FUNCTION OF LEAF HAMAMELITOL AS A COMPATIBLE SOLUTE DURING WATER-STRESS TREATMENT OF HEDERA-HELIX L

Hamamelitol is an unusual branched-chain sugar alcohol previously sugg ested to function as a leaf compatible solute, In this study, we have examined the leaf metabolism and intracellular compartmentalization of hamamelitol and other soluble sugars during long-term mater stress tr eatment of Hedera helix (English ivy), Total leaf hamamelitol content was relatively low in greenhouse control plants, but increased 2-fold during water stress treatment to levels approaching those observed in field-grown plants (6-7 mu mol g(-1) fresh weight), Using density grad ient fractionation with non-aqueous solvents, we showed that hamamelit ol occurs primarily in the cytoplasm and vacuoles of leaf mesophyll ce lls, During water stress treatment most of the increase in leaf hamamd itol occurred in the mesophyll cytoplasm, compensating osmotically for a decrease in cytoplasmic sucrose concentration, The maximum concentr ation of cytoplasmic hamamelitol was 155 mol m(-3) and occurred in fie ld-grown plants, Labelling experiments showed that hamamelitol is slow ly synthesized from (CO2)-C-14 in leaves of H. helix, but is very long -lived (estimated t(1/2) of 4 years), Together, these data indicate th at hamamelitol probably functions during long-term stress conditions a s an osmotically active, compatible solute in plant leaves, We suggest that the signal for enhanced accumulation of hamamelitol during the w ater stress treatment was initiated by decreased plant growth and incr eased leaf sucrose hydrolysis.