Mesophyll conductance limits photosynthesis and relates to anatomical traits in high-elevation plants in the Andes

Plants face harsher conditions with increasing elevation, including shorter growing seasons, lower temperatures, and reduced gas pressure. This often leads to increased leaf mass per area, suggesting greater limitation to photosynthesis due to decreased mesophyll conductance. However, some species maintain consistent photosynthetic rates at higher elevations, suggesting compensatory mechanisms. In the central Chile Andes, high-elevation habitats present cold temperatures with no soil moisture deficits, whereas low-elevations experience warm temperatures and summer droughts. Zonal plants adapt to these changes, whereas azonal plants grow near water sources and avoid drought. We assessed how elevation affects photosynthesis and its limitations in these plant-types, together with the role of leaf internal anatomy. This was done with gas exchange and chlorophyll fluorescence analyses, along with measurements of leaf inner structure, on zonal and azonal species growing at 2600 and 3550 m a.s.l. Results showed that whilst photosynthesis decreased with elevation in azonal plants, zonal plants showed no change, with mesophyll conductance being a primary limitation, influenced by chloroplast arrangement rather that cell wall thickness. This affects carbon acquisition in high-elevation plants due to low gas pressure and light availability.