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

Our previous reformulation of Cowan and Farquhar's optimality hypothesis of stomatal regulation has resulted in models for photosynthesis and transpir ation which have been readily testable against field data. When analysing t he water use efficiency implied by our previous reformulation of the optima lity hypothesis of stomatal regulation, we discovered an unexpected propert y: when stomatal regulation is active, a linear relationship could be found between transpiration and a term involving water vapour deficit and photos ynthesis. This prediction gives rise to a novel test which requires no para meter estimation. We conducted such a test in Scots pine, utilising ca 10 0 00 measurements of CO2 exchange, transpiration, temperature, PAR, and water vapour concentration, taken at the SMEAR I measuring station in Finnish La pland. 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.