DOI: 10.5061/dryad.pc866t1nk
title: Systemic effects of rising atmospheric vapor pressure deficit on plant physiology and productivity

description: Earth is currently undergoing a global increase in
 atmospheric vapor pressure deficit (VPD), a trend which
 is expected to continue as climate warms.  This
 phenomenon has been associated with productivity decreases in ecosystems
 and yield penalties in crops, with these losses
 attributed to photosynthetic limitations arising from
 decreased stomatal conductance.  Such VPD
 increases, however, have occurred over decades, which
 raises the possibility that stomatal
 acclimation to VPD plays an important role
 in determining plant productivity under high
 VPD.  Furthermore, evidence points to more far-ranging
 and complex effects of elevated VPD on plant physiology,
 extending to the anatomical, biochemical and developmental levels, which
 could vary substantially across species.  Because
 these complex effects are typically not considered in
 modelling frameworks, we
 conducted a quantitative literature review
 documenting temperature-independent VPD effects on 112 species
 and 59 traits and physiological
 variables, in order to develop an integrated and
 mechanistic physiological
 framework.  We found
 that VPD increase reduced yield and
 primary
 productivity, an effect that was
 partially mediated by stomatal acclimation, and also linked
 with changes in leaf anatomy, nutrient and
 hormonal status.  The productivity
 decrease was also associated with negative
 effects on reroductive development, and changes
 in architecture and growth rates that could decrease
 the evaporative surface or minimize embolism
 risk.  Cross-species quantitative relationships were
 found between levels
 of VPD increase and
 trait responses, and we
 found differences across plant
 groups, indicating that future VPD impacts will depend on community assembly and crop functional diversity.  Our analysis confirms predictions arising from the hydraulic corollary to Darcy's law, outlines a systemic physiological framework of plant responses to rising VPD, and provides recommendations for future research to better understand and mitigate VPD-mediated climate change effects on ecosystems and agro-systems.
description: 1. Literature search strategy and selection criteria
 The databases Scopus® and Web of Science® were searched between
 March 30, 2018 and May 13, 2018. The search included the search terms:
 “VPD”, “vapour pressure deficit”, “vapor pressure deficit”, “evaporative
 demand”, “acclimation humidity”, “acclimation VPD”, “relative humidity
 acclimation”, “relative humidity adaptation”, “air humidity acclimation”,
 “air humidity adaptation”, “stomata humidity”, “air humidity”, “relative
 humidity”, “humidity photosynthesis”.  These broad searches resulted in a
 total of 9245 records.  The vast majority of the initial records were
 excluded, as they reflected research themes outside of the scope of the
 investigation (detailed in Supporting Information Appendix: Figure
 S1).  The remaining 104 papers addressed the longer-term effects of VPD on
 various plant traits and physiological variables.  Effects were considered
 longer-term if these two conditions were fulfilled: 1) the rationale of
 the study was to investigate longer-term effects of VPD (i.e.
 acclimation); and 2) differential VPD treatments were sustained for two
 days or more.   2. Data extraction from
 records Data extraction from the core 104 papers was
 undertaken to perform quantitative analyses and enable synthesis of the
 literature.  To perform quantitative analyses, data from each paper were
 either directly extracted from text and tables or were extracted by
 digitizing graphs using the online platform WebPlotDigitizer, version 4.1
 (https://automeris.io/WebPlotDigitizer).  Each record from the 104 papers was scrutinized to extract the following metadata: year of publication, country of origin, species name (as reported in the paper), cultivar or ecotype name (if applicable), type of growth environment (field, greenhouse, growth chamber, room), soil medium (e.g., artificial soil, hydroponics, topsoil mixture, native soil), control and high VPD (kPa), nighttime temperature (T, °C), daytime T (°C), photoperiod (h), photosynthetically active radiation (PAR, μmol m<sup>-2 s-1</sup>), atmospheric CO2 concentration (ppm), plant age when the experiment was initiated in days (d), and duration of the VPD treatment (d).  Daytime/nighttime VPD could be extracted from most papers (n = 98), with the exception of a set of papers from two research groups.  Information about the response of the traits and physiological variables of interest to the VPD treatment was extracted from each paper, leading to the identification of a total of 59 variables.  We extracted the following information: the trait/variable means observed at each VPD treatment, the sample size and the standard deviation .  This information was extracted even if the VPD effect was found to be non-significant (i.e., P &gt; 0.05).