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Abstract
The effects of water stress on internal water potential components and specific physiological processes were investigated in field grown potatoes (Solanum tuberosum L. cv. Viking). Leaf water potential (ψ leaf) as estimated by the pressure chamber, was not directly related to soil water potential (ψ soil) until a specific minimum ψ soil was attained. Subsequently ψ leaf did not increase in response to increases in ψ soil. Water stress affected physiological processes such as stomatal resistance, photosynthesis and enzyme activity. A decline in ψ leaf was apparently responsible for increased stomatal resistance and decreases in photosynthetic rates. The activities of ribulose diphosphate carboxylase and phosphoenolpyruvate carboxylase decreased as ψ leaf declined. The relationship between water stress and physiological processes and the inability of ψ leaf to respond to increases in ψ soil after a maximum stress may partially explain the extreme sensitivity of potatoes to even mild water stress.