Patterns of variation in meristem allocation across genotypes and species in monocarpic Brassicaceae

The three principal functions of axillary meristems in plants are branching or vegetative growth (G), reproduction (R), and inactivity (I). The purpose of this study was to test for pattern in the trade-off between these three meristem functions across monocarpic species and genotypes (of one species) in the family Brassicaceae and to examine the relationship between meristem allocation and other fitness components: plant height, plant size (vegetative biomass), and reproductive biomass allocation. Results from three independent tests showed a strong positive relationship between % G and % R meristem functions (and a correspondingly strong: negative relationship between % I and % R meristem functions and between % I and % G meristem functions) at three different taxonomic Levels: across ten genotypes (inbred lines) of Arabidopsis thaliana, across five species of Brassica, and across 13 species of Brassicaceae growing in natural populations. We interpret these results in terms of adaptive strategies along light gradients. Since there is only one reproductive opportunity in monocarpic (annual/biennial) species, lifetime seed production should be maximized by the total number of meristems that can be developed into flowers (R meristems) within one or two years. This should be maximized by high branching intensity (high allocation to G meristem function) under relatively weak competition for light, thus explaining the strong positive relationship between % G and % R meristem functions. Under more intense competition for light, however. lifetime seed production should be maximized by a taller, less branched growth form through higher allocation to I meristem function (imposed by stronger apical dominance). The detected trade-offs therefore may be interpreted as the cost of reproduction (high % R meristem function) and/or branching (high % G meristem function) measured as reduced apical dominance (low % I meristem function). Alternatively, the data may be interpreted as the cost of apical dominance (high % I meristem function) measured as reduced branching intensity (low % G meristem Function) and reduced reproductive effort (low % R meristem function). Significant relationships between meristem allocation and other fitness components were detected only in A. thaliana genotypes: reproductive biomass allocation was best predicted by % R and % G meristem functions whereas both height and vegetative biomass were best predicted by % I meristem Function.