Tropical trees in South America “drink” in the dark — and carry a genomic secret that could unlock drought-resistant crops
Around 1800, Alexander von Humboldt placed a tropical tree leaf in sunlit water and watched — and waited. No oxygen bubbles appeared. Every other plant he’d tested behaved differently in light. This one simply didn’t.
The observation went unresolved for roughly two centuries. Now, a team of researchers at the University of Vienna has finally traced the answer back to its source — and what they found in these trees’ genomes turned out to be far stranger and more intricate than the puzzle Humboldt left behind.
A 200-year-old mystery finally gets a genomic answer
Humboldt’s puzzling leaf belonged to a Clusia tree — a tropical genus with a radically different relationship with sunlight. Most plants open their stomata during the day to absorb CO₂ and release oxygen. Clusia trees do the opposite. Those tiny pores stay firmly shut while the sun is up, preventing water loss through evaporation. Instead, CO₂ is absorbed at night, bound chemically, and stored as malic acid — ready to fuel photosynthesis once daylight arrives.
This strategy is called CAM photosynthesis, short for Crassulacean Acid Metabolism. It’s a well-documented survival mechanism, common in cacti and succulents. What makes Clusia exceptional is that it contains the only known trees that use it — and its species span the entire photosynthetic spectrum, from ordinary daytime CO₂ absorption all the way to strong, committed CAM.
That range made Clusia an irresistible subject for researchers at the University of Vienna. Their central question: how did such different CAM strategies emerge within a single genus? The answer was buried deep in the trees’ genomes.
Three trees, three genomes, three very different strategies
The team focused on three Clusia species, each representing a distinct photosynthetic approach. Clusia rosea is a strong CAM practitioner, storing large amounts of malic acid every night. Clusia minor takes a more conditional approach, switching CAM on mainly under stress. Clusia major runs both systems simultaneously, blending conventional C3 photosynthesis with CAM in a hybrid strategy.
To understand why, researchers combined full genome sequencing with real-environment greenhouse monitoring. Water availability was varied, and each tree was tracked throughout the day — measuring physiology alongside gene activity, protein levels, and metabolic products. The result was a comprehensive, multi-layered portrait of how each species actually functions.
The differences were striking, and traceable. Physiological gaps between the three species weren’t random; they mapped directly onto distinct patterns of gene activity and metabolism, giving scientists a rare opportunity to connect genomic structure with observable plant behavior.
Ancient genome duplications rewired photosynthesis over millions of years
The deeper explanation lies in evolutionary deep time. All three Clusia species are ancient polyploids — their entire genomes were duplicated at some point in their evolutionary past. That kind of wholesale multiplication isn’t unusual in plants. What happened next, however, shaped everything.
Over millions of years, those enlarged genomes were gradually reorganized through a process called diploidization. Gene copies were lost, silenced, or repurposed — not simply carried along as redundant extras, but transformed by the reshuffling. “In the process, gene copies are lost, deactivated or take on new functions,” explains lead author Hannes Kramml. Co-lead author Johannes Herpell adds that genes crucial for nocturnal CO₂ storage in CAM metabolism were particularly affected by this reorganization.
The result wasn’t one version of CAM but several, each shaped by a different trajectory of genomic change. Study leader Wolfram Weckwerth put it plainly: “The genomes have not simply multiplied; over millions of years, they have been reorganized, reduced and functionally rewired.” That plasticity, he argues, explains why different Clusia species ended up with such different photosynthetic strategies despite sharing a common ancestor.
What Clusia’s survival trick could mean for future crops
The implications reach well beyond tropical forests. CAM plants use substantially less water than those relying on conventional photosynthesis — a meaningful advantage as climate change intensifies drought conditions across agricultural regions worldwide. That efficiency has long attracted the attention of crop scientists.
The newly mapped genomic data from Clusia could help researchers identify the specific metabolic pathways responsible for efficient CO₂ fixation and high water-use efficiency. Knowing which genes drive strong CAM, and how those genes were assembled through genome reorganization, gives scientists a more detailed blueprint than they’ve previously had. Two distinct questions — which genes matter, and how they got there — now have clearer answers.
The pathway from tropical tree genome to drought-resistant wheat or maize is long. This is foundational research; it won’t produce a new crop variety tomorrow. But it does provide a detailed new map of the evolutionary machinery behind one of nature’s most effective water-saving strategies.
As droughts grow more frequent and arable land more fragile, that map only becomes more valuable. Researchers will likely use these Clusia genomes as a reference point for identifying analogous genetic features in other species — and potentially for guiding targeted efforts to engineer greater water efficiency into staple crops. The trees Humboldt puzzled over two centuries ago may yet have something practical to offer a world that’s running short on rain.
More information about this exciting discovery is available here: Hannes M. Kramml, Johannes B. Herpell, Clara Priemer, Zoe Wessely, Florian Schindler, Andreas Berger, Maximilian Kellner, Stefan Plott, Ágnes Dohovits, Tamara Schmidt, Peter Kerpan, Leila Afjehi-Sadat, Palak Chaturvedi, Arindam Ghatak, Martin Brenner, Iro Pierides, Lena Fragner, Eva M. Temsch, Fabio Trevisan, Menriti Ibrahim, Felix Fromwald, Anke Bellaire, Oleg Simakov, Werner Huber, Ulrich Lüttge, Ovidiu Paun, Susann Wicke, Hanna Weiss-Schneeweiss, Gert Bachmann, Wolfram Weckwerth. Clusia genomes shed light on the evolution and diversity of crassulacean acid metabolism physiotypes. Nature Communications, 2026; 17 (1) DOI: 10.1038/s41467-026-71958-z
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