Rooted to the spot in their watery world, aquatic plants depend on the fluid flowing around their leaves to deliver the carbon dioxide(CO2) that they need for photosynthesis. Josef Ackerman at the University of Guelph, Canada, has pondered for some time how the complex flow of water around plants' leaves affects this delivery. While some plants have flat leaves, others have twisted or crinkled leaves, which led Ackerman and his postdoc Gregory Nishihara to wonder if this influenced water flow over the leaves' surface, and CO2 delivery. Nishihara and Ackerman decided to find out by looking at how the speed of water flow, CO2concentration and leaf shape influences the rate of photosynthesis, and oxygen production, in two closely related aquatic plants: the flat-leaved Vallisneria americana and the spiral-leaved V. spiralis(p. 522).

First the team cut 8 cm leaf samples from both plants, fastening them horizontally to a wire stand in a flow chamber that pumped water over the leaves at different speeds. To find out how fast the plants were photosynthesising at different water speeds and CO2 concentrations,they adjusted the CO2 concentration in the water using a chemical buffer, and positioned microsensors very close to the leaves' surface to measure how much oxygen they produced.

At a higher CO2 concentration of 17.1 mmol m–3,the twisted V. spiralis produced more oxygen, suggesting that this plant has a greater affinity for CO2 and boosts its rate of photosynthesis when there is a glut of the gas dissolved in the water. Wondering how water speed affected the rate of photosynthesis, they found that oxygen production in both species levelled off when the water speed reached 4.4 cm s–1, showing that both plants can't photosynthesise any faster at higher water speeds, despite faster CO2 delivery.

However when the team dropped the CO2 concentration by ten times to 1.7 mmol m–3, V. spiralis produced the same amount of oxygen as V. americana. It's strange that they behave differently at one concentration and not at the other' says Ackerman. The next surprise came when the team discovered that both plants produced a different amount of oxygen in different areas of the leaf. 1 cm from the end of the leaf section facing into the water flow, oxygen production levelled off as water speed went up, showing that photosynthesis couldn't happen any faster.

However, 7 cm from the end of the leaf, 6 cm downstream, oxygen production kept rising with increasing water speed, suggesting that this leaf area used every last scrap of CO2 available in the water and that the rate of photosynthesis outpaced the rate of CO2 delivery. This means that the assumption that CO2 concentration stays constant as the water flows over the surface of the leaf probably isn't correct, Nishihara and Ackerman note.

But what about the twist? Unsure if the differences in oxygen production were due to physiology or leaf shape, the team twisted V. americanaleaves before fastening them to the stand, and found that this didn't affect the results. However that isn't the end of the story – Ackerman hopes that future experiments will uncover whether there are any more twists to this tale. Then, he says, the real challenge will be working out what is going on when the plants are intact and moving in the water flow'.

Nishihara, G. N. and Ackerman, J. D. (
2007
). The interaction of CO2 concentration and spatial location on O2 flux and mass transport in the freshwater macrophytes Vallisneria spiralis and V. americana.
J. Exp. Biol.
210
,
522
-532.