INTRODUCTION

Changes in the physical environment, particularly in relation to temperature, light, humidity and gaseous atmosphere, limit the growth of all organisms and promote entry into a long-term persistent, “dormant” state. Of these, probably moisture and temperature are the most important. At high humidities (>99%), organisms tend to grow at an unhindered rate. However, dehydration towards 90% relative humidity (RH) severely reduces metabolism and growth of plants, fungi, yeast and animals. For example, nematodes coil and Fusarium hyphae become stationary when RH is reduced (45). To facilitate survival of such high levels of desiccation, a complex sequence of biochemical changes is needed that safely down regulates biosynthetic pathways. For example in seeds, the accumulation of pernicious compounds (such as free radicals/activated oxygen and lipid peroxidation products) is avoided, and/or counteracted by the accumulation of protectants like tocopherols (60). In seeds, the point of growth cessation has been estimated to be around–2 to–4 MPa (∼99% RH). Seeds fail to germinate below this water potential, although metabolism, e.g., respiration, only ceases at about –15 MPa (∼90% RH) (73). Below this water potential, insufficient water means relative metabolic stasis and enhanced longevity as further moisture is withdrawn down to around –250 MPa (∼15% RH). It is not just the seeds (sporophytes) of many higher plants which can tolerate such an environment, but also vegetative tissue (gametophyte) of a small number of plants (the resurrection plants).