Ephemeral pools as stressful and isolated habitats for the endemic aquatic resurrection plant Chamaegigas intrepidus

Abstract

Ephemeral pools are widespread in (semi-)arid regions, where they form unique, mostly isolated habitats in an otherwise dry landscape matrix. They are charac- terized by extremely variable conditions, the most important stress factors being (1) fre- quent and fast desiccation and rehydration, (2) long dry seasons, (3) intensive solar irradi- ation and high temperatures during the dry season, (4) large diurnal oscillations of O2 and CO2 concentration and pH in the pool water, (5) extreme nutrient shortage, especially nitrogen. One functional plant type being highly adapted to these stressful conditions are aquatic resurrection plants, i. e. poikilohydric cormophytes like Chamaegigas intrepidus (Scrophulariaceae). This species grows endemically in ephemeral rock pools on granite outcrops in Central Namibia. Ch. intrepidus shows a number of anatomical, biochemical and physiological adaptations to the complex of extreme habitat conditions such as con- tractive xylem, velamen radicum, accumulation of abscisic acid, dehydrins and carbohy- drates during desiccation. Nitrogen deficiency is partly avoided by acquisition of amino acids and an efficient utilisation of urea. Chamaegigas populations on single inselbergs are genetically very highly isolated, whereas gene flow between sub-populations from different pools on one inselberg is rather high. In accordance with the predominantly outcrossing breeding behaviour, most genetic diversity is found within a pool. A habitat model showed clear niche partitioning between Ch. intrepidus and the less desiccation- tolerant species Limosella grandiflora with respect to depth of the pools. The specific constitutive dehydration tolerance mechanisms displayed by Ch. intrepidus are discussed as a vital adaptation to the low predictability of the occurrence and duration of periods favourable for growth and reproduction.