Monday, 6 October 2008
George R. Brown Convention Center, Exhibit Hall E
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 (gstom) is the mechanism responsible for decreased CER under increasing VPD in the absence of other limiting factors. Understanding the relationship between CER and gstom 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(gstom):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(gstom) 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(gstom) and VPD did not explain the causal mechanism behind CER response to VPD. In contrast to CER responses, species ln(gstom) 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(gstom) responsiveness among species implied that factors other than gstom contribute to CER responsiveness to VPD. We suggest further exploration of this phenomenon to improve biomass production modeling capabilities.