Ben jij ook niet jezelf in de ochtend tot je een kopje koffie op hebt? Helaas staat dit dagelijks ritueel onder druk. Koffieplanten krijgen het namelijk moeilijk door het veranderend klimaat. Gelukkig is er hoop: door de bladeren van 58 wilde koffiesoorten in Afrika te bestuderen, zocht een student aan de KU Leuven in samenwerking met de Plantentuin van Meise uit hoe we onze koffieplanten weerbaarder kunnen maken.
Een gevarieerd kopje koffie
Sterkere koffieplanten kweken is alleen mogelijk als we genoeg weten over welke kenmerken geschikt zijn in welk klimaat, in welke koffiesoorten we die kenmerken kunnen vinden en hoe die kenmerken geëvolueerd zijn doorheen de geschiedenis. Voor de commerciële koffieteelt worden namelijk slechts twee of drie soorten gebruikt, maar er zijn ook meer dan honderd wilde koffiesoorten, die gebruikt kunnen worden in kruisingen en kweekprogramma’s.
Waarom bladeren?
Aangezien bladeren een grote invloed hebben op de gezondheid van een plant, zijn bladkenmerken een ideaal doelwit om te optimaliseren. Niet alle bladeren zijn gelijk: sommige soorten hebben grotere of dikkere bladeren dan andere. Daarnaast kan ook de hoeveelheid of de grootte van de huidmondjes verschillen: dit zijn microscopische poriën op het blad die de plant gebruikt om CO2 op te nemen voor de fotosynthese. Door deze poriën verdampt er ook water naar de omgeving, waardoor een grotere CO2-opname ook gepaard gaat met een groter risico op uitdroging. Dat is belangrijk om te weten, aangezien de regio’s waar koffie groeit steeds droger worden door de klimaatsverandering. Door te onderzoeken of deze evolutionaire verschillen in bladkenmerken tussen soorten ontstaan zijn als reactie op bepaalde klimaten, kunnen we meer te weten komen over welke kenmerken het best aangepast zijn aan bepaalde omgevingen.
Grote bladeren, veel grote huidmondjes
Ten eerste toont het onderzoek aan dat er best wel wat variatie zit in de bladkenmerken van de koffiesoorten. Dat is goed nieuws, want dat betekent dat kweekprogramma’s voldoende gevarieerd plantenmateriaal hebben om mee te werken. Daarnaast was er ook een duidelijke afweging tussen de grootte van de huidmondjes en hun dichtheid, ofwel de hoeveelheid huidmondjes per vierkante millimeter. Dat is logisch, want hoe groter de huidmondjes, hoe minder er in een vierkante millimeter passen. Wel opvallend was het feit dat in soorten met grotere bladeren, zowel de grootte als de dichtheid van de huidmondjes kon toenemen. Soorten met grote bladeren komen ook voor in vochtigere omgevingen, zo blijkt uit het onderzoek. Dat wil dus zeggen dat als het risico op uitdroging kleiner is, de planten een groter deel van hun bladoppervlak kunnen opvullen met huidmondjes. Zo kunnen ze dus meer CO2 opnemen om te groeien.
De evolutie van koffie in het wild
Het onderzoek wil ook bekijken hoe deze bladkenmerken precies geëvolueerd zijn doorheen het Coffea-genus, het genus dat alle koffiesoorten omvat. Hoe zijn de verschillende koffiesoorten gediversifieerd, d.w.z. van een gemeenschappelijke voorouder weg geëvolueerd tot verschillende soorten? Was er ongeremde evolutie in een bepaalde richting, was de evolutie willekeurig of werd de diversificatie op een of andere manier afgeremd? Als we de evolutionaire modellen mogen geloven, dan lijkt er inderdaad een bepaalde afremming te zijn geweest in de diversificatie. Die afremming kan ontstaan door natuurlijke selectie, ofwel omdat er bepaalde genetische beperkingen zijn die ervoor zorgen dat extreme kenmerken (zoals heel grote of kleine bladeren, of heel veel of weinig huidmondjes) niet kunnen evolueren. Toch blijken twee heel nauw verwante eilandsoorten, waarvan er één op Mauritius en één op Réunion voorkomt, sterk verschillende kenmerken te hebben. Die soorten zijn nog maar redelijk recent van elkaar afgesplitst, wat er dus op wijst dat snelle diversificatie wel degelijk mogelijk is en dat genetische beperkingen geen grote rol gespeeld hebben in de evolutie van het genus. Er moet dus stabiliserende selectie zijn opgetreden, waardoor extreme kenmerken minder voordelig waren dan meer gematigde kenmerken.
Koffie is niet alleen iets om gezellig te drinken thuis of in een koffiebar – de teelt van koffie is ook een bron van inkomsten voor miljoenen mensen. Om koffietelers een toekomst te geven, is het belangrijk dat onze koffieteelt in stand gehouden wordt. Dit onderzoek biedt belangrijke inzichten in de evolutie van het koffie-genus en brengt wat hoop voor toekomstige koffieproductie. Hoe de inzichten in de toekomst zullen worden toegepast, is echter nog koffiedik kijken.
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