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Summary Solute contributions to the callus Ψ s were assessed in loblolly pine ( Pinus taeda L.) callus while growing on medium at 2 Ψ w 's over a 5-week period. Murashige and Skoog medium with 30 g 1- 1 sucrose was used to produce a high Ψ w of −0.4 MPa (H) and the same medium was used to create a moderate Ψ w of −0.7 MPa (M) by adding 10% (w/v) polyethylene glycol (PEG, MW = 8000). Callus growth, Ψ w , Ψ s , and 4 solute classes (potassium, malate, soluble carbohydrates and amino acids) were measured and Ψ p (Ψ w -Ψ s ) was calculated. Callus Ψ p was maintained over time on both H and M media. H-callus Ψ w approached equilibrium with that of the external H medium after 5 weeks, whereas M-callus Ψ w was lower than that of the M medium. After 5 weeks on either medium, no increase was observed in the level of total solutes, when expressed per unit of either tissue DW or FW. At 5 weeks, the 4 solute classes were calculated to account for 87% of the H-callus Ψ s and 42% of the M-callus Ψ s . Apoplastic PEG accounted for an additional 14% (0.21 MPa) of theM-callus Ψ s . H-callus Ψ s increased with time because of increased water uptake, whereas M-callus Ψ s decreased because of PEG accumulation and probable PEG blockage of Mcallus hydration. Osmotic adjustment was not observed in this callus system. |
