The significance of water co-transport for sustaining transpirational water flow in plants: a quantitative approach.

In a recent Opinion paper, Wegner (Journal of Experimental Botany 65, 381-392, 2014) adapts a concept developed for water flow in animal tissues to propose a model, which can explain the loading of water into the root xylem against a difference in water potential (Ψ) between the xylem parenchyma cell (more negative Ψ) and the xylem vessel (less negative Ψ). In this model, the transport of water is energized through the co-transport of ions such as K(+) and Cl(-) through plasma membrane-located transporters. The emphasis of the model is on the thermodynamic feasibility of the co-transport mechanism per se. However, what is lacking is a quantitative evaluation of the energy input required at the organismal level to sustain such a co-transport mechanism in the face of considerable net (transpirational) flows of water through the system. Here, we use a ratio of 500 water molecules being co-transported for every pair of K(+) and Cl(-) ions, as proposed for the animal system, to calculate the energy required to sustain daytime and night-time transpirational water flow in barley plants through a water co-transport mechanism. We compare this energy with the total daily net input of energy through photosynthetic carbon assimilation. Water co-transport can facilitate the filling of xylem against a difference in Ψ of 1.0MPa and puts a minor drain on the energy budget of the plant. Based on these findings it cannot be excluded that water co-transport in plants contributes significantly to xylem filling during night-time and possibly also daytime transpiration.