Enhanced drought tolerance of a soil-dwelling springtail by pre-acclimation to a mild drought stress.

The springtail Folsomia candida has a highly permeable cuticle, but is able to survive several weeks at 98.2%RH. This corresponds to a water potential deficit of about 17bars between the environment and the normal osmotic pressure of the body fluids of this animal. Recent studies have shown a water vapour absorption mechanism by accumulation of sugars and polyols (SP) in F. candida, which explains how this species can survive dehydrating conditions. In the present study, adult F. candida were pre-acclimated at 98.2%RH to induce the accumulation of SP, and were subsequently exposed for additional desiccating conditions from 98 to 94%RH. Activity level, water content, osmotic pressure of body fluids and SP composition were investigated. After the desiccation period, the animals were rehydrated at 100%RH and survival was assessed. The results showed that F. candida survived a more severe drought stress when it had been pre-acclimated to 98.2%RH before exposure to lower humidity. This species was able to maintain hyperosmosity to the surroundings at 95.5%RH, suggesting that it can absorb water vapour down to this limit. Below this limit, trehalose levels increased while myo-inositol levels decreased. We propose that this is a change of survival strategy where F. candida at mild desiccation levels seek to retain water by colligative means (remain hyperosmotic), but at severe desiccation levels switches to an anhydrobiotic strategy.