Moisture content of the dried leaf is critical to desiccation tolerance in detached leaves of the resurrection plant Boea hygroscopica

In our experimental conditions detached leaves of the resurrection plant Boea hygroscopica survived equilibration to 65-80% RH (Relative Humidity), but not to very low RH (close to 0%). The first aim of our research was to determine whether sensitivity to equilibration to very low RH depends on the rate of the drying process or on the very low final MC (Moisture Content) attained, The second aim of our research was to determine ABA content of leaves exposed to the two drying processes: a first step towards understanding whether ABA is involved in the tolerance mechanism of Boea hygroscopica. Detached leaves were equilibrated either to 1.4 or to 60-70% RH or to various temporal combinations of these two RH. ABA content was monitored during drying. Dehydrated leaves were imbibed in liquid water either directly or after a slow rehydration at 98% RH. Tolerance was assessed after 48 h imbibition in liquid water. The low final MC attained (about 3%) and not the rate of drying was responsible of the sensitivity of leaves equilibrated to 1.4% RH. Slow rehydration attained better recovery, but it was not able to allow full "resurrection" thus suggesting that a plain biophysical liquid-crystalline to gel phase transition of the membrane lipid bilayer could not fully account for the lethal damage of the very low MC. The conclusions relative to the first part of our research was of primary importance in interpreting results concerning ABA variations during the two drying treatments, ABA showed a very similar transient increase when excised leaves were dried at either 1.4% RH (sensitive leaves) or at 60-70% RH (tolerant leaves). However we cannot exclude that the transient increase of the hormone is a necessary component of the desiccation tolerance mechanisms in detached leaves of Boea hygroscopica: the extremely low MC reached by equilibration to 1.4% RH may impair the mechanism itself.