Ogburn, Matthew , Edwards, Erika .
Anatomy of leaf succulence in the clade Portulacineae + Molluginaceae: evolutionary jumps into novel phenotypic space.
Succulent water storage is a prominent plant adaptation throughout the arid zones of the world, but the evolution of this complex trait has been little studied. To better approach the question in a comparative phylogenetic context, we developed a rapid proxy for tissue succulence, saturated water content (SWC), and verified its utility through its relationship to relative capacitance and other water use traits. Using SWC measured across large numbers of taxa we then examined hypotheses pertaining to the evolution of succulence in the clade Portulacineae + Molluginaceae, a widespread group featuring considerable variation in water storage tissues. Using trait MEDUSA, we found a significant shift in evolutionary rate of leaf SWC at the basal node of Portulacineae, supporting the observation that this clade in particular has undergone extensive phenotypic diversification with regard to succulence. To test the hypothesis that increasing succulence engenders trade-offs with other aspects of leaf function,we examined the correlation between leaf SWC and other leaf anatomical traits. We found SWC was positively associated with increased distance between veins and from veins to intercellular airspaces in taxa with two-dimensional venation systems in the leaves, but that these relationships were broken in taxa with leaf veins ramifying in three dimensions. We hypothesize that low vein density and large distances to intercellular air spaces in two-dimensional leaves present an upper constraint on the possible extent of succulence, and that the evolution of three-dimensional venation may provide an escape from this constraint into novel phenotypic space. An alternative hypothesis states that three-dimensional venation is simply a developmental artifact of producing a terete leaf, which features a lower surface area-to-volume ratio and is therefore an optimal way of attaining a high degree succulence. However, the occurrence in Portulacineae of distinct types of three-dimensional venation argues against this second hypothesis. We recovered 5-6 independent origins of three-dimensional leaf venation across Portulacineae, and this leaf trait may provide a mechanism underlying the increased evolutionary rates of SWC in this group.
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1 - Brown University, Ecology & Evolutionary Biology, 80 Waterman St., Box G-W, Providence, RI, 02912, USA
2 - Brown University, Department Of Ecology And Evolutionary Biology, 80 Waterman St Box G-W, Providence, RI, 02912, USA
Presentation Type: Oral Paper:Papers for Topics
Date: Tuesday, July 10th, 2012
Time: 8:45 AM