Plant-water relations of the mangrove species Rhizophora stylosa: a unique story
Bomen kopje-onder en met de voeten in de zee
Net zoals de echte Robinson Crusoe onder ons weet dat dorst lessen met zeewater alles behalve slim is, vermijden ook bomen dit zodat ze niet gaan uitdrogen. Net zoals wij moeten slapen om rust en herstel een kans te geven, groeien en herstellen bomen tijdens de nacht. En net zoals wij, moeten bomen ook ademen en iedereen weet dat ademen onder water niet echt goed lukt. Maar wat als ik je zou vertellen dat er een boom bestaat die zich van al het voorgaande helemaal niets aantrekt? Maak kennis met de mangroveboom Rhizophora stylosa.
Wat doe je als boom als je de helft van de dag kopje-onder staat en je wortels zich heel de dag in een zoute grond bevinden? Mangrovebomen staan erom bekend dat ze na vele aanpassingen en evoluties in deze omstandigheden kunnen overleven.
Mangroves worden gekenmerkt door het feit dat ze op regelmatige basis worden overspoeld door zeewater. Het zout uit dit water zorgt voor een extreem zoute bodem. Bovendien sluit het water alle zuurstoftoevoer naar de wortels af. Net zoals voor mensen is water voor bomen onmisbaar. Daarom zijn de wortels zo belangrijk voor bomen en vergen ze veel energie van de boom. Deze energie kan, net zoals bij de mensen, enkel worden geleverd door te ademen en hiervoor is natuurlijk zuurstof nodig. Om te kunnen ademen wanneer de wortels overspoeld zijn door de zee hebben mangrovebomen grote holtes in de wortels ontwikkeld die dienst doen als luchtreservoir. Mangrovebomen maken veel wortels boven de grond aan om, wanneer de zee wegtrekt, zoveel mogelijk te kunnen ademen en zuurstof op te slaan. Dit geeft mangroves hun typische uitzicht met wortels die meer weg hebben van tentakels, zichtbaar op Figuren 1 en 2.
Zo vermijden mangrovebomen om in ademnood te komen, maar dan hebben ze nog steeds de nog grotere zoutuitdagingen niet opgelost. Onaangepaste bomen zouden leeglopen omdat zout in de bodem water aanzuigt. Mangrovebomen ontwikkelen echter een nog sterkere zuigkracht in de bladeren om toch water te kunnen opnemen. Daarenboven werken hun wortels als filters waardoor er relatief zoet water in de boom circuleert. Waarom moeten bomen zoveel water drinken? Net zoals een mens, transpireert een boom ook. Door te zweten kunnen bomen hun bladeren koelen. Hoe beter ze hun bladeren kunnen afkoelen, hoe efficiënter het fotosynthetisch apparaat, de motor van de boom, kan werken. Om voldoende te kunnen afkoelen moet er dus ook voldoende water beschikbaar zijn om te transpireren.
Mangrovebomen hebben na vele miljoenen jaren van evolutie allerhande opmerkelijke aanpassingen ondergaan om zo efficiënt mogelijk om te gaan met water. Net zoals je zuinige auto's hebt, zijn mangrovebomen zuinige bomen. Ze verbruiken weinig water per gram voedingsstof die ze produceren. Op die manier moeten ze minder grote hoeveelheden water opzuigen en ondervinden ze minder problemen met het zoute water waarin ze groeien.
Dit onderzoek heeft zich gericht op één specifieke mangrovesoort, Rhizophora stylosa. Deze soort heeft nog vele geheimen en dit onderzoek ontrafelde welke specifieke aanpassingen deze boom heeft ondergaan om te overleven in gebieden die worden overspoeld door het getij. De combinatie van een intense subtropische zon en een zoute bodem zorgt immers voor een enorme droogtestress. De intense zon zorgt voor veel transpiratie die nodig is om de bladeren koel te houden. De wateropname die hierdoor nodig is wordt evenwel sterk bemoeilijkt door het zout dat aanwezig is in de bodem. De soort Rhizophora stylosa heeft echter een zeer elegante oplossing gevonden om geen al te grote problemen te ondervinden onder deze stressvolle omstandigheden.
Maar laat ons eerst eens kijken hoe gewone bomen, die niet geconfronteerd worden met zoutstress, groeien. Naast jaarlijkse groeipatronen, waarbij jaarringen een gekend voorbeeld zijn, bestaan ook dagelijkse groeipatronen. Alle bomen krimpen en zwellen dag in dag uit. Nochtans zie je geen verschil in de dikte van een boom wanneer je een boom 's morgens of 's avonds bekijkt. Dit komt omdat deze dagelijkse groeipatronen zich afspelen in de grootteorde van een duizendste van een millimeter: niet meteen zichtbaar voor het blote oog, maar wel detecteerbaar door supergevoelige sensoren die deze verschillen kunnen meten.
Het klassieke groeipatroon, dat bij bijna alle bomen waargenomen wordt, is een zwelfase tijdens de nacht en een krimpfase overdag, in het zwart afgebeeld in Figuur 3. De oorzaak van deze krimp ligt bij het waterverlies, veroorzaakt door transpiratie. Wanneer de zon ondergaat, stopt de transpiratie en zorgen de wortels ervoor dat er genoeg water wordt opgenomen zodat de boom kan zwellen en dus groeien.
Het groeipatroon van Rhizophora stylosa is echter helemaal anders, zoals in het rood afgebeeld in Figuur 3. De stam zwelt 's morgens en krimpt tijdens de namiddag, terwijl de stamdiameter 's nachts helemaal niet verandert. Om dit onverwachte resultaat te verklaren moest een unieke hypothese bedacht worden.
De hypothese stelt dat deze boom de suikers die aangemaakt worden tijdens de fotosynthese anders gebruikt en transporteert. Suikers zijn, net zoals bij de mens, de basisbouwstenen voor de boom. De suikers gevormd door Rhizophora stylosa hebben een osmotische werking wat wil zeggen dat ze water aanzuigen in de plantcellen. Het water dat in de boom omhoog gezogen wordt kan zo in belangrijke mate naar de cellen gaan in plaats van door transpiratie te verdampen via de bladeren en verloren te gaan in de atmosfeer. Cellen die voldoende water hebben functioneren veel beter en door ervoor te zorgen dat er genoeg water in de cellen aanwezig is op het moment dat er veel zonne-energie is, kan deze boom efficiënt groeien. Dit verklaart dus waarom het de perfecte keuze is voor een mangroveboom om overdag te groeien in plaats van 's nachts.
Dit verhaal bevestigt eens te meer dat bomen in staat zijn om zeer dynamisch te reageren op veranderende omstandigheden, ondanks hun statisch voorkomen. Hoe langer hoe meer worden er unieke processen en nieuwe mechanismen ontdekt die verklaren hoe bomen zelfs in de meest barre condities kunnen overleven. Genoeg redenen dus om onderzoek te blijven doen naar de geheimen van de plantenwereld.
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