Weeding the Gems

Plants contain a continuous water column from the roots, where water is absorbed, to the leaves, where water is lost through evaporation via the stomata. When a plant’s cells require water, opening stomata makes water potential in the xylem strongly negative and water is pulled from the soil into the roots and into the xylem strongly and quickly. However, the water potential gradient can be too steep, causing cavitation (bubbles) in the xylem, which slows down water transport. The optimum transpiration rate occurs when water potential and cavitation are balanced in the right way. According to research published recently in New Phytologist, plants are able to maintain a transpiration rate very close to the maximum theoretical transpiration potential, allowing partial cavitation but not letting it limit hydraulic conductivity.

Here, Manzoni et al. from Amilcare Porporato’s group at Duke University, compared the theoretical optimum transpiration rate with actual transpiration ates in a number of tree species (grouped into boreal, temperate, Mediterranean, tropical dry, and tropical moist species). Their parameters for calculating the theoretical optimum were extensive, including soil water potential, xylem hydraulic conductivity, and canopy height.

The actual maximum transpiration rate of these species was then collected from published papers, and sorted according to climate and the conditions under which the analysis was done. Only the data from well-irrigated systems was used. The average observed peak transpiration rate was close to the theoretical maximum transpiration rate, and both were fairly conserved among plant types of a similar size in a particular climate.

Plant biologists contributed to this paper, but the first and corresponding authors Stefano Manzoni and Amilcare Porporato are both engineers. Their final objective was to find out if water uptake in plants can be made more efficient in order to improve carbon assimilation by plants, yet they did not attempt to understand the complex molecular interactions and signalling exchanges involved in plant uptake of water. They derived a model to calculate the maximum theoretical transpiration rate.

This modelling approach is useful when approaching synthetic biology, which can be defined as the design or re-design of biological systems. In this case, a model and literature search was enough to show that this particular process cannot easily be improved, at least under certain conditions. If water uptake is to be optimised, the next step the authors recommend is more complicated modelling of transpiration rate under conditions of varying water availability. If a rate-limiting factor is identified, biologists will be able to focus on it straight away.

Teaching resources: The classic transpiration classroom activity is using a measuring cylinder and plastic tube filled with water, attached to a branch or leaf cut underwater so the water column isn’t broken, to watch water uptake. If the living plant material is in different environments, water will move up the column at different speeds. A video from Paul Anderson on Share My Lesson demonstrates it.

Highlighted article: Manzoni, S., Vico, G., Katul, G., Palmroth, S., Jackson, R. B. and Porporato, A. (2013) Hydraulic limits on maximum plant transpiration and the emergence of the safety–efficiency trade-off. New Phytologist. Doi: 10.1111/nph.12126

Image credit: Summer Rain by Eryk Klucinski via Stock.xchng.