Water transport from stem to stomata: The coordination of hydraulic and gas exchange traits across 33 subtropical woody species

Authors: LIU, XIAORONG - CHINESE ACADEMY OF SCIENCES; LIU, HUI - CHINESE ACADEMY OF SCIENCES; Gleason, Sean; ZHU, SHIDAN - GUANGXI UNIVERSITY; HE, PENGCHENG - CHINESE ACADEMY OF SCIENCES; HOU, HAO - CHINESE ACADEMY OF SCIENCES; LI, RONGHUA - CHINESE ACADEMY OF SCIENCES; YE, QING - CHINESE ACADEMY OF SCIENCES

Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2019
Publication Date: 7/24/2019
Citation: Liu, X., Liu, H., Gleason, S.M., Zhu, S., He, P., Hou, H., Li, R., Ye, Q. 2019. Water transport from stem to stomata: The coordination of hydraulic and gas exchange traits across 33 subtropical woody species. Tree Physiology. 39(10):1665-1674. https://doi.org/10.1093/treephys/tpz076.
DOI: https://doi.org/10.1093/treephys/tpz076

Interpretive Summary: Plants must transport water from the soil to the leaves so that photosynthesis can take place. As such, we should logically expect the water transport capacities of xylem tissue (the water-transporting tissue) in roots, stems, and leaves to be roughly aligned with one another. However, despite this logical expectation, there exists conflicting evidence on the strength of the alignment between stems, leaves, and the measured rates of water loss from the leaf surface. This suggests that further clarification is needed to support current conceptual models of water transport in vascular species. Towards this goal, we measured the capacities of stems and leaves to transport water across 33 vascular species. Furthermore, we measured water transport in leaves through both the xylem tissue as well as the water transport pathway after water leaves the xylem ("outside xylem" transport). We found that the maximal rates of water transport through the xylem of stems and leaves were strongly aligned with the rates of water loss from the leaf surface. In contrast to this, the maximal rates of water transport through the "outside xylem" tissues of leaves was not strongly aligned with water transport through stems nor directly with rates of water loss from the leaf surface. These results suggest that measurements of water transport in leaves must include transport through xylem tissues as well tissues beyond (i.e., "outside") the xylem tissue if we are to understand the consequences (and benefits) of water transport on photosynthesis and growth.

Technical Abstract: Positive coordination between stem xylem-specific hydraulic conductivity (Kstem) and stomatal conductance (gs) has been identified in previous studies, however, coordination between Kstem and leaf hydraulic conductance (Kleaf), as well as between Kleaf and gs has not always been consistent. This suggests that there is need to improve our understanding of the hydraulic linkage among stems, leaves and stomata. Here, hydraulic traits (Kstem, Kleaf and gs) were measured for 33 co-occurring subtropical forest species. Kleaf was divided into two components: leaf hydraulic conductance inside the xylem (Kleaf-x) and outside the xylem (Kleaf-ox). We found that Kstem was positively correlated with Kleaf-x, but not with Kleaf-ox or Kleaf, despite Kstem, Kleaf, Kleaf-x and Kleaf-ox all being positively correlated with gs. Path analysis revealed significant and direct effects of Kstem and Kleaf-x on gs, whereas Kleaf-ox did not directly affect gs. Our results demonstrate that the hydraulic linkage from stem to stomata is weakened by the leaf water pathway outside the xylem, and that there is a tight linkage among Kstem, Kleaf-x, and gs. It is therefore essential to dissect water transport within a leaf into xylem and outside-xylem components to develop an accurate understanding of hydraulic coordination among stems, leaves and stomata.