Changes in the Responses of Leaf Gas Exchange to Temperature and Photosynthesis Model Parameters in Four C₃ Species...

Substantial variation in the temperature dependence of parameters of the Farquhar, von Caemmerer, and Berry C₃ photosynthesis model, as well as those of in vitro Rubisco kinetic characteristics, have been observed in controlled conditions but have seldom been systematically examined in the field. In this work, A vs. C_i curves were measured over a 15 or 20 °C range of temperature in four C₃ species growing outdoors on two occasions about three weeks apart early in the growing season and also once near mid-season when air temperatures were more stable. The two early season occasions were chosen for having contrasting temperatures for 3 to 4 days preceding the measurements. Low temperatures (mean maximum/minimum temperatures of 19/11 °C) resulted in higher values of the V_Cmax of Rubisco and J_max at a given measurement temperature in most species compared with higher temperatures (max/min 31/25 °C). The apparent activation energy of V_Cmax of Rubisco ranged from 56 to 82 kJ mol^-1, and that of electron transport (J_max) ranged from 28 to 56 kJ mol^-1 across species and temperatures. In three of the four species, the activation energy of V_Cmax decreased and that of J_max increased after the cooler temperatures. Stomatal conductance measured at 20 and 25 °C increased strongly with the prior warm temperatures in all species. Measurements made near mid-season, after a period of relatively stable temperatures (mean maximum/minimum temperatures of 27/18 °C), also indicated a wide range of values of the activation energies of V_Cmax and J_max among these species.