The dynamics of stem water storage can provide insights into a tree’s capacity to face imbalances between water supply and demand, and thus maintain hydraulic function and growth under fluctuating environmental conditions. Changes in stem radius due to early morning water depletion and night time refilling of storage tissues is generally well documented. This study seeks to understand the unusual daytime refilling of stem elastic storage tissues present in the mangrove Avicennia marina (Forssk.) Vierh, which deviates from most terrestrial trees and is hypothesized to be due to endogenous osmotic adjustment. We explored the relationship of this pattern with other water-related physiological processes and environmental conditions, and investigated the seasonal changes in the timing and time lags of peak swelling at different stem heights, in order to understand the “peristaltic” depletion of internally stored water.
Results/Conclusions
Our findings show that daytime stem swelling occurs year-round, even on days when leaf water potentials dropped to values lower than -4 MPa. The amplitude of stem swelling was strongly positively correlated to daily light sums but rarely to measures of water availability in air and soil. There was also a clear seasonal reversal in the timing and direction of the “peristaltic” depletion of water along the stem, with an earlier onset of shrinking in the upper (median = 10:00 h) than in the lower stem (median = 12:00 h) in winter, but an earlier onset of shrinking in the lower (median=08:00 h) than in the upper stem (median=11:00 h) in summer. This time lag was closely correlated to daily temperature, with a clear switch in the direction of stem shrinking at the start of the growing season. We propose that sugar loading/unloading and changes in source-sink activity play a role in the endogenous osmotic adjustment responsible for daytime stem swelling and the seasonal switch in the direction of peristaltic water storage depletion.