Modelling water use efficiency in a dynamic environment: An example using Arabidopsis thaliana

Intrinsic water use efficiency (W<inf>i</inf>), the ratio of net CO<inf>2</inf> assimilation (A) over stomatal conductance to water vapour (g<inf>s</inf>), is a complex trait used to assess plant performance. Improving W<inf>i</inf> could lead in theory to higher productivity or reduced water usage by the plant, but the physiological traits for improvement and their combined effects on W<inf>i</inf> have not been clearly identified. Under fluctuating light intensity, the temporal response of g<inf>s</inf> is an order of magnitude slower than A, which results in rapid variations in W<inf>i</inf>. Compared to traditional approaches, our new model scales stoma behaviour at the leaf level to predict g<inf>s</inf> and A during a diurnal period, reproducing natural fluctuations of light intensity, in order to dissect W<inf>i</inf> into traits of interest. The results confirmed the importance of stomatal density and photosynthetic capacity on W<inf>i</inf> but also revealed the importance of incomplete stomatal closure under dark conditions as well as stomatal sensitivity to light intensity. The observed continuous decrease of A and g<inf>s</inf> over the diurnal period was successfully described by negative feedback of the accumulation of photosynthetic products. Investigation into the impact of leaf anatomy on temporal responses of A, g<inf>s</inf> and W<inf>i</inf> revealed that a high density of stomata produces the most rapid response of g<inf>s</inf> but may result in lower W<inf>i</inf>.