5. Conclusion
This work has refined our understanding of radial transport dynamics in
tree stems. Girdling was used to generate experimental data to derive a
detailed mechanistic model, that disentangles dimensional changes driven
by cell expansion, hydraulic pressure, osmotic potential and radial
transport processes in xylem and bark over 24-hour periods. In contrast
to the paradigm that radial growth of stems mostly occurs during the
night, we found that growth of E. tereticornis was continuous
during day and night. Moreover, we documented asynchrony, where xylem
expansion was greatest during the mid-day and bark expansion was
greatest at night-time. Girdling accelerated stem growth above the
girdle, particularly during the daytime, suggesting that increased
carbon supply (including NSC) plays a critical role in the regulation of
diurnal changes in stem diameter due to osmotic adjustment in bark and
in xylem ray cells. Based on the observed responses to girdling, we
devised a novel regulatory framework that helps explain patterns of
contraction and expansion of bark and xylem. Applying this mechanistic
model to better understand the hydraulic-metabolite interdependent
processes that generate irreversible growth in stems of other tree
species would be the logical next step to better understand the radial
daily growth in trees more broadly.