The interplay between hydraulic capacitance and stomatal regulation strategy affects soil–plant hydraulics and transpiration

Plant water storage contributes to transpiration, but it is unclear how its relevance in supporting transpiration depends on the stringency of stomatal regulation. Here, we show the compounding effect of stomatal regulation and hydraulic capacitance on plant water use, by means of a soil-plant hydraulic model and measurements of leaf water potential, sap flow, stomatal conductance and capacitance in beech and spruce in the field. We found that large capacitance led to a large buffering effect on leaf water potential, explained by increasing amounts of transpiration sourced from internal plant water storage. However, the extent to which capacitance allows plants to sustain transpiration depends on the stringency of stomatal regulation. For stomata that limit leaf water potential at a fixed threshold (as observed in spruce), large capacitance increased transpiration throughout all soil water conditions. By contrast, for flexible stomatal regulation mechanisms optimizing transpiration over leaf water potential (as observed in beech), large capacitance caused stomata to close earlier in the day under wet soil conditions. Our findings suggest a trade-off between developing tissues that can store large water volumes and stomatal regulation mechanisms that allow leaf water potential to reach more negative values during periods of high transpiration demand.