Could anyone explain the theory behind oxygen uptake and release in seagrass?

Question

Through respiration and photosynthesis, seagrass acquired oxygen and release it back to the environment. The leaves and roots have special tissues that aids these process. What is the function of aerenchyma and lacunae? What is the function of the airspaces around the leaves and roots? How does the oxygen uptake and release in seagrass relate to the efficiency of root to absorb nutrients from the surrounding?

Answer 1

Leaf photosynthesis takes place almost exclusively within the epidermis, the outermost layer of leaf cells, not the central mesophyll (as in typical land plants). Epidermal cells have well developed chloroplasts and many organelles associated with active metabolism, whereas mesophyll cells of seagrasses possess very few chloroplasts and appear to store starch grains.

The roots and rhizomes occur in an anoxic (anaerobic) environment, so that oxygenation is required for proper and rapid growth of anchoring organs. In seagrasses, oxygen generated inside chloroplasts during the light reaction of photosynthesis diffuses most easily to the center of the leaf, where there are large intercellular air chambers (lacunae), in a tissue type called aerenchyma. Use of oxygen by the roots creates a sharp oxygen gradient, from high levels in leaf aerenchyma (greater than 30% in these lacunae) to near zero in growing roots; hence, via mass flow oxygen diffuses from leaf to stem to root and rhizomes through aerenchyma air spaces. At each node there occurs a diaphragm, a wall of cells that appears to block passage of air molecules into the next segment (internode), but this is merely an illusion, because there are air gaps large enough between cells of the diaphragm that no blockage likely occurs in the pathway (based on calculated resistances).

Because seagrass communities are highly productive, exceeding that of most crop plants, plant biologists have attempted to study in detail how carbon dioxide is delivered to the leaf chloroplasts across the waxy cuticle. Investigators know that at a typical ocean water pH of 8.2, as much as 99% of the carbon dioxide is present instead in the form of bicarbonate, HCO3. Speculation has been that bicarbonate, because it is so abundant, could be the carbon-containing molecule used. The best models to date, however, have revealed that carbon dioxide, a much smaller molecule, diffuses much more rapidly across the unstirred layer next to the leaf and through cutinized cell walls than does bicarbonate, so carbon dioxide is now taken to be the chief gas used to enter the leaf, even if present at a very low concentration.