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Responses of C4 Grasses to Atmospheric CO2 Enrichment. II. Effect of Water Stress¹

Changes in leaf water potential, photosynthesis, and stomatal conductance in response to atmospheric CO_s enrichment at two watering regimes were studied in :Four species of C₄ grasses: Echinochloa crusgalli, Digitaria sanguinalis, Eleusine indica, and Setaria faberi. Plants were grown in controlled environment chambers at 350 and 675 µmol mol^-1 CO₂ with 1000 µmol m^-2 s-¹ photosynthetic photon tux density (PPFD). One group plants of each of the COs concentrations was subjected to water stress by withholding irrigation for 10 days starting 21 days after planting. Water potentials of the leaves of plants grown at 350 #tool mol^-1 CO₂ declined more rapidly and reached lower values at the end of the stress period than the water potential of those grown at 675 µmol mol^-1 CO₂. High COs-grown plants maintained higher turgor pressures than low CO₂-grown plants. All plants had lower rates of photosynthesis during water stress compared to non-stressed conditions. However, the net photosynthetic rates of the plants grown at high CO₂ concentration did not decline as rapidly as the rates of plants grown at low CO₂ when subjected to water stress. The maintenance of turgor along with CO₂ enrichment during water stress development in plant leaves may be responsible for the higher rates of net photosynthesis observed in high CO₂-grown plants compared to those in low CO₂-grown plants.

Key Words: Stomatal conductance • Photosynthesis • Leaf water potential • CO₂ x water stress • C₄ weeds

² Sr. res. Scientist and plant physiologist, Botany Dep. Phytotron and USDA, ARS, Duke Univ., Durham, NC, respectively.

Received for publication August 17, 1984.