Measurement of Leaf Hydraulic Conductance and Stomatal Conductance and Their Responses to Irradiance and Dehydration Using the Evaporative Flux Method (EFM)

Abstract

Water is a key resource, and the plant water transport system sets limits on maximum growth and drought tolerance. When plants open their stomata to achieve a high stomatal conductance (g s ) to capture CO 2 for photosynthesis, water is lost by transpiration 1,2 . Water evaporating from the airspaces is replaced from cell walls, in turn drawing water from the xylem of leaf veins, in turn drawing from xylem in the stems and roots. As water is pulled through the system, it experiences hydraulic resistance, creating tension throughout the system and a low leaf water potential (Ψ leaf ). The leaf itself is a critical bottleneck in the whole plant system, accounting for on average 30% of the plant hydraulic resistance 3 . Leaf hydraulic conductance (K leaf = 1/ leaf hydraulic resistance) is the ratio of the water flow rate to the water potential gradient across the leaf, and summarizes the behavior of a complex system: water moves through the petiole and through several orders of veins, exits into the bundle sheath and passes through or around mesophyll cells before evaporating into the airspace and being transpired from the stomata. K leaf is of strong interest as an important physiological trait to compare species, quantifying the effectiveness of the leaf structure and physiology for water transport, and a key variable to investigate for its relationship to variation in structure (e.g., in leaf venation architecture) and its impacts on photosynthetic gas exchange. Further, K leaf responds strongly to the internal and external leaf environment 3 . K leaf can increase dramatically with irradiance apparently due to changes in the expression and activation of aquaporins, the proteins involved in water transport through membranes 4 , and K leaf declines strongly during drought, due to cavitation and/or collapse of xylem conduits, and/or loss of permeability in the extra-xylem tissues due to mesophyll and bundle sheath cell shrinkage or aquaporin deactivation [5][6][7][8][9][10] . Because K leaf can constrain g s and photosynthetic rate across species in well watered conditions and during drought, and thus limit whole-plant performance they may possibly determine species distributions especially as droughts increase in frequency and severity [11][12][13][14] . We present a simple method for simultaneous determination of K leaf and g s on excised leaves. A transpiring leaf is connected by its petiole to tubing running to a water source on a balance. The loss of water from the balance is recorded to calculate the flow rate through the leaf. When steady state transpiration (E, mmol • m -2 • s -1) is reached, g s is determined by dividing by vapor pressure deficit, and K leaf by dividing by the water potential driving force determined using a pressure chamber (K leaf = E /-Δψ leaf , MPa) 15 . This method can be used to assess K leaf responses to different irradiances and the vulnerability of K leaf to dehydration 14,16,17 .

Abscisic Acid Down-Regulates Hydraulic Conductance of Grapevine Leaves in Isohydric Genotypes Only

“…However, K leaf values were 3-fold higher when measured with the HPFM as compared to the EFM, which suggests that K leaf might be overestimated by the HPFM because of the higher hydrostatic pressure imposed to water within the leaf while negative pressures develops in transpiring leaves. For this reason, the EFM most likely mimicked the natural pathway of water in leaves through the transpiration flow (Sack and Scoffoni, 2012), even though the flow rate was of the same order of magnitude whatever the method used (between 0.5 and 3 mmol m 22 s…”

“…The leaf is thus flooded with a liquid solution and leaf airspaces might rapidly become infiltrated, implying novel pathway for water movement, hence yielding K leaf values not reflecting in vivo context . By contrast, the EFM was proposed to more closely follow the natural pathway of water in leaves through the transpiration flow (Sack and Scoffoni, 2012). Despite differences in K leaf magnitude, both methods and both independent experiments consistently evidenced a marked difference in K leaf measured in control conditions between cultivars.…”

“…transpiration rate E leaf ) divided by the leaf water potential (Sack et al, 2002) has long been a matter of debate when exploring the relationship between E leaf and K leaf . By contrast with other methods that use pressurized water to force water flow through the leaf, the EFM respects the native paths of water flow together with the range of negative values for water potentials within the leaf (Sack and Scoffoni, 2012). However, calculation of K leaf rests on E leaf that causes some partial correlation between variables and hinders the analysis of their relationship.…”

Growth responses of Lantana (Lantana camara L.) varieties to varying water availability and light conditions

“…Stomatal conductance was measured (n = 6) at the 10 th wk during midday using a porometer (AP4-UM-3, Cambridge, UK). Stomatal resistance was considered as the reciprocal value of stomatal conductance (Sack & Scoffoni, 2012;Goyal & Sharma, 2015). At the end of 12 wks plants were uprooted carefully and average length and width of cleaned root mass was recorded.…”

Remote Estimation of a Managed Pine Forest Evapotranspiration with Geospatial Technology

“…Local measurement of stomatal conductance is performed with an indirect optical method (Brauman et al, 2012) or with a semi-direct method using a vapor pressure deficit algorithm (Pearcy et al, 1989;Sack and Scoffoni, 2012), but these methods are costly and time consuming. Stomatal conductance of pine needles has been measured and used for estimating and modeling transpiration of pine forests (McCarthy et al, 1991;Amatya et al, 1996;Amatya and Skaggs, 2001).…”