Surprising implications of the optimality hypothesis of stomatal regulation gain support in a field test

Abstract

Our previous reformulation of Cowan and Farquhar’s optimality hypothesis of stomatal reg-ulation has resulted in models for photosynthesis and transpiration which have been readily testable against field data. When analysing the water use efficiency implied by our previous reformulation of the optimality hypothesis of stomatal regulation, we discovered an unexpected property: when stomatal reg-ulation is active, a linear relationship could be found between transpiration and a term involving water vapour deficit and photosynthesis. This prediction gives rise to a novel test which requires no parameter estimation. We conducted such a test in Scots pine, utilising ca 10 000 measurements of CO₂ exchange, transpiration, temperature, PAR, and water vapour concentration, taken at the SMEAR I measuring station in Finnish Lapland. As predicted, on clear and sunny days the correlation coefficient of the linear relationship was as high as 0.99, corroborating our formulation of the optimality hypothesis.