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A pot experiment was conducted to investigate the effects of soil water stress on gas exchange and stomatal response of the upper fully expanded leaf of the root parasitic weed Striga hermonthica and that of its sorghum host. Photosynthetic rate of S. hermonthica was less affected by water stress than sorghum, but the ratio of respiration rate to photosynthetic rate increased considerably under water stress. Thus, under water stress, growth of S. hermonthica may rely more on heterotrophic carbon from host to compensate the low autotrophic carbon gain. Water stress-induced reductions in transpiration rate and stomatal conductance were more severe in infected sorghum than in S. hermonthica. Water stress-induced reduction in stomatal aperture of the leaf was greater in infected sorghum than in S. hermonthica. These results indicated that higher transpiration rate of S. hermonthica under water stress, achieved through greater stomatal aperture, may be related to maintenance of water and solute movement from the host to the parasite.