In this review, water deficit remains one of the major constraints on plant productivity under rising temperatures, increasing atmospheric aridity, and the growing frequency of extreme weather events. Photosynthesis responds to drought not simply as a consequence of reduced water availability, but as a systemic disturbance of hydraulic, carbon, energy, and redox balance. At the early stage of stress, the decline in net CO2 assimilation is determined mainly by stomatal closure and reduced stomatal conductance; however, as water deficit intensifies, the roles of mesophyll conductance, biochemical capacity for CO2 assimilation, ribulose-1,5-bisphosphate regeneration, and thylakoid membrane function become increasingly important. In parallel, reduced chloroplastic CO2 availability enhances photoprotective processes, alters ATP/NADPH balance, and increases the importance of photorespiration, mitochondrial respiration, and antioxidant defense. This review synthesizes current concepts of ABA -dependent and ABA - independent stomatal closure signaling, non -stomatal limitations of photosynthesis, photochemical regulation, and the roles of photorespiration and redox homeostasis, with particular emphasis on the diagnostic value of integrating gas exchange with PAM - fluorometry. Drought tolerance is interpreted as a multicomponent trait emerging from the coordination of molecular, physiological, morphological, and recovery related processes across levels of plant organization.