JXB Advance Access originally published online on September 16, 2009
Journal of Experimental Botany 2009 60(15):4397-4410; doi:10.1093/jxb/erp276
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RESEARCH PAPER |
Functional diversity in gravitropic reaction among tropical seedlings in relation to ecological and developmental traits
1INRA, UMR Ecologie des Forêts de Guyane, BP 709, 97310 Kourou, France
2CIRAD, UMR Ecologie des Forêts de Guyane, BP 709, 97310 Kourou, France
3AgroParisTech, UMR 1092, Laboratoire d'Etude des Ressources Forêt Bois (LERFOB), ENGREF, 14 rue Girardet, F-54000 Nancy, France
4INRA, UMR 1092, Laboratoire d'Etude des Ressources Forêt Bois (LERFOB), Centre de Nancy, F-54280 Champenoux, France
* Present address and to whom correspondence should be sent. LMGC, Place E. Bataillon, cc 048, 34095 Montpellier, France. E-mail: t_almeras{at}hotmail.com
Gravitropism is necessary for plants to control the orientation of their axes while they grow in height. In woody plants, stem re-orientations are costly because they are achieved through diameter growth. The functional diversity of gravitropism was studied to check if the mechanisms involved and their efficiency may contribute to the differentiation of height growth strategies between forest tree species at the seedling stage. Seedlings of eight tropical species were grown tilted in a greenhouse, and their up-righting movement and diameter growth were measured over three months. Morphological, anatomical, and biomechanical traits were measured at the end of the survey. Curvature analysis was used to analyse the up-righting response along the stems. Variations in stem curvature depend on diameter growth, size effects, the increase in self-weight, and the efficiency of the gravitropic reaction. A biomechanical model was used to separate these contributions. Results showed that (i) gravitropic movements were based on a common mechanism associated to similar dynamic patterns, (ii) clear differences in efficiency (defined as the change in curvature achieved during an elementary diameter increment for a given stem diameter) existed between species, (iii) the equilibrium angle of the stem and the anatomical characters associated with the efficiency of the reaction also differed between species, and (iv) the differences in gravitropic reaction were related to the light requirements: heliophilic species, compared to more shade-tolerant species, had a larger efficiency and an equilibrium angle closer to vertical. This suggests that traits determining the gravitropic reaction are related to the strategy of light interception and may contribute to the differentiation of ecological strategies promoting the maintenance of biodiversity in tropical rainforests.
Key words: Biomechanics, French Guiana, functional diversity, gravitropism, reaction wood, tropical rainforest
Received 31 July 2009; Accepted 17 August 2009