ROOT-ZONE SALINITY ALTERS RAFFINOSE OLIGOSACCHARIDE METABOLISM AND TRANSPORT IN COLEUS

Exposure of variegated coleus (Coleus blumei Benth.) plants to a salin e root-zone environment (60 mM NaCl:12 mM CaCl2) resulted in a signifi cant decline in elongation growth rate over the 30-d experimental peri od. During the initial 5 to 10 d of exposure, mature source leaves sho wed strongly diminished rates of photosynthesis, which gradually recov ered to close to the control rates by the end of the experiment. In gr een leaf tissues, starch levels showed the same transient decline and recovery pattern. Low starch levels were accompanied by the appearance of several novel carbohydrates, including high-molecular-weight raffi nose family oligosaccharides (RFOs) with a degree of polymerization (D P) of 5 to 8, and an O-methylated inositol (OMI). New enzyme activitie s, including galactan:galactan galactosyltransferase, for the synthesi s of high-DP RFOs and myo-inositol 6-O-methyltransferase for O-methyla tion of myo-inositol, were induced by salinity stress. Phloem-sap anal ysis showed that in the stressed condition substantially more sucrose than RFO was exported, as was the OMI. In white sink tissues these phl oem sugars were used to synthesize high-DP RFOs but not OMIs. In sink tissues galactan:galactan galactosyltransferase but not myo-inositol 6 -O-methyltransferase was induced by salinity stress. Models reflecting the changes in carbohydrate metabolism in source and sink tissues in response to salinity stress are presented.