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Environmental and Stress Physiology

Osmotic Adjustment in Sorghum

II. Relationship to Gas Exchange Rates

Department of Agronomy, Horticulture, and Entomology, Texas Tech University, Lubbock, Texas 79409-4169

Lowering of the solute potential by osmotic adjustment (OA) has been proposed to allow maintenance of leaf turgor potential (_p), stomatal conductance (g), and photosynthesis (A) at low leaf water potential. However, literature concerning the role of OA in the maintenance of g and A under water stress is limited and often contradictory. The objective of this experiment was to examine the association of OA with g and A in grain sorghum (Sorghum bicolor L. Moench). A single sorghum hybrid (cv ATx623 x RTx430) was studied under field conditions using four different water supplies. Diurnal and midday water potential, solute potential, _p, OA, g, and A were measured during preflowering and grain-filling growth stages. A second experiment was conducted under greenhouse conditions. Two sorghum genotypes (BTx623 and BTx378) differing in their g and A responses to plant water stress were compared for their OA capacity during a water deficit cycle imposed from the beginning of panicle initiation through flowering. Under both field and greenhouse conditions, g and A rapidly declined with increased water stress despite the occurrence of OA. Under greenhouse conditions, BTx623 maintained significantly higher g and A than BTx378 during the water stress cycle. However, no significant differences in OA or _p existed between the two genotypes, indicating that OA was not associated with differences observed in g and A between these genotypes. We conclude that the response of g and A to water stress was not directly associated with OA and certainly was not maintained by OA.

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