The Response of Photosynthesis and Transpiration to Rising Atmospheric CO2 Concentrations Under High Temperatures.

Climate change will include correlated increases in atmospheric CO₂ concentration [CO₂ ] and temperature. The response of Sorghum is  referred to here and used to quantitatively examine the implications of concurrent increase in temperature and [CO₂].  Measurements  of leaf photosynthesis in growth chambers show an  increase in phtosynthesis, with elevation of [CO₂ ] from 400 to 1000 µmol mol^-1. The increase in [CO₂ ]increased  photosynthesis by about 25% at 30ᵒC. But  at 40°C, no difference between 400 and 1000 µmol  mol^-1 was detected . Probably because high temperatures  increased the ratio of photorespiratory loss of carbon to photosynthetic gain whilst rising [CO₂ ] had an opposing effect. Experimental evidence concerning the effects of elevated [CO₂ ] on transpiration and water use efficiency is only beginning to accumulate.  It is concluded from this study  that,  an increase in transpiration  is possible in response to increased atmospheric CO2 levels. With elevation of [CO₂ ] from 400 to 1000 µmol mol^-1 transpiration increased by  more than 10% at both 30 and 40ᵒC. but the absolute values were higher at 40ᵒC. The mechanisms by which elevated CO2 may cause a change in these measures are discussed in terms of stomatal conductance.  .Water use efficiency (WUE) was calculated here as the net gain of CO₂ per unit H₂O transpired. In this study it  was respectively 0.55 and 0.47 mol CO₂ per one kg of water for 400 and 1000 µmol mol^-1  CO₂  at temperature of 30ᵒC. It is concluded that although the data contributed to our knowledge on the interactions among  increased temperature, elevated CO2 and WUE , substantially more data is required before reliable conclusions can be made.