A relationship between humidity response, growth form and photosynthetic operating point in C₃ plants

Gas exchange experiments were performed with 13 plant species that differ from each other in growth-form and natural habitat. These comprised three herbaceous species, two ferns, two temperate deciduous trees, five rainforest trees and one liana from wet tropical forest. The aims were to investigate whether plants of similar growth-form and from similar habitats tended to respond similarly to a change in leaf-to-air vapour pressure difference (VPD), and to compare their ratio of intercellular to ambient partial pressures of CO₂ for given conditions. Leaves were subjected to a step change in VPD and the initial and final steady rates of transpiration were used to calculate an index of sensitivity, φ, which enabled comparison of species. The results suggest that species of similar growth-form and habitat respond similarly to increasing VPD, with the temperate deciduous trees undergoing a greater reduction in stomatal conductance than the herbaceous plants in well-watered soil. Also, for these experimental conditions, the ratio of leaf internal to ambient CO₂ partial pressure (p(i)/p(a)) was positively correlated with both CO₂ assimilation rate and stomatal insensitivity to VPD, across the 13 species. The results are discussed in terms of growth strategies and possible advantages and limitations of hydraulic systems in different plants.