Wetland Plant Adaptations

Wetland environment stresses on plants

–Water regime

–Wetlands: periodic drying

–Terrestrial: periods of flooding.

–Temperature: extremes on the wetland surface due to the shallow water.

–Anoxia leads to

–changes in nutrient supply

–toxic levels of some substances

–inefficient anaerobic respiration

–Salt

Adaptations can be broadly classified as those that enable the organism to tolerate stress and those that enable it to regulate stress.

•Tolerators (resisters) have functional modifications that enable them to survive and often to function efficiently in the presence of stress.

•Regulators (avoiders) actively avoid stress or modify it to minimize its effects.

•In a freshwater aquatic or soil environment, the osmotic concentration of the cytoplasm in living cells is higher than that of the surrounding medium. This enables the cells to develop turgor, that is, to absorb water until the turgor pressure of the cytoplasm is balanced by the resistance of their cell membranes and walls. •In coastal wetlands, organisms must cope with high and variable external salt concentrations. •The dangers of salts are twofold:

•osmotic•direct toxicity.

•The immediate effect of an increase in salt concentration in a cell's environment is osmotic. If the osmotic potential surrounding the cell is higher than that of the cell cytoplasm, water flows out of the cell and the cytoplasm dehydrates. This is a rapid reaction that can occur in a matter of minutes and is often lethal to the cell.

Anoxia

Flood-tolerant species (hydrophytes) possess a range of adaptations that enable them either to tolerate stresses or to avoid them. There are several adaptations by hydrophytes that allow them to tolerate anoxia in wetland soils. These adaptations can be grouped into two main categories

1) Morphological

2) Physiological

1) Structural Adaptations (morphological) a. Aerenchyma

b. Special organs or responses

Adventitious roots

Stem elongation

Hypertrophied lenticels

Pneumatophores

2) Physiological adaptations a. Anaerobic respiration

b. Malate production

Hormones

Hormonal changes, especially the concentration of ethylene in hypoxic tissues, initiate structural adaptations. Ethylene stimulates cellulase activity in the cortical cells of number of plant species, with the subsequent collapse and disintegration of cell walls. In addition to aerenchyma development, ethylene has been reported to stimulate the formation of adventitious roots, which develop in both flood-tolerant trees and herbaceous species and flood-intolerant plants just above the anaerobic zone when these plants are flooded. These roots are able to function normally in an aerobic environment.

Adventitious Roots

Stem Elongation•stimulated by submergence

•in some aquatic and semiaquatic plants

-Floating Heart (Nymphoides peltata) right

-Wild Rice (Oryza sativa)

-Bald Cypress (Taxodium

distichum).

AerenchymaAllows the diffusion of oxygen from the aerial portions of the

plant into the roots.

Root aerenchyma

Root aerenchyma

Lenticels

Pneumatophores

Bald Cypress

Cypress Knees

Cypress Knees

Shallow Fibrous Root Systems

Mangroves

Dacey described an adaptation that increases the oxygen supply to the floating-leaved water lily (Nuphar luteum). Increased air flow to the rhizosphere is related to increased leaf surface temperature which creates a humidity-induced pressurization.

Prop Roots

Red mangrove (Rhizophora spp.) grows on arched prop roots in topical and subtropical tidal swamps around the world. These have numerous small lenticels above the tide level which terminate in long, spongy, air-filled, submerged roots.