Longevity of dry Myrothamnus flabellifolius in simulated field conditions

This study tested the length of time the desiccation-tolerant Myrothamnus flabellifolius could remain in the dry state under simulated field conditions, without losing viability. Dry plants were kept at 50% relative humidity, with a 25/16degreesC day/night temperature regime and a daylight intensity of 1200 mumol.m(-2) s(-1). At three monthly intervals plants were rehydrated and the ability to resume respiration, photosynthesis, protein synthesis and transcription was assessed and changes in subcellular organisation and plant growth regulator content (zeatin, zeatin riboside and abscisic acid) monitored. Plants survived in the dry state for only one year. The ability to resurrect metabolism in existing leaves was lost after nine months, after which survival occurred due to regrowth of new leaves from meristems. There was little evidence of subcellular damage in leaf tissues of plants kept dry for up to six months. These plants recovered respiration before the onset of translation (and thus repair) suggesting considerable subcellular protection of this metabolism against desiccation damage. Furthermore, full recovery of metabolism, including photosynthesis, occurred before the onset of transcription in these plants. Some subcellular damage occurred in plants maintained dry for nine months. There was increased electrolyte leakage indicative of membrane damage which was repaired with the onset of protein synthesis. Since this repair occurred before the onset of transcription, it is likely that stored mRNA, present in the dry leaves, was used. Recovery of photosynthesis in plants dried for 9 months was delayed until the onset of transcription. We propose that mRNA for recovery of this metabolism is not stored during drying and thus damage to the photosynthetic apparatus can be repaired only upon de novo transcription of the genome. In leaves that were able to resurrect, cytokinin content increased transiently just prior to onset of chlorophyll biosynthesis and translation, and abscisic acid content increased just prior to the onset of transcription. These plant growth regulator changes did not occur in leaves which did not resurrect, but we cannot distinguish whether it was the lack of signal, or the extent of tissue damage (or both) which prevented the repair and recovery of metabolism in these tissues.