The effect of dehydration and rehydration at pre and post-flowering stages on the water relations, gas exchange characteristics (stomata conductance, photosynthesis and Fv/Fm (quantum efficiency) ratio), pigment compositions (chlorophyll and carotenoid contents) and water use efficiency on maize (cv Melkassa-2) and sorghum (cv Macia) were investigated with the objectives of understanding the physiological basis of drought resistance mechanisms and investigating whether there were differential responses in some of the physiological traits of drought resistance and recovery upon rehydration of maize and sorghum. The study was carried out in a controlled environment growth chamber under constant environmental conditions (12/12 h day/night, 28-32/17°C day/night temperature, 60-80% RH and PPFD of 1200-1400 μ mol m-2s-1). Both species showed reduced gs (stomatal conductance) in response to dehydration to reduce water loss over a range of relative water contents during both developmental stages. In maize, stomata appeared to be closed earlier and completely, while partial stomatal closure at relatively higher relative water contents appeared to have occurred in sorghum. gs recovery occurred following pre-flowering rehydration to the control level in both species only. The response of all other gas exchange characteristics (Pn and Fv/Fm) and water use efficiency followed similar trends to that of gs both in maize and sorghum at pre and post-flowering dehydration and rehydration. Dehydration also led to a decrease in Fv/Fm ratios as compared to the control plants in both species. Both species, however, exhibited similar rates of Fv/Fm ratios during pre and post-flowering dehydration. Fv/Fm ratios appeared to be affected more during post than pre-flowering dehydration in both species. Fv/Fm ratios of both species were recovered following pre-flowering rehydration but only maize recovered from post-flowering rehydration.
Key words: Chlorophyll, carotenoid, dehydration, electrolyte leakage, rehydration, stomatal conductance.
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