ENGLISH ABSTRACT: The bright colouration of involucral leaves in Leucadendron is unfortunately transient in nature. Undesirable colour changes render this cut flower unmarketable, resulting in a considerable loss of profit. A deeper understanding of the mechanism leading to colour change is needed to form the framework on which future manipulation strategies can be built.
Yellow Leucadendron possess the ability to degreen and regreen naturally, a phenomenon linked to the controlled degradation of chlorophyll and the lesser degradation of carotenoids, which then impart the yellow colour. This colour change is directly linked to the development of the inflorescence. Involucral leaves degreen towards anthesis and are entirely yellow at full bloom. They begin to regreen again when the last florets on the cone have wilted. Deconing before flowering completely inhibits the colour change. Deconing at full bloom, results in leaves regreening sooner. Therefore the inflorescence appears to be the origin of the cue for colour change. Any factor that expedites the death of the florets, results in sooner regreening of involucral leaves. Ultra-structurally, the degreening and regreening resulted from a transdifferentiation of mature chloroplasts to gerontoplast-like plastids, which upon regreening completely redifferentiated into fully functional chloroplasts.
In the red Leucadendron cultivar Safari Sunset, the photosynthetic pigment degradation pattern is identical to that of yellow cultivars. However, colour expression is complexed by the presence of anthocyanins. Anthocyanin concentration was shown to be directly related to the opening of the flower head rather than to the phenological development of the inflorescence. With opening, the previously shaded inner involucral leaf surfaces are exposed to higher levels of irradiance and respond by turning red, presumably for photoprotection. Similar to yellow cultivars, any factor leading to the death of the florets before flowering, not only prevents the degreening of involucral leaves, but also prevents the opening of the flower head and therefore the associated change in anthocyanin levels. The ecological significance of regreening was also investigated. What does a female Leucadendron plant stand to gain by regreening rather than discarding the involucral leaves? Regreened involucral leaves were shown not to play a significant role in providing photosynthates for the developing cone. Although the presence of regreened involucral leaves were shown to provide protection against high irradiance and radiant heating of the cone, they were not essential to ensure survival of the cone. The small floral bracts were shown to be very capable of adaptation. The most plausible reason for regreening is therefore assumed to be based on a cost-benefit relationship. As most Leucadendron are adapted to grow on very nutrient poor soils, the question should maybe be rephrased. Why waste valuable resources? Sclerophyllous leaves, like the involucral leaves, are costly to make and therefore reusing, rather than discarding them does seem a sensible strategy for survival. AFRIKAANSE OPSOMMING: Leucadendron snyblomme word gekenmerk deur die helder kleure van hul omwindselblare. Die helder kleure is egter slegs vir 'n kort periode aanwesig waarna die snyblomme onbemarkbaar word, met aansienlike verlies aan potensiele inkomste. Die ontwikkeling van manipulasies ten einde die bemarkbare periode van Leucadendron te verleng, berus op die verkryging van 'n dieper insig in die meganisme van kleurverandering.
Die kleurveranderinge van geel Leucadendron omwindselblare is te wyte aan 'n unieke vermoë tot die gereguleerde degradasie en heropbou van chlorofiele en karotenoiede onder direkte beheer van die ontwikkelende bloeiwyse. Met die aanvang van blom, lei groter proporsionele degradasie van chlorofiele tot geleidelike vergeling van omwindselblare. Die hele blomhofie verkry uiteindelik met volblom 'n helder geel kleur. Sodra die laaste blommetjies doodgaan, neem chlorofiel- en karotenoiedsintese weer in aanvang en binnekort is die omwindselblare weer net so groen soos voor die aanvang van blom. Die geel verkleuring kan verhoed word deur die keël voor blom uit te breek. Enige faktor wat die dood van die blommetjies versnel, asook die uitbreek van keël tydens volblom, lei tot die vroeëre aanvang van vergroening. Die degradasie van plastiedpigmente hang nou saam met die differensiasie van volwasse chloroplaste tot gerontoplast-agtige plastiede wat op hul beurt weer tydens vergroening tot volkome funksionele chloroplaste herdifferensieer.
Soortgelyk aan geel Leucadendron kultivars, vind die veranderinge in plastiedpigmente ook plaas tydens blom van die rooi kultivar, Safari Sunset. Kleurveranderinge in 'Safari Sunset' is egter meer ingewikkeld vanweë die aanwesigheid van variërende konsentrasies antosianiene. Antosianienkonsentrasies en rooi kleur neem toe tydens blom vanwee die blootstelling van die beskutte adaksiale binnekante van omwindselblare aan hoe irradiasie met die oopvou van die blomhofie. Die akkumulasie van antosianiene het moontlik 'n fotobeskermende funksie. Kleurveranderinge in 'Safari Sunset' kan, soos in geel kultivars, voorkom word deur blom te verhoed. Antosianiensintese word voorkom deurdat die blomhofie geslote bly en is nie direk gekoppel aan blom soos wat met plastiedpigmente die geval is nie.
Die belang van vergroening is ondersoek na aanleiding van die vraag oor wat dit 'n vroulike Leucadendron baat om omwindselblare te behou na die afloop van blom? Die bydrae van foto-assimilasie deur omwindselblare tot die ontwikkeling van keels is beperk. Alhoewel omwindselblare wel keels teen hoe irradiasie en stralingsverhitting beskerm, is die blomskutblare in staat om aan te pas by hierdie kondisies. Die mees waarskynlike verklaring vir die behoud van die omwindselblare na blom berus moontlik op 'n koste-voordele verwantskap. Alhoewel nie essensieel nie, is die beperkte bydrae van die omwindselblare na die afloop van blom tot die oorlewing en welstand van die keel waarskynlik genoegsaam om hul behoud te regverdig. Verskeie Leucadendron spesies groei in gronde wat baie arm is aan nutriente. Sklerefiele blare, soos die van Leucadendron, is verder duur om te vervaardig. Dit maak dus sin om hulle vir meer as een funksie te herontplooi eerder as om hulpbronne te belê in meer gespesialiseerde en minder durende blombykomstighede. Dus dui die behoud van omwindselblare dalk op 'n strategie wat gemik is op die behoud en besparing van beperkte hulpbronne. Description:
Thesis (PhD(Agric)--University of Stellenbosch, 2010.
Doctoral Degrees (Horticulture)