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.