CARBOHYDRATE-COMPOSITION AND INVERTASE ACTIVITY OF THE LEAFY LIVERWORT PORELLA-PLATYPHYLLA

The major soluble carbohydrates in the desiccation-tolerant leafy live rwort Porella platyphylla (L.) Lindb. are sucrose and a homologous ser ies of fructans including the trisaccharide 1-kestose. Exogenous gluco se and fructose (10 mol m(-3)) did not affect the composition of the s oluble carbohydrate pool. Sucrose caused an increase in the fructan po ol. Sucrose also inhibited photosynthetic oxygen evolution and respira tion. The fructan pool was maintained in preference to sucrose during dark starvation. Low temperature and low water potential increased the fructan pool whereas desiccation increased the proportion of high mol ecular weight fructan. Acid invertase activity was detected in a taxon omically diverse range of liverworts but was very low or undetectable in a range of mosses. The invertase activity from P. platyphylla was p artially purified by ammonium sulphate precipitation. The reaction pro ducts of the partially purified enzyme were equimolar glucose and fruc tose. Kestose and higher DP fructans were not detected suggesting that , at least under the assay conditions used, the enzyme does not have s ucrose:sucrose fructosyl transferase activity. The pH optimum was 4.5- 5 and the K-m for sucrose was 1.7 mol m(-3) Pyridoxal hydrochloride (5 mol m(-3)) caused 50 % inhibition. The coexistence of sucrose and inv ertase suggests that either the invertase is inactive in vivo or is in a different subcellular compartment from sucrose. The pH response sho ws that it would have very low activity at cytosolic pH. A large acidi c vacuole was detected in P. platyphylla leaf cells by neutral-red sta ining in which either invertase or sucrose could be sequestered. Rehyd rating desiccated P. platyphylla for 10 min resulted in a 60 % loss of extractable invertase activity. By contrast, extractable malate dehyd rogenase activity increased during rehydration. Rehydrating desiccated leaves caused an increase in glucose and fructose suggesting that the sucrose pool was susceptible to invertase at this time. It is suggest ed that the partial inactivation of invertase during rehydration minim izes sucrose hydrolysis while membrane structure and subcellular compa rtmentation are re-established.