Stress alters the role of silicon in controlling plant water movement

One function of plant Si is ameliorating stress, including drought and salinity stress, which can induce active Si uptake in addition to passive uptake via transpiration. However, the interactions and feedbacks between stress, water movement, and Si uptake remain unknown.
To examine this gap, we compiled papers reporting transpiration and/or stomatal conductance of plants exposed to stresses while varying Si availability.
Our meta-analysis (34 studies, excluding rice) showed that stress alters the role of Si in controlling water movement across diverse plant groups. Increased Si availability significantly increased water movement in stressed plants, particularly stomatal conductance (p<0.001, k=84) in plants exposed to salinity (p<0.05, k=20) and drought (p<0.05, k=45) stress.
This signal of increased conductance was most apparent in C4 plants (p<0.001, k=41) and Poales (p<0.001, k=47). These findings have implications for plants under increasing water and salinity stress, particularly for Poales, where survival in affected ecosystems could be mediated by soil Si availability, and in agricultural systems, supplying Si to water-stressed plants could increase productivity. 
Intriguingly, Si addition to unstressed plants had no consistent impact on water movement, with reduction of water movement with Si addition to unstressed plants in 50% of studies, mostly those involving non-Poales species. This is an important first broad-scale Si cost quantification, as the costs of Si for plants have remained stubbornly mysterious, hampering evolutionary and functional understanding of plant Si use.

Methods
We systematically identified papers testing or reporting the impact of Si availability on plant conductance and transpiration by searching the Web of Science database, for TOPIC: (silicon OR silica) AND TOPIC: (transpiration OR stomatal conductance) AND TOPIC: (plant OR leaf OR leaf OR foliar) and NOT TOPIC (rice) in November 2021. We excluded rice from our analyses. We extracted values for stomatal conductance and transpiration, for experiments in which plants were grown with and without Si addition (+/- Si) and with or without stress (+/- stress) applied to the plants. We converted reported water movement values to stomatal conductance (mmol H2O m2 s-1) and transpiration rate (mol H2O m2 s-1) as appropriate, and all measures of variance to standard deviations.

We used the ‘metafor’ package (Viechtbauer, 2010) in R (R Core Team, 2022) to calculate the effect size (Hedges’ D) of the impact of silicon addition on one response (e.g. conductance or transpiration) at a time for individual studies. This effect size measure compares two means using a pooled standard deviation and bias correction and is a measure of the number of standard deviations by which the means differ (Hedges & Olkin, 1985). We considered stress and unstressed treatments separately. The effect sizes calculated from individual studies were then used to conduct our analyses. The raw effect sizes, prior to analysis, are reported here.  

Viechtbauer, W. (2010). Conducting Meta-analyses in R with the metafor package. Journal of Statistical Software, 36(3), 1–48.
R Core Team (2022). R: A language and environment for statistical computing. Foundation for Statistical Computing, Vienna, Austria. https://www.R-project.org/.