Coupled effects of soil and atmospheric drying on soil-plant hydraulics of sorghum

Transpiration response of plants during both soil drying and increasing vapor pressure deficit (VPD) have been thoroughly studied separately. However, the interactive effects of both on soil-plant hydraulics remain largely unknown. In this study, we tested the combined effects of soil and atmospheric drying on soil-plant hydraulics of sorghum.

Sorghum plants were grown in sandy soil under well-watered conditions with a daily VPD increment, increasing in five steps from 0.5 to 3.7 kPa. After 30 days, the soil was dried over five days. We measured transpiration rate (E), soil water content (θ), soil and leaf water potential (ψsoil, ψleaf) both under wet soil and during soil drying. A soil-plant hydraulic model was used to reproduce the data and provide further insight to disentangle soil and atmospheric effects.

Both soil drying and VPD affected the relation between transpiration rate and leaf water potential. In wet soil conditions, the E (ψleaf-x) relation was linear even at high VPD. During soil drying, this relation was linear in relatively low VPD conditions (0.5 – 2.5 kPa) but exhibited a non-linear relation under relatively high VPD (2.5 – 3.7 kPa). This response was also reflected in a breakpoint of soil-plant conductance at around 2.5 kPa VPD, resulting in a decrease in transpiration. 

We conclude that decreasing soil water status has a stronger impact on soil-plant conductance and water uptake than increasing VPD. Furthermore, the modeling revealed the importance of understanding how soil parameters are changed by the presence of plants, especially during soil drying. We suggest that for a holistic understanding of plant response to drought, more emphasis would need to be given to the interactions between VPD and soil drying, as the effects of VPD become increasingly important with soil drying.