282 K. A. CLENDENNING, T. E. BROWN, E. E. WALLDOV 



OSMOTIC PRESSURE 



The leaf sap of land plants is eharacterized by widely different and 

 fluctuating osmotic pressures (8,9). The osmotic pressures to which 

 chloroplasts are subjected within a single leaf are far from constant. 

 Gradients exist betwe(Mi diftVicnt leaf tissues, the osmotic pressure 

 regularly being higher in palisade than in spongy parenchyma (8). 

 The osmotic pressure of the leaf sap undergo(>s diurnal and seasonal 

 changes, and also varies with the leaf position, growth environment, 

 and species (8,9). According to MeClendon (4), it has been known 

 for almost seventy years that the swelling of isolated chloroplasts can 

 be prevented by sucrose solutions. A maceration medium of con- 

 stant composition matches the fluctuating natural environment of 

 leaf chloroplasts only sporadically, even in leaves of a single species. 

 We have compared the photochemical activity of chloroplasts iso- 

 lated in neutral 30% Carbowax 4000, 0.5 M sucrose, and 0.05 M 

 phosphate, using "active" sources ranging from the "leathery" 

 leaves of several trees to simple aquatic plants. The measured water 

 contents ranged from 46% in GledUsia to 94% in Lemna (Table I). 

 The stabilizing effect of Carbowax was apparent in chloroplasts ob- 

 tained from all these diverse sources, provided limiting amounts of 

 chloroplasts weie used in the activity measurements. The photo- 

 chemical activities of chloroplasts isolated in 0.5 M sucrose and 0.05 

 M phosphate were approximately the same. The much higher activity 

 of the maple and black locust chloroplasts in Carbowax vs. sucrose is 

 attributed to Carbowax-tannin interaction. The proportionately 

 smaller enhancement of the initial Hill reaction rate in chloroplasts 

 from species of low soluble tannin content is attributed to the sta- 

 bilizing action of Carbowax via the cytoplasm. Elodea is one of the 

 several plants which have yielded active chloroplasts in one survey 

 (4) and not another (2) ; this may be a result of varying concentra- 

 tions of natural inhibitors. The Elodea samples used in the present 

 study yielded active chloroplasts consistently, with evidence of the 

 usual preservation effect of Carbowax. Several new sources of "active" 

 chloroplasts (woody and herbaceous legumes) are reported inciden- 

 tally in the accompanying table. 



The natural inhibitors discussed above represent those which have 

 been positively identified in a survey of a few dozen species. Many 

 additional types might be disclosed by further study. The best plant 

 sources of chloroplasts, mitochondria, and soluble enzymes are 

 those which are consistently free of irreversible inhibitors. 



