American Journal of Botany最新文献

Why did it take so long for angiosperms to diversify after they arose? Here I consider the indirect but potentially crucial impact of the “photosynthetic revolution” on plant–herbivore coevolution. Increased vein density in fossil leaves implies a doubling in photosynthesis 125–100 million years ago. Higher photosynthetic rates increase the opportunity cost of anti-herbivore defenses, favoring shifts to chemically diverse, low-cost, low-molecular-weight qualitative toxins (e.g., alkaloids) from chemically stereotyped, high-cost, high-molecular-weight quantitative toxins (e.g., tannins). Given the greater functional significance of incremental changes in defensive compounds of lower molecular weight, shifts to qualitative toxins should accelerate plant-herbivore coevolution and species diversification. The large genome and cell sizes of ferns and gymnosperms should drive lower rates of coevolutionary diversification by decreasing vein density and photosynthetic rates; high vein density found in many euasterids, eurosids, and monocots should drive higher diversification rates. This theory might also explain the general restriction of qualitative toxins to herbaceous plants, given the higher photosynthetic rates of herbs vs. woody plants. Lower hydraulic limitation and selection for small genomes in short, fast-growing, short-lived plants should foster evolution of small cells, fine vein networks, high leaf N levels and photosynthetic rates, reliance on qualitative toxins, and high speciation rates.

Climate change and land-use alterations are driving forest fires to unprecedented frequencies and intensities worldwide. Even wet tropical forests—historically rarely subjected to fire—are increasingly experiencing fire disturbances. The impact of wildfires on these forests is likely large, since many of their tree species are not adapted to fire. The extent of the consequences depends on complex feedback mechanisms between fire and vegetation, with plant functional traits playing a critical role. However, how different traits may drive fire–vegetation dynamics in such ecosystems is poorly understood due to limited consolidation of relevant data. This uncertainty leaves the future of tropical forests in question. In this review, we explore how functional traits influence the feedback between fire and vegetation. Specifically, we examine how functional traits of species shape (1) fire regimes (here considered in terms of fire frequency and intensity) through fuel characteristics, (2) their resistance to fire, and (3) their recovery after fire. Resilience is conceptualized as resistance to biomass loss and capacity for post-disturbance biomass recovery. We provide a comprehensive overview of these traits and 12 mechanisms involved. Since the relative importance of these traits and mechanisms for fire dynamics and species’ fire resilience remains unknown, we conclude by outlining possible scenarios for how novel fire regimes might affect tropical forest resilience. We also identify knowledge gaps and avenues for future research to improve our understanding of fire–vegetation dynamics in tropical forests.

Flower colors brighten our natural world. How and why have they evolved? How might ongoing global warming alter their evolutionary trajectories? In this review, I examine the influence of ambient temperature on the evolution of flower color. Given the wide body of literature on pollinator-mediated selection, I restricted the review to temperature-mediated selection and interactions between temperature and two other abiotic factors, drought and light. I focus on flavonoid-based colors because they are widespread, and their biosynthetic pathway is well characterized. Accumulated data suggest that temperature has been a selective factor in determining large- and small-scale geographic patterns in species having genetically fixed flower color and in species with temperature-sensitive plasticity in color. However, it is also clear that we have much to learn about direct and indirect selection on flower color related to ambient temperature and temperature's contributions to phylogenetic color patterns. Therefore, I conclude with questions to help advance understanding about temperature's role in past evolution and present and future changes arising from global warming.

Premise: The coloration of flowers is caused by wavelength-selective absorption by pigments and scattering of light by floral structures. Although the molecular, physiological, and chemical properties of floral pigments have been studied in considerable detail, how floral structures contribute to the visual signal remains largely unknown. A flower can be considered as a stack of layers, where each layer is characterized by specific pigmentation and scattering properties. Quantifying the contribution of different floral layers to visual signalling aids our understanding of the origin and maintenance of Earth's resplendent flora.

Methods: We quantified the contribution of the cuticle, epidermal cell layer, and the mesophyll to the reflection of light by flowers for nasturtium (Tropaeolum majus), a St. John's wort hybrid (Hypericum 'Hidcote'), and the large-flowered evening primrose (Oenothera glazioviana). The obtained experimental and modelling data allowed the quantification of the absorption and scattering of light by different floral layers.

Results: The mesophyll was by far the strongest reflecting layer. The reflectance from the epidermal layer was minor, and the cuticle reflected a very small percentage of the total. The strong scattering by the mesophyll is caused by its inhomogeneity and thickness.

Conclusions: The strong light-scattering by the mesophyll crucially determines a flower's visual signal, whereas the cuticle and epidermal cells contribute less than generally assumed. Mesophyll thickness, cell properties and inhomogeneity, including porosity, are essential components of the flower's optical toolkit.