The effects of daylength and spectral quality on assimilate partitioning and leaf carbohydrate content should be considered when conducting controlled environment experiments or comparing results between studies obtained under different lighting conditions. Changes in partitioning may indicate alterations to photoregulatory processes within the source leaf rather than disruptions in sink strength. Moreover, it may be possible to use photoregulatory responses of assimilate partitioning to probe mechanisms of growth and development involving translocation of carbon or adaptation to environmental factors such as elevated CO2. It may also be possible to steer assimilate partitioning for the benefit of controlled environment agriculture using energy-efficient manipulations such as daylength extensions with dim irradiances, end-of-day alterations in light quality, or shifting plants between different spectral qualities as a part of phasic control of growth and development. Note that high starch levels measured on a one-time basis provide little information, since it is the proportion of photosynthate stored as starch that is meaningful. Large differences in starch content can result from small changes in partitioning integrated over several days. Rate information is required

Britz, Steve J.

English

NASA Technical Reports Server

REGULATION OF ASSIMILATE PARTITIONING BY DAYLENGTH AND
SPECTRAL QUALITY
Steve J. Britz
USDA-Climate Stress Laboratory, Beltsville MD 20705-2350, USA
/
N96- 18125
INTRODUCTION
Photosynthesis is the process by which plants utilize light energy to assimilate and transform
carbon dioxide into products that support growth and development. The preceding review
provides an excellent summary of photosynthetic mechanisms and diurnal patterns of carbon
metabolism with emphasis on the importance of gradual changes in photosynthetically-active
radiation at dawn and dusk (Geiger, this volume). In addition to these direct effects of irradiance,
there are indirect effects of light period duration and spectral quality on carbohydrate
metabolism and assimilate partitioning. Both daylength and spectral quality trigger
developmental phenomena such as flowering (e.g., photoperiodism; Deitzer, this volume) and
shade avoidance responses (Pausch et al., 1991), but their effects on partitioning of
photoassimilates in leaves are less well known. Moreover, the adaptive significance to the plants
of such effects is sometimes not clear.
DAYLENGTH
The light period normally occupies only part of the 24 h cycle, but photosynthesis during the
light must support the carbon requirements of the plant during the dark as well. Thus,
photosynthetic productivity frequently exceeds the capacity of the plant to transport and/or
utilize the products of photosynthesis during the light period alone. Excess capacity is often
stored in leaves or other tissues as polymers of glucose or other sugars (e.g., starch, sucrose,
fructans). Temporary storage of photosynthate as large molecular weight compounds provides
immediate benefits for photosynthesis, since it releases phosphate that would otherwise be
sequestered in phosphorylated sugars (potentially inhibiting photosynthesis).
However, carbohydrate storage serves another important purpose. Many plants accumulate large
amounts of starch or other carbohydrates in photosynthetic tissues during the light and then
breakdown and utilize this material in the dark. This temporal redistribution of photosynthetic
products allows plants to support growth and respiration during long dark periods. Mutants
unable to accumulate starch are disadvantaged when grown under light-dark cycles as compared
to continuous light (Caspar et al., 1985).
Early experiments conducted in greenhouses indicated that plants.accumulated a greater
proportion of photosynthate as starch under short day conditions (Challa, 1976). Subsequent
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experimentswerelargelyperformedin controlledenvironmentchambersanddocumentedthat
similarresponsesto daylengthcouldbeobservedin awiderangeof speciesandthatplantscould
adaptto suddenchangesin daylength,sometimeswithin24h of the switch(Britz, 1990a).Note
thatphotosynthatepartitioninginto starchwasapproximatelyhalvedwhensoybeanplantswere
transferredfrom a 11.5h daylengthinto a 16h daylength(Table1;Britz, unpublished ata).
Partitioningundera 7 h daylength,however,wassimilarto thatunder11.5h, indicatingthe
transitionbetweenshortandlong-dayresponsewasbetween1I.5 and16h. In severalwell-
documentedcases,daylengthregulationof assimilatepartitionwasdemonstratedto resultfrom
timingof darkperioddurationinvolvingcircadianrhythmsinitiatedat thetransitionbetween
light anddarkperiods(Blitz et al., 1987).Detectionof the light-darktransitionapparentlywas
perceivedby non-photosyntheticphotoreceptorscapableof suppressingrhythmsabovecertain
low irradiances(Britz, 1986;Britz, 1991).
TABLE 1 Effect of Daylength on Carbohydrate Allocation in Soybean
Daylength Treatment* Leaf Number ** Starch Accumulation
(percent of photosynthesis) * **
11.5 h _ 7h TF 3 34.7
TF 4 36.3
ll.5h_ ll.5h TF 3 35.3
TF 4 32.9
11.5 h _ 16h TF 3 18.7
TF 4 19.7
* Plants were grown (Chatterton and Silvius, 1981) for 24 days at a daylength of 11.5 h (12.5 h
dark period) and shifted for 4 days to the indicated daylength prior to measurement. ** Third and
fourth trifoliolate leaves (TF3 and TF4, respectively). *** Rates of starch accumulation were
determined under growth conditions between 1 and 6 h after lights-on and referenced to rates of
intact leaf net photosynthesis expressed as carbohydrate assimilation (Britz, 1990b).
In spite of the early greenhouse work, some researchers (e.g., Geiger et al., 1985) speculated that
the daylength response was peculiar to the complex lighting manipulations used in controlled
environments (e.g., Britz et al., 1985). However, an extensive series of greenhouse experiments
conducted with natural daylight at 12 intervals during a growing season showed that the
proportion of assimilate partitioned into starch (TF4, 4th trifoliolate) increased steadily under
standard measurement conditions as prior daylength shortened between the summer solstice and
the autumnal equinox (Britz, 1990b). About one-third of photosynthate was stored as starch at
midsummer, but this fraction increased to 80% in early autumn. Temperature in the greenhouse
was controlled with a heat pump, so the effect of this variable was minimized. Growth intervals
were adjusted so that TF4 of comparable developmental status (i.e., plastochron), but differing in
2O
daylengthhistory,wereobtainedfor eachharvest.In fact,photosyntheticratesof TF4measured
understandardconditionsdeclinedonlybyabout10%at laterharvestsin thefall.
Increasedpartitioninginto leafstarchwasobservedundershortdaysat theendof thegrowing
season,in spiteof thefact that dailyintegralsof photosynthetically-activeradiationwere
reducedby50%andthat plantswerefilling podsat theaxilof TF4.Theseresultssuggesthat
daylengtheffectsonassimilatepartitioningwithina sourceleafmaytakeprecedenceover the
demandof nearbysinks.It mayalsoexplainwhy soybeanseeddevelopmentis sometimesfound
to besinklimited,whileleavesmayatthe sametimecontainhigh levelsof starch(Streeterand
Jeffers,1979).Clearly,regulationof assimilatepartitioningbyfactorsoperatingat the levelof
the leafcanbeanimportantcomponentof overallplantproductivity.
SPECTRALQUALITY
It hasbeenknownfor sometimethat spectralqualityaffectsplanttissuecomposition.In
particular,carbohydratelevelsarehigher,whileproteinandaminoacidsarelower,in plants
raisedunderred-biasedascomparedto blue-biasedspectra(e.g.,WarringtonandMitchell,
1976).It is importantto determineif photosynthatepartitioningcontributesto morphologicaland
physiologicaladaptationto alteredspectralquality(e.g.,canopyshade).A crucialquestionis
whetherspectralqualityaffectsphotosynthatepartitioningdirectlyat the levelof sourceleaf
metabolismor indirectlyasaresultof photomorphogeneticeffectson thestrengthof developing
sinks.For example,highstarchcontentin thefirst leafof cucumberwasshownto correlatewell
with thegrowth of thedevelopingthird leafleafascontrolledbybluelight and/orultraviolet-B
radiation(Britz andAdamse,1994).It seemslikely thatstarchcontentin thefirst leafwasan
indicatorof sinkdemand.
Soybeansraisedunderrelativelyhighphotosynthetically-activeradiationfromblue-deficient
low pressuresodium(LPS) lampsmanifestedmanyof thecharacteristicsof shadeplants(Britz
andSager,1990).Theleavescontainedbaseline(i.e., end-of-night)starchlevelsthreefold
higherthanplantsraisedunderbroadspectrumfluorescentlight.Moreover,35%more
photosynthatewaspartitionedinto starchandsugarduringthefirst half of the light period,
apparentlycausinga declinein exportfrom 52to 37%of photosynthate(Table2; Britz and
Sager,1990).Someof theretainedcarbonmayhavebeenusedto supportleafgrowth at the
expenseof root growth(Table2). Highratiosof total leafareato total dry mattercompensated
reducedphotosynthesisonanareabasisandmaintainedsimilartotal RelativeGrowthRates
underthetwo differentspectralqualityconditions(Table2).Note that netphotosynthesis(total
leafbasis!)wasequalfor first trifoliolateleavesmeasuredundergrowth conditionsfor thetwo
differentlight qualitieseventhoughtheareaof leavesfromblue-deficientconditionswasmuch
greater.Thesedataconfirmtheimportanceof generatinghighleafareaandsuggesthat changes
in sourceleafpartitioningmaybeaform of resourcerationingthat maintainshigh
photosynthesisunderperceivedshadeconditions.
21
TABLE 2 Photoassimilation, Export and Growth Parameters in Soybean
Parameter
Broad Spectrum
Fluorescent Lamps
Blue-deficient Low
Pressure Sodium Lamps
First Trifoliolate Leaf*
Leaf Area (dm 2)
Net Photosynthesis (mg-C leaflh -i)
Starch + Soluble Sugar Accumulation
(percent of net photosynthesis)
Export
(percent of net photosynthesis)
Relative Growth Rates**
Total Cry Matter (g g-i d-l)
Leaf Dry Matter (g gl d 1)
Stem Dry Matter (g g_ d_)
Root Dry Matter (g g-1 d-l)
Leaf Area (dm 2 dm "2d "l)
0.559 b*** 0.656 a
3.46 a 3.47 a
34 46
52 37
0.226 ab 0.218 b
0.195 b 0.212 b
0.252 a 0.230 ab
0.253 a 0.208 b
0.157 c 0.202 b
Leaf Area Ratio (dm z g-l)
14 days 2.09 a 2.19 a
18 days 1.59 b 2.07 a
*Determined 16 days after planting.
**Determined 14 to 18 days after planting.
***Values followed by different letters are significantly different at the 5% confidence level.
More detailed experiments with younger soybean seedlings (8 to 10 days after planting) revealed
significant reductions in the partitioning of _4C-labelled photosynthate to the roots of plants
transferred from blue-sufficient to blue-deficient lighting (Verkleij and Britz, unpublished data).
Alterations in translocation preceded discernible changes in the partitioning of growth to the root
but were accompanied by only small changes in primary leaf assimilate accumulation, raising
questions about the cause-and-effect relationship between leaf carbohydrate storage and growth
patterns. Under these conditions, high levels of leaf starch were shown to result from small and
gradual increases in the proportion of photosynthate stored as starch during the light coupled
with small reductions in the amount of starch broken down in the dark.
22
CONCLUSIONS
Theeffectsof daylengthandspectralqualityonassimilatepartitioningandleafcarbohydrate
contentshouldbeconsideredwhenconductingcontrolledenvironmentexperimentsor
comparingresultsbetweenstudiesobtainedunderdifferentlightingconditions.Changesin
partitioningmayindicatealterationsto photoregulatoryprocesseswithin the sourceleafrather
thandisruptionsin sinkstrength.Moreover,it maybepossibleto usephotoregulatoryresponses
of assimilatepartitioningto probemechanismsof growthanddevelopmentinvolving
translocationof carbonor adaptationto environmentalfactorssuchaselevatedCO> It mayalso
bepossibleto steerassimilatepartitioningfor thebenefitof controlledenvironmentagriculture
usingenergy-efficientmanipulationsuchasdaylengthextensionswith dimirradiances,
end-of-dayalterationsin light quality,or shiftingplantsbetweendifferentspectralqualitiesasa
partof phasiccontrolof growthanddevelopment.Note that highstarchlevelsmeasuredona
one-timebasisprovidelittle information,sinceit is theproportionof photosynthatestoredas
starchthatis meaningful.Largedifferencesin starchcontentcanresultfrom smallchangesin
partitioningintegratedoverseveraldays.Rateinformationis required.
REFERENCES
Britz, S.J.1986.Theroleof circadianrhythmsin thephotoperiodicresponse
of photosynthatepartitioningin Sorghum leaves: a progress report, p. 527-534. In: J.
Cronshaw, W.J. Lucas, and R.T. Giaquinta (eds.). Phloem Transport. Liss, New York.
Britz, S.J. 1990a. Photoperiodic and thermoperiodic regulationof assimilate
partitioning into storage carbohydrates (starch and sugar) in leaves of crop plants, p.
853-866. In: D.K. Hayes, JE. Pauly, and J.R Reiter (eds.). Chronobiology: Its role in
Clinical Medicine, General Biology, and Agriculture, Part. B. Wiley-Liss, New York.
Britz, S.J. 1990b. Regulation of photosynthate partitioning into starch in soybean leaves.
Response to natural daylight. Plant Physiol. 94:350-356.
Britz, S.J. 1991. Setting the clocks that time photosynthate partitioning into
starch and stored soluble sugar in Sorghum: response to spectral quality, p. 265-274. In:
J.L. Bonnemain, S. Delrot, W.J. Lucas, and J. Dainty (eds:). Recent Advances in Phloem
Transport and Assimilate Compartmentation. Ouest Editions, Nantes, France.
Britz, S.J. and P. Adamse. 1994. UV-B induced increase in specific leaf weight
of cucumber as a consequence of increased starch content. Photochem Photobioi., in
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Britz, S.J., W.E. Hungerford, and D.R. Lee. 1985. Photoperiodic regulation of
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23
Britz, S.J., W.E. Hungerford, and D.R. Lee. 1987. Rhythms during extended dark
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sugars in subsequent light periods in leaves of Sorghum. Planta. 171:339-345.
Britz, S.J. and J.C. Sager. 1990. Photomorphogenesis and photoassimilation in
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Caspar, T., S.C. Huber, and C. Sommervile. 1985. Alterations in growth, photosynthesis, and
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Regulation of assimilate partitioning by daylength and spectral quality

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