Mangrove Roots Pushing and Maybe Pulling

Last week John and I tested Red Mangrove branchlets for sun-generated pressure and found it, the pressure presumably forcing air down to the roots submerged in deep, wet, tidal, de-oxygenated mud.    Most branches when cut off from other species show a distinct negative pressure, suction, the opposite of Red Mangrove.

Red Mangrove fruits and embryos by John Bradford.

We hoped the roots would show continuity with the forced-air-ventilation from above by being positively pressurized too.   We connected a couple cut-off Red Mangrove roots to a pressure meter.

Under bright sunshine the pressure climbs dramatically as shown below:

Pressure increasing for 150 seconds in bright sunshine.

When the tree is shaded, we could not find pressure.  Sunshine was required.  Look at the correspondence between light intensity and pressure below:

Blue line is pressure.  Rusty line is light intensity.

While we were in a Mangrove habitat today, we thought too about Black Mangrove, which aerates its mud-buried roots with little chimneys called dead man’s fingers.   On a whim we connected the hose and meter to the top of a dead man’s finger, where air goes in.   We wondered if the air entry is passive (with no pressure change) or, by contrast, if it is “sucked” in somehow.

In the graph below see a sharp and brief yet sustained pressure drop upon connecting the meter to a dead man’s finger.  At first superficial blush it looks like maybe air is pulled in.   It would take a lot more tests to make a case for anything, and explaining it in terms of how a plant is built is going to be a challenge, although conceivable.   As explained above, cut-off branches usually generation suction, perhaps in the case extending all the way into the roots.   Guess we’ll have to explore that a little more.

Black Mangrove. Apparent suction at top of dead-man’s finger root “chimney.”