The objectives of this study were to determine the influence of sublethal water stress on the physiology of loblolly and Aleppo pine. Gas exchange characteristics, uncoupled thylakoid electron transport capacity, and needle osmotic potentials were measured. Seedlings of both species were watered only when their needle water potential fell below -1.8 and -2.2 MPa respectively (water stress conditioning) or were kept well watered (controls). After 10 weeks of water stress conditioning, both regimes were allowed to dry down. During this period of increasing water stress, photosynthesis in both species was determined at 5 different cuvette CO₂ concentrations (approximately 200, 330, 500, 650, and 800 ppm). With Aleppo pine only, mesophyll resistances and stomatal limitations to gas exchange were estimated. Thylakoids were extracted from both species and their activity was measured in a liquid phase O₂ electrode (Hansatech L^td) as rate of O₂ consumption. Methyl Viologen (1,1’ -dimethyl -4, 4’- bipyridinium ion ) was included in the reaction medium. Photosynthesis decreased with increasing water stress but fell more slowly in the conditioned seedlings. As water stress increased, total resistance to CO₂ exchange increased for both regimes to a much higher level than explained by stomatal resistance alone. In the conditioned seedlings, resistances increased less precipitously than in controls. Osmotic adjustment as measured with thermocouple psychrometers occurred in both species. Decreases in photosynthesis (both species) and increases in mesophyll resistance (Aleppo pine) were not accompanied by a decrease in whole chain uncoupled electron transport capacity. / Master of Science