title: Reduced plant water status under sub-ambient pCO2 limits plant productivity in the wild progenitors of C3 and C4 cereals.
abstract: The reduction of plant productivity by low atmospheric CO2partial pressure (pCO2) during the last glacial period is proposed as a limiting factor for the establishment of agriculture. Supporting this hypothesis, previous work has shown that glacial pCO2limits biomass in the wild progenitors of C3and C4founder crops, in part due to the direct effects of glacial pCO2on photosynthesis. Here, we investigate the indirect role of pCO2mediated via water status, hypothesizing that faster soil water depletion at glacial (18 Pa) compared to post-glacial (27 Pa) pCO2, due to greater stomatal conductance, feeds back to limit photosynthesis during drying cycles.
  
  
  
  We grew four wild progenitors of C3and C4crops at glacial and post-glacial pCO2and investigated physiological changes in gas exchange, canopy transpiration, soil water content and water potential between regular watering events. Growth parameters including leaf area were measured.
  
  
  
  Initial transpiration rates were higher at glacial pCO2due to greater stomatal conductance. However, stomatal conductance declined more rapidly over the soil drying cycle in glacial pCO2and was associated with decreased intercellular pCO2and lower photosynthesis. Soil water content was similar between pCO2levels as larger leaf areas at post-glacial pCO2offset the slower depletion of water. Instead the feedback could be linked to reduced plant water status. Particularly in the C4plants, soil-leaf water potential gradients were greater at 18 Pa compared with 27 Pa pCO2, suggesting an increased ratio of leaf evaporative demand to supply.
  
  
  
  Reduced plant water status appeared to cause a negative feedback on stomatal aperture in plants at glacial pCO2, thereby reducing photosynthesis. The effects were stronger in C4species, providing a mechanism for reduced biomass at 18 Pa. These results have added significance when set against the drier climate of the glacial period.
authors: Department of Animal and Plant Sciences, Alfred Denny Building, University of Sheffield, Western Bank, Sheffield S10 2TN, UK j.cunniff@cabi.org.
authors: Jennifer
authors: J
authors: Cunniff
authors: Institute of Archaeology, 36 Beaumont St, Oxford OX1 2PG, UK.
authors: Michael
authors: M
authors: Charles
authors: Department of Archaeology, Northgate House, University of Sheffield, West Street, Sheffield S1 4ET, UK.
authors: Glynis
authors: G
authors: Jones
authors: Department of Animal and Plant Sciences, Alfred Denny Building, University of Sheffield, Western Bank, Sheffield S10 2TN, UK.
authors: Colin P
authors: CP
authors: Osborne
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism; D001467:Hordeum / Q000254:growth & development* / Q000378:metabolism / Q000502:physiology; D008897:Panicum / Q000254:growth & development* / Q000378:metabolism / Q000502:physiology; D010788:Photosynthesis / Q000502:physiology; D018515:Plant Leaves / Q000502:physiology; D018526:Plant Transpiration / Q000502:physiology; D032983:Setaria Plant / Q000254:growth & development* / Q000378:metabolism / Q000502:physiology; D014908:Triticum / Q000254:growth & development* / Q000378:metabolism / Q000502:physiology; D014867:Water / Q000378:metabolism
chemical_list: D014867:Water; D002245:Carbon Dioxide
keywords: C3 photosynthesis; C4 photosynthesis; Hordeum spontaneum; Panicum miliaceum var. ruderale; Setaria viridis; Triticum boeoticum; crop progenitors.; origin of agriculture; sub-ambient pCO2; water relations