C4 photosynthesis in Paulownia? A case of inaccurate citations

Societal Impact StatementC(4) photosynthesis is an ultra-efficient mode of photosynthesis found in some of our most productive crop species yet is notably rare in trees. Given C-4 photosynthesis is associated with high yield in herbaceous species, especially under hot and dry conditions, C-4 trees may seem an attractive prospect for biomass production and carbon sequestration in a rapidly changing climate. This may explain why some in the literature have optimistically linked C-4 photosynthesis with the exceptionally fast-growing tree Paulownia. However, this claim is lacking in evidence and represents an example of poor citation practices leading to the spread of misinformation. The rapid growth of trees in genus Paulownia (Paulowniaceae) has been attributed in the literature to their use of C-4 photosynthesis, a complex trait that confers increased photosynthetic efficiency under certain environmental conditions. After careful examination of citations used to support the idea that Paulownia species use C-4 photosynthesis, we find that there is no data underpinning this claim. Despite this, many investment schemes utilise information about the physiology of Paulownia, including photosynthetic type, to legitimise the use of Paulownia trees for financial investment and carbon offsetting.This study uses leaf physiology, anatomy and stable isotope data to determine whether or not three species in Paulownia (Paulownia tomentosa, Paulownia fortunei and Paulownia kawakamii) use C-4 photosynthesis. These data are compared with existing data for C-3 and C-4 woody species in the literature.We show that the leaf physiology, anatomy and stable isotope phenotypes of the three Paulownia trees considered in the study are not consistent with those of C-4 plants.Our findings highlight how inaccurate citation of scientific findings can contribute to the spread of misinformation beyond the scientific community, as some of those promoting investments in Paulownia plantations reference the photosynthetic superiority of Paulownia as a means to legitimise its use in carbon offsetting.