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.

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^-2 s-1), 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 > 0.05).