Cotton Squares Research Articles

Sublethal effects of malathion on boll weevil (Coleoptera: Curculionidae) fecundity when maintained on cotton squares or artificial diet

AbstractMated 3‐day‐old female boll weevils, Anthonomus grandis grandis Boheman, reared from field‐infested cotton (Gossypium hirsutum L.) squares were topically treated with an estimated LD50 of malathion (2 μg) to assess its effects on fecundity, oviposition, and body fat condition. Two different food sources, cotton squares and artificial diet, were assessed in malathion‐treated and nontreated (control) weevils. The LD50 caused ∼50% mortality in the square‐fed malathion treatment, but the artificial diet‐fed malathion‐treated weevils were less susceptible. LD50 survivors fed on the squares produced ≥ 9 times more chorionated eggs in the ovaries and oviposited ≥ 19‐fold more than survivors fed artificial diet, regardless of the malathion treatment. Boll weevils that survived a 2 μg LD50 malathion and also fed squares were ∼4.5‐fold leaner than diet‐fed weevils. Our findings demonstrate that non‐resistant boll weevils surviving a sublethal dose of malathion will reproduce without any delay or significant loss in fecundity, and the food source for which boll weevils are maintained when conducting these assays will directly affect the results. The significance of these findings and how they are related to the final stages of eradicating the boll weevil from the US are discussed.

Stable isotope tracer marking of individual boll weevils

Stable isotope markers have been used to study animal nutrition for several decades and more recently to study the foraging and cultural habits of imported fire ants. In this work, we have extended that effort to evaluate the potential for marking boll weevils, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), with the rare earth element samarium to aid in studies of insect invasion and pest eradication protocols. Neutron activation analysis (NAA) was performed on the marked boll weevils as well as plant material from the cotton squares on which the insects were fed. Samarium levels in non-dosed insects average about 20 ng/g or about 100 pg total element per insect. Our computed average determination limit was 36 pg samarium/weevil. The determination limit for cotton plant squares and leaves averaged 3.5 ng/g and 8.2 ng/g, respectively. These initial results indicate the NAA method is capable of identifying individual marked insects which have assimilated 1 ng of samarium, a ten-fold increase in content over average blank values.

Assessing nest building in mice

For small rodents, nests are important in heat conservation as well as reproduction and shelter. Nesting is easily measured in the home cages of mice, particularly with the advent of pressed cotton materials. The mice first shred the tightly packed material, then arrange it into a nest. Published studies have often used materials such as hay, twine or tissues, sometimes preshredded, and have assigned scores of the quality of the resulting nest with rather rudimentary rating scales; e.g., 0, no nest; 1, flat nest; 2, nest covering the mouse. The protocol described here uses pressed cotton squares and a definitive 5-point nest-rating scale. Any unshredded material left after a bout of nesting can also be weighed, providing a semi-independent objective assay of nesting ability. Nesting has been shown to be sensitive to brain lesions, pharmacological agents and genetic mutations. This is a simple, cheap and easily done test that, along with other tests of species-typical behavior, is a sensitive assay for identifying previously unknown behavioral phenotypes. The test needs to be done overnight, but it should take no more than 5 minutes to set up plus 1 minute to assess one nest and weigh the untorn residue.

An experiment using neutron activation analysis and a rare earth element to mark cotton plants and two insects that feed on them

Studies on insect dispersal and other behaviors can benefit from using markers that will not alter flight and fitness. Rare earth elements, such as samarium (Sm), have been used as ingested markers of some insects and detected using neutron activation analysis (NAA). In this study, samarium nitrate hexahydrate was mixed into artificial diet for boll weevils, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), at different dosages and in water used to irrigate cotton, Gossypium hirsutum L. Samarium was detected in adult boll weevils fed on the samarium-labeled diet, but not after 5 or 10 days of being switched to non-labeled diet, even if the insects were given labeled diet for as long as 7 consecutive days. Introduced in irrigation water, 1% samarium (m/m) was detectable in cotton squares and leaf tissue. However, boll weevil adults fed samarium-labeled squares did not retain detectable levels of samarium, nor did boll weevil adults reared to adulthood from samarium-labeled squares. Fourth instar beet armyworms, Spodoptera exigua (Hübner) (Noctuidae: Lepidoptera), fed on samarium-labeled cotton leaves obtained enough samarium for NAA detection, but adult moths reared from them did not have detectable amounts of samarium. Although samarium can be useful as a marker when insects are presented with a continuous pulse of the label, elements that are assimilated by the insect would be more useful if a continuous infusion of the marker cannot be provided.

Stage‐dependent feeding behavior by western tarnished plant bugs influences flower bud abscission in cotton

AbstractUnexplained variability in the relationship between the number of herbivores in a field and the amount of crop damage can arise if there is a large amount of variation among herbivore individuals in the amount of feeding damage each generates. In California, populations of the western tarnished plant bug, Lygus hesperus Knight (Heteroptera: Miridae), produce highly variable levels of damage to cotton plants (Gossypium hirsutum L.) (Malvaceae), even when found at low densities. Because L. hesperus populations are also highly variable in their overall stage structure, we hypothesize that differences in crop damage might result from varying impact by each L. hesperus stage on cotton flower buds (termed squares). Laboratory measurements of L. hesperus mouth‐parts and distance to anther sacs, a preferred feeding site, revealed that 1st−3rd instar L. hesperus nymphs will not be able to feed on anther sacs of larger squares (over 8 mm in length) but will be able to feed on squares that are most sensitive to L. hesperus damage (<7 mm). Because even the 1st instars can feed on the most sensitive ‘pinhead’ squares, size constraints do not rule out damaging effects from the youngest L. hesperus. Laboratory observations revealed that later developmental stages, and adults, spend more time feeding on cotton squares relative to 2nd and 3rd instars. In addition, a field experiment revealed no effect of 2nd instars on square retention (relative to control cages) but did reveal a significant decrease in square retention generated by adult L. hesperus (4th instar L. hesperus resulted in an intermediate level of square retention). In a final study we sampled L. hesperus stage structure and density across 38 cotton fields. Multiple regression revealed that the densities of 1st−3rd instars of L. hesperus are not correlated with anther sac damage or square retention. However, in 2 years 4th and 5th instars were positively correlated with anther sac damage and negatively correlated with square retention. In the a third year, adult L. hesperus showed correlations in the same direction, across fields and across sites within fields. Overall, these results suggest that the adults and the largest nymphs of L. hesperus (4th and 5th instars) are particularly damaging to cotton squares, with the 1st−3rd instars of L. hesperus causing little damage to plants.

Relationships of Different Cotton Square Sizes to Boll Weevil (Coleoptera: Curculionidae) Feeding and Oviposition in Field Conditions

Feeding and oviposition preferences of the boll weevil, Anthonomus grandis grandis Boheman, for four different cotton square size classes in field conditions of the Lower Rio Grande Valley of Texas were studied during 2002 and 2003. Percentages of large (5.5-8-mm-diameter) squares used for oviposition and feeding were greater than pinhead or match-head squares. The preference for large squares as food and associated accelerated fecundity explain the substantial boll weevil population buildups that occur after large squares form. Medium-sized (3-5.5-mm-diameter) squares also were used but less than large squares. Feeding and oviposition on pinhead (1-2-mm-diameter) and match-head (2-3-mm-diameter) squares were negligible. Although planting date did not affect oviposition or feeding preferences for squares larger than pinhead and match-head sizes, the least amount of either damage to large squares was found in the earliest plantings during both years. This study indicates that pinhead and match-head squares, regardless of planting date, do not require pesticide applications to protect against boll weevil feeding and oviposition.

Relationships of Different Cotton Square Sizes to Boll Weevil (Coleoptera: Curculionidae) Feeding and Oviposition in Field Conditions

Relationships of Different Cotton Square Sizes to Boll Weevil (Coleoptera: Curculionidae) Feeding and Oviposition in Field Conditions

Physiological causes of cotton fruit abscission under conditions of high temperature and enhanced ultraviolet‐B radiation

An experiment was conducted in sunlit controlled environment growth chambers to determine the physiological mechanisms of fruit abscission of cotton (Gossypium hirsutum L. cv. NuCOTN 33B) grown in high temperature and enhanced ultraviolet (UV)‐B radiation. Six treatments included two levels of optimum (30/22°C) and high (36/28°C) day/night temperatures and three levels of biologically effective UV‐B radiation (0, 7, and 14 kJ m−2 per day). Both the temperature and UV‐B treatments were imposed from seedling emergence through 79 days after emergence (DAE). High temperature did not negatively affect either leaf net photosynthetic rates (Pn) or abscission of cotton squares (floral buds with bracts) but significantly decreased boll retention. Plants exposed to 7 kJ UV‐B radiation retained 56% less bolls than the 0 kJ UV‐B control plants at 79 DAE, despite no significant differences in leaf Pn measured at squaring and flowering. At 53 DAE, leaf Pn of plants grown in high UV‐B radiation (14 kJ m−2 per day) decreased by 11%, whereas total non‐structural carbohydrate (TNC) concentrations in the leaves, floral buds, and young bolls decreased by 34, 32, and 20%, respectively, compared with the control plants. The high UV‐B radiation significantly increased square abscission. Square abscission was not related to leaf TNC concentration but closely correlated with TNC in floral buds (r = −0.68, P < 0.001). Young boll abscission was highly correlated with TNC concentrations in both the leaves (r = −0.40, P < 0.01) and the bolls (r = −0.80, P < 0.001). Our results indicate that non‐structural carbohydrate limitation in reproductive parts was a major factor associated with fruit abscission of cotton grown under high temperature and enhanced UV‐B radiation conditions.

Comparison of Lygus elisus Van Duzee and Lygus hesperus Knight (Hemiptera: Miridae) Feeding Injury to Pre-bloom Cotton

Feeding injury and abscission of 6th, 9th and 11th node, first-position cotton squares exposed to Lygus hesperus Knight and L. elisus Van Duzee for 24 h were investigated in field and laboratory studies in 2001 and 2002. Square abscission was not significantly different for L. elisus or L. hesperus in six separate field trials over 2 yrs; however, the control was significantly lower in abscission in two of the six trials. Final square abscission averaged across the 2 yrs was 82.5% for the control, 90.0% for L. hesperus, and 86.3% for L. elisus. Square injury, as measured by estimating the surface area affected by Lygus feeding, was significantly higher for L. elisus compared with L. hesperus in three of the six of trials (6th node in 2002, 11th node for 2001 and 2002) indicating that, in some cases, L. elisus damage is higher than L. hesperus. However, when seed-cotton yield was obtained from squares that did not abcise, there was no significant difference for the control or the two Lygus species. This result was similar to other published studies of infesting cotton with L. lineolaris (Palisot de Beauvois) where yield compensation negated any differences in yield when infested cotton was compared to the control. Lygus elisus, a little known and often misidentified plant bug pest of Texas High Plains cotton, should be considered as damaging as L. hesperus.

Deterrent effects of four neem-based formulations on gravid female boll weevil (Coleoptera: Curculionidae) feeding and oviposition on cotton squares.

Three commercial neem-based insecticides, Agroneem, Ecozin, and Neemix, and a neem seed extract formulation, bitters, containing 1,036, 16,506, 471, and 223 microg/ml azadirachtin, respectively, were assessed for feeding and oviposition deterrence against gravid female boll weevils, Anthonomus grandis grandis Boheman, in the laboratory. In choice assays, excised cotton squares dipped in the separate formulations were first physically contacted by the weevils' tarsi or antennae fewer times than nontreated control squares. In choice and no-choice assays, each formulation repelled the weevils for > or = 90 min. After 24 h in the choice assays, feeding punctures on the squares treated with Agroneem, Ecozin, or bitters were significantly fewer compared with controls. Egg punctures on the Ecozin- and the bitters-treated squares were significantly fewer than on control squares after 24 h. In the no-choice assay, no significant difference was detected. Aging the formulations under outdoor conditions for 24 h before weevils were exposed resulted in 46-60% and 62-82% reductions in feeding and oviposition punctures, respectively, compared with controls. Agroneem- and bitters-treated squares had > 37% fewer feeding punctures after being aged for 48 h. No significant difference was detected after 72 h of aging. Because the deterrence of the gravid female boll weevils was not correlated with amounts of azadirachtin, azadirachtin does not seem to be the only, or the most influential, component of neem that induced the observed deterrence.

Deterrent Effects of Four Neem-Based Formulations on Gravid Female Boll Weevil (Coleoptera: Curculionidae) Feeding and Oviposition on Cotton Squares

Three commercial neem-based insecticides, Agroneem, Ecozin, and Neemix, and a neem seed extract formulation, bitters, containing 1,036, 16,506, 471, and 223 μg/ml azadirachtin, respectively, were assessed for feeding and oviposition deterrence against gravid female boll weevils, Anthonomus grandis grandis Boheman, in the laboratory. In choice assays, excised cotton squares dipped in the separate formulations were first physically contacted by the weevils’ tarsi or antennae fewer times than nontreated control squares. In choice and no-choice assays, each formulation repelled the weevils for ≥90 min. After 24 h in the choice assays, feeding punctures on the squares treated with Agroneem, Ecozin, or bitters were significantly fewer compared with controls. Egg punctures on the Ecozin- and the bitters-treated squares were significantly fewer than on control squares after 24 h. In the no-choice assay, no significant difference was detected. Aging the formulations under outdoor conditions for 24 h before weevils were exposed resulted in 46–60% and 62–82% reductions in feeding and oviposition punctures, respectively, compared with controls. Agroneem- and bitters-treated squares had >37% fewer feeding punctures after being aged for 48 h. No significant difference was detected after 72 h of aging. Because the deterrence of the gravid female boll weevils was not correlated with amounts of azadirachtin, azadirachtin does not seem to be the only, or the most influential, component of neem that induced the observed deterrence.

Timing Defoliation Applications for Maximum Yields and Optimum Quality in Cotton Containing a Fruiting Gap

Stresses during cotton (Gossypium hirsutum L.) square and boll formation can result in fruit abscission from several continuous nodes, resulting in a fruiting gap on the plant. This gap may cause a shift in benchmarks for timing various agronomic practices aimed at maximizing cotton yield and optimizing quality. A cotton defoliation timing study was performed in 1999, 2000, and 2001 to (i) see if the creation of a fruiting gap would influence defoliation timing and to (ii) compare the use of the open boll percentage at defoliation (OBPD), nodes above cracked boll (NACB), and micronaire readings at defoliation as tools for timing defoliation. In late July or early August each year, a fruiting gap was created by physically removing fruit from several continuous nodes on plants. Plots were defoliated on the basis of various OBPD values. Upon defoliation, OBPD and NACB were taken and lint was retained to determine the micronaire at defoliation. In both 1999 and 2000, there was a yield advantage to delaying defoliation beyond 60 OBPD in treatments containing a gap. However, in 2001 there was no yield advantage to delaying defoliation. This may have been due to optimal late-season growing conditions experienced in 2001. In years when micronaire readings are high, data suggested cotton not containing a fruiting gap is more likely to be above discount levels for high micronaire and should not be defoliated past 60 OBPD. In both 1999 and 2000, trends confirmed a direct relationship between OBPD and both yield and micronaire. Overall, these studies demonstrated that in some years, where no fruiting gaps exist, it might be possible to terminate cotton before the 60% open boll recommendation without sacrificing yields. These results would allow farmers to shift defoliation, and hence harvest, to a time when there are fewer risks of quality-based discounts. While a significant interaction prohibited the examination of the use of micronaire at defoliation as a technique for timing defoliation, there did not appear to be a significant advantage to using either NACB or OBPD for timing defoliation.

Timing Defoliation Applications for Maximum Yields and Optimum Quality in Cotton Containing a Fruiting Gap

Stresses during cotton (Gossypium hirsutum L.) square and boll formation can result in fruit abscission from several continuous nodes, resulting in a fruiting gap on the plant. This gap may cause a shift in benchmarks for timing various agronomic practices aimed at maximizing cotton yield and optimizing quality. A cotton defoliation timing study was performed in 1999, 2000, and 2001 to (i) see if the creation of a fruiting gap would influence defoliation timing and to (ii) compare the use of the open boll percentage at defoliation (OBPD), nodes above cracked boll (NACB), and micronaire readings at defoliation as tools for timing defoliation. In late July or early August each year, a fruiting gap was created by physically removing fruit from several continuous nodes on plants. Plots were defoliated on the basis of various OBPD values. Upon defoliation, OBPD and NACB were taken and lint was retained to determine the micronaire at defoliation. In both 1999 and 2000, there was a yield advantage to delaying defoliation beyond 60 OBPD in treatments containing a gap. However, in 2001 there was no yield advantage to delaying defoliation. This may have been due to optimal late‐season growing conditions experienced in 2001. In years when micronaire readings are high, data suggested cotton not containing a fruiting gap is more likely to be above discount levels for high micronaire and should not be defoliated past 60 OBPD. In both 1999 and 2000, trends confirmed a direct relationship between OBPD and both yield and micronaire. Overall, these studies demonstrated that in some years, where no fruiting gaps exist, it might be possible to terminate cotton before the 60% open boll recommendation without sacrificing yields. These results would allow farmers to shift defoliation, and hence harvest, to a time when there are fewer risks of quality‐based discounts. While a significant interaction prohibited the examination of the use of micronaire at defoliation as a technique for timing defoliation, there did not appear to be a significant advantage to using either NACB or OBPD for timing defoliation.

Mechanical damage in cotton buds caused by the boll weevil

The boll weevil (Anthonomus grandis Boheman) causes high levels of bud abscission in cotton plants due to feeding or oviposition punctures. It has been reported that abscission is mainly due to enzymes present in the insect's saliva, but mechanical damage could also contribute to square abscission. The objective of this paper was to undertake an analysis of the morphological damages caused by the insect in cotton squares using microscopy. Anthers and ovules are the main target of boll weevil feeding. The process initiates by perforation of young sepal and petal tissues and proceeds with subsequent alimentation on stamen and ovary leading to abscission of floral structures.

Boll Weevil (Coleoptera: Curculionidae) Feeding and Reproduction as Functions of Cotton Square Availability

Abstract The influence of food item availability on boll weevil, Anthonomus grandis grandis Boheman, feeding and reproduction was determined by providing different numbers of cotton squares (flower buds) to individual weevils. Squares were replaced daily after a 5-d feeding and mating conditioning period. The number of lifetime punctures produced by boll weevil females and males increased with square availability. The total number of punctures caused by boll weevil females was 2.7-fold higher than that caused by males. Fecundity was significantly higher in the 10:1, 15:1, and 20:1 (squares:female) treatments than in the 1:1 treatment. The relationship between eggs laid per day and the square to female ratio significantly changed over the life of the female, with the largest differences among treatments occurring in the first 3 wk of adulthood. Survival of weevil progeny to adulthood was about two-fold higher in the 10:1, 15:1, and 20:1 treatments than in the 1:1 and 5:1 treatments. When each boll weevil f...

Effects of Weeds on Selected Arthropod Herbivore and Natural Enemy Populations, and on Cotton Growth and Yield

Vegetative diversification with weeds can enhance natural enemy populations and suppress pest-related damage in various crops. Weedy and weed-free cotton (Gossypium hirsutum L.) plots were used to study the effects of weediness on selected herbivorous arthropod groups, including the boll weevil (Anthonomus grandis grandis Boheman), and natural enemies, boll weevil-induced injury to cotton squares, and cotton plant growth and yield in the Lower Rio Grande Valley of Texas, during 2000 and 2001. The presence of weeds was associated with greater populations of 9 of the 11 prey arthropod groups, and 9 of the 13 natural enemy arthropod groups counted in this study. These trends were mostly evident late in the season when weed biomass was greatest. Weed-free cotton harbored more cotton aphids (Aphis gossypii Glover), early in the season and silverleaf whiteflies (Bemisia argentifolii Bellows and Perring) later in the season than weedy cotton on some of the sampling dates. Diversity (Shannon’s index) within the selected arthropod groups counted in this study was significantly greater in dvac samples from the weed foliage than from weed-free cotton plants during both years, and diversity on weedy cotton plants was greater than on weed-free cotton plants during 2000. Boll weevil oviposition injury to squares was unaffected by weeds, but the higher weed-associated predator populations mainly occurred after most squares had become less vulnerable bolls. Weed competition resulted in lower lint yields of 89% and 32% in the 2 yr.

Effects of kaolin-based particle film application on boll weevil (Coleoptera: Curculionidae) injury to cotton.

This study examined a non-insecticidal tactic for suppressing boll weevil, Anthonomus grandis grandis Boheman, damage to cotton, Gossypium hirsutum L. In cage assays, kaolin, a reflective white mineral, applied to excised cotton squares or to the cotton foliage, initially resulted in lower levels of boll weevil injury to squares than nontreated squares. Boll weevil oviposition and feeding on kaolin-treated squares and squares on kaolin-treated cotton plants increased when nontreated squares and cotton plants were in short supply. A laboratory assay and field trials suggested that boll weevils distinguished between cotton plots based on color differences caused by kaolin and this appeared to influence levels of damage to squares. Random sampling in small plots indicated that oviposition damage to squares in plots treated with kaolin was reduced (P < 0.05) compared with nontreated controls, except when rain washed the kaolin off the foliage. Lint yield differences were not detected between the small plots, but the kaolin-treated small plots yielded as much as 2.36 times more cotton lint than a large but unreplicated adjacent nontreated control plot, and up to 1.39 times more than another large but unreplicated adjacent plot sprayed twice with preemptive applications of azinphosmethyl when cotton squares were first developing (pinhead stage). Potentially important avenues for future research on boll weevil injury suppression using kaolin are discussed.

Evaluation of Catolaccus grandis (Burks) (Hym., Pteromalidae) as a biological control agent against cotton boll weevil

Abstract: An evaluation of augmentative releases carried out at the Agreste site of the state of Paraiba, Brazil, provided significant insight into the ecology and potential impact of Catolaccus grandis (Burks) against the boll weevil, Anthonomus grandis Boheman in that cotton agroecosystem. The rate of increase in density of C. grandis was higher than the boll weevil. Catolaccus grandis showed ability to effectively search and reproduce within the release environment and revealed pronounced host and habitat preferences. Parasitism by C. grandis was largely confined to third instar weevil larvae, the majority of which (86.9%) occurred in abcised cotton squares. Catolaccus grandis inflicted significant mortality on third instar weevil larvae in the plot resulting in a significant level of suppression. The net effect was a higher boll weevil mortality in the release plot in comparison with the control. The use of augmentative releases of C. grandis has a very high potential for supplementing and enhancing available technology for suppressing boll weevil populations in the Agreste Paraiba.

Dynamics of non-structural carbohydrates in developing leaves, bracts and floral buds of cotton

Development of cotton ( Gossypium hirsutum L.) squares (i.e. floral buds with bracts) is fundamental for yield formation. A 2-year field study was conducted to determine dry weight (DW) accumulations of cotton leaves, floral bracts and floral buds, and the changes in concentrations of non-structural carbohydrates (hexoses, sucrose and starch) in these tissues during square ontogeny as affected by fruiting positions within the plant canopy. During square development, DW accumulation of a subtending sympodial leaf and floral bracts followed a sigmoid growth curve with increasing square age, whereas the DW increase of a floral bud followed an exponential curve. Main-stem node (Node 8, 10 or 12) and branch position (proximal vs. distal) within a plant canopy significantly affected DW accumulations of the leaf, bracts and floral bud. Starch was the dominant non-structural carbohydrate in the three tissues, accounting for more than 65% of total non-structural carbohydrates (TNC). Subtending leaf TNC increased as square age increased. The bracts exhibited a smaller change in TNC than leaves. Non-structural carbohydrate concentration was the lowest in 10-day-old floral buds, and had little change during the first 15 days of square development. Within 5 days prior to anthesis, the floral-bud TNC increased dramatically, tripling at the time of floral anthesis compared with 15-day-old floral buds. Square age and fruiting position significantly affected non-structural carbohydrate concentrations of subtending leaves, bracts, and floral buds. The correlation did not exist between final boll retention and non-structural carbohydrate concentrations of floral buds at different fruiting positions under normal growth conditions. The pattern of floral-bud non-structural carbohydrates during square ontogeny suggests that major events in carbohydrate metabolism occur just prior to anthesis.

Photosynthetic Capacity and Carbon Contribution of Leaves and Bracts to Developing Floral Buds in Cotton

During ontogeny of Gossypium hirsutum L. floral buds (squares), increases in area and dry mass (DM) of floral bracts and the subtending sympodial leaf followed a sigmoid growth curve with increasing square age. The maximum growth rates of the bract area and bract DM occurred between 15 and 20 d after square first appearance (3 mm in diameter). Net photosynthetic rate (PN) of the sympodial leaf at first fruiting branch position of main-stem node 10 reached a maximum when the subtended square developed into a white flower. Floral bracts had much lower PN and higher dark respiration than the subtending leaf. The amount of 14CO2 fixation by the bracts of a 20-d-old square was only 22 % of the subtending leaf, but 56 % of 14C-assimilate in the floral bud was accumulated from the bracts, 27 % from the subtending leaf, and only 17 % from the main-stem leaf at 6 h after 14C feeding these source s. Hence floral bracts play an important role in the carbon supply of developing cotton squares.