Contrasting effects of elevated CO2 and water deficit on two native herbs

This study investigated the effects of carbon dioxide (CO2) enrichment and soil water deficit on the water use efficiency (WUE) and growth of Sanguisorba minor Scop. (salad burnet) and Anthyllis vulneraria L. (kidney vetch), growing in controlled environments. Instantaneous WUE (IWUE) increased in both species in elevated CO2, with a higher average increase in unwatered (UW) A. vulneraria over the drying cycle. Total plant WUE of A. vulneraria increased in elevated CO2 and under water deficit: the UW plants in elevated CO2 had higher WUE and reduced water loss. By contrast, thee was only an effect of water supply on S. minor: total plant WUE increased and water loss decreased in the UW plants in both CO2 treatments. Total apparent root length (ARL) of both species increased with CO2 enrichment and in UW S. minor total ARL was increased. By contrast, for A. vulneraria, total ARL of UW plants increased in ambient CO2, but decreased in elevated CO2 as compared with well-watered (WW) plants. Shoot dry weight (SDW) and root dry weight increased in both species (WW and UW) with CO2 enrichment. For UW S. minor, SDW decreased relative to WW plants in both CO2 treatments. By contrast, ANOVA showed no significant effect of water supply on SDW of A. vulneraria. Leaflet length increased in both species in elevated CO2 and decreased following drought. Cell wall tensiometric extensibility (%P) increased in expanding leaves of S. minor in elevated CO2 and for both species %P decreased in the UW plants as compared with those WW. Leaf water potential (Psi) of both species was lower in growing leaves of WW plants in elevated CO2. Water deficit reduced the Psi of growing leaves in both CO2 treatments. The different responses of these species suggest that in a drier, enriched CO2 environment survival in a community might depend on their ability to maintain growth at the same time as conserving water.