The wonders of leaf hydraulics: efficiency-safety trade offs in woody plants

It is estimated that each year, more than 40 trillion tons of water move through plant leaves. Plants take up water, which moves through the plant, and then some of it evaporates on the leaf surface. Whole-plant hydraulics is an understudied system despite its importance in plant functioning and drought resistance. 

Dr Shi-Dan Zhu and Yong-Qiang Wang from Guangxi University and colleagues from the South China Botanical Garden investigated the relationships between leaf biomechanical (e.g. thickness, force to punch), structural (e.g. vein density) and leaf hydraulic conductance of 30 subtropical woody species. 

The researchers found biomechanical resistance correlated with hydraulic safety and outside-xylem leaf hydraulic conductance but not with leaf anatomical traits. These findings are surprising as they go against the previously thought, “the bigger and tougher the leaves are, the more resistant the leaves are against leaf hydraulic risk (e.g. wilting due to drought)” rule. 

First, plant anatomy comes with its own vocabulary. Below is a brief overview of some terms related to hydraulics. 

Leaf hydraulic conductance (K_leaf) is a measure of how efficiently water is transported through the leaf, determined as the ratio of water flow rate (F_leaf) through the leaf. Leaf hydraulic conductance varies more than 65-fold across species, reflecting the diversity of the petiole and leaf anatomy of different plants. Botanists can use hydraulic safety margin (HSM) to assess the degree of hydraulic risk (e.g. wilting due to drought) for a species. HSM is defined as the difference between the minimum water potential and water potential causing 50% loss of hydraulic conductivity. Scientists quantify leaf biomechanical resistance (LBR) in terms of leaf force to punch (F_p) and force to tear (F_t) per unit fracture length or per unit width, which reflect the structural and material resistance of leaves against herbivores and physical damage. In the leaves and roots, water moves through both the xylem and living cells outside the xylem.

Zhu, Wang and colleagues collected plant samples from 30 woody species around the Dinghushan Forest Ecosystem Research Station in China to investigate the relationships between leaf biomechanical resistance, leaf hydraulic conductance and leaf anatomy. 

There is a significant difference in rainfall between the wet and dry seasons in this area. The team measured the minimal leaf water potential in the dry season whilst they measured all the other traits (e.g., force to tear, force to punch, leaf mass per unit area, vein density, leaf water potential, K_leaf) in the wet season.

Zhu, Wang and colleagues found a leaf hydraulic efficiency-safety trade-off amongst the sampled woody plant species. Species with higher leaf biomechanical resistance (LBR; more resistant to punching and tearing) had greater leaf hydraulic safety, but leaf hydraulic conductance (K_leaf; water flow rate efficiency) did not correlate with LBR. 

“The findings of this study provide insights into the biomechanics–safety–efficiency–elasticity relationships in leaves,” Zhu, Wang and colleagues write.

Whilst there was no overall correlation between biomechanical resistance and leaf hydraulic conductance in the xylem (e.g., through the veins), there was a correlation outside the xylem (e.g. intercellular air spaces). It was only recently that researchers realised the importance of outside-xylem hydraulic conductance, especially when it comes to drought response. During drought, the leaf hydraulic conductance declines mainly because of mesophyll turgor loss and the collapse of minor-vein conduits. Once the water status drops below a critical threshold, the leaves are damaged and senesce due to hydraulic dysfunction. 

“Further attention should thus be focused on the outside-xylem pathways of leaves with respect to determining the correlation between leaf hydraulics and biomechanics in more species and biomes,” recommend the researchers.

This study highlights the need for basic science and how much we still do not understand about plant hydraulics.