were tested with reps within years; sampling dates were treated as subplots nested within the whole plots of cultivar and K rates. A polynomial regression was used to compare total lint yields to the change in stem starch.

RESULTS AND DISCUSSION Potassium Fertility The soil test K (Table 1) was an indicator for the amount of K fertilization needed according to the Mehlich I test. When soil K concentrations are high (180–358 kg K ha21), application of 56 kg K ha21 was recommended for cotton production in Tennessee (Savoy and Joines, 2001). For soil test K levels above 358 kg K ha21, no K fertilization was recommended (Savoy and Joines, 2001). By these criteria, study plots receiving 56 kg K ha21 yr21 had recommended rates (13) of K fertilization, but plots receiving 112 kg K ha21 yr21 had twice the recommended rate (23) of K fertilization, according to soil test K results (Table 1).

Carbohydrate Partitioning Analysis of variance of the fixed effects on monosaccharides, sucrose, and starch concentrations are summarized in Table 2. Cultivars had significant effects on all nonstructural carbohydrates, whereas K fertilization had significant effects only on the monosaccharides. The sampling date had significant (P , 0.05) effects on all carbohydrates except glucose (P 5 0.056). The year 3 cultivar 3 date interaction was significant only in the soluble sugars—glucose, fructose and sucrose. Across years, monosaccharide concentrations in stem tissue were lower (4.2 g kg21 dwt) at 112 kg K ha21 yr21 Table 1. Potassium fertility in test plots relative to extension guidelines for cotton fertilization in Tennessee, 2003–2005. Year

Soil test K 21

2003 2004 2005

kg K ha 230 337 245 444 198 343

21

Fertilization treatments 21

21

kg K ha yr 56 112 56 112 56 112

Extension recommendations kg K ha 56 56 56 0 56 56

K fertilization in test plots

Table 2. P values from analysis of variance of fixed effects on soluble sugars and starch concentrations in cotton stem tissue. Source of variance

Glucose

Year Cultivar Year 3 cultivar K Year 3 K Cultivar 3 K Year 3 cultivar 3 K Date Year 3 date Cultivar 3 date Year 3 cultivar 3 date K 3 date Year 3 K 3 date Cultivar 3 K 3 date Year 3 cultivar 3 K 3 date

0.018 ,0.001 0.426 ,0.001 0.357 0.548 0.041 0.056 ,0.001 0.408 ,0.001 0.172 0.231 0.085 0.975

Starch

P.F ,0.001 0.163 0.241 0.003 0.458 0.002 0.037 0.304 0.014 0.220 0.675 0.331 0.249 0.053 ,0.001 ,0.001 ,0.001 ,0.001 0.382 0.189 0.002 0.001 0.581 0.232 0.335 0.601 0.045 0.020 0.940 0.688

,0.001 ,0.001 0.021 0.285 0.652 0.869 0.718 ,0.001 ,0.001 ,0.001 0.314 0.042 0.378 0.915 0.701

Table 3. Glucose and fructose concentrations in cotton stem tissue at two K fertilization rates, averaged across cultivars, 2003–2005. Year

K rate 21

is 13 extension recommendations for soil test of 180– † 56 kg K ha 21 358 kg K ha (Savoy and Joines, 2001). 21 ‡ 112 kg K ha is 23 extension recommendations for soil test of 180– 21 358 kg K ha (Savoy and Joines, 2001).

Sucrose

when compared with 56 kg K ha21 yr21 (4.9 g kg21 dwt) (Table 3). These data may indicate that more cellulose synthesis occurred in stem tissue with 23 K fertility. The significant year 3 K interaction was due to K rates significantly influencing fructose concentrations only in 2003 (Table 3). There was no significant response to K in sucrose or starch concentrations (Table 2). There was a significant year 3 treatment interaction in the 3-yr total soluble sugars. When 2003 and 2004 were analyzed separately from 2005, there was no significant year 3 treatment interaction. Therefore, sugar concentration data for 2003 and 2004 were combined and shown separately from 2005 in Fig. 1. Across years and K rates, the more indeterminate cultivar, DP555, had higher concentrations of soluble sugars at early bloom (48.7 g kg21 dwt) compared with PM1218 (43.7 g kg21 dwt) (Fig. 1), whereas PM1218 had higher concentrations of starch in stem tissue (71.3 g kg 21 dwt) compared with DP555 (37.9 g kg21 dwt) (Fig. 2). By cutout, however, both cultivars had equivalent concentrations of total soluble sugars and starch. The soluble sugars decreased in both cultivars between early bloom and cutout, except in 2005, when concentrations remained the same in PM1218, but accumulated in DP555. There was a significant year 3 cultivar interaction in starch (Table 2). The analysis showed the 2003 starch concentrations to be different than 2004 and 2005. In 2003, starch concentrations of both cultivars declined during boll filling, whereas in 2004 and 2005 the con-

21

13† 23‡ 13 .23 13 23

Fructose

– – 2003 2003 2004 2004 2005 2005

kg K ha yr 56 112 56 112 56 112 56 112

Glucose 21

Fructose 21

mg g

dry wt.

4.9a† 4.2b

4.4a 4.1b

5.3a 4.5bc 5.2ab 4.2c 4.4bc 3.9c

4.6b 3.5c 2.6d 2.3d 6.2a 6.4a

† Within groups, means followed by the same letter do not differ at P 5 0.05.

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a

ab bc

cd

40

d

d

d

20

0

t

u uto

B C C Fig. 1. Soluble sugar concentrations in stem tissue of two cotton cultivars at two growth stages in 2003–2004 and in 2005. Within year or year-group, means with the same letter do not differ significantly at P 5 0.05. B

centrations accumulated (Fig. 2). The starch concentrations are consistent with observations by Wells (2002) and Viator et al. (2005), who also found year 3 cultivar interactions. Differences between years in the accumulation and depletion of starch may be due to variations in sampling date and possibly environmental conditions between years. The relationship between lint yield and the accumulation or depletion of starch reserves across years, K rates, and cultivars were examined by polynomial regression (Fig. 3). The significant polynomial regression indicates that as lint yields increased, starch reserves were not depleted as hypothesized, but rather accumulated. Within each year, there was no significant correlation of lint yield with change in stem starch between early bloom and cutout. The results suggest that stem starch reserves tended to increase in years with higher lint yields, and to decrease in a year (2003) with lower lint yields. Another possible factor that may have influenced the 2003 starch was excessive rainfall occurred immediately after planting (Fig. 4), which delayed plant growth and development. 140

2003

120

2000

1600

-100

-50

0

50

100

150

-1

Change in Stem Starch (mg g dwt) Fig. 3. Change in stem starch from early bloom to cutout relative to lint yield formation in two cultivars in 2003, 2004, and 2005.

We speculate that the amount of starch present in the lower stem of the two cultivars in this study may be directly related to their root systems. Whereas PM1218 may store starch in the lower stem due to less root growth, DP555’s sucrose levels may be an indication of movement to the roots for starch storage or continued root growth at early bloom. Halevy (1976) found that a more determinate cultivar had decreased root/shoot ratio relative to an indeterminate cultivar. Brouder and Cassman (1990) found that root extension after peak bloom was higher in a K-efficient cultivar, giving it more 50 2003 2004 2005

40 30 20 10 0

2004/2005

Paymaster PM1218 BG/RR Deltapine DP555 BG/RR

2003 2004 2005 2 y=1642 + 2.62x + 0.028x

2400

1200 -150

t

m loo

u uto

Lint Yield (kg ha-1)

60 cd

2800

2005

Paymaster PM1218BG/RR Deltapine DP555BG/RR

Rainfall + Irrigation (cm)

2003-04

a

100

a

80

a

b

60

c c

b

40 c

20 0

t t m tou tou oo Bl Cu Cu Fig. 2. Starch concentrations in stem tissue of two cotton cultivars at two growth stages in 2003, and in 2004 –2005. Within year or year-group, means with the same letter do not differ significantly at P 5 0.05. m

oo

Bl

Accumulated Degree Days (base 15.6°C)

Total soluble sugar (mg g-1 dry wt)

80

m loo

Starch (mg g-1 dry wt)



600

400

200

0 Early bloomPlantingEarly bloom Cutout

CutoutDefoliation

Fig. 4. Rainfall 1 irrigation and accumulated degree days (base 15.6°C) for three periods of plant growth and development in 2003, 2004, and 2005.

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Table 4. P values from analysis of variance of fixed effects on total aboveground dry weight, percentage reproductive dry weight, total lint yield, and percentage first harvest. Source of variance

Total dry wt.

Percentage reproductive dry wt.

Year Cultivar Year 3 cultivar K Year 3 K Cultivar 3 K Year 3 cultivar 3 K Date Year 3 date Cultivar 3 date Year 3 cultivar 3 date K 3 date Year 3 K 3 date Cultivar 3 K 3 date Year 3 cultivar 3 K 3 date

0.385 0.006 0.672 0.782 0.737 0.686 0.723 ,0.001 0.003 0.925 0.374 0.818 0.614 0.916 0.927

,0.001 ,0.001 0.039 0.005 0.294 0.234 0.498 ,0.001 ,0.001 0.018 0.510 0.296 0.655 0.383 0.800

Total lint yield

Percentage first harvest

Pr . F ,0.001 0.001 0.001 0.002 0.372 0.004 0.111 – – – – – – – –

0.308 ,0.001 0.006 0.037 0.557 0.328 0.890 – – – – – – – –

root surface area than a less efficient cultivar, indicating that K response differences were associated with differences in determinacy of root growth.

Biomass Partitioning, Earliness, and Lint Yield Significance levels indicated by analyses of variance of fixed effects on biomass, lint yields, and earliness of maturity are shown in Table 4. Potassium influenced percentage dry weight in reproductive organs, lint yield, and earliness, whereas cultivars had significant effects on biomass partitioning, lint yields, and earliness. Across sampling dates and K rates, total aboveground biomass was significantly higher in the more determinate culti-

var, PM1218, with 69.7 g plant21 compared with DP555’s 60.4 g plant21 (Table 5). The more determinate cultivar was also higher in percentage reproductive dry weight than was DP555, implying less vegetative growth after first bloom (Table 5), consistent with its determinacy. Total aboveground biomass was not affected by K, but it significantly affected percentage reproductive biomass. Across sampling dates and K rates, the 23 rate, 112 kg K ha21 yr21, decreased percentage reproductive dry weight from 31.7 to 28.7% in the indeterminate cultivar (Table 5), indicating delayed maturity. Results confirm Gwathmey (2005) findings that additional K resulted in more aboveground biomass partitioned to vegetative parts in DP555 than in the more determinate cultivar. Results are also consistent with Pettigrew et al. (2005), who found that K deficiency increased the proportion of dry matter in reproductive organs slightly by cutout, although total aboveground dry weight was not affected by K. However, the present study also revealed a difference in partitioning response to K between the two cultivars. Tennessee cotton variety trials have shown that the more determinate cultivar, PM1218, matures earlier than indeterminate DP555 (Gwathmey et al., 2003). The earliness difference is expected since a more determinate cultivar produces less vegetative biomass and fewer new fruiting sites after first bloom; it partitions more to reproductive structures. This maturity difference predicted by biomass partitioning during boll filling was also observed in the percentage first harvest of this study (Table 6). Potassium had significant effects on earliness, showing a delay in maturity at the 112 kg K ha21 rate, 79.9% compared with 84.4% at the 56 kg K ha21 rate.

Table 5. Total aboveground dry weight and percentage reproductive dry weight of two cultivars with two K fertilization treatments sampled at early bloom and cutout, in 2003–2005. Year

Cultivar

K rate 21

2003 2004 2005

– – –

kg K ha – – –

yr

Sampling date 21

Total dry wt. 21

Reproductive dry wt.

– – –

g plant 61.8a† 66.7a 66.7a

% 41.4a 30.6b 28.3b

– –

PM1218BG/RR DP555BG/RR

– –

– –

69.7a 60.4b

36.8a 30.2b

2003 2003 2004 2004 2005 2005

PM1218BG/RR DP555BG/RR PM1218BG/RR DP555BG/RR PM1218BG/RR DP555BG/RR

– – – – – –

– – – – – –

66.5a 57.0a 69.4a 63.8a 72.9a 60.3a

46.1a 36.8b 33.5bc 28.2de 30.9cd 25.6e

– –

– –

56 112

– –

64.6a 65.5a

34.6a 32.5b

– – – –

PM1218BG/RR PM1218BG/RR DP555BG/RR DP555BG/RR

56 112 56 112

– – – –

68.6a 70.7a 60.6a 60.1a

37.5a 36.2a 31.7b 28.7c

– –

– –

– –

early bloom cutout

37.8b 92.3a

12.3b 54.7a

– – – –


– –

early bloom cutout early bloom cutout

42.5b 96.8a 33.0b 87.8a

15.1c 58.7a 9.5d 50.8b

–

† Within groups, means followed by the same letter do not differ at P 5 0.05.


Table 6. Percentage first harvest and total lint yields of two cultivars with K fertilization treatments in 2003–2005. Year

Cultivar

K rate 21


2003 2004 2005

– – –

kg K ha – – –

yr

21

First harvest

Total lint yield

% 78.9a† 83.1a 84.5a

kg ha 1591b 2170a 1694b

21

– –

PM1218BG/RR DP555BG/RR

– –

88.7a 75.6b

1874a 1762b

2003 2003 2004 2004 2005 2005

PM1218BG/RR DP555BG/RR PM1218BG/RR DP555BG/RR PM1218BG/RR DP555BG/RR

– – – – – –

90.0ab 67.8c 88.9a 77.3bc 87.2ab 81.8ab

1609d 1571d 2055b 2285a 1767c 1621d

– –

– –

56 112

84.4a 79.9b

1773b 1863a

– – – –


56 112 56 112

89.9a 87.5a 78.8b 72.4b

1676b 1846a 1869a 1877a

† Within groups, means followed by the same letter do not differ at p 5 0.05.

Gwathmey and Howard (1998) also observed that additional K delayed maturity, but their study compared deficient and adequate K rates only. Pettigrew et al. (2005) found that K deficiency resulted in earlier maturity in only two of nine genotypes examined. Growth of their two genotypes was not characterized, but one of the two cultivars (FM832) was relatively late maturing while the other (SG747) was relatively early. Potassium and cultivars had significant effects on lint yields, and there was a significant cultivar 3 K interaction (Table 4). The total lint yield showed a significant response to additional K only in the more determinate cultivar, PM1218, which had lower yields than DP555 at the 56 kg K ha21 rate (1676 and 1869 kg ha21, respectively) (Table 6). The two cultivars produced similar yields at the higher K rate. This observation indicates that the indeterminate cultivar produced higher yields with limited K fertility than the more determinate cultivar. This result is consistent with Pettigrew et al. (2005), who found that lint yield decreased only in one genotype (PM1218) under K deficiency, unlike eight other genotypes representing a range of maturity and growth characteristics in which K did not affect yield. These authors offered no explanation for the PM1218 response in their 3-yr study. Our results are also consistent with Tupper et al. (1996), who concluded that earlier maturing cultivars require higher rates of fertilizer K to maximize yield. We speculate that two factors may have contributed to the differences in cultivar yield response to additional K in the present study. One may have been the lower photosynthetic source/sink ratio at cutout in the more determinate PM1218, which placed increasing demand on limited plant K to support remobilization and transport of storage carbohydrates to developing bolls. In addition, the reproductive sink strength of PM1218 may have been stronger than that of DP555 due to its higher seed oil content (USDA, 2003). Another factor may involve limitations on root K uptake due to a more deter-

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minate root growth habit in PM1218 grown with no tillage. Root growth was not evaluated in this study, but cultivar differences in root growth found by Brouder and Cassman (1990) indicated that K response differences were associated with differences in determinacy of root growth. Such differences may be amplified to the extent that root growth was impeded by long-term notillage methods. Varco (2000) speculated that cotton uptake of K fertilizer applied to no-tilled ground may be less efficient than when fertilizer is incorporated, due to surface stratification. He suggested that higher rates of K fertilization may be required to optimize fertilizer use efficiency with no tillage than with conventional tillage practices. Consistent with Essington et al. (2002), these results suggest that a higher K fertilizer rate than currently recommended (Savoy and Joines, 2001) is needed to optimize lint production in the more determinate cultivar under no tilled conditions. In summary, additional K did not promote remobilization of stored starch reserves in either cultivar. Carbohydrate partitioning differences between cultivars were seen at the start of reproductive development when the more determinate cultivar had accumulated more starch in stem tissue, while the more indeterminate cultivar had higher sucrose concentration. These differences had diminished by cutout, however, with no evidence of greater remobilization of stored starch in the more determinate cultivar. Starch dynamics seemed to be more influenced by year than by cultivar or K. The more determinate cultivar did benefit from the additional K, which increased total lint yields; however, the percentage reproductive dry weight was not affected by additional K in the determinate cultivar, while it decreased in the more indeterminate cultivar. This result implies a cultivar difference in biomass partitioning response to additional K, as the vegetative growth response of the more indeterminate cultivar further delayed its maturity. Widespread planting of early maturing, determinate cultivars with no-tillage indicates that further research is needed to optimize their K nutrition. ACKNOWLEDGMENTS The authors gratefully acknowledge the assistance of Dr. Arnold Saxton for statistical support, Carl Michaud for his aid in field work, and Dr. Don Howard for his review of the manuscript.

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