Sunburned Tops, Soggy Feet

Aquatic Emergent Plants

South Florida is rich in plants growing with their roots submerged at least part of the year, and their tops periscopes above water.   Many unrelated plants have evolved this lifestyle separately.   The convergences and divergences in unrelated species united under one extreme lifestyle makes it all intriguing. And they range the spectrum from fundamentally landlubbers able to withstand temporary flooding to floaters able to withstand temporary stranding.

Pickerel weed emergent. By John Bradford.

Emergents come from all corners of the plant world, including ferns and fern allies (Isoetes) to flowering plants from numerous families.   Monocots rule the marsh, with such representaives as arrow arums, arrowheads, cattails, golden clubs, grasses, pickerel weeds, rushes, sedges, and more.

Many emergents imperialize large spaces by subterranean rhizome growth.    You might say with limitless sun and usually unlimited water, two arenas for competition might be nutrients and space.   Some marshes house carpets of a single species, or multiple carpets of multiple species pushin’ and shovin’.  Sometimes competing species intermix, sometimes they occupy pure stands.

Sagittaria graminea by John Bradford.

Sunken rhizomes are useful beyond colonization.  Water levels fluctuate seasonally.  Emergents may spend months high and dry where risks include drying, sun, fire, grazing…or the other extreme, flooding.

Hey—where did the water go?

Apocalyptics like underground refuge.  Some species double down to face dryageddon by making thick starch-storing rhizomes, or tubers such as water chestnuts, chufa, and sagittaria (aka “duck potato”).  We’re ready, come hell or high water!

This photo shows subterranean “be ready” starch storage in Sagittaria.  The dark stained area in the rhizome shows starch storage.  Starch reserves stop abruptly at the leaf bases.

Sagittaria rhizome sliced to show starch storage.  The starch is stained with purple.

Emergent usually have ductwork called aerenchyma (air-EN-cah mah).   If your feet are under oxygen-starved mud and your top is above water, ventilation shafts help,  “air” channels from leaves to roots. The snorkel analogy is too simplistic, however.   The pipes are not generally open to the outside air.   They usually are more or less closed, sometimes pressurized. Let me explain:

The leaves make oxygen as a waste gas, and require carbon dioxide to make sugar.   The submerged roots do the opposite…they use oxygen for respiration while shedding carbon dioxide.  They make what the leaves need and take in what the leaves shed.   And the reverse prevails up in the leaves.  So it makes sense for  the leaves and roots to trade gases.   A closed pipe system allows the leaves and roots to decontaminate and feed each other…oxygen moving down where needed and carbon dioxide moving up where needed.  Too much outside venting may interfere with such self-contained exchange.

Sagittaria leaf stalk cut lengthwise to show air channels.

Sagitaria leaf stalk cut across to show air channels.

A plant physiologist studying cattail found the carbon dioxide concentration in the aerenchyma to be 10 times the atmospheric concentration. To the foliage that’s a photosynthesis supercharger.   John and I measured carbon dioxide released from cut aerenchyma at the base of a sagittaria.   The technique would necessarily under-estimate the concentration, yet our reading rose to 5683 parts per million,  even higher than the 10-fold increase in cattail, given that the atmospheric carbon dioxide concentration is a bit over 400 parts per million.

Okay then, does the leaf aerenchyma pipe oxygen rootward?   We cut off several leaves and inserted their stalks (petioles) into a sealed test chamber with an oxygen sensor. The atmospheric oxygen level outside the chamber as 208,284 parts per million in contrast with  224,389 inside.      Yes, oxygen from the leaf photosynthesis is clearly southbound down the leafstalk air channels.

The species we tested Sagittaria lancifolia has thick leathery leaves you might expect on a desert plant or on a high dry epiphyte, not where water seems unlimited.

Emergents often resemble plants of arid circumstances.   Many have thick resistant blades, or nearly leafless photosynthetic stems, or pencil-shaped leaves with minimal surface area.    Easy to explain at a “hunch” level:  ready for seasonal dry times, robust to relentless marshland sun,   and perhaps root- impaired by suffocation in standing water and waterlogged mud.

The Everglades marsh ecosystem under natural conditions was famously nutrient-limited, most notably phosphorus.   Modern pollution high in phosphorus and other nutrients disrupts the natural species balance.

To turn to the other end of the nutrient spectrum, marshes gobble up unwelcome nitrogen, phosphorus, and other nutrients from sewage effluent.  Visit such water treatment sites as Wakodahatchee or Green Cay wetlands, and see the nutrient-loving species composition of hyper-enriched waters.  Broadleaf arrowhead abounds, true to its reputation as a nutrient vacuum.

Nutrient-loving aquatic species at Wakodahatchee Wetlands.