Adriana Bochicchio

Antioxidant system in Boea hygroscopica: Changes in response to desiccation and rehydration

Abstract In leaves of Boea hygroscopica subjected to either rapid or slow dehydration and rehydration, the response to H 2 O 2 production was studied by monitoring the changes in the amounts of ascorbic and dehydroascorbic acids as well as the amounts of reduced and oxidized glutathione and related enzyme activities. Boea hygroscopica is a resurrection plant in which drought tolerance depends on the rate of desiccation. For this reason, excised leaves of B. hygroscopica were subjected either to rapid or slow dehydration, by a 144-hr exposure to 0% or 80% relative humidity, respectively. Slowly dried leaves, rehydrated for 36 hr, were able to resume their activities completely, while rapidly dried leaves were not able to revive. After dehydration, H 2 O 2 production decreased both in slowly and in rapidly dried leaves, whereas the levels of total ascorbate and glutathione became two and 50 times as high, respectively, as in the control. During slow drying, reduced glutathione was oxidized, since its content in slowly dried leaves was 50% lower than in rapidly dried leaves. The levels of soluble proteins were found to be, respectively, five and six times as high in rapidly and slowly dried leaves as in the control and rehydrated leaves. The electrophoretic patterns of soluble proteins were similar in both dehydrated leaves, but different from control and rehydrated leaves, which also presented similar patterns. The specific activities of dehydroascorbate reductase (EC 1.8.5.1), glutathione reductase (EC 1.6.4.2) and ascorbate peroxidase (EC 1.11.1.11) also had the same values in both slowly and rapidly dried leaves. Following rehydration, oxidative processes were intensified: H 2 O 2 levels and the oxidation of ascorbate and glutathione mostly increased during this period in which the functioning of the ascorbate/glutathione cycle was induced.

Changes of low-molecular weight substances in boea hygroscopica in response to desiccation and rehydration

Abstract Boea hygroscopica is a poikilohydrous plant. This and similar species are also called desiccation-tolerant or resurrection plants. They have the unique ability to revive from an extreme air-dry condition. Samples of fresh, dried and rehydrated leaves were analysed for the major classes of organic substances like sugars, alditols, inositols, fatty acids, amino acids, betaines, phytosterols and others. The compounds which accumulated in desiccated leaves were sucrose and polyunsaturated C 18 acids. Abietanes and pimaranes were also detected. Monosaccharides, which are present in fresh leaves, disappeared, whilst phytosterols, stearic and oleic acids decreased to low levels; paimitic acid was unchanged. Rehydration of dried leaves seemed to restore the chemical composition of unstressed leaves. Betaines were not detected in any of the plant samples examined.

Desiccation Tolerance in Immature Embryos of Maize: Sucrose, Raffinose and the ABA—Sucrose Relation

Our previous experiments on desiccation tolerance in immature embryos of maize cast doubt on a crucial role of high concentrations of sucrose and high raffinose to sucrose mass ratio. In the present work we report experiments aimed at gaining a further insight into the above mentioned subject and to ascertain whether endogenous ABA may stimulate the accumulation of sucrose. Immature and desiccation sensitive embryos of maize were isolated and exposed to different drying conditions either leading or not leading to desiccation tolerance acquisition and allowing for different ABA and sugar responses. ABA concentrations as well as sugar concentrations were compared in embryos undergoing such different dehydration treatments. We conclude that in maize embryos: 1) sucrose, even at high concentrations, is not the sole factor responsible for desiccation tolerance; 2) high sucrose concentrations do not appear to be an absolute requirement of desiccation tolerance; 3) a high (>0.05) raffinose to sucrose mass ratio is not a prerequisite of desiccation tolerance; 4) ABA doesn’t seem to stimulate sucrose accumulation. We suggest that desiccation tolerance can occur even in the absence of raffinose.

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.

Sugar biotransformations by fungi on leaves of the resurrection plant Sporobolus stapfianus

Abstract Fresh, dried and rehydrated leaf samples of the resurrection plant, Sporobolus stapfianus , were analysed for their contents of low- M r substances. Glucose, fructose and sucrose were the prevailing sugars in aqueous extracts from fresh leaves, whereas sucrose accumulated in dried leaves. After 72 hr rehydration, an unusual content of mannitol and trehalose was observed. The involvement of fungal species, able to perform sugar biotransformations in rehydrated leaves, is reported.