Elucidation of the Relationships Between Vapor Pressure Deficit, Leaf Gas Exchange, and Stomatal Conductance.

Plant biomass production rates depend on carbon assimilation, the rate of which is determined by a plant’s CO₂ exchange rate (CER). When vapor pressure deficit (VPD) values rise CER slows, even when soil moisture is adequate.  We hypothesize that a decrease in stomatal conductance (g_stom) is the mechanism responsible for decreased CER under increasing VPD in the absence of other limiting factors. Understanding the relationship between CER and g_stom response to changing VPD is key to understanding plant productivity in diverse environments. A firm understanding of these relationships is essential to developing an effective model to predict plant biomass production. We compared the relationship between CER:VPD and ln(g_stom):VPD responses for two C₄ annual species: maize (Zea mays L.) and grain sorghum (Sorghum bicolor L. Moench), and one C₃ perennial species: giant reed (Arundo donax).  All species showed negative CER and ln(g_stom) responses to increased VPD.   Maize showed the greatest CER:VPD slope (-5.38 mmol m^-2 s^-1 kPa^-1), followed by sorghum with -3.95 and giant reed with -2.07 (P<0.001).  The observed relationships between ln(g_stom) and VPD did not explain the causal mechanism behind CER response to VPD. In contrast to CER responses, species ln(g_stom) responses to VPD showed maize and sorghum had similar slopes (-0.26 and -0.24 mmol m^-2 s^-1 kPa^-1, respectively; Tukey’s = 0.08) while giant reed had the greatest slope (-0.57; Tukey’s <0.001 for maize and sorghum comparisons). Thus, the discrepancies between CER responsiveness and ln(g_stom) responsiveness among species implied that factors other than g_stom contribute to CER responsiveness to VPD. We suggest further exploration of this phenomenon to improve biomass production modeling capabilities.