Photosynthesis, water use and growth of a C4 grass stand at high CO2 concentration

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Abstract
Leaf photosynthesis rate of the C4 species Paspalum plicatulum Michx was virtually CO2-saturated at normal atmospheric CO2 concentration but transpiration decreased as CO2 was increased above normal concentrations thereby increasing transpiration efficiency. To test whether this leaf response led growth to be CO2-sensitive when water supply was restricted, plants were grown in sealed pots of soil as miniature swards. Water was supplied either daily to maintain a constant water table, or at three growth restricting levels on a 5-day drying cycle. Plants were either in a cabinet with normal air (340 μmol (CO2) mol-1 (air)) or with 250 μmol mol-1 enrichment. Harvesting was by several cycles of defoliation. With abundant water supply high CO2 concentration did not cause increased growth, but it did not cause an increase in growth over a wide range of growth-limiting water supplies either. Only when water supply was less than 30–50% of the amount used by the stand with a water-table was there evidence that dry weight growth was enhanced by high CO2. In addition, with successive regrowth, the enhancing effect under a regime of minimal water allocations, became attenuated. Examination of leaf gas exchange, growth and water use data showed that in the long term stomatal conductance responses were of little significance in matching plant water use to low water allocation; regulation of leaf area was the mechanism through which consumption matched supply. Since high CO2 effects operate principally via stomatal conductance in C4 species, we postulate that for this species higher CO2 concentrations expected globally in future will not have much effect on long term growth.