The role of the plant hydraulic conductance for the transpiration rate response to increasing VPD

A restricted transpiration rate (TR) response to rising vapor pressure deficit (VPD) is often considered a useful ‘trait’ for soil water conservation under circumstances of terminal drought. The plant hydraulic conductance (Kp) is debated to play a role in shaping water use under atmospheric drought. Here, we investigated whether a limited TR response to elevated VPD defines as a ‘trait’ (or whether it is rather the product of several interacting traits). Further, we aimed to identify which role limitations in Kp play for plant water use regulation during increasing VPD. To achieve that, we tested whether a reduction in Kp would lead to an altered TR-VPD response.

We exposed five individual maize (Zea mays L.) plants to rising VPD up to 2.3 kPa in a climate chamber in wet soil conditions while simultaneously and continuously monitoring whole plant TR with balances and stem water potential (Ψ_stem) dynamics using optical dendrometers. Kp was calculated from the slope between TR and Ψ_stem in the linear part of the relation. To achieve a reduction in Kp, we destructively cut the root system in several places. The TR-VPD profile was measured on: (1) intact plants, (2) plants of which the root system was cut, and (3) disturbed plants after five days of recovery.

In undisturbed conditions, plants transpired linearly until a VPD of 0.9 kPa, upon which TR increased to a lesser extent. In damaged plants, TR was restricted at comparable VPD but subsequently decreased with rising VPD beyond this threshold. Despite strong stomatal regulation, Ψ_stem declined further and became more negative compared to undisturbed plants.  Kp was reduced by merely 5% due to root cutting. Upon five days of recovery, plants transpired at a relatively lower initial rate compared to undisturbed conditions, but linearly with rising VPD.

Root cutting and recovery created new phenotypes of the same plant in terms of water-use regulation during atmospheric drought, implying (i) the difficulty of phenotyping for, and (ii) the context-dependency of water use responses. Root cutting leading only to a minor decrease in Kp indicates that root length does not linearly correlate with Kp. Given the minimal decline in Kp, we suggest that the strong change in the TR-VPD response that was associated with root cutting might be attributed to changes in the plant’s capacitive storage, modulating short-term Ψ_stem dynamics and stomatal response.