Identification of the Phloem Translocated Carbohydrate in Idria Columnaris (Boojum Tree)

Identification of the Phloem Translocated Carbohydrate in Idria columnaris (Boojum tree)

Item Type text; Article

Authors Miller, W. B.

Publisher College of Agriculture, University of Arizona (Tucson, AZ)

Journal Turfgrass and Ornamentals Research Summary

Download date 30/09/2021 00:38:03

Link to Item http://hdl.handle.net/10150/216070 Identification of the Phloem Translocated Carbohydrate in Idria columnaris (Boojum Tree) W B. Miller

ABSTRACT

Sucrose was identified by high performance liquid chromatography (HPLC) as the only phloem - mobile carbohydratein the Boojum tree.This result has implications for carbohydrate metabolism in the desert adapted Boojum and ocotillo, as discussed below. INTRODUCTION

TheFouquieriaceaefamilyisrepresentedinArizona,California,New Mexico,Baja peninsula,andnorthernMexico by twogenera:Fouquieria(ocotillo),andIdria,orboojum. The ocotillo(Fouquierias_plendens Engelm.)isa prominent woody plantof the Sonoran and Chihuahuadeserts. Theplantischaracterizedbyitsmultiplethorny,cane -likebranches whicharisefromashort,inconspicuoustrunk. Itsflamingredinflorescenceisafamiliar sightinthesouthwesterndesertregioninthespring. Therangeoftheboojum(Idria columnaris Kellog)issmaller than thatof theocotillo, being centered on the Baja peninsula of Mexico,althoughitiscommonly seeninbotanical gardens and arboretainsouthern and central Arizona.

Extremelydroughttolerant,thesespeciesare knownfortheirabilitytoshedleaves toavoid water loss during periods of drought. Thisis an obvious advantage, considering the harshenvironmentinwhichtheythrive. Preliminaryobservationsbytheauthoridentified regionsofstarchaccumulationunderthethorny barkoftheocotillo,agreeingwithearlier anatomicalstudiesfromthe1930s. Thisstarchprobablyservesasareservecarbohydrate from which the plant may draw upon during leaf growth.In the case of immature (expanding) leaves, carbohydrate import is crucial untilleaves become photosynthetically competent. Sucroseservesastheprimarytranslocatedcarbohydrateinthemajorityofhigherplants, althoughpolyolssuchasmannitol,sorbitol,andgalactitolarethemajortranslocatable sugarsinseveralcommerciallyimportantvegetable,ornamentalandpomologicalspecies. It is not known what carbohydrate serves as the translocated sugar in ocotillo and Boojum.

When a leafis removed fromitsparentplant, the leaf phloem tissueusually becomes clogged duetotheformationof callose"plugs ". This presentsa problem,asthe phloem mobilematerialscannotbeeasilycollected. By theuseofethylenediaminetetraaceticacid (EDTA) bathingsolutions,however, phloemintegrityismaintained. Inthismanner, phloem translocatedsubstances,includingcarbohydrates,maybecollectedforanalysisand identification. Thisapproach was usedasafirststepinaninvestigationofcarbohydrate metabolisminthe Fouquieriaceae. Thisreportdescribesstudiesundertakentodetermine the identity of the main translocated carbohydrate in Idria columnaris.

93 MATERIALS AND METHODS

Two experimentswereconductedduringtheSpringof1988. Boojumleaveswere collected from the northernmost plant in the Joseph Wood Krutch garden on the UA campus. Shootswithmatureleaveswerecollectedandheldinwaterduringtransporttothe laboratory. Leaves(averaging 48 mg freshweighteach)wereseveredatthebaseofthe pedicel,andfloatedondistilledwateruntiluse(lessthan0.5hour). Leaves wereplaced upright in small vials with leaf bases submerged in 1 ml of 10 mM Na2EDTA at pH 7.0.At various timeintervals,thebathingsolutionwasremovedandsaved, thenreplacedwithfresh solution.

Inthesecondexperiment,differentbathingsolutionsexpectedtoeitherallowof inhibitphloemexudation were prepared. The threebathingsolutions wereused:1)distilled water; 2) 10 mM Na2EDTA at pH 7.0; and 3) 10 mM Na2EDTA plus 20 mM CaCI. Excess calcium was used in the third solution to chelate the EDTA and make it unavailable to the phloem.Bathing solutionswereremovedatirregularintervalsovera24 -hour period,withfreshbathing solution added each time.

In each experiment, the individualbathing solutions collected over time were processed for soluble carbohydrate analysis by high performance liquid chromatography (HPLC). Samples were injectedinto the HPLC, and soluble sugars separated and quantitated bydifferentialrefractometry. Theidentityandquantityofelutingsugarswasdetermined by comparing retention times to known standards.

RESULTS AND DISCUSSION In both experiments, sucrose was the only carbohydrate appearing in the EDTA exudation solution. Inthesecondexperiment,leavesincubatedwithdistilledwateror EDTA plus excesscalciumdidnot exude any sucrose, while the EDTA treatmentsteadily exuded sucrose (Fig.1 and Fig.2). This suggested that leaf phloem clogged in the absence of EDTA, either from its complete absence or from chelation due to excess calcium.EDTA appeared to maintain functional leaf phloem that was capable of long term sucrose exudation.

Therateandcumulativeamountofsucroseexudedwassimilarbetweenthetwo experiments. Inthefirstexperiment,eachleaf exudedapproximately280 µgofsucrosein 24hours(Fig.2);inthesecond experiment,thecumulativeexudationwasapproximately 220agperleaf. HPLC analysisoftheleavesindicatedmanysolublecarbohydrateswere actuallypresentintheleaves(Figure2). None ofthesecarbohydrates wereseeninthe bathingsolutions,however,indicatingthatthecumulativeincreasesinthemassofsucrose exuded was not a function of damaged and "leaking"cellsatthe base of theleaf. Had this been the case, evidence of other carbohydrates would have been seen in the bathing solution.

Theseexperimentshavebeenthefirststepindeterminingthephysiological mechanismsofcarbohydratestorageandremobilizationintheFouquieriaceae family. Confirmationis needed,butpresumablysucrosewouldalsobethetranslocatedsugarin ocotillo as well. With the knowledge gained from this work, we may speculate on the pathway of sucrose metabolism leadingtostarchstorageinthe stems of boojum andocotillo. After sucroseisexported from a mature leaf to storage cellsin the stem,itmust be hydrolyzed to itscomponenthexoses(glucoseandfructose)priortostarchsynthesis. Theresulting hexoses arethen available toenterthestarch biosynthetic pathway, resulting in accumulation of starch in the ocotillo stem.

Theoppositesequencewouldoccurduringleafgrowth. Afterreceiptofadequate water,aseriesofeventsmightbeinitiatedas follows: 1) stemstarchreservesare hydrolyzed;2)sucroseissynthesizedwithinstemcells;3)sucroseisexportedintothe young,expandingleaves;and4)sucroseishydrolyzedintothecomponenthexosesthatare readily available for leaf growth and cell expansion.

94 Calcium

EDTA

LEAF

Figure1. HPLC traces of exudates collected from leaves heldiriwater, leaves held in EDTA plus excess calcium, and EDTA.A representative chromatogram of leaf tissueis also given at bottom.Key: S = sucrose; G = glucose; F = fructose; U = unknown; O = oligosaccharide.

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0 0 5 10 15 20 25 30 HOURS OF EXUDATION Figure 2. Cumulative exudation of sucrose from leaves held in EDTA, water, or excess calcium. Data from both experiments are shown.