Osmotic adjustment: a drought tolerance mechanism in sorghum
Girma, Fekade Selassie
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Lack of an adequate water supply is the primary limitation to plant productivity world-wide. Grain sorghum (Sorghum bicolor L. Moench) production occurs predominantly in semiarid regions of the world where random drought is common. A number of drought resistance mechanisms are known to exist among plant species which allow adaptation to water deficits. Osmotic adjustment, the lowering of osmotic potential (øn) by net solute accumulation, is a drought resistance mechanism that is proposed to allow tolerance of tissue dessication by maintaining turgor and those developmental and physiological processes dependent upon turgor (cell expansion, stomatal opening, chloroplast activity, etc.). Despite considerable evidence for the lowering of øn due to water stress, limited information is available on the mechanisms involved and the association between osmotic adjustment and plant performance in response to water deficits. A field experiment was conducted to determine the mechanisms involved in øn changes and the association between osmotic adjustment and physiological parameters in grain sorghum. Degrees of plant water stress were induced by varying the soil water supply through differential irrigation. Leaf water potential (øw ), øn, stomatal conductance, and photosynthesis were measured diurnally on selected days prior to flowering and during grain filling stage. Partitioning of the total diurnal change in øn indicated that dehydration and net solute accumulation were the major mechanisms for diurnal change in øn. Osmotic adjustment maintained turgor pressure over a wide range of øw. Despite the maintenance of turgor pressure by osmotic adjustment, stomatal conductance and photosynthesis declined as øw decreased. Results from this study indicate that osmotic adjustment does not maintain stomatal conductance and photosynthesis in grain sorghum.