The major soluble carbohydrates in the desiccation-tolerant leafy liverwort
Porella platyphylla (L.) Lindb. are
sucrose and a homologous series of fructans including the trisaccharide
1-kestose. Exogenous glucose and fructose (10 mol m−3)
did not affect the composition of the soluble carbohydrate pool. Sucrose
caused an increase in the fructan pool. Sucrose also inhibited photosynthetic
oxygen evolution and respiration. 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
molecular weight fructan. Acid invertase
activity was detected in a taxonomically diverse range of liverworts
but was very low or undetectable in a range
of mosses. The invertase activity from P. platyphylla was
partially purified by ammonium sulphate precipitation.
The reaction products of the partially purified enzyme were equimolar
glucose and fructose. Kestose and higher
DP fructans were not detected suggesting that, at least under the assay
used, the enzyme does not have sucrose[ratio ]sucrose fructosyl transferase
activity. The pH optimum was 4·5–5 and the Km
for sucrose was 1·7 mol m−3.
Pyridoxal hydrochloride (5 mol m−3) caused 50% inhibition.
The coexistence of sucrose and invertase suggests
that either the invertase is inactive in vivo or is in a
different subcellular compartment from sucrose. The pH
response shows that it would have very low activity at cytosolic pH. A
large acidic vacuole was detected in P. platyphylla leaf cells
by neutral-red staining in which either invertase or sucrose could be sequestered.
Rehydrating desiccated P. platyphylla for 10 min resulted in a
60% loss of extractable invertase activity. By
contrast, extractable malate dehydrogenase 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 suggested that the partial inactivation of invertase during
rehydration minimizes sucrose hydrolysis while
membrane structure and subcellular compartmentation are re-established.