Relative sensitivity of photosynthetic assimilation and translocation of carbon to water stress.

The relationship between photosynthesis and translocation rate changes as affected by water stress intensity and stage of plant development was evaluated in cotton and sorghum, representing a C(3) and a C(4) photosynthetic type, respectively. Photosynthetic rates were reduced as midday leaf water potentials declined from -14 to -27 bars in both species. Sorghum maintained higher photosynthesis and translocation rates compared to cotton at comparable leaf water potentials; however, the rate of change per bar decline in water potential was greater in sorghum than in cotton. Photosynthetic rates were reduced with increasing water stress prior to any significant change in translocation rates suggesting that photosynthesis is the more sensitive of the two processes. Severe water stress, corresponding to leaf water potentials of -27 bars, did not completely inhibit either photosynthesis or translocation.