H. de Kroon

Plant hormones regulate fast shoot elongation under water: From genes to communities

Flooding affects the abundance and distribution of plant species worldwide. Many plants are damaged or even killed by flooding events due to the associated oxygen deprivation in cells. Stimulated shoot elongation is an important adaptive mode that can restore contact of leaves with the atmosphere above the water surface. This strongly im- proves inward diffusion of oxygen and the rate of photosynthesis. Fast elongation of sub- merged petioles of the model plant Rumex palustris involves the integrated action of the plant hormones ethylene, auxin, gibberellin, and abscisic acid. The closely related Rumex acetosa is unable to switch on petiole elongation when submerged. In a comparative study of these two Rumex species, we found that the response to the gaseous phytohormone ethylene, which accumulates in plant tissues during submergence, explains their contrasting elongation behavior. In order to study the importance of this shoot elongation response in the distributional patterns of plants in natural floodplains, we quantified the ethylene- induced elongation response of 22 plant species occurring in the Rhine River floodplain. These results were compared with the results of a multivariate analysis based on 84 veg- etation surveys performed in the same area. The species compositions of the surveys were grouped along two environmental gradients: flooding duration and soil dehydration after the floodwater subsided. If we superimpose the ethylene-induced elongation capacity on these vegetation data, it becomes clear that the capacity to elongate upon exposure to ethylene positively correlates with flooding duration and negatively with soil dehydration. Based on this analysis, we conclude that the capacity to elongate is an important selective trait in field distribution patterns of plants in flood-prone environments. Fast shoot elon- gation under water seems to be a favorable trait only in environments with shallow and prolonged flooding events, while costs associated with this response prevent its expression in sites with deep floods, sites with floods short in duration, or in sites in which flood water recedes rapidly. The approach outlined in this paper may be more widely applicable in ecological studies that aim to understand the functional relationship between plant traits and species distributions along environmental gradients.

Unravelling below-ground plant distributions: a real-time polymerase chain reaction method for quantifying species proportions in mixed root samples

Knowledge on below‐ground plant distributions is almost lacking to date, despite the fact that such information would be very valuable in understanding below‐ground competition and species‐specific interactions, processes that are expected to shape community structure. Methods available so far for below‐ground species determination have drawbacks that we tried to challenge. Some methods make use of differences in the chemical composition between species, but this is highly variable upon environmental factors. DNA‐based techniques — far less dependent on chemical composition — such as polymerase chain reaction on internal transcribed spacer (ITS) primers can so far only determine presence–absence of a species in a mixed root sample. Here, we present a quantitative DNA‐based technique that allows investigation of relative species abundances in experimental mixed root samples. We used quantitative real‐time polymerase chain reaction (PCR) on species‐specific markers obtained from intersimple sequence repeat (ISSR) analyses in root samples. This molecular technique is novel in the field of root ecology and its development overcame three challenges: (i) determination of species‐specific DNA fragments, (ii) development and optimization of the real time PCR protocol, (iii) designing a data treatment method based on a modified delta–delta‐cycle threshold (CT) analysis. The method gained robustness from using relative DNA abundances in species mixtures rather than absolute concentration readings. This requires accurate multispecies reference series as a calibration. Test samples with different known biomass ratios of all species showed proof of concept of this method. The pros and contras of this method are discussed in the light of its contribution to advancing ecological research on below‐ground plant–plant interactions.

Plant populations track rather than buffer climate fluctuations

Climate change not only affects mean temperature and precipitation but also exacerbates temporal fluctuations in these conditions. However, we know relatively little about how species respond to such climate fluctuations, with respect to variation in vital rates (i.e. survival, growth and reproduction of individuals) and population fluctuations. We examine whether populations display evidence of buffering against environmental variation in one of two ways: (1) through negative covariances among vital rates, or (2) reduction of variation in those vital rates to which population growth is most sensitive. We analyse time series of demographic data for 40 plant species and show that there is no evidence for either of these mechanisms. In species in which there is evidence for vital rate covariation, positive covariances between reproduction and survival rates predominate, and tend to magnify the effect of variability. Increasing climate variability is therefore expected to increase population fluctuations and extinction risks.

MODULE RESPONSES IN A TROPICAL FOREST TREE ANALYZED WITH A MATRIX MODEL

Module dynamics were studied for the shade-tolerant canopy tree species Vouacapoua americana in a French Guiana rain forest. A module life cycle graph was constructed, including all the possible transitions between four module states: apically growing (G), apically dormant (D), apically arrested (A), and branching (J). Transitions (module level) were translated to the module population growth rate λ (tree level) and related to the variance in λ among 18 different trees. This variance was also related to light availability (1–60% of ambient PAR) and tree height (5–30 m). Three module life cycle pathways (or loops) were dominant in their contributions to λ: persistent apical dormancy (DD), biannual apical growth (GDG), and biannual branching by dormant modules (JDJ). This suggests that biannual or even slower module production rates predominate in the module life cycle. The positive covariance between biannual loops seems the result of synchronization in apical and axillary activity. Slow production ra...

Selection on phenotypic plasticity of morphological traits in response to flooding and competition in the clonal shore plant Ranunculus reptans

Adaptive evolution of phenotypic plasticity requires that plastic genotypes have the highest global fitness. We studied selection by spatial heterogeneity of interspecific competition and flooding, and by temporal heterogeneity of flooding on morphological plasticity of 52 genotypes of the clonal shore plant Ranunculus reptans. Competition reduced clone size, rosette size, leaf length and stolon internode thickness. Flooding had similar effects and reduced competition. Differences in selection between environments imply potential for either local adaptation or for indirect evolution of phenotypic plasticity. We also detected direct selection for plastic reductions in internode length in response to flooding and for a plastic increase in internode length in response to competition. Plastic responses of some morphological traits to flooding were in line with selection thereon, suggesting that they indeed are adaptive and might have evolved in response to direct selection on plasticity.

Environmental factors constraining adventitious root formation during flooding of solanum dulcamara

Flooding is a compound stress, imposing strong limitations on plant development. The expression of adaptive traits that alleviate flooding stress may be constrained if floodwater levels are too deep. For instance, adventitious root outgrowth is typically less profound in completely submerged plants than in partially submerged plants, suggesting additional constraints in full submergence. As both oxygen and carbohydrates are typically limited resources under submergence, we tested the effects of oxygen concentration in the floodwater and carbohydrate status of the plants on flooding-induced adventitious root formation in Solanum dulcamara L. Partially submerged plants continued to form adventitious roots in low-oxygen floodwater, whereas completely submerged plants developed hardly any roots, even in floodwater with twice the ambient oxygen concentration. This suggests that contact with the atmosphere, enabling internal aeration, is much more important to optimal adventitious root formation than floodwater oxygen concentrations. If plants were depleted of carbohydrates before flooding, adventitious root formation in partial submergence was poor, unless high light was provided. Thus, either stored or newly produced carbohydrates can fuel adventitious root formation. These results imply that the impact of an environmental stress factor like flooding on plant performance may strongly depend on the interplay with other environmental factors.

Designing and Evaluating a Context-based Lesson Sequence Promoting Conceptual Coherence in Biology

Context-based education, in which students deal with biological concepts in a meaningful way, is showing promise in promoting the development of students’ conceptual coherence. However, literature gives little guidance about how this kind of education should be designed. Therefore, our study aims at designing and evaluating the practicability of a context-based biology lesson sequence. Four design principles for conceptual coherence were defined: build upon familiar concepts; focus on core concepts; stimulate establishing connections between concepts; and reflect on these connections. These design principles have been elaborated into a lesson sequence about concepts related to cellular metabolism and the relevant connections between the concepts have been visualised in a reference concept map. The activities of teacher and students that were expected to contribute in establishing these connections were described in a research scenario. The lesson sequence was conducted in a 10th-grade class of 21 students, aged 15–16, in senior general secondary education. Data were collected from video-recordings in the classroom. The observed activities of the teacher and students were compared with the intended activities. The findings show that a research scenario is a powerful tool to systematically evaluate the design and to provide information for improving it.

Benefits of flooding-induced aquatic adventitious roots depend on the duration of submergence: linking plant performance to root functioning

Background and Aims Temporal flooding is a common environmental stress for terrestrial plants. Aquatic adventitious roots (aquatic roots) are commonly formed in flooding-tolerant plant species and are generally assumed to be beneficial for plant growth by supporting water and nutrient uptake during partial flooding. However, the actual contribution of these roots to plant performance under flooding has hardly been quantified. As the investment into aquatic root development in terms of carbohydrates may be costly, these costs may - depending on the specific environmental conditions - offset the beneficial effects of aquatic roots. This study tested the hypothesis that the balance between potential costs and benefits depends on the duration of flooding, as the benefits are expected to outweigh the costs in long-term but not in short-term flooding. Methods The contribution of aquatic roots to plant performance was tested in Solanum dulcamara during 1-4 weeks of partial submergence and by experimentally manipulating root production. Nutrient uptake by aquatic roots, transpiration and photosynthesis were measured in plants differing in aquatic root development to assess the specific function of these roots. Key Results As predicted, flooded plants benefited from the presence of aquatic roots. The results showed that this was probably due to the contribution of roots to resource uptake. However, these beneficial effects were only present in long-term but not in short-term flooding. This relationship could be explained by the correlation between nutrient uptake and the flooding duration-dependent size of the aquatic root system. Conclusions The results indicate that aquatic root formation is likely to be selected for in habitats characterized by long-term flooding. This study also revealed only limited costs associated with adventitious root formation, which may explain the maintenance of the ability to produce aquatic roots in habitats characterized by very rare or short flooding events.