The viviparous spikerush (Eleocharis vivipara Link) is found in and around marshes and swamps of the southern U.S. This small fine member of the Cyperaceae or sedge family is a cousin to the huge tule (Scirpus californicus, Plant of the Week, November 29, 2004). The spikerush consists of a clump of thin culms. The tip of the culm may produce a spike of flowers or, like its name suggests, it may reproduce by growing a plantlet.
When inundated, the branching tips of viviparous spikerush form dense mats in the water. On moist soil, the plants form colonies that cover the ground like a lawn. But water levels fluctuate through the year and a viviparous spikerush that was on soil may be submerged for months after heavy rains. That is when the spikerush does an amazing thing. Although the plants do not outwardly appear very different, they are dimorphic—one form terrestrial and another aquatic.
Osamu Ueno, Muneaki Samejima, Shoshi Muto and Shigetoh Miyachi found that when submerged, the viviparous spikerush has a C3 photosynthetic pathway. This is the most straight forward method of photosynthesis. All of the processes take place within the same cell; each cell containing chlorophyll takes up carbon dioxide and produces sugar independent of the other cells.
The C3 pathway has its problems. When stressed by too little water or too high temperatures, the pathway fails. An enzyme called Rubisco (ribulose 1,5-bisphosphate carboxylase-oxidase) becomes saturated with too much oxygen and reverses the processes. The plant loses sugar molecules and energy. C3 plants are water-spenders and usually limited to temperate zones, as annuals during cool seasons in tropical areas, or as aquatics.
When water levels fall and the viviparous spikerush is now growing terrestrially, it produces new culms. These have a different anatomy and a different photosynthetic pathway. The anatomy includes a new system of cells called the bundle-sheath or Kranz (wreath formation) with cells filled with chloroplasts. The pathway is called C4 since it includes an extra step.
Carbon dioxide is fixed in the mesophyll (middle leaf cells) and transported to the cells of the Kranz. Since the mesophyll is constantly feeding CO2 in one direction, the Rubisco can only drive the chemical reaction in a positive, sugar-producing direction. The more efficient pathway saves water and energy, and works during higher temperatures.
(Compiled from: "Eleocharis vivipara", R.P. Wunderlin and B.F. Hansen, Atlas of Florida Vascular Plants, Institute for Systematic Botany, University of South Florida, Tampa, 2004 and "Photosynthetic characteristics of an amphibious plant, Eleocharis vivipara: Expression of C4 an C3 modes in contrasting environments", Osama Ueno, Muneaki Samejima, Shoshi Muto, and Shigetoh Miyachi, Proceedings of the National Academy of Sciences, Vol 85, pp 6733-7637, September 1988)
