the number of squares floral buds with bracts retained by the plants. Therefore, a better under-
standing of the initiation, differentiation and devel- opment of cotton squares, and the characteristics
of their physiology and biochemistry during devel- opment are important for improving cotton man-
agement.
The development of cotton squares is a funda- mental step of fruit development. However, most
studies on squares have emphasized the relation between morphology and abscission McMichael
and Guinn, 1980; Ungar et al., 1989. We know much less about the physiology of square forma-
tion and growth than any other part of the cotton fruiting cycle Stewart, 1986. Since only limited
studies have been carried out on the physiology of square development, there are many phenomena
and questions about cotton fruit development that are not completely understood. For instance, why
do cotton plants shed young squares and young bolls, but not white flowers? Why is the abscission
of small squares greater than that of large squares? How do the carbohydrate and mineral nutrient
profiles of squares change during development? Are there differences in carbohydrate concentrations in
floral buds and bracts located at different fruiting positions?
The endogenous hormone balance, photosyn- thetic assimilate supply, and mineral nutrient status
of plants are major physiological factors con- trolling fruit abscission Benedict, 1984. Environ-
mental factors inducing fruit abscission include adverse photosynthetic photon flux density, tem-
perature, moisture, and nutrition Guinn, 1974, 1982. McMichael and Guinn 1980 suggested that
the sensitivity of cotton square abscission to water stress was greatest during the first week after the
squares become visible. Later studies showed that squares smaller than 1 cm in size were more
sensitive to stress environments inducing abscission than the squares larger than 1 cm in size Guinn,
1982; Ungar et al., 1989.
Development of young squares into flowers is an important process of yield development. However,
squares often fail to form flowers. If this failure occurs with too many squares abscised, crop yield
can be reduced and maturity can be delayed. The hypothesis of this study is that the non-structural
carbohydrate contents of floral buds may be used to explain the differences in the boll retention at
different main-stem nodes and fruiting positions. Since the physiological reasons for that failure are
not well understood and because there is little information on this topic, the objective of this
research was to characterize leaf and square carbo- hydrate concentrations as a function of square age,
sympodial branch position i.e. proximal vs. distal and vertical main-stem node i.e. Node 8, 10 or 12
within the canopy.
2. Materials and methods
2
.
1
. Plant culture The cotton cultivar Deltapine 20 was machine-
planted on a moderately well-drained Captina Typic Fragiudult silt loam soil at the Arkansas
Agricultural Research and Extension Center, Uni- versity of Arkansas in Fayetteville, Arkansas, USA
on 4 June 1993 and 17 May 1994. Preplant fertilizer was applied at a rate of 45-30-75 kg N-P-K ha
− 1
, and an additional side-dressing of 56 kg N ha
− 1
as ammonium nitrate was applied on 13 July 1993 and
28 June 1994 early square stage. Control of insects and weeds and furrow irrigation were performed as
needed during the growing seasons to minimize plant stress and optimize yield. The field was
divided into three blocks replications, each with two plots for sampling and for determining final
boll retention and yield parameters at different fruiting positions. Each plot consisted of ten rows
spaced 1 m apart 15 m in length and oriented in a north – south direction. Plots were hand-thinned
to a density of nine plants m
− 1
row when the seedlings had three true leaves.
The samples of leaves and squares for non-struc- tural carbohydrate analysis were collected from the
three plots. The squares were obtained from Branch Positions 1, 2 and 3 within a selected sympodium
fruiting branch Fig. 1. The morphological devel- opment of squares and the analysis of non-struc-
tural carbohydrates focused on Branch Positions 1, 2 and 3 of Node 8 and Branch Positions 1 and 2
of Nodes 10 and 12 in 1993, and Branch Positions 1, 2 and 3 of Node 10 in 1994.
2
.
2
. Sample collection When the squares at the selected positions first
became visible about 3 mm in diameter, they were individually labeled by tagging main-stem
leaves at the respective nodes with dated jewelers tags. Approximately 900 squares at each position
from three replications were labeled on the same day. Squares at tagging were considered day 0 in
age. During development of the squares, 30 tagged squares were randomly collected at 900 h
central daylight saving time CDST on each sam- pling date at 5, 10, 15, 20, or 25 days of age until
the squares developed into white flowers. Addi- tionally, in 1994 the sympodial leaves subtending
Positions 1, 2 and 3 along the sympodium at Node 10 were collected simultaneously with
square sampling. The dates of tagging and sam- pling squares at each node and sympodial branch
position are shown in Table 1.
The fresh-tissue samples were placed into plas- tic bags and taken to the laboratory on ice.
Squares were separated into bracts and floral buds. The floral buds, bracts, and leaves were
dried at 90°C for 30 min, followed by 72 h at 70°C in a forced-draft oven before weighing. The
entire process from sample collection until initial drying was completed within 2 h to minimize
error in measured sugars. The dried tissues were ground and passed through a 0.5-mm sieve for
analysis of non-structural carbohydrates hexoses, sucrose and starch. In this study, hexose concen-
tration represents the sum of glucose and fructose concentrations.
2
.
3
. Non-structural carbohydrate measurements A modification of the method of Hendrix
1993 was used to extract and quantify the con- centrations of non-structural carbohydrates in the
leaves, bracts, and floral buds. Details of entire processes of non-structural carbohydrate extrac-
tion and measurement are the same as previously described Zhao and Oosterhuis, 1998. The sum
of hexoses, sucrose and starch was defined as total non-structural carbohydrates TNC.
2
.
4
. Data analysis The dry weight DW accumulations of leaves,
bracts and floral buds, as well as patterns of changes in non-structural carbohydrate concen-
trations of these tissues, were plotted as a function of square age. All data were analyzed by analysis
of variance with the ANOVA procedures of the statistical analysis system SAS Inst., Cary, NC.
Fig. 1. Diagrammatic representation of a cotton plant and detailed inset of a sympodial branch. The cotton plant left shows the main-stem nodes, one monopodial and 11 sympodial branches. The main-stem nodes are numbered starting from cotyledonal node
Node 0, and the positions sampled in the plant canopy are shown. Asterisks and arrows show the specific sampling positions in 1993 and 1994, respectively. The sympodial branch at main-stem Node 10 right shows the relative size and positions of a white
flower Position 1 and two squares Positions 2 and 3, the main-stem leaf, and the subtending sympodial leaves.
Table 1 The dates of tagging squares at different main-stem nodes and fruiting positions, sampling times, and heat units accumulated during
the period of square development in 1993 and 1994 Sampling monthday
Tagging monthday Heat units
b
°C Node-position
a
5 Days 10 Days
15 Days 20 Days
25 Days Flower
1993 723
728 82
87 812
813 317
8-1 718
730 84
89 814
725 819
8-2 819
311 731
8-3 85
810 815
820 825
825 311
728 82
87 812
10-1 817
723 817
309 83
88 813
818 729
823 10-2
823 314
728 12-1
82 87
812 817
– 821
301 88
12-2 813
83 818
823 828
828 308
1994 10-1
76 711
716 721
726 731
83 273
718 723
728 10-2
82 713
87 810
265 723
728 82
87 718
812 10-3
815 266
a
The two numbers represent main-stem node and sympodial branch position, respectively.
b
Daily heat units °C = [T
max
+ T
min
2]−15°C, where T
max
and T
min
are the maximum and minimum daily temperatures in degrees Celsius, respectively.
Data of Branch Positions 1 and 2 at Node 10 for each year and non-structural carbohydrates of the
three tissues of leaves, bracts, and floral buds in 1994 were also performed with the ANOVA to
determine differences between years and among tissues. Correlation coefficients between final boll
retention and floral-bud non-structural carbohy- drate content at different positions were also cal-
culated. The analyses of variance combined over the 2 years of this study indicated significant
differences between years for non-structural car- bohydrates of the bracts and floral buds. There-
fore, data were interpreted individually for each year. When the F-value was significant at P 5
0.05, the LSD test P = 0.05 was performed.
3. Results
