Constant hydraulic supply enables optical monitoring of transpiration in a grass, a herb, and a conifer.

Plant transpiration is an inevitable consequence of photosynthesis and has a huge impact on the terrestrial carbon and water cycle, yet accurate and continuous monitoring of its dynamics is still challenging. Under well-watered conditions, canopy transpiration (Ec) could potentially be continuously calculated from stem water potential (Ψstem), but only if the root to stem hydraulic conductance (Kr-s) remains constant and plant capacitance is relatively small. We tested whether such an approach is viable by investigating whether Kr-s remains constant under a wide range of daytime transpiration rates in non-water-stressed plants. Optical dendrometers were used to continuously monitor tissue shrinkage, an accurate proxy of Ψstem, while Ec was manipulated in three species with contrasting morphological, anatomical, and phylogenetic identities: Tanacetum cinerariifolium, Zea mays, and Callitris rhomboidea. In all species, we found Kr-s to remain constant across a wide range of Ec, meaning that the dynamics of Ψstem could be used to monitor Ec. This was evidenced by the close agreement between measured Ec and that predicted from optically measured Ψstem. These results suggest that optical dendrometers enable both plant hydration and Ec to be monitored non-invasively and continuously in a range of woody and herbaceous species. This technique presents new opportunities to monitor transpiration under laboratory and field conditions in a diversity of woody, herbaceous, and grassy species.