SOLUTES CONTRIBUTING TO OSMOTIC POTENTIAL IN YOUNG VERSUS MATURE LEAVES OF CACAO SEEDLINGS

The concentrations of sugars, inorganic ions and amino acids were measured at apical and basal leaf positions within both the new (expanding) leaf flush and previous (mature) flush of 8-month-old cacao (Theobroma cacao L.) seedlings. The osmotic potential of leaves of the previous flush was approximately 0.1 MPa lower (more negative) than that measured in the new flush. No differences were observed, however, for leaf water potential between leaves of the new flush and previous flush. Fructose and glucose were the predominant sugars in cell sap of apical leaves of the new flush, with sucrose accounting for only 11% of the total concentration (mM L-1) of sugars measured. The relative proportions of monosaccharides and sucrose were markedly different in basal leaves of the new flush, however, with sucrose being the predominant sugar and glucose plus fructose being present at less than one-fourth the concentration measured in apical leaves. Sucrose was the predominant sugar in leaves of the previous flush, and concentrations in younger and older leaves were not different. Potassium made the largest single contribution to the osmolality of cell sap, accounting for 38 to 61% of the total solutes measured. The contribution of amino acids to total osmolality was small, relative to that of sugars or K+. Alanine, asparagine (+ aspartate), gamma-aminobutyrate, glutamine (+ glutamate), and serine together accounted for more than 90% of the osmolality generated by 14 amino acids analyzed. Sugars, inorganic ions and amino acids measured analytically together accounted for 79 to 84% of the osmolality of expressed sap as measured by vapor pressure osmometry.