Large Day to Day Variation in VCmax During Mid-Season in Soybean and Wheat Crops Related to Water Vapor Pressure Deficit

Most modelling of photosynthesis of higher plants utilizes the Farquhar, Von Caemmerer, Berry photosynthesis model of C3 photosynthesis. At current air levels of CO2, C3 photosynthesis at high photosynthetic photon flux density is most often limited by the maximum capacity of RuBisco carboxylation, VCmax. This study tested whether experimentally determined values of VCmax corrected for variation in measurement temperature were consistent from week to week during mid-season in two major C3 crop species, soybean and wheat. Gas exchange was measured on single upper canopy leaves near mid-day on sunny days, at the current air temperature and humidity, every 3-9 days during mid-season for two years and was used to estimate VCmax. It was found that VCmax values corrected to the same temperature sometimes varied by a factor of about 1.5 in both species from day to day. Variation in VCmax was not associated with prior 4-day means of temperature, or cloudiness, or with day of year in either species. However, VCmax in both species and years was significantly negatively correlated with the leaf to air difference in water vapor pressure (VPD) at the time of measurement. The results indicate a strong negative impact of high VPD on mid-day leaf photosynthesis which was not evident from sub-stomatal CO2 concentrations, which varied little from day to day. Because VPD increases with global warming, these results indicate that increasing VPD could diminish crop growth beyond any direct effects of warming.