T HE PRESENT investigation involves the studies of physiological responses of A. raddiana (savi) Brenan and A. nilotica (L.) seedlings under extreme arid conditions. Experiments were performed in a hyper arid environment to study the effects of drought stress using different water regimes at 12%, 9%, 6%, 4% and 2%. Photosynthesis and transpiration rate were measured under full Photosynthetic Active Radiationrange (0-2500μmols -1 m -2 ) and instantaneous water use efficiency was calculated.A. raddiana and A. nilotica showed maximum photosynthesis rate under 4% and 12% soil moisture content, respectively at high Photosynthetic Active Radiation levels, maximum transpiration rate of A. raddiana recorded at 4% soil moisture content and at 9% soil moisture content in A. nilotica at highest Photosynthetic Active Radiation. The maximum instantaneous water use efficiency was noticed in A. raddiana at 12% soil moisture content, while A. nilotica showed maximum instantaneous water use efficiency at 6% soil moisture content at high Photosynthetic Active Radiation level.A. raddiana acted as water spender ideal desert plant at high Photosynthetic Active Radiation. Other wise A. nilotica maximised photosynthesis rate and minimised transpiration rate giving maximum instantaneous water use efficiency at high Photosynthetic Active Radiation and low soil moisture content levels.

The present investigation involves the studies of water use efficiency of Acacia seyal (Del.) seedlings under extreme arid conditions. Experiments were performed in a hyper arid environment to study the effects of drought stress using different water regimes at 12%, 9%, 6%, 4% and 2%. Calculated instantaneous water use efficiency was measured under full Photosynthetic Active Radiation range (0-2500μmols-1 m-2). Acacia seyal showed maximum photosynthesis rate at 9% soil moisture content, and at high Photosynthetic Active Radiation levels. Maximum transpiration rate recorded at 9% soil moisture content at highest Photosynthetic Active Radiation. The maximum instantaneous water use efficiency was noticed at 4% soil moisture content at high Photosynthetic Active Radiation level. Acacia seyal maximized photosynthesis rate and minimized transpiration rate, giving maximum instantaneous water use efficiency at the high Photosynthetic Active Radiation and low soil moisture content levels.