ROOT-SYSTEM TOPOLOGY OF PLANTS FROM HABITATS DIFFERING IN SOIL RESOURCE AVAILABILITY

1. We examined the branching patterns of plant root systems to test th e expectation that these differ both in a species-specific fashion in species from habitats differing in soil resource availability and as a plastic response to varying resource levels. 2. We grew 17 species of winter annual plants in a common greenhouse environment at two soil r esource levels. Water and all nutrients were varied in tandem. The spe cies were from collections made at two stabilized sand dune sites in I srael; one a resource-poor desert site, the other a relatively rich me diterranean site. 3. Branching pattern was quantified by Fitter's alti tude topological index. High values of this index indicate a herringbo ne branching pattern, with root systems consisting of a main axis and primary laterals only. Low values indicate a more dichotomous branchin g pattern. 4. Dicots showed the predicted plastic response, tending to wards more herringbone root systems with decreased resource availabili ty. Dicots from the lower-resource desert site had more herringbone ro ot systems than those from the richer mediterranean site, although thi s difference was significant only when the species were grown under lo w-resource conditions. 5. Grasses were relatively invariant, with clos e to maximally herringbone topology and did not show any significant p lastic response or differences between species of different habitats.