effects of shading on sink capacity and yield components of cotton in controlled environments

J. Dusserre et al.Effects of shading on cotton yield

Original article

Effects of shading on sink capacity and yieldcomponents of cotton in controlled environments

Julie DUSSERREa*, Yves CROZATb, Fernand R. WAREMBOURGc, Michael DINGKUHNa

aCirad-amis, Equipe Ecotrop, Programme Agronomie, TA 40/01, Avenue Agropolis, 34398 Montpellier Cedex 5, Franceb École Supérieure d’Agriculture, 55 rue Rabelais, BP 748, 49007 Angers Cedex 01, France

c Centre d’Écologie Fonctionnelle et Évolutive (CNRS), 1919 route de Mende, BP 5051, 34033 Montpellier Cedex 1, France

(Received 31 May 2001; accepted 13 November 2001)

Abstract – This study analysed effects of a simulated shade treatment on interactions among reproductive sinks, using pruned (mono-culm) plants in growth chambers. Plants were grown under non-limiting water supply and 600 µmol.m–2.s–1 PAR, reduced by 40% to si-mulate shade. Shade was imposed from 7th anthesis onwards, and plant transpiration, dw distribution and phenology were monitored.An index of apparent competition for assimilates (IAC) was calculated by dividing boll load by plant growth rate. Within the inflores-cence, shading affected all components of a cascade of events determining sink capacity, including carpel growth, seed number, boll abs-cission, final seed-cotton dw and partitioning between grain and fibre. In addition to sink down-sizing (abscission, seed number), shadealso increased boll growth duration, spreading the demand for assimilates over time. Fibre quality was poor in such situations. Shade alsoreduced boll appearance rates, thus limiting the number of competing sinks. Organ appearance rates, boll duration and abscission rates werecorrelated with IAC. The future use of an IAC in the analysis and prediction of sink behaviour in complex plant systems is discussed.

light resources / competition among sinks / assimilate partitioning / source–sink relationships

Résumé – Effets de l’ombrage sur les ajustements de puits et les composantes du rendement du cotonnier en milieu contrôlé.Cette étude analyse les effets d’un ombrage simulé sur les interactions entre puits reproductifs, en utilisant des cotonniers élagués, neprésentant qu’une seule série d’organes reproducteurs. Les plantes ont été cultivées en milieu contrôlé sous régime hydrique non limi-tant. Un ombrage, réduisant de 40 % le niveau lumineux (600 µmol.m–2.s–1 chez le témoin), a été imposé à partir de la 7e anthèse. Dessuivis de la transpiration, de la distribution de matière sèche et de la phénologie ont été réalisés. Un indice de compétition apparente pour lesassimilats (ICA) a été calculé en divisant la charge en capsules par le taux de croissance de la plante. A l’échelle de l’inflorescence, l’om-brage affecte la cascade de processus déterminant la capacité de puits, incluant la croissance des carpelles, le nombre de graines, l’abscis-sion de la capsule, la masse finale de coton-graine par graine et la fraction de fibres. En plus d’une réduction de la capacité des puits(abscission, nombre de graines), l’ombrage augmente la durée de développement de la capsule, étalant dans le temps la demande pour lesassimilats. Ce phénomène provoque une réduction de la qualité des fibres. L’ombrage réduit également le taux d’apparition des capsules, li-mitant ainsi le nombres de puits en compétition. Les taux d’apparition des capsules, de durée de développement des capsules et d’abscissionsont corrélés avec l’ICA. L’utilisation future de l’ICA dans l’étude des puits au sein de systèmes de plantes complexes est discutée.

ressources lumineuse / compétition entre puits / répartition des assimilats / relation source–puits

Communicated by Gede Wibawa (Palembang, Indonesia)

Agronomie 22 (2002) 307–320 307© INRA, EDP Sciences, 2002DOI: 10.1051/agro:2001009

* Correspondence and [email protected]

1. INTRODUCTION

Shading, and the associated reduction in assimilatesupply, reduces fibre yield and quality, and promotesshedding of reproductive organs in cotton [6, 10, 30]. Ascotton is an indeterminate plant, vegetative developmentcontinues during formation of reproductive structures.Also in contrast to cereals, the reproductive sink loadcontinues to evolve. To avoid excessive internal compe-tition for resources ultimately affecting the production ofviable seed (and, from the producer’s viewpoint, poor fi-bre yield and quality), the plant needs to adjust sink loadto variable resources.

The fruiting sites are produced as a function of the de-velopment rate of the meristems of the main stem. Mostcotton cultivars exhibit cycles of growth, flowering andfruit retention. A pronounced decrease in the frequencyof antheses and in structural growth (plant height, leafarea index), and an increased boll abscission rate, are ob-served once the plant attains a critical boll load [9, 12, 15,27] and are commonly referred to as ‘cut-out’ [27]. Thelevel of internal competition for resources is thus deter-mined by the balance between cumulative sink capacityand the ability of the plant to meet this demand.

Cotton fibre yield is the product of the number of bollsproduced, the dry weight (dw) of each boll, and the dwpercentage of fibre contained within each boll. Improvedlight resources have been shown to increase fibre yieldthrough boll number per plant [28]. Zhao and Oosterhuis[42] demonstrated that of the three main yield compo-nents, the number of bolls per unit ground area was themost sensitive to shade, although boll weight also de-creased. Increases in fibre yield were obtained on cottonplants grown under CO2 enriched atmospheres(630 µL.L–1 [10, 23]). On the other hand, doubling ofCO2 concentration (from 360 to 720 µL.L–1) did not af-fect boll maturation period, size or growth rates, or any ofthe fibre parameters [33]. Low light treatments reducedfibre strength and maturity, resulting in poor fibre qual-ity, while increasing fibre length [6, 29], but enhancedlight resources did not consistently improve fibre quality[30]. Such effects of light intensity on quality parametersare mediated by assimilate availability, as evidenced bypositive correlations between photosynthetic rates and fi-bre strength and micronaire [31]. Only little attention hasbeen given to fibre yield per boll, and observations arecontradictory because of the complex interactions be-tween yield components and between bolls at differentdevelopmental stages [35].

The comprehensive information available on the ef-fects of cultural practices and environments on cottongrowth and yield has been used to develop integrated,physiological simulation models [1, 11, 13, 14, 24]. Thecomprehensive model, COTONS, presented by Jallas[16] integrates cultural practices, plant phenology andmorphogenesis, canopy architecture, and competitionamong multiple sinks. It also highlights current knowl-edge gaps, namely, with regards to the plasticity of re-source partitioning within a boll, the variability of bollgrowth duration and the physiological determinants of fi-bre quality.

The present study seeks to explore further the under-pinning paradigm of the COTONS model, namely, thehypothesis that competition for assimilates within theplant has strong morphogenetic effects in terms of sinksize relationships and sink appearance rates. Our specificobjective was to describe the various mechanisms of sinkadjustment under differential light resources with partic-ular emphasis on yield components (boll number, seednumber per boll, weight distribution within the boll andfibre quality). We then sought to explain the appearanceand development rates and the size of sinks with the re-source restrictions imposed and with an index of appar-ent competition (IAC), based on the observed ratiobetween current boll load and whole plant dry matter ac-quisition rate. Experiments were conducted in controlledenvironments and on pruned plants in order to simplifyplant architecture and limit abscission.

2. MATERIALS AND METHODS

2.1. Genetic material, plant culture and pruning

The high-yielding cotton cultivar DES 119 (GossypiumhirsutumL.) from the Delta Branch, Mississippi Agricul-tural and Forestry Experiment Station [2] was used. DES119 has been described as having large fruits with a largecarpel, producing more fibre per fruit and larger grains aswell as a large number of fertile seeds and ‘motes’(aborted seed) compared to many other cultivars [34].

Four seeds were sown per 5-L pot containing a mix-ture of 30% loamy sand, 50% commercial humic gardensoil with 12% organic matter, and 20% sand. Pots werecovered with aluminium foil to prevent evaporation, andirrigated daily to field capacity with a complete culturesolution. Before each irrigation, pots were weighed andthe volume of irrigation water added to replace the

308 J. Dusserre et al.

quantity lost. This procedure served to calculate planttranspiration.

Conditions in the growth chambers were 27/24oCday/night temperature and relative air humidity between70 and 80%. HQI NDL lamps provided 600 µmol.m–2.s–1

PAR at the level of the plant tops (1 m from source, ad-justed as plants grew) with 12 h day length. Seedlingemergence was observed at 2–3 days after sowing. Plantswere thinned to one per pot at the two-leaf stage while se-lecting for uniformity.

On each node, vegetative branches were clipped uponemergence, and all fruiting buds were excised except thefirst on the fruiting branch. The fruiting branch too wasclipped after the second fruiting position. The objectivewas to simplify plant architecture and limit shedding byestablishing a mono-culm plant with a single series ofsuccessively appearing inflorescences.

2.2. Experimental design

Two experiments were conducted in growth chambersusing differential light treatments imposed during repro-ductive development. The first (Exp. 1) was initiated onOctober 27, 1999, and carried through to 1960oCd aftersowing. (Thermal time was calculated for each day as thedifference between mean air temperature within the can-opy and the base temperature, estimated at 13oC.) Tenplants (pots) were raised for each of 2 light treatments, 6of which were used for destructive sampling at 1030,1205 and 1455oCd (2 replicates), and 4 were used forcontinuous, non-destructive observations, followed bydestructive sampling at the end of the experiments (dw ofvegetative parts including roots; dw of carpel, grains andfibre for all bolls; seed number and fibre quality parame-ters for all bolls). Bolls maturing before the end of the ex-periment were harvested upon dehiscence.

Pots were rotated 3 times a week throughout the ex-periment to avoid location effects. At 1030oCd, when the7th inflorescence had flowered, light resources were de-creased by 40% in the simulated shade treatment by in-creasing the distance from the light source. Thedifferential treatments were carried through to the end ofthe experiment (30th anthesis and 15th dehiscence in thefull light treatment). Air temperature was monitored withthermocouples at the top and within the canopy, indicat-ing very small effects of treatment (<0.3oC).

Experiment 2 served to analyse the short-term effectsof shade on assimilation and assimilate distribution

within the plant, using plants raised as described for thefull light treatment in Experiment 1. Seeds were sown onMarch 17, 1999 (label 1) and July 7, 1999 (label 2). Thetwo labelling dates were replicated twice (one plant perdate, replicate and treatment). Label 1 was at 1250oCdafter sowing (16th anthesis) and label 2 at 1050oCd (11thanthesis). Labelling was performed with14CO2 for 11 hduring the photoperiod, with shaded and non-shadedplants sharing the same atmosphere. During labelling,the incident PAR was reduced by 60% for shaded plantsusing nets. The chase period was 48 h under full lightconditions.

2.3. 14C-labelling and detection

14CO2-labeling was carried out according toWarembourg [38] and Warembourg et al. [39]. Wholeplants were placed in a gas-tight chamber for a day (8 ha.m. to 7 h p.m.), and exposed to14CO2 generated fromNa2

14CO3 by adding sulphuric acid. The CO2 concentra-tion was maintained at 350 ppm using an IR gas analyserby automatic addition of14CO2. Temperature was main-tained at 27oC by circulating refrigerated air. After label-ling and the chase period, all bolls were collected and theplant divided into roots (extracted from soil by sievingand washing), stems and leaves. Each boll was separatedinto carpel, grains and fibre. The plant fractions wereoven-dried for 4 days at 70oC, weighed, and the seedscounted. After grinding, two subsamples per fractionwere analysed for total C using the dry combustionmethod [3]. This method provided aliquots for two14Cmeasurements by scintillation counting. Results were ex-pressed as the mean of the four measurements per frac-tion.

2.4. Plant mapping

Fruiting sites were referred to by node number on themain stem, starting from the cotyledonary node (nodezero). For each fruiting site, the dates of flower bud ap-pearance (as visible to the naked eye without dissection),anthesis (opening of the flower) and boll opening(dehiscence), and date of abscission if any were re-corded. Thermal time (oCd) after anthesis was used as thetime scale for boll development, and calculated for eachday as the difference between the mean air temperatureand the base temperature, estimated at 13oC. Base

Effects of shading on cotton yield 309

temperatures between 10 and 15.3oC have been reportedfor cotton, with 13oC frequently being cited for temper-ate environments [12, 22, 40].

2.5. Fibre growth and quality

To fit growth curves to fibre dw kinetics derived fromdestructive sampling (Exp. 1), a 3-parameter logisticmodel was used:

y = A / (1+ (x/x0)B)

where y is fibre weight per seed, x is the time variable(oCd), A is the upper asymptote, x0 is the point of maxi-mum rate on the logistic curve, B describes a rate param-eter. Graphic representations and regression analyseswere carried out with SigmaPlot 4.0 for Windows (JandelScientific).

Parameters of fibre quality were determined on eachboll by AFIS (Zellweger-Uster Advanced Fibre Informa-tion System, Knoxville, USA). The system module forlength and maturity was used to determine the mean fibrelength, 2.5% span length, short fibre content(<12.7 mm), fineness and maturity ratio (indicating thefibre cavity is filled with cellulose).

2.6. Calculation of an index of apparentcompetition (IAC)

A plant level index describing the apparent level ofcompetition for assimilates was calculated by dividingthe number of growing bolls per plant (sink load) by thecurrent dw growth rate of the entire plant (indicative ofsource) (Fig. 1). Plant growth rate was assumed to beproportional to water use (which was known on a dailybasis). The conversion of water use to dw growth wasperformed with an empirical coefficient for water use ef-ficiency (WUE) derived from observed changes in dw(linear regression across 4 dates) and cumulative wateruse for the same period. Mean WUE for the two treat-ments was 7.46 mg (dw).g–1 (water), with no significantdifferences between the two treatments (P > 0.05).

3. RESULTS AND DISCUSSION

3.1. Growth rates and14C partitioningbetween vegetative and reproductive organs

In Experiment 1, shading of 40% reduced plantgrowth rates, as estimated from destructive sampling andinterpolation using observed whole-plant transpirationrates (Fig. 1ab), by about 40% throughout reproductivedevelopment. Significant reductions in vegetative dw


Fig. 1.Calculation of an index of apparent competition (IAC) onthe basis of the demand-supply ratio (number of growing bolls /plant growth rate), Exp. 1. (a) Cumulative transpiration, totalplant dw and calculated kinetics of dw using an empirical WUEof 7.46 mg (dw).g–1 (water). (b) Calculated time courses ofplant growth rate (g dw.oCd–1). (c) Time course of IAC(dimensionless). Arrow indicates onset of shading for the shadetreatment.

(including roots) were only observed 75 d after the onsetof shading, whereas the dw of reproductive organs wasalready reduced after 15 d, but not thereafter (Tab. I). Itappears therefore that shading initially affected growthof reproductive organs more than that of vegetative or-gans, followed by the opposite effect as the plant ad-justed to the reduced light. This led to a significant buttransitory reduction of the dw fraction for reproductiveorgans 15 days after the onset of shading, which disap-peared thereafter.

In Experiment 2, shading of 60% reduced assimilationof 14CO2, measured during the 11 hours following the on-set of shading, by about half (Tab. II). Partitioning of thelabelled assimilates during the same period was not af-fected by light level. In both labelling experiments, theproportion of 14C received by the reproductive organs(relative to total) was about 80%. At these stages of de-velopment (1015oCd and 1250oCd after sowing), with

an already significant boll load, the sink strength ofreproductive organs was thus greater than that of vegeta-tive organs.

The shade treatment had minimal effect on the distri-bution of dry matter and14C-assimilates between vegeta-tive and reproductive plant parts during fruit set. This isconsistent with studies of effects of shade on partitioningbetween vegetative and reproductive structures on otherindeterminate flowering plants, as reported by Egli [7]for soybean and Jeuffroy and Warembourg [19] for greenpea. Even when the cause of growth reduction imposedon cotton was not shade but drought or nutrient defi-ciency, the dw fractions in leaves, stems and bolls at theend of the seasons were fairly constant over a wide rangeof plant dw [4].

These results indicate that the balance between vege-tative and reproductive sinks in cotton is largely unaf-fected by resource restrictions, involving either a


Table I. Vegetative and reproductive dry matter per plant (g) at four dates after the onset of shading (Exp. 1). Comparison between con-trol and shaded plants.

Treatment Days /oCdafter sowing

Days after onsetof shading

Vegetative organs Reproductive organs

Dry matter (g) Dry matter (g) Fraction of total

Control 70 / 1030 0 98.7 ± 2.2a ab 11.8± 1.0 a 0.11 ± 0.01 a

Shade 90.7 ± 0.6 a 11.1 ± 3.0 a 0.11 ± 0.03 a

Control 85 / 1205 15 102.2 ± 3.3 a 35.8 ± 0.8 a 0.26 ± <0.01 a

Shade 92.7 ± 5.4 a 24.3 ± 1.2 b 0.21 ± <0.01 b

Control 105 / 1455 35 116.4 ± 2.0 a 75.2 ± 5.9 a 0.39 ± 0.02 a

Shade 93.1 ± 15.7 a 58.9 ± 10.1 a 0.39 ± <0.01 a

Control 145 / 1960 75 150.5 ± 9.6 a 116.1 ± 6.1 a 0.44 ± 0.03 a

Shade 119.0 ± 4.6 b 109.1 ± 1.5 a 0.48 ± 0.01 aa Standard error for means of 2 plants for the first three dates, and for 4 plants for the last date.b For a given date, means followed by the same letter are not significantly different at the 5% level.

Table II. Total assimilated radioactivity per plant and relative distribution of14C between vegetative (stem, leaves, roots) and reproduc-tive organs, at two dates of labelling (Exp. 2). Comparison between control and shaded plants.

Treatment Total activity per plant(106 dpm)

Reproductive organs(% of total activity)

Label 11015oCd after sowing

ControlShade

22.44 ± 8.5a

9.82 ± 2.578.8 ± 3.481.7 ± 3.5

Label 21250oCd after sowing

ControlShade

66.72 ± 4.433.66 ± 2.7

82.5 ± 1.184.5 ± 2.8

a Standard error for means for two plants.

downward adjustment of the demand, or incomplete sat-isfaction of all or some of the demand functions.

3.2. Rate of production of fruiting sites

Regardless of light treatment, new flower buds ap-peared continuously from 500oCd until the end of the ex-periment (2000oCd) (Exp. 2; Fig. 2a). The appearancerate of buds, however, was not constant even in the con-stant light environment, and exhibited a marked reduc-tion between 900 and 1600oCd. The bud appearance ratewas lowest at about 1500oCd, coinciding with the timeof greatest competition for resources caused by a heavy

boll load (Fig. 1c: index of apparent competition, IAC,calculated as boll load / growth rate). This temporaryslowdown in bud appearance rate, frequently associatedwith boll abscission and reduced internode elongation, iscalled the cut-out period [27].

Differential light treatments were implemented at1030oCd and did not affect the time and intensity of thecut-out observed at 1500oCd (Fig. 2a). The recovery ofbud initiation rates after cut-out, however, was delayedby the shade treatment. Across light treatments and de-velopmental stages of the crop, a highly significant cor-relation (P < 0.001) was found between bud appearancerate and IAC (Fig. 2b). Consequently, the plant re-sponded to increased inter-organ competition for assimi-lates by reducing the rate of appearance of new sinks,regardless of whether reduced assimilation (shading) orincreased boll load caused the competition. This result isconsistent with results reported by Pettigrew [28] oncotton, indicating that increasing light (by opening thecanopy or using reflectors) increases production ofharvestable bolls.

3.3. Abscission rates

In the course of Experiment 1, about 3 inflorescencesper plant were shed, irrespective of treatment (Fig. 3a).Abscission occurred earlier in the shaded than in the fulllight treatment, however, and abscission rates were high-est during the cut-out period (about 1500oCd) whencompetition for resources was strongest (Fig. 1c) andbud appearance rates were lowest (Fig. 2a). The proba-bility for a given boll position to be aborted at any timebefore boll maturity was negatively correlated(P < 0.001) with the bud appearance rate observed atflowering (Fig. 3b). A similar but not so good correlationwas observed between abscission probability and thecompetition index IAC observed at flowering (data notpresented).

The relatively low abscission rates in this study com-pared to those encountered on similarly pruned plants un-der field conditions (Dusserre, unpublished) wereprobably due to the complete absence of water deficit.

The actual time of shedding of a given inflorescencewas not related to current competition levels, but to theage of the organ. The abscission probability is maximalduring the first 80oCd after bud appearance and around160oCd after anthesis [5].


Fig. 2. Effects of shade on the appearance rate of flower buds,Exp. 1. (a) Time courses of cumulative flower bud appearanceand derived flower bud appearance rates. Vertical bars indicatestandard errors of means for 4 plants. Arrow indicates onset ofshading. (b) Relationship between flower bud appearance rateand index of apparent competition (IAC). Solid line, second-or-der correlation with R2=0.79 andP < 0.001; broken line, 0.05probability confidence interval.

3.4. Seed number per boll

The average seed number per boll was the same for theshade and full light treatment at about 28 (Fig. 4a). In thefull-light treatment, seed numbers were markedly de-pressed for inflorescences having flowered very early(about 820oCd after sowing) and near the cut-out period.

Effects of the shading treatment were complex. Whenthe onset of shading coincided with anthesis, seed num-ber per boll was significantly reduced (P < 0.05). How-ever, when shading was imposed more than 40oCd

before or after anthesis, effects were either positive or ab-sent, but never negative. We interpret the depression ofseed number caused by shading at anthesis as a direct ef-fect of assimilate deprivation at the level of the inflores-cence concerned, whereas the general trend towardspositive effects of shading when imposed long before orafter anthesis of the boll in question may have indirectcauses. Whenever the anthesis of a particular inflores-cence coincided with the onset of shading, other inflores-cences were either at pre-anthesis stages or alreadymaturing. We therefore hypothesise that any increase inboll seed number by shading was due to compensatorysink adjustments in other organs, such as abscission, budappearance rates, seed number or filling rates.

Shade is known to reduce seed number per fruit ingreen pea [18] and soybean [8]. By contrast, shading didnot significantly affect cotton boll seed number in a studyby Zhao and Oosterhuis [41]. Our results suggest that as-similate resources do affect seed number in cotton, butthis effect may be masked by compensatory sink adjust-ments at the scale of the whole plant. More researchwould be needed to confirm this hypothesis.

Seed number per boll was the main determinant of thefinal boll dw (Fig. 4b). Despite a reduction of the finaldw of some bolls on shaded plants with high seed num-bers, covariance analysis of the correlation boll dw vs.seed number did not indicate any significant differencebetween the two treatments.

In the14C labelling experiments, the fraction of activ-ity imported by bolls was proportional to their seed num-ber fraction, relative to the total seed number per plant(Fig. 4c). In this analysis, however, all bolls youngerthan 160 oCd, counting from anthesis, were disregardedbecause this fraction included an unknown number ofbolls destined for abscission, and bolls whose seed num-ber was not yet fixed. In legumes, the end of the periodduring which abortions are probable is called the FinalStage in Seed Abortion (FSSA [32]), and corresponds tothe end of embryo cellular divisions [25].

3.5. Components and determinants of final bolldry weight

In terms of dw, bolls mainly consist of the carpels,grains and the fibres attached to the grains. Carpelgrowth begins before anthesis and overlaps partly withthe growth of grain and fibre, which both begin shortlyafter anthesis (data not presented). Shade reduced the fi-nal carpel dw significantly when applied from about


Fig. 3. Effects of shade on abscission rate, Exp. 1. (a) Timecourses of cumulative flower bud abscissions (means per plant).Arrow indicates onset of shading. (b) Relationship betweenabscission probability (for any given position on the stem) andflower bud appearance rate (at the time of flowering on that posi-tion). Solid line, second-order correlation with R2 = 0.61 andP < 0.001; broken line, 0.05 probability confidence interval.

50 oCd after anthesis onwards, or earlier (Fig. 5a). Shadeapplied at a later date had no effect on the final carpel dw,probably because the organ was already fully differenti-ated. By contrast, shade reduced fibre dw per boll only

when applied at or after flowering (Fig. 5b). Conse-quently, a shade treatment initiated before anthesis hadno effect on the boll’s fibre production, despite the factthat the shade treatment continued throughout the boll’sdevelopment. This observation may be partly explainedby the reduction in carpel growth under shade, thus bene-fiting fibre growth; and partly by the previous observa-tion that seed number was not negatively affected byshade except when its onset coincided with flowering(Fig. 4a), thus not affecting potential fibre mass per boll.

The dw ratio between fibres and seed-cotton, indica-tive of dw partitioning between fibres and grains, variedstrongly with boll position and treatment (Fig. 5c). Un-der constant light resources, the ratio was constant atabout 0.41 for all inflorescences flowering before1200oCd after sowing and then dropped significantly,probably due to increasing competition caused by bollload (Fig. 1c). Shade reduced significantly the fibre:seed-cotton dw ratio when applied at flowering (of theparticular boll position) or later (Fig. 5c). Consequently,less fibre was produced per unit seed dw when light re-sources were reduced during the growth of grains andfibres. By contrast, there was a (statistically not signifi-cant) trend towards higher fibre: seed-cotton dw ratioswhen the shade treatment was initiated well beforeanthesis (of a particular boll position), possibly due tocompensatory sink adjustments in reproductive organson the same plant having a different developmentalstage.

Figures 5a to 5c and Figure 4a permit the estimation ofboll development phases sensitive to assimilate short-falls, while considering only the negative effects on or-gan dw (supposed to be direct), and not the positiveeffects (possibly resulting from sink adjustments hap-pening simultaneously elsewhere in the plant). Carpeldw was sensitive to shade throughout its differentiationand growth, from at least –400oCd to +100oCd afteranthesis of the corresponding inflorescence (Fig. 5a).Seed number per boll was sensitive to shade at and


Fig. 4.Effects of shade on seed number per boll, Exp. 1 and 2.(a) Changes of the number of seeds per boll as a function of bollage (oCd after anthesis) at the onset of shading and plant age (oCdafter sowing) at anthesis. Vertical bars indicate standard error ofmeans for 4 plants (Exp. 1). (b) Relationship between boll dw atdehiscence and number of seeds per boll. Lines indicate linearcorrelations, control R2 = 0.94 and shade R2 = 0.62 (P < 0.001 inboth cases) (Exp. 1). (c) Relationship between14C activity perboll (fraction of total in bolls) and seed number per boll (fractionof total in bolls). Exp. 2 (label 1 and label 2 combined).

shortly after anthesis (0 to +40oCd after anthesis,Fig. 4a). Fibre dw per boll, however, showed two sensi-tive phases, the first (A) at and shortly after flowering(reflecting the impact of reduced seed number) and thesecond (B), at +100 to 180oCd after anthesis. During thelatter phase, characterised by structural growth, priorityis apparently given to grain over fibre developmentwhenever the two sinks compete directly. Phases A and Btogether (0 to 180oCd after anthesis) coincide roughlywith the abscission phase, and end at the FSSA stage.This stage is followed by the filling period, characterisedby a major increase in dw [35].

The final dw of bolls (carpel, Fig. 5a; grain, not pre-sented; and fibre, Fig. 5b) depended not only on treat-ment but also on boll position on the stem, as evidentfrom observations in the constant light environment. Thelast (topmost) three positions showed a markedly re-duced boll dw. This is consistent with the inferior bollsize and fibre quality near the end of the season reportedby Kerby and Ruppenicker [20] and Jenkins et al. [17].Bolls tend to be largest when associated with parts of theplant carrying the largest leaf area [26].

3.6. Fibre growth

Fibre dw per seed followed a typical sigmoid growthcurve, beginning at anthesis and ending at bolldehiscence (600 to 700oCd after anthesis) (Fig. 6ab).Shade during that period generally reduced fibre growthrates but did not affect the final fibre dw when the shadetreatment began before anthesis, because lower growthrates were compensated by a longer growth duration(Fig. 6a). When the onset of shading fell into a periodsignificantly later than anthesis, however, no such com-pensatory effect took place and the final fibre dw was re-duced (Fig. 6b). These observations were consistent withthe differential effects of timing of shading presented inFigure 5b.


Fig. 5.Effects of shade on dw of boll components at dehiscence,Exp. 1. (a) Carpel dw as affected by shade treatment and boll age(oCd after anthesis) at the onset of shading. (b) Fibre dw per bollas affected by shade and boll age (oCd after anthesis) at the onsetof shading. (c) Dry weight ratio between fibre and seed-cotton(grain + fibre) as affected by shade and boll age (oCd afteranthesis) at the onset of shading. Vertical bars indicate standarderror of means for 4 plants.

3.7. Variability of boll growth duration

Under constant light, the duration of boll developmentdepended strongly on boll position on the plant. A reduc-tion of the length of the boll growth duration was ob-served from the lower to the upper nodes. Thisphenomenon has been described by Trent et al. [36]. Onthe other hand, a longer growth period, or a delay in boll

dehiscence, was observed in response to the shade treat-ment (Fig. 7a).

Across treatments, a significant (P < 0.05), positivecorrelation was observed between boll growth durationand the mean IAC calculated for the filling period(180oCd after anthesis to dehiscence) (Fig. 7b). It istherefore possible that competition for resources causedby shading increased the duration from anthesis todehiscence, thus distributing the boll’s demand for


Fig. 6. Time courses of fibre dw per seed, Exp. 1. (a) Timecourses for fibre dw per seed observed on bolls receiving differ-ential shade treatment from 36oCd before anthesis onwards. Lo-gistic regressions using y=A/1(x/x0)B): Control R2=0.99,P < 0.0001, A=0.082 (g), B=–2.87, x0=259 (oCd); ShadeR2=0.98, P < 0.0001, A=0.092 (g), B=–2.33, x0=375 (oCd).(b) Time courses for fibre dw per seed observed on bolls receiv-ing differential shade treatment from 178oCd after anthesisonwards. Logistic regressions using y=A/1(x/x0)B): ControlR2=0.95, P < 0.0001, A=0.089 (g), B=–2.36, x0=324 (oCd);Shade R2=0.78,P < 0.0001, A=0.07 (g), B=–2.10, x0=311 (oCd).

Fig. 7.Effects of shade on boll growth duration (oCd, anthesis todehiscence), Exp. 1. (a) Boll growth duration as affected byshade treatment and boll age (oCd after anthesis) at the onset ofshading. Vertical bars indicate standard error of means for4 plants. (b) Relationship between boll growth duration andindex of apparent competition (IAC; mean value during boll fill-ing period). Solid line, linear correlation with R2=0.64 andP < 0.001; broken line, confidence interval at 0.05 probability.

assimilates over a longer period. No such correlation wasfound, however, when the IAC was calculated for earlierperiods of boll development (e.g., 0 to 180oCd afteranthesis). Since the choice of reference period for IACcalculation was somewhat arbitrary and affected the re-sults of the analysis strongly, a delaying effect of compe-tition on boll dehiscence cannot be considered anestablished fact and requires further study.

An alternative explanation for the delay in dehiscencecaused by shading is temperature. Although temperaturewas fully controlled in the growth chambers, and plantswere kept at a constant distance from the light source, it ispossible that differential illumination affected tissuetemperature. As generally observed for developmentalprocesses, boll growth duration decreases as temperatureincreases [33, 37]. However, the daytime temperature inthe present study was 27oC, which is about the optimumtemperature for boll development [21, 33], thus render-ing temperature an unlikely cause of the observations.Furthermore, field experiments in Thailand not reportedhere (Dusserre, unpublished) based on a similar designindicated very similar effects of shading on boll growthduration, although air temperatures were well above thephysiological optimum for cotton development.

The variability of the boll growth duration did notonly affect fibre dw, but also the fibre quality. Across thelight treatments and boll positions on the plant, a nega-tive, linear correlation was found between boll growthduration and the maturity ratio (y = 1.33 – 0.0008x, R2 =0.51,P < 0.001); and a positive, linear correlation wasfound with the 2.5% span length (y = 24.93 + 0.022x,R2 = 0.37,P < 0.001). The maturity ratio and 2.5% spanlength are commercial fibre quality parameters describ-ing the filling of the fibre’s cavity and the length of theirlongest fraction, respectively.

Our results indicate that increased boll growth dura-tion caused by shade (Fig. 7a) was associated with a de-crease in fibre quality (correlations cited above), withinternal competition for resources (Fig. 7b) probably be-ing the common cause of both. Eaton and Ergle [6] ob-served that a 70% reduction in light resources reducedfibre strength and maturity but increased fibre length.Light enhancement, however, did not consistently im-prove fibre quality. In another study, a 30% shade treat-ment reduced both fibre strength and micronaire (anestimate of secondary wall deposition) by 6% [30]. Fur-thermore, a positive correlation was found between as-similation rates, fibre strength and micronaire [31], thussupporting our conclusion that plant-level competitionfor assimilates affects fibre quality.

3.8. Critique of the approach

This study demonstrated that assimilate limitationsdue to reduced light levels caused three types of sink ad-justments, (1) the elimination of individual sinks (bolls,seeds), (2) the down-sizing of individual sinks (final dwof carpel, grain and fibre) and (3) a slowdown of devel-opment processes (reduction of bud appearance rates, de-layed dehiscence of bolls), thus distributing assimilatedemand over a longer period. These adjustments must beinterpreted at the level of the entire plant, which keepsproducing new sinks while satisfying the demand of theexisting ones.

Our experimental system sought to simplify thesecomplex interactions by reducing the plant to a mono-culm system (one line of sequential appearances) whileallowing inter-sink competition to take effect. The prun-ing also served to limit boll abscissions, which can bedisruptive if massive, to a minimum. For the same rea-son, reductions in light resources were applied at a timewhen boll load was significant but not yet at cut-outlevel. These carefully chosen manipulations render theexperimental system distant from production situations,but permit the analysis of the mechanisms governing sinkinteractions.

A word of caution is also necessary with regards towhole-plant development processes confounding com-parisons between different bolls, which appeared se-quentially and were therefore not only affected bycurrent competition relationships, but also by plant age.This is particularly evident with respect to boll duration,which decreases from bottom to top positions [36], whilecompetition (cumulative boll load) increases. Increasedcompetition introduced by shade treatment, however,had the opposite effect (Fig. 7a). Consequently, the mostmeaningful comparisons between bolls on a plant (withrespect to effects of shade application at a given time) aremade between bolls having appeared not too far apart(e.g., in Fig. 5).

With the same logic, it should be noted that some ofthe effects provoked by shading were probably quite in-direct and thus, inaccessible to analysis. This is evidentlythe case in Figure 4a (seed number per boll), where seednumbers were significantly reduced when shading beganat the time of their formation (around flowering; directeffect), but increased when the inflorescence was still400oCd away from flowering at the onset of shading (in-direct effect). Such indirect, positive effects of shadingon yield components were also observed for the final fi-bre dw per seed (Fig. 5c), and were probably caused by


cumulative effects of sink adjustments leading to over-compensation for the stress.

The calculation of an index of apparent competition(IAC: ratio between boll load and whole-plant growthrate) turned out to be a useful reference to test for trophiccontrol of sink adjustments and development rates. Nei-ther the absolute boll load nor assimilation rates alone,but only the ratio between them characterises a currentlevel of competition for resources. The limitations of thissimple approach reside, on the sink side, in (1) the non-consideration of gradual (non-abortive) sink adjustmentsand (2) the different sink strength of bolls havingdifferent developmental stages (particularly duringoleosynthesis in grains, an energy intensive process).Models using a plant level competition index to predictsink adjustments need to take into account these consid-erations.

3.9. Synthesis of development processes and sinkinteractions under growth limitation

The diagram in Figure 8 shows hypothetical competi-tion effects on sink adjustments. At the level of an indi-vidual boll, a cascade of adjustments occurs beginning

with the dw growth of the carpel (which may actuallyrepresent a reserve for future seed-cotton growth), fol-lowed by the seed number determined soon after flower-ing, potential abscission, and growth of seed cotton withthe associated partitioning between grain and fibre. Eachof these processes is sensitive to resources during a par-ticular period (estimates shown in Fig. 8), some of themoverlapping in time. The time at which fibre quality is de-termined is difficult to estimate. The fibre maturity ratioand fineness values observed on shaded plants were al-ways inferior to those of control (data not presented). Notreatment effects were observed on mean fibre length,but superior values for span length and short fibre contentwere observed for the shade treatment (data not pre-sented).

In addition to these quantitative components of yieldformation, two temporal ones were also identified, therate of appearance of new inflorescences and boll dura-tion. For each of these quantitative and temporal compo-nents, significant effects of shading were observed.These effects were generally of alleviating nature, thusdown-sizing the sink or “stretching” it over time (as dis-cussed further above, we disregard very indirect, positiveeffects probably caused by over-compensation). In somecases, correlations were found between these adaptive


Fig. 8.Conceptual diagram of competition effects on sink adjustments. Solid arrows with solid lines indicate direct evidence (significantcorrelation between sink adjustment and index of apparent competition (IAC)), solid arrows with broken line indicate indirect evidence(significant effects of shade treatment). Periods of sensitivity to competition are indicated asoCd relative to anthesis (daa, days afteranthesis).

effects and IAC, the downward adjustments of sinks gen-erally being associated with high levels of competition.Such correlation analyses across different developmentalstages of the crop, however, highlight the limitations ofthe experimental design, which confounds crop ontogenywith the development of individual inflorescences. Mod-ified experiments will be needed to further substantiateplant level competition effects on sink adjustments.

4. CONCLUSION

This study analysed effects of a simulated shade treat-ment on cotton fibre production and quality, with empha-sis on the interactions between reproductive sinks. At thelevel of the inflorescence, shading affected all compo-nents of a cascade of development processes determiningsink capacity, including (in approximate sequence) car-pel growth, seed number, boll abscission, the final seed-cotton dw and dw partitioning between grain and fibre.Some of these effects, which took place during more orless distinct periods of boll development, can be inter-preted as adaptive adjustments of sink size (e.g.,abscission and seed number). In addition to sink down-sizing, shade also increased boll growth duration,thereby spreading the demand for assimilates over time.Increased boll duration caused by shade was associatedwith a significant decrease in fibre quality, but not neces-sarily quantity. At the level of the entire plant, shade in-duced a reduction in boll appearance rates, thus limitingthe number of competing sinks.

Some of the multiple adjustments of sink timing/dura-tion and size could be related to the apparent level ofcompetition for assimilates at the plant level (ratio de-mand/supply). This was particularly evident for organinitiation rates and abscission. These results suggest thatthe systematic use of a competition index to analyse andpredict sink behaviour in complex plant systems is prom-ising, particularly for crop modelling.

Our study provided an inventory of sink adjustmentmechanisms in cotton under assimilate shortage. The rel-ative importance of the different mechanisms, however,requires additional research on plant populations underfield conditions, where inter-plant competition interactswith the intra-plant competition investigated here.

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effects of shading on sink capacity and yield components of cotton in controlled environments

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