The Sites of Evaporation within Leaves

Abstract

The sites of evaporation within leaves are unknown, but they have drawn attention for decades due to their perceived implications for many factors, including patterns of leaf isotopic enrichment, the maintenance of mesophyll water status, stomatal regulation, and the interpretation of measured stomatal and leaf hydraulic conductances. We used a spatially explicit model of coupled water and heat transport outside the xylem, MOFLO 2.0, to map the distribution of net evaporation across leaf tissues in relation to anatomy and environmental parameters. Our results corroborate earlier predictions that most evaporation occurs from the epidermis at low light and moderate humidity but that the mesophyll contributes substantially when the leaf center is warmed by light absorption, and more so under high humidity. We also found that the bundle sheath provides a significant minority of evaporation (15% in darkness and 18% in high light), that the vertical center of amphistomatous leaves supports net condensation, and that vertical temperature gradients caused by light absorption vary over 10-fold across species, reaching 0.3°C. We show that several hypotheses that depend on the evaporating sites require revision in light of our findings, including that experimental measurements of stomatal and hydraulic conductances should be affected directly by changes in the location of the evaporating sites. We propose a new conceptual model that accounts for mixed-phase water transport outside the xylem. These conclusions have far-reaching implications for inferences in leaf hydraulics, gas exchange, water use, and isotope physiology.The pathways for water transport through a plant are often perceived to end at the sites of evaporation outside the xylem within leaves (Holmgren et al

Unexpected Connections between Humidity and Ion Transport Discovered Using a Model to Bridge Guard Cell-to-Leaf Scales

“…The second model posits evaporation from the guard cells directly, leading to their water loss and stomatal closure. However, a growing body of evidence indicates that the bulk of the evaporation within the leaf occurs well away from the guard cells, dominated instead by evaporation from the vascular tissue, nearby mesophyll and pavement cells (Pieruschka et al, 2010;Rockwell et al, 2014;Buckley et al, 2017). These studies imply that water in the guard cell is maintained primarily by equilibration with water in the vapor phase within the adjacent substomatal cavity and the stomatal pore rather than by liquid flow through the cell wall and neighboring cells.…”

“…Similarly, we define this exchange to take place with the potential of liquid water averaged over the guard cell surface. Beyond exchange with the guard cell, the p concept does not specify the tissues from which the bulk of evaporation occurs and accommodates evaporation from the mesophyll as well as from the inner surface of the epidermis (Buckley et al, 2017). Since the conductivity for liquid water to the stomatal pore will be much lower than that for water vapor within the leaf air space, cells at different points in the leaf along the transpiration path will exchange with different partial pressures of water in the vapor phase.…”

“…Thus, in the steady state, w p can be sought through the balance between diffusion through the pore and delivery from the sites of evaporation (see Appendix). As noted above, evaporation within the leaf is a complex function of factors that include water delivery through the xylem, and its transfer across the evaporative surfaces of the surrounding mesophyll and nearby epidermal cells (Sack and Holbrook, 2006;Rockwell et al, 2014;Buckley et al, 2017). The evaporative surface area, A W (Figure 1), is thought to vary substantially with hydraulic conductance through the xylem (Rockwell et al, 2014) as well as environmental factors, including w air (Buckley et al, 2017).…”

“…To accommodate evidence of evaporation away from the guard cells (Pieruschka et al, 2010;Rockwell et al, 2014;Buckley et al, 2017), we assume that water in the guard cell is maintained by equilibration with water in the vapor phase within the adjacent substomatal cavity and the stomatal pore. No system undergoing steady state flux can be fully at equilibrium, but it is an adequate approximation for evaporation, water vapor diffusion, and exchange inside a leaf.…”

Stomatal Response to Humidity: Blurring the Boundary between Active and Passive Movement

“…In these circumstances, the assumption leads to an overestimation of the VPD across the stomatal pore and, hence, an underestimation of the true g s (Buckley et al, 2017), potentially accounting for the apparent drop in g s with VPD steps observed by Merilo et al (2018). To eliminate this explanation, alternative porometry techniques (Farquhar and Raschke, 1978;Pantin et al, 2013) are needed to assess g s independent of evapotranspiration.…”