The Responses of Cytochrome Redox State and Energy Metabolism to Dehydration Support a Role for Cytoplasmic Viscosity in Desiccation Tolerance1

To characterize the depression of metabolism in anhydrobiotes, the redox state of cytochromes and energy metabolism were studied during dehydration of soaked cowpea (Vigna unguiculata) cotyledons and pollens of Typha latifolia and Impatiens glandulifera. Between water contents (WC) of 1.0 and 0.6 g H2O/g dry weight (g/g), viscosity as measured by electron spin resonance spectroscopy increased from 0.15 to 0.27 poise. This initial water loss was accompanied by a 50% decrease in respiration rates, whereas the adenylate energy charge remained constant at 0.8, and cytochrome c oxidase (COX) remained fully oxidized. From WC of 0.6 to 0.2 g/g, viscosity increased exponentially. The adenylate energy charge declined to 0.4 in seeds and 0.2 in pollen, whereas COX became progressively reduced. At WC of less than 0.2 g/g, COX remained fully reduced, whereas respiration ceased. When dried under N2, COX remained 63% reduced in cotyledons until WC was 0.7 g/g and was fully reduced at 0.2 g/g. During drying under pure O2, the pattern of COX reduction was similar to that of air-dried tissues, although the maximum reduction was 70% in dried tissues. Thus, at WC of less than 0.6 g/g, the reduction of COX probably originates from a decreased O2 availability as a result of the increased viscosity and impeded diffusion. We suggest that viscosity is a valuable parameter to characterize the relation between desiccation and decrease in metabolism. The implications for desiccation tolerance are discussed.

Drying Increases Intracellular Partitioning of Amphiphilic Substances into the Lipid Phase1 : Impact on Membrane Permeability and Significance for Desiccation Tolerance

Previously we proposed that endogenous amphiphilic substances may partition from the aqueous cytoplasm into the lipid phase during dehydration of desiccation-tolerant organ(ism)s and vice versa during rehydration. Their perturbing presence in membranes could thus explain the transient leakage from imbibing organisms. To study the mechanism of this phenomenon, amphiphilic nitroxide spin probes were introduced into the pollen of a model organism, Typha latifolia, and their partitioning behavior during dehydration and rehydration was analyzed by electron paramagnetic resonance spectroscopy. In hydrated pollen the spin probes mainly occurred in the aqueous phase; during dehydration, however, the amphiphilic spin probes partitioned into the lipid phase and had disappeared from the aqueous phase below 0.4 g water g−1 dry weight. During rehydration the probes reappeared in the aqueous phase above 0.4 g water g−1 dry weight. The partitioning back into the cytoplasm coincided with the decrease of the initially high plasma membrane permeability. A charged polar spin probe was trapped in the cytoplasm during drying. Liposome experiments showed that partitioning of an amphiphilic spin probe into the bilayer during dehydration caused transient leakage during rehydration. This was also observed with endogenous amphipaths that were extracted from pollen, implying similar partitioning behavior. In view of the fluidizing effect on membranes and the antioxidant properties of many endogenous amphipaths, we suggest that partitioning with drying may be pivotal to desiccation tolerance, despite the risk of imbibitional leakage.

Influence of Water Content and Temperature on Molecular Mobility and Intracellular Glasses in Seeds and Pollen1

Although the occurrence of intracellular glasses in seeds and pollen has been established, physical properties such as rotational correlation times and viscosity have not been studied extensively. Using electron paramagnetic resonance spectroscopy, we examined changes in the molecular mobility of the hydrophilic nitroxide spin probe 3-carboxy-proxyl during melting of intracellular glasses in axes of pea (Pisum sativum L.) seeds and cattail (Typha latifolia L.) pollen. The rotational correlation time of the spin probe in intracellular glasses of both organisms was approximately 10−3 s. Using the distance between the outer extrema of the electron paramagnetic resonance spectrum (2Azz) as a measure of molecular mobility, we found a sharp increase in mobility at a definite temperature during heating. This temperature increased with decreasing water content of the samples. Differential scanning calorimetry data on these samples indicated that this sharp increase corresponded to melting of the glassy matrix. Molecular mobility was found to be inversely correlated with storage stability. With decreasing water content, the molecular mobility reached a minimum, and increased again at very low water content. Minimum mobility and maximum storage stability occurred at a similar water content. This correlation suggests that storage stability might be at least partially controlled by molecular mobility. At low temperatures, when storage longevity cannot be determined on a realistic time scale, 2Azz measurements can provide an estimate of the optimum storage conditions.