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Title: STOMATAL SENSITIVITY TO VAPOR PRESSURE DIFFERENCE OVER A SUBAMBIENT TO ELEVATED CO2 GRADIENT IN A C3/C4 GRASSLAND

Author
item MAHERALI, HAFIZ - DUKE UNIVERSITY
item Johnson, Hyrum
item JACKSON, ROBERT - DUKE UNIVERSITY

Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2003
Publication Date: 11/22/2003
Citation: Maherali, H., Johnson, H.B., Jackson, R.B. 2003. Stomatal sensitivity to vapor pressure difference over a subambient to elevated CO2 gradient in a C3/C4 grassland. Plant Cell and Environment. 26:1297-1306.

Interpretive Summary: Increasing amounts of carbon dioxide (CO2) in the atmosphere is the hallmark issue of global change debates. Because CO2 is a biologically active gas, it is hypothesized that the current rise in atmospheric CO2 will modify processes that affect productivity, stability and species composition of extensive natural grasslands. This report addresses the effect of CO2 on two plant processes, photosynthesis (plant carbon gain) and transpiration (plant water use), as observed in the field in a unique CO2 gradient system. Two representative grasses, Japanese brome (Bromus japonicus) a spring growing C3 type and King Ranch bluestem (Bothriochola ischaemum) a summer growing C4 type, were studied. Stomata, the pores in plant leaves through which water is lost in transpiration and carbon dioxide (CO2) enters in photosynthesis, change their conductance to CO2 and water vapor by opening and closing in response to changes in humidity and CO2 concentration. In both species conductance decreased with increasing CO2 and with lower humidity. Lower conductance reduced photosynthesis in both grasses. Effects of CO2 on stomata and photosynthesis depended on humidity in Japanese brome but did not depend on humidity in King Ranch bluestem. These differences in species performance should be included in predictive global change models.

Technical Abstract: We examined the response of stomatal conductance (gs) to increasing leaf-to-air vapor pressure difference (D) in early season C3 (Bromus japonicus) and late season C4 (Bothriochloa ischaemum) grasses grown in the field across a range of CO2 (200-550 umol mol-1). Stomatal sensitivity to D was calculated as the slope of the response of gs to the natural log of externally manipulated D (dgs/dlnD). Increasing D and CO2 significantly reduced gs in both species. Increasing CO2 caused a significant decrease in stomatal sensitivity to D in Br. japonicus, but not in Bo. ischaemum. The decrease in stomatal sensitivity to D at high CO2 for Br. japonicus fit theoretical expectations of a hydraulic model of stomatal regulation, in which gs varies to maintain constant leaf water potential. The weaker stomatal sensitivity to D in Bo. ischaemum suggested that stomatal regulation of leaf water potential was poor in this species, or that non-hydraulic signals influenced guard cell behavior. The decline in gs with increasing D reduced net photosynthesis (A) in both species, but increased stomatal limitation of photosynthesis only in Br. japonicus. Rising CO2 had the greatest effect on gs and A in Br. japonicus at low D. In contrast, the strength of stomatal and photosynthesis responses to CO2 were not affected by D in Bo. ischaemum. Carbon and water dynamics in this grassland are dominated by the seasonal transition from early C3 to late C4 species. Interspecific variation in the response of stomatal conductance to D therefore has implications for predicting seasonal ecosystem responses to CO2.