REGULAR ARTICLE Early root growth plasticity in seedlings of three Mediterranean woody species Francisco M. Padilla Æ Juan de Dios Miranda Æ Francisco I. Pugnaire Received: 23 March 2007 / Accepted: 10 May 2007 / Published online: 5 June 2007 Ó Springer Science+Business Media B.V. 2007 Abstract Since very young seedlings are sensitive to dehydration, soil desiccation is often responsible for seedling death in water-stressed environments. Roots play a major role in overcoming water stress and plant establishment, thus early root development in response to limited water availability becomes a strategy that may ensure seedling recruitment. We explored whether different water availabilities altered growth patterns of very young seedlings, focussing on root elongation, and hypothesized that seedling responses would depend on species-specific drought tolerance and seed size. We carried out a greenhouse experiment exposing 2-week-old seedlings of three Mediterranean shrubland species, the drought-toler- ant and small-seeded Genista umbellata (L’He ´r.) Dum. Cours. and Lycium intricatum Boiss., and the drought-sensitive, large-seeded Retama sphaerocar- pa (L.) Boiss., to two watering quantities and monitored plant and root growth weekly in glass cases for 5 weeks. We found that at such early stages, reduced water quantity enhanced root growth in all three species, regardless of drought tolerance and seed size, although root plasticity was the highest in the small-seeded and drought-tolerant Genista. In contrast, shoot elongation and mass allocation, root-to-shoot mass (R:S) ratio, was unaffected by watering. Seedlings responded to lower water avail- ability with faster root elongation rate and greater absorptive root surface, which can account for the enhanced relative growth rate (RGR) of the small- seeded Genista and Lycium under reduced watering. By contrast, a larger root absorptive surface did not lead to higher RGR in the large-seeded Retama probably because of its greater independence from external mineral resources. Our data evidence the importance of water availability on the initial devel- opmental stages of these three species regardless of seed size and drought tolerance. Root growth can be interpreted as an adaptive strategy to deal with drying soils and decreasing soil moisture since larger roots enable to exploit unexplored soil areas of soil, which may ensure recruitment success. Keywords Cotyledons  Drought  Mediterranean ecosystems  Recruitment  Root growth  Seedlings Introduction Plant communities are shaped by germination and recruitment processes (Donovan et al. 1993), which ultimately affect community composition and struc- ture (Grubb 1977; Harper 1977). Plants do not actively choose the habitat they grow in (Bazzaz Responsible Editor: Tibor Kalapos. F. M. Padilla (&)  J. Miranda  F. I. Pugnaire Estacio ´n Experimental de Zonas A ´ ridas, Consejo Superior de Investigaciones Cientı ´ficas, C/ General Segura 1, 04001 Almeria, Spain e-mail: fpadilla@eeza.csic.es 123 Plant Soil (2007) 296:103–113 DOI 10.1007/s11104-007-9294-5