Monitoring and modeling of tree bark turgor in transport phloem

Background/Question/Methods

Currently, phloem transport in plants under field conditions is not well understood. This is largely the result of the lack of techniques suitable for the measurement of the physiological properties of phloem.

We present a model that interprets the changes in xylem diameter and live bark thickness and separates the components responsible for such changes. We test the predictions from this model on data from a variety of model systems, including boreal pine, desert pine and junipers and mediterranean eucalypts.

The model separates the live bark thickness variations caused by bark water capacitance from a residual signal interpreted to indicate the turgor changes in the bark.

Results/Conclusions

 The predictions from the model are consistent with processes related to phloem transport. At the diurnal scale, this signal is related to patterns of photosynthetic activity and phloem loading. At the seasonal scale, bark turgor shows changes following with droughts and rainfall events, consistent with physiological predictions for the desert pine and juniper. Daily cumulative totals of this turgor term were related to daily cumulative totals of canopy photosynthesis for the boreal pine. For the mediterranean eucalypts, treatment differences were seen between  trees kept under high temperatures vs control and trees kept under drought stress versus control.

The model parameter representing radial hydraulic conductance between phloem and xylem showed a temperature dependence consistent with the temperature-driven changes in water viscosity.

We propose that this model has potential for the continuous field monitoring of tree phloem function.