The responses of cytochrome redox state and energy metabolism to dehydration support a role for cytoplasmic viscosity in desiccation tolerance

To characterize the depression of metabolism in anhydrobiotes, the redox st ate of cytochromes and energy metabolism were studied during dehydration of soaked cowpea (Vigna unguiculata) cotyledons and pollens of Typha latifoli a 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 sp ectroscopy increased from 0.15 to 0.27 poise. This initial water loss was a ccompanied by a 50% decrease in respiration rates, whereas the adenylate en ergy charge remained constant at 0.8, and cytochrome c oxidase (COX) remain ed fully oxidized. From WC of 0.6 to 0.2 g/g, viscosity increased exponenti ally. The adenylate energy charge declined to 0.4 in seeds and 0.2 in polle n, whereas COX became progressively reduced. At WC of less than 0.2 g/g, CO X remained fully reduced, whereas respiration ceased. When dried under N-2, 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 O-2, the pattern of COX reduct ion was similar to that of air-dried tissues, although the maximum reductio n was 70% in dried tissues. Thus, at WC of less than 0.6 g/g, the reduction of COX probably originates from a decreased O-2 availability as a result o f the increased viscosity and impeded diffusion. We suggest that viscosity is a valuable parameter to characterize the relation between desiccation an d decrease in metabolism. The implications for desiccation tolerance are di scussed.