Hormonal profiles in dormant turions of 22 aquatic plant species: do they reflect functional or taxonomic traits?

BACKGROUND AND AIMS: Turions are vegetative, dormant overwintering organs formed in aquatic plants in response to unfavourable ecological conditions. Contents of cytokinin (CK), auxin metabolites and abscisic acid (ABA) as main growth and development regulators were compared in innately dormant autumnal turions of 22 aquatic plant species of different functional ecological or taxonomic groups with those in non-dormant winter apices in three aquatic species and with those in spring turions of four species after their overwintering. METHODS: The hormones were analysed in miniature turion samples using ultraperformance liquid chromatography coupled with triple quadrupole mass spectrometry. KEY RESULTS: In innately dormant turions, the total contents of each of the four main CK types, biologically active forms and total CKs differed by two to three orders of magnitude across 22 species; the proportion of active CK forms was 0.18-67 %. Similarly, the content of four auxin forms was extremely variable and the IAA proportion as the active form was 0.014-99 %. The ABA content varied from almost zero to 54 µmol kg-1 dry weight and after overwintering it usually significantly decreased. Of all functional traits studied, hormone profiles depended most on the place of turion sprouting (surface vs bottom) and we suggest that this trait is crucial for turion ecophysiology. CONCLUSIONS: The key role of ABA in regulating turion dormancy was confirmed. However, the highly variable pattern of the ABA content in innately dormant and in overwintered turions indicates that the hormonal mechanism regulating the innate dormancy and its breaking in turions is not uniform within aquatic plants.

Genome size is strongly linked to carbohydrate storage and weakly linked to root sprouting ability in herbs.

BACKGROUND AND AIMS: Several lines of evidence indicate that carbohydrate storage in plant below-ground organs might be positively related to genome size because both these plant properties represent resource sinks and can affect cell size, cell cycle time, water-use efficiency and plant growth. However, plants adapted to disturbance, such as root sprouters, could be an exception because their strategy would require higher carbohydrate reserves to fuel biomass production but small genomes to complete their cell cycles faster. METHODS: We used data from a field survey to test the relationship between genome size and the probability of root sprouting ability in 172 Central European herbaceous species. Additionally, we conducted a pot experiment with 19 herbaceous species with different sprouting ability (nine congeneric pairs plus one species), and measured root non-structural carbohydrate concentrations and pools at the end of a growing season. KEY RESULTS: In the Central European flora, the probability of root sprouting ability was lower in large-genome species but this pattern was weak. In the pot experiment, both total non-structural and water-soluble carbohydrates (mainly fructans) were positively and non-linearly related to genome size, regardless of sprouting strategy. The concentrations of mono- and disaccharides and all carbohydrate pools showed no link to genome size, and starch was absent in large-genome species. The link between genome size and carbohydrate storage was less apparent at a small phylogenetic scale because we only observed a higher carbohydrate concentration in species with larger genomes for four of the species pairs. CONCLUSIONS: Root sprouters may have smaller genomes because of their frequent occurrence in dry and open habitats. Large-genome species with presumably large cells and vacuoles could accumulate more water-soluble carbohydrates at the end of the growing season to fuel their growth and perhaps protect vulnerable organs from freezing early in the next season.

What determines root-sprouting ability: Injury or phytohormones?

PREMISE: Root-sprouting (RS) is an evolutionarily independent alternative to axillary stem branching for a plant to attain its architecture. Root-sprouting plants are better adapted to disturbance than non-RS plants, and their vigor is frequently boosted by biomass removal. Nevertheless, RS plants are rarer than plants that are not root-sprouters, possibly because they must overcome developmental barriers such as intrinsic phytohormonal balance or because RS ability is conditioned by injury to the plant body. The objective of this study was to identify whether phytohormones or injury enable RS. METHODS: In a greenhouse experiment, growth variables, root respiration, and phytohormones were analyzed in two closely related clonal herbs that differ in RS ability (spontaneously RS Inula britannica and rhizomatous non-RS I. salicina) with and without severe biomass removal. RESULTS: As previously reported, I. britannica is a root-sprouter, but injury did not boost its RS ability. Root respiration did not differ between the two species and decreased continuously with time irrespectively of injury, but their phytohormone profiles differed significantly. In RS species, the auxins-to-cytokinins ratio was low, and injury further decreased it. CONCLUSIONS: This first attempt to test drivers behind different plant growth forms suggests that intrinsic phytohormone regulation, especially the auxins-to-cytokinins ratio, might be behind RS ability. Injury, causing a phytohormonal imbalance, seems to be less important in spontaneously RS species than expected for RS species in general.

Stoichiometry versus ecology: the relationships between genome size and guanine-cytosine content, and tissue nitrogen and phosphorus in grassland herbs.

BACKGROUND AND AIMS: Plant tissue nitrogen (N) and phosphorus (P) and genome traits, such as genome size and guanine-cytosine (GC) content, scale with growth or metabolic rates and are linked to plant ecological strategy spectra. Tissue NP stoichiometry and genome traits are reported to affect plant growth, metabolic rates or ecological strategies in contrasting ways, although the elemental costs for building and maintaining DNA are typically overlooked. METHODS: We formulated stoichiometry- and ecology-based predictions on the relationship between genome size and GC content to tissue N, P and N : P and tested them on a set of 130 herbaceous species from a temperate grassland using ordinary, phylogenetic and quantile regression. KEY RESULTS: Genome size was only negatively linked to plant N and N : P in species with very small genomes. We found no link between genome size and plant P. GC content was negatively linked to plant N and P but we found these significant links consistently in both GC-rich and GC-poor species. Finally, GC content correlated positively with plant N : P but only in species with GC-rich genomes. CONCLUSIONS: Our results provide stronger support for the ecology-based predictions than the stoichiometry-based predictions, and for the links between GC content and plant N and P stoichiometry than for genome size. We argue that the theories of plant metabolic rates and ecological strategies (resource-acquisitive vs. conservative or ruderal vs. stress-tolerator spectra) better explain interspecific genome-NP stoichiometry relationships at the tissue level (although relatively weakly) than the stoichiometric theory based on the elemental costs for building and maintaining DNA.

Variability in mycorrhizal status of plant species is much larger within than between plots in grassland and coastal habitats.

Community-level studies linking plant mycorrhizal status to environment usually do not account for within-plot mycorrhizal status variability; thus, patterns of plant mycorrhizal status diversity are largely unknown. Here, we assessed the relative importance of within- and between-plot variability components in mycorrhizal status and examined how plant mycorrhizal status diversity is related to soil nutrient availability. We hypothesised larger between-plot variability in mycorrhizal status and higher plant mycorrhizal status diversity in P-poor soils. To test these hypotheses, we used plant phylogenies, vegetation, soil and plant mycorrhizal status data from Czech semi-natural grasslands and Scottish coastal habitats. We divided plant mycorrhizal status diversity into divergence and evenness and tested their relations to soil P, K, Ca and Mg. Within-plot variability component of mycorrhizal status was always, on average, at least 2.2 times larger than between-plot variability in our datasets. Plant mycorrhizal status divergence was positively related to Ca (in both datasets) and Mg (only in grasslands and when accounting for phylogeny). In grasslands, the relationship between Mg and plant mycorrhizal status evenness was negative when accounting for phylogeny, while it was positive when not accounting for phylogeny. Plant mycorrhizal status diversity was not linked to P and its relation to K was inconsistent. Our results suggest that high Ca in the soil can promote coexistence of mycorrhizal, facultatively mycorrhizal and non-mycorrhizal plant species. We encourage future studies to also focus on within-plot variability in mycorrhizal status, because it appears to be highly relevant in herbaceous systems.

Mycorrhizal status is a poor predictor of the distribution of herbaceous species along the gradient of soil nutrient availability in coastal and grassland habitats.

Plant mycorrhizal status (a trait indicating the ability to form mycorrhizas) can be a useful plant trait for predicting changes in vegetation influenced by increased fertility. Mycorrhizal fungi enhance nutrient uptake and are expected to provide a competitive advantage for plants growing in nutrient-poor soils; while in nutrient-rich soils, mycorrhizal symbiosis may be disadvantageous. Some studies in natural systems have shown that mycorrhizal plants can be more frequent in P and N-poor soils (low nutrient availability) or Ca and Mg-high (high pH) soils, but empirical support is still not clear. Using vegetation and soil data from Scottish coastal habitats, and Latvian and Czech grasslands, we examined whether there is a link between plant mycorrhizal status and plant-available P, N, Ca and Mg. We performed the max test analysis (to examine the central tendency) and a combination of quantile regression and meta-analysis (to examine tendencies in different quantiles) on both community and plant species data combined with plant phylogenies. We consistently found no changes in mycorrhizal status at the community and species levels along the gradients of plant-available P, N, Ca and Mg in the central tendency and in almost all quantiles across all datasets. Thus, we found no support for the hypotheses that herbaceous species which are able to form mycorrhizas are more frequent in nutrient-poor and high pH environments. Obligatory, facultatively and non-mycorrhizal herbaceous species appear to assemble randomly along the gradients of nutrient availability in several European herbaceous habitats, suggesting that all these strategies perform similarly under non-extreme soil nutrient conditions.

Clade composition of a plant community indicates its phylogenetic diversity.

Phylogenetic diversity quantification is based on indices computed from phylogenetic distances among species, which are derived from phylogenetic trees. This approach requires phylogenetic expertise and available molecular data, or a fully sampled synthesis-based phylogeny. Here, we propose and evaluate a simpler alternative approach based on taxonomic coding. We developed metrics, the clade indices, based on information about clade proportions in communities and species richness of a community or a clade, which do not require phylogenies. Using vegetation records from herbaceous plots from Central Europe and simulated vegetation plots based on a megaphylogeny of vascular plants, we examined fit accuracy of our proposed indices for all dimensions of phylogenetic diversity (richness, divergence, and regularity). For real vegetation data, the clade indices fitted phylogeny-based metrics very accurately (explanatory power was usually higher than 80% for phylogenetic richness, almost always higher than 90% for phylogenetic divergence, and often higher than 70% for phylogenetic regularity). For phylogenetic regularity, fit accuracy was habitat and species richness dependent. For phylogenetic richness and divergence, the clade indices performed consistently. In simulated datasets, fit accuracy of all clade indices increased with increasing species richness, suggesting better precision in species-rich habitats and at larger spatial scales. Fit accuracy for phylogenetic divergence and regularity was unreliable at large phylogenetic scales, suggesting inadvisability of our method in habitats including many distantly related lineages. The clade indices are promising alternative measures for all projects with a phylogenetic framework, which can trade-off a little precision for a significant speed-up and simplification, such as macroecological analyses or where phylogenetic data is incomplete.