关于秋的美文2Whydoleaveschangecolorinthefall?asksPatriciaBrown,ofNewYorkCity.Autumn'scooldaysaretrimmedwithdeepblueskiesandgoldenlight,andbrilliantleavesofyellow,orangeandred.Leaveschangingcolorinthefallareatree'swayofpreparingforlongwinter,ratherlikeweputupstormwindowsandpullwarmclothesandblanketsoutofstorage.Insummer,theleavesontreeslikepinoaksandsugarmaplesaregreenbecausetheyarechock-fullofthegreenpigmentchlorophyll.Treesneedsunlighttoproducechlorophyll.Inturn,chlorophyllusessunlight'senergytosplitwater(H2O)intohydrogenandoxygen.Meanwhile,leavesalsoabsorbcarbondioxidegasfromtheair.Theendproductsofleafchemistry:carbohydrates(homemadeplantfoodforthetree),andoxygen,releasedintotheair(thegasweneedtobreathe).Thewholeprocessiscalledphotosynthesis.Alongwithgreenchlorophyll,mostleavesalsocontainyellow,orangeandred-orangepigmentscelledcarotenoids.Treesdon'tneedlighttomakecarotenoids.Botanistscallthem"helperpigments,"becausecarotenoidsabsorbsomesunlightand(nicely)passtheenergyalongtochlorophyll.Wedon'tseemuchofthesedeputypigments(carotene,lycopene,andxanthophyll)insummer,becausetheyaremaskedbyabundantgreenchlorophyll.Buttheever-shorteningdaysoffallmeanlessdaylightandcolderweather.Theaveragetreeisrushingtosaveallthenutrientsitcanforitswinterhibernation.Nitrogenandphosphorusarepulledfromleavesforstorageinbranches.Alayerofcorkycellsgrowsbetweentheleaves'stemsandtheirbranches,reducingtheleaves'supplyofnutrientsandwater.Withdiminishedsunlight,water,andnutrients,chlorophyllsynthesisslows.Old,worn-outchlorophyllbreaksdownattheusualrate--ironically,sunlightdestroysit--soeachleaf'sstockgraduallydwindles.Andasthegreenfades,yellowandorangeemergefromhiding.Unlikethegreenandyellowpigments,redandpurplepigments(anthocyanins,partoftheflavonoidclass)actuallyforminleavesintheautumn,tintingleavesscarletandburgundy.Botanistshavelongwonderedwhysometreesaregeneticallyprogrammedtomanufactureanthocyaninsinthefall.Newresearchindicatesthatanthocyaninsmaybeatree'sownsunscreen.Anthocyaninsaremadeinaleaf'ssugarysap,withthehelpoflotsofsunandcooltemperatures.Botaniststhinkthatanthocyaninsshieldtheleaves'fadingphotosynthesisfactoriesfromtoomuchsunlight,ratherlikethepigmentmelaninprotectsourskinfromthesun.Whiletheredpigmentsactasashield,thetreefeverishlybreaksdownandpullsnutrientsoutofleavesandintoitslimbsandtrunkbeforeleavesdropordie.AnthocyaninsmayalsoactlikeVitaminCorE,scavengingso-called"freeradicals"beforetheycandooxidizingdamagetoafallleaf'sfragilestructure.Upperandouterleavestendtobereddest,sincetheyaremostexposedtosunlightandcold.Insometrees,likesugarmaples,theredsoftheanthocyaninscombinedwiththeyellowsofthecarotenoidsmakeespeciallybrilliantorangeleaves.
