2.8 Stem hydraulic model
The model developed by Mencuccini et al. (2017) separates the relative dimensional changes of D b caused by irreversible cambial growth (D G) (plastic growth in the terminology of Cosgrove 2005) from those due to reversible elastic changes. The latter are the result of transpiration-driven, negative hydraulic pressure changes in xylem (D bΨP,x) and changes in the bark osmotic pressure (D bΨΠ,b) caused by shifts in the balance between phloem carbohydrate supply and radial growth. Note that here and later on in the text, variablesD bΨP,x andD bΨΠ,b must be read as the component of bark diameter D b affected by changes in xylem pressure potential ΨP,x and bark osmotic potential ΨΠ,b, respectively. The same applies for theD x variables presented later on. However, the Mencuccini et al. (2017) model did not account for lateral solute fluxes between bark and xylem via rays. That is, whereasD bΨP,x (i.e., transpiration-driven) does account for lateral water fluxes,D bΨΠ,b (osmotic pressure-driven) only accounts for osmotic changes within bark. Here, we expand the model to incorporate an additional lateral osmotic transfer function; please refer to Supporting Information (Method S1, Table S1) and Mencuccini et al. (2017, 2013) for full details of the model.
Here we assumed that:
Radial transfer of osmotica between bark and xylem determines reversible changes of bark thickness.
The reversible radius change of xylem is the sum of two independent processes, one driven by the shrinkage/expansion of dead conduits and live cells under negative pressure (D xΨP,x) and one driven by shrinkage/expansion of the live xylem parenchyma cells (i.e., rays) caused by radial solute transport to/from bark (D xΨΠ,b) with subsequent impacts on accumulation/release of symplastic water in/from rays.
3) The rate of radial solute transfer to/from bark is identical in magnitude, but opposite in sign, to the rate of radial solute transfer from/to xylem. This assumption is only approximately correct, since a fraction of the solutes transported radially from bark into xylem-bound rays will form the building blocks of irreversible growth in the cambial growth zone.
4) No net transfer from soluble to non-soluble carbohydrates orvice versa during this transfer.