Protection mechanisms against excess light in the resurrection plants Craterostigma wilmsii and Xerophyta viscosa

Mechanisms of avoidance and protection against light damage were studied in the resurrection plants Craterostigma wilmsii and Xerophyta viscosa. In C. wilmsii, a combination of both physical and chemical changes appeared to afford protection against free radical damage. During dehydration leaves curled inwards, and the abaxial surface became exposed to light. The tissue became purple/brown in colour, this coinciding with a three-fold increase in anthocyanin content and a 30% decline in chlorophyll content. Thus light-chlorophyll interactions are progressively reduced as chlorophyll became masked by anthocyanins in abaxial layers and shaded in the adaxial layers. Ascorbate peroxidase (AP) activity increased during this process but declined when the leaf was desiccated (5% RWC). During rehydration leaves uncurled and the potential for normal light-chlorophyll interaction was possible before full hydration had occurred. Superoxide dismutase (SOD) and glutathione reductase (GR) activities increased markedly during this stage, possibly affording free radical protection until full hydration and metabolic recovery had occurred. In contrast, the leaves of X. viscosa did not curl, but light-chlorophyll interactions were minimised by the loss of chlorophyll and dismantling of thylakoid membranes. During dehydration, free radical protection was afforded by a four-fold increase in anthocyanin content and increased activities of AP, GR and SOD. These declined during rehydration. It is suggested that potential free radical damage may be avoided by the persistence of anthocyanins during the period of thylakoid membrane re-assembly and full chlorophyll restitution which only occurred once the leaves were fully rehydrated.