Effect of shock and gradual drought by PEG on callus growth and proline accumulation in sour orange (Citrus x aurantium)

The responses of sour orange (Citrus x aurantium) callus to different concentration of polyethylene glycol (PEG), induced water stress including callus growth, water content and proline accumulation was studied. To study callus growth in response to two different treatment of PEG-8000 (shock and gradual), 100 mg callus was grown in test tube containing 12.5 ml of solid media supplemented with PEG (MW 8000) at 0, 2, 4, 6, and 8 g/L. Results revealed that increasing water stress initiated by increasing PEG concentration caused a progressive reduction in callus fresh weigh. A significant reduction in callus weight and water content were noticed in response to 4 g/L PEG but the inhibitory concentration was identified to be 8 g/L PEG, whereas, a significant increase in callus weight and water content were noticed in 0, and 2 g/L PEG. Increasing PEG concentration in cultured media caused increase in proline accumulation. In conclusion, this study showed that gradual treatment was better than shock treatment in all studied parameters.