Seed size, number and strategies in annual plants: a comparative functional analysis and synthesis.

BACKGROUND AND AIMS: Plants depend fundamentally on establishment from seed. However, protocols in trait-based ecology currently estimate seed size but not seed number. This can be rectified. For annuals, seed number should simply be a positive function of vegetative biomass and a negative function of seed size. METHODS: Using published values of comparative seed number as the 'gold standard' and a large functional database, comparative seed yield and number per plant and per m2 were predicted by multiple regression. Subsequently, ecological variation in each was explored for English and Spanish habitats, newly calculated C-S-R strategies and changed abundance in the British flora. KEY RESULTS: As predicted, comparative seed mass yield per plant was consistently a positive function of plant size and competitive ability, and largely independent of seed size. Regressions estimating comparative seed number included, additionally, seed size as a negative function. Relationships differed numerically between regions, habitats and C-S-R strategies. Moreover, some species differed in life history over their geographical range. Comparative seed yield per m2 was positively correlated with FAO crop yield, and increasing British annuals produced numerous seeds. Nevertheless, predicted values must be viewed as comparative rather than absolute: they varied according to the 'gold standard' predictor used. Moreover, regressions estimating comparative seed yield per m2 achieved low precision. CONCLUSIONS: For the first time, estimates of comparative seed yield and number for >800 annuals and their predictor equations have been produced and the ecological importance of these regenerative traits has been illustrated. 'Regenerative trait-based ecology' remains in its infancy, with work needed on determinate vs. indeterminate flowering ('bet-hedging'), C-S-R methodologies, phylogeny, comparative seed yield per m2 and changing life history. Nevertheless, this has been a positive start and readers are invited to use estimates for >800 annuals, in the Supplementary data, to help advance 'regenerative trait-based ecology' to the next level.

Cereal progenitors differ in stand harvest characteristics from related wild grasses.

The domestication of crops in the Fertile Crescent began approximately 10,000 years ago indicating a change from a hunter-gatherer lifestyle to a sedentary, agriculture-based existence. The exploitation of wild plants changed during this transition, such that a small number of crops were domesticated from the broader range of species gathered from the wild. However, the reasons for this change are unclear.Previous studies have shown unexpectedly that crop progenitors are not consistently higher yielding than related wild grass species, when growing without competition. In this study, we replicate more closely natural competition within wild stands, using two greenhouse experiments to investigate whether cereal progenitors exhibit a greater seed yield per unit area than related wild species that were not domesticated.Stands of cereal progenitors do not provide a greater total seed yield per unit ground area than related wild species, but these crop progenitors do have greater reproductive efficiency than closely related wild species, with nearly twice the harvest index (the ratio of harvested seeds to total shoot dry mass).These differences arise because the progenitors have greater seed yield per tiller than closely related wild species, due to larger individual seed size but no reduction in seed number per tiller. The harvest characteristics of cereal progenitors may have made them a more attractive prospect than closely related wild species for the early cultivators who first planted these species, or could suggest an ecological filtering mechanism. Synthesis. Overall, we show that the maintenance of a high harvest index under competition, the packaging of seed in large tillers, and large seeds, consistently distinguish crop progenitors from closely related wild grass species. However, the archaeological significance of these findings remains unclear, since a number of more distantly related species, including wild oats, have an equally high or higher harvest index and yield than some of the progenitor species. Domestication of the earliest cereal crops from the pool of wild species available cannot therefore be explained solely by species differences in yield and harvest characteristics, and must also consider other plant traits.

Trade-offs between seed and leaf size (seed-phytomer-leaf theory): functional glue linking regenerative with life history strategies and taxonomy with ecology?

Background and Aims: While the 'worldwide leaf economics spectrum' (Wright IJ, Reich PB, Westoby M, et al. 2004. The worldwide leaf economics spectrum. Nature : 821-827) defines mineral nutrient relationships in plants, no unifying functional consensus links size attributes. Here, the focus is upon leaf size, a much-studied plant trait that scales positively with habitat quality and components of plant size. The objective is to show that this wide range of relationships is explicable in terms of a seed-phytomer-leaf (SPL) theoretical model defining leaf size in terms of trade-offs involving the size, growth rate and number of the building blocks (phytomers) of which the young shoot is constructed. Methods: Functional data for 2400+ species and English and Spanish vegetation surveys were used to explore interrelationships between leaf area, leaf width, canopy height, seed mass and leaf dry matter content (LDMC). Key Results: Leaf area was a consistent function of canopy height, LDMC and seed mass. Additionally, size traits are partially uncoupled. First, broad laminas help confer competitive exclusion while morphologically large leaves can, through dissection, be functionally small. Secondly, leaf size scales positively with plant size but many of the largest-leaved species are of medium height with basally supported leaves. Thirdly, photosynthetic stems may represent a functionally viable alternative to 'small seeds + large leaves' in disturbed, fertile habitats and 'large seeds + small leaves' in infertile ones. Conclusions: Although key elements defining the juvenile growth phase remain unmeasured, our results broadly support SPL theory in that phytometer and leaf size are a product of the size of the initial shoot meristem (≅ seed mass) and the duration and quality of juvenile growth. These allometrically constrained traits combine to confer ecological specialization on individual species. Equally, they appear conservatively expressed within major taxa. Thus, 'evolutionary canalization' sensu Stebbins (Stebbins GL. 1974. Flowering plants: evolution above the species level . Cambridge, MA: Belknap Press) is perhaps associated with both seed and leaf development, and major taxa appear routinely specialized with respect to ecologically important size-related traits.