Do changes in water levels in wetlands give plants a backbone?

The usual rule whenever a headline asks a silly question is that answer is no, and that’s the same here because plants don’t have backs. However research by Hamann and Puijalon does show that emergence due to falling water levels can cause a biomechanical response.

The stresses for aquatic and terrestrial plants differ, because aquatic plants have the support of water to give them buoyancy. Hamann and Puijalon point out, if a plant can float then the chief stress it will have is tension as it is dragged by the local current. What it needs is an anchor and flexibility to cope with the forces on it. A terrestrial plant in contrast feels gravity much more. It has to support its own weight. Wind can put a plant in tension, but the force of gravity can compress some tissues. So the mechanical needs of a plant out of water are different to those in water.

Diagram of forces on aquatic and terrestrial plants
Schematic overview of the main forces (thick arrows) and stresses (thin arrows) acting on plants in aquatic and terrestrial environments. In the aquatic environment, buoyant plants withstand the drag forces resulting from current flow through tension (σ +). In the terrestrial environment, in addition to the force of gravity, self-supportive plants withstand drag forces induced by wind through bending (a combination of tension σ + and compression σ −). Diagram by Elena Hamann and Sara Puijalon.

This is a problem for a plant that is happily sat in water, until there’s a drought. When the water goes the plants are faced with a major change in environment. Can they change their physical structure to cope? Hamann and Puijalon expected that plants could increase their cross-sectional area and the proportion of strengthening tissue in their stems to increase strength. They also expected the stems to become stiffer.

They looked at a wide variety of species Berula erecta (Hudson) Coville, Hippuris vulgaris L., Juncus articulatus L., Lythrum salicaria L., Mentha aquatica L., Myosotis scorpioides L., Nuphar lutea L. and Sparganium emersum Rehmann. The plants were growing in wetlands along the Ain and the Rhône in eastern France. One set of plants was picked from submerged conditions and the other from close by in emergent conditions, to keep the population and growing conditions as similar as possible. They then tested the plants for strength and flexibility and examined them physically.

They found that usually there were clear differences between submerged and emergent plants. Stems and petioles (the branch connecting a leaf to a stem) were generally longer in emergent plants, but some had the opposite response. Likewise the cross-sectional area varied, but not in a clear way. Some species thickened their stems in emergent conditions, other were thicker in submerged conditions. Likewise the results for tensile strength and Young’s Modulus varied between plants, so there was no consistent effect by emergence. The only thing that did change consistently was that all plants had an increase in dry matter content in emergent conditions.

The overall result was that emergent plants did have a greater capacity for stiffness, but they seem to have developed it in different ways.

It’s not currently clear what the costs and benefits are from building a better emergent form. Emergence might put plants into a better location to reproduce and photosynthesise. However, it could be the costs of building the stiffer structures are overall a disadvantage. Also, when the plant is re-submerged the terrestrial adaptations could be create more stresses as they’re adapted to resist the wind, not move with the water current.

The research shows that there are clear differences to be measured, but these alone will not tell us how the plants are equipped to respond to increased variability in water levels in the future. It’s also not clear why some plants change one way but others take another route. Clearly there are subtle differences in the costs and benefits for building a plant in a specific way, but it’s not clear what. Hamann and Puijalon’s research shows there are a few interesting research projects that you could develop in the future around wetlands.

Hamann E. & Puijalon S. (2013). Biomechanical responses of aquatic plants to aerial conditions, Annals of Botany, 112 (9) 1869-1878. DOI: http://dx.doi.org/10.1093/aob/mct221