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

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. Ci curves were measured over a 15 or 20 degrees 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 degrees C) resulted in higher values of the VCmax of Rubisco and Jmax at a given measurement temperature in most species compared with higher temperatures (max/min 31/25 degrees C). The apparent activation energy of VCmax of Rubisco ranged from 56 to 82 kJ mol-1, and that of electron transport (Jmax) ranged from 28 to 56 kJ mol-1 across species and temperatures. In three of the four species, the activation energy of VCmax decreased and that of Jmax increased after the cooler temperatures. Stomatal conductance measured at 20 and 25 degrees 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 degrees C), also indicated a wide range of values of the activation energies of VCmax and Jmax among these species.