What is dry? Exploring metabolism and molecular mobility at extremely low water contents

Desiccation tolerance is defined as ‘the ability of tissues to survive loss of 95% of cellular water or dehydration to tissue water contents of ≤0.1 g H2O g–1 dry mass (DM)’. This trait is common in reproductive structures such as seeds, but relatively rare in vegetative tissues, occurring in only 330 (0.86%) species of vascular plants (Box 1) (Proctor and Pence, 2002). Among non-tracheophytes, it is present in virtually all lichens (Kranner et al., 2009) and in 210 (1.04%) bryophytes (Wood, 2007). The ability to survive such extreme water loss has invoked much scientific interest which, among other things, has facilitated applications in germplasm conservation via seed storage (Bewley et al., 2013) and, in the case of vegetative desiccation tolerance, towards production of extremely drought-tolerant crops for food security in a hotter dryer future (Hilhorst and Farrant, 2018).