Ecological significance of determinate primary root growth: inter- and intra-specific differences in two species of Gymnocalycium (Cactaceae) along elevation gradients

Abstract

Determinate primary root growth in Cactaceae has been interpreted as an adaptation to dry environments. However, little information is available regarding ecological patterns of this process. Our research question was whether primary root length is affected by the determinate growth pattern of the plant or following predictions from resource allocation theories. We analyzed the presence of apical meristem exhaustion and patterns of primary root length (PRL), days until the end of growth (day's post-germination, DPG) and seed mass in Gymnocalycium monvillei and G. quehlianum, which present different elevation distributions and wide elevation ranges. We analyzed five elevation provenances for G. monvillei (878, 1250, 1555, 1940 and 2230 m a.s.l.), and three for G. quehlianum (610, 950 and 1250 m a.s.l.). One hundred seeds per species per altitude were set to germinate in vertical petri dishes. We measured PRL and DPG in each seedling and also seed mass. Both species present determinate growth and PRL varied between species and among populations. PRL was higher in G. quehlianum. DPG was related to differences between species in PRL: roots of G. quehlianum grow for a longer period. In both species we found differences among elevation provenances, with higher PRL at the extremes of the distribution. Among elevation provenances, DPG was significantly related to PRL in G. monvillei, and marginally significantly related in G. quehlianum. Seed size was not related to differences in PRL between species or among elevation provenances. The comparison between species and among elevation provenances suggests that a higher PRL would be related to more extreme environments; this assumption agrees with plant resource allocation theories, which predict a lower shoot : root ratio with increasingly stressful environments.