Pronounced drought tolerance characterizes the early life stages of the epiphytic bromeliad Tillandsia flexuosa

Germination and seedling performance may set the limits for plant distributions, particularly in stressful habitats. Stressful conditions at these early stages may be avoided by opportunistic germination and growth, or may be tolerated. Many epiphytic plants are frequently exposed to severe drought. Adult epiphytes endure such dry periods in various ways, but little is known about strategies employed during germination and early life. Epiphyte seedlings could show either opportunistic fast growth to quickly attain the benefits of being larger or inherently slow growth and early drought tolerance. Here we address the question: which of these strategies characterizes the early life stages of the epiphytic bromeliad Tillandsia flexuosa, a species typical for dry tropical habitats? We studied growth and drought tolerance of germinating seeds, of the emerging seedlings and of 2-month-old seedlings under controlled conditions. Additionally, we studied drought hardening in 6-month-old seedlings. Germination of T. flexuosa was reduced by intermittent dry periods. However, compared to the congeneric T. fasciculata, which typically occurs in wetter habitats, the depression of germination by drought was small. Seedling growth was fastest at intermediate moisture levels: both prolonged drought and continuous moisture depressed growth. Prolonged drought had a less negative effect on drought-hardened seedlings than on previously well-watered seedlings. After a 3-week drought treatment the previously well-watered seedlings had lost their growth advantage entirely. Had drought continued, they would have probably been starved, indicated by the low level of their non-structural carbohydrate pool. Tillandsia flexuosa employs a stress-tolerance strategy both during germination and during the seedling stage. In its epiphytic habitat this strategy is clearly adaptive, considering the predictable briefness of moisture availability throughout the year and the low competition pressure that allows the very slow growth typically seen in adults and seedlings. These conditions characterize not only the dry-forest habitat of T. flexuosa, but all exposed epiphytic growing sites, so we expect this early stress-tolerance to be common among epiphytes in general. Still, a lower stress tolerance in species from wetter habitats may at least partly explain why T. flexuosa shares its dry-forest habitat with so few other vascular epiphytes.