Anita Roth-Nebelsick

Modelling of stomatal density response to atmospheric CO2

Stomatal density tends to vary inversely with changes in atmospheric CO(2) concentration (C(a)). This phenomenon is of significance due to: (i) the current anthropogenic rise in C(a) and its impact on vegetation, and (ii) the potential applicability for reconstructing palaeoatmospheric C(a) by using fossil plant remains. It is generally assumed that the inverse change of stomatal density with C(a) represents an adaptation of epidermal gas conductance to varying C(a). Reconstruction of fossil C(a) by using stomatal density is usually based on empirical curves which are obtained by greenhouse experiments or the study of herbarium material. In this contribution, a model describing the stomatal density response to changes in C(a) is introduced. It is based on the diffusion of water vapour and CO(2), photosynthesis and an optimisation principle concerning gas exchange and water availability. The model considers both aspects of stomatal conductance: degree of stomatal aperture and stomatal density. It is shown that stomatal aperture and stomatal density response can be separated with stomatal aperture representing a short-term response and stomatal density a long-term response. The model also demonstrates how the stomatal density response to C(a) is modulated by environmental factors. This in turn implies that reliable reconstructions of ancient C(a) require additional information concerning temperature and humidity of the considered sites. Finally, a sensitivity analysis was carried out for the relationship between stomatal density and C(a) in order to identify critical parameters (= small parameter changes lead to significant changes of the results). Stomatal pore geometry (pore size and depth) represents a critical parameter. In palaeoclimatic studies, pore geometry should therefore also be considered.

The dynamics of gas bubbles in conduits of vascular plants and implications for embolism repair

Pressure-induced tensions in the xylem, the water conducting tissue of vascular plants, can lead to embolism in the water-conducting cells. The details and mechanisms of embolism repair in vascular plants are still not well understood. In particular, experimental results which indicate that embolism repair may occur during xylem tension cause great problems with respect to current paradigms of plant water transport. The present paper deals with a theoretical analysis of interfacial effects at the pits (pores in the conduit walls), because it was suggested that gas-water interfaces at the pit pores may be involved in the repair process by hydraulically isolating the embolized conduit. The temporal behaviour of bubbles at the pit pores was especially studied since the question of whether these pit bubbles are able to persist is of crucial importance for the suggested mechanism to work. The results indicate that (1) the physical preconditions which are necessary for the suggested mechanism appear to be satisfied, (2) pit bubbles can achieve temporal stability and therefore persist and (3) dissolving of bubbles in the conduit lumen may lead to the final breakdown of the hydraulic isolation. The whole process is, however, complex and strongly dependent on the detailed anatomy of the pit and the contact angle.

Reconstructing atmospheric carbon dioxide with stomata: possibilities and limitations of a botanical pCO2-sensor

Stomatal frequency is often observed to vary inversely with atmospheric CO2 concentration (pCO2). The response is due to (1) individual phenotypic plasticity and (2) evolutionary change, depending on the time scale. Evolutionary responses occur more frequently than individual responses and individual responses are more pronounced under subambient pCO2 levels than under elevated pCO2 (“CO2 ceiling”). The evolutionary response appears therefore to be a valuable device for determining past pCO2. Since tree leaves often represent a conspicuous and rich resource of fossil material, they are increasingly important in this respect. Additionally, certain tree species are considered to represent “living fossils” and therefore valuable sources of ancient stomatal data. There are, however, numerous difficulties which have to be considered such as: (1) high variance of the data, especially for fossil material, (2) interspecific differences of the response, (3) the CO2 ceiling and (4) differences between short-term and long-term responses. Whereas the qualitative pCO2 signal of stomatal frequency appears to be reliable, quantitative pCO2 reconstruction has to be performed with caution. The results of a number of studies which used stomatal frequency as a pCO2 sensor demonstrate good agreement with the results obtained with other proxy data. Current techniques are based on “transfer functions” which calibrate the fossil data with extant material. It is suggested that a mechanistic approach including physical as well as physiological processes could improve pCO2 reconstruction. Furthermore, the topic of the influence of pCO2 on stomatal frequency is significant not only for reconstructing past pCO2 but also with respect to the climate-biosphere interrelationship.

Morphometric analysis of Rhynia and Asteroxylon: testing functional aspects of early land plant evolution

Abstract New morphometric data gathered from cross-sections of two Lower Devonian land plants (Rhynia gwynne-vaughanii and Asteroxylon mackiei) are interpreted in terms of the evolution of the function of vascular bundles in early land plants. The following conclusions can be drawn from these new data: (1) The ratio of the cross-sectional area of the xylem (representing the conducting volume supplying the axis with water) to the xylem perimeter (representing the “contact area” between xylem and parenchyma through which water leaves the xylem and enters the parenchyma) is not constant for Rhynia axes, almost constant for Asteroxylon axes, and different between Rhynia and Asteroxylon. Thus, Bower´s hypothesis that the ratio of cross-sectional area of the xylem to xylem perimeter is constant during ontogenetic development is true for Asteroxylon. That this ratio is constant during phylogeny, however, is not supported by our data. (2) The ratio between cross-sectional area of xylem to parenchyma is higher in Asteroxylon than in Rhynia. (3) As predicted by previous computer simulations, the ratio of the xylem perimeter to the axis perimeter plays a major role in determining water transport performance of the transpiring axis. This ratio is constant within ontogeny but is different in Asteroxylon and Rhynia. In Asteroxylon axes, this ratio is about twice as large as in Rhynia axes. (4) Contrary to the expectations, the distance between the outermost layer of the xylem and the transpiring surface, which represents the low-conductivity pathway through the parenchyma, appears not to be a limiting factor for the water transport in axes of Rhynia and Asteroxylon. (5) From the analysis of the geometric parameters, it is evident that Rhynia and Asteroxylon with their distinct stelar geometries represent two different constructional types for which no transitional stages are known.

Leaf Surface Wettability and Implications for Drop Shedding and Evaporation from Forest Canopies

Wettability and retention capacity of leaf surfaces are parameters that contribute to interception of rain, fog or dew by forest canopies. Contrary to common expectation, hydrophobicity or wettability of a leaf do not dictate the stickiness of drops to leaves. Crucial for the adhesion of drops is the contact angle hysteresis, the difference between leading edge contact angle and trailing edge contact angle for a running drop. Other parameters that are dependent on the static contact angle are the maximum volume of drops that can stick to the surface and the persistence of an adhering drop with respect to evaporation. Adaption of contact angle and contact angle hysteresis allow one to pursue different strategies of drop control, for example efficient water shedding or maximum retention of adhering water. Efficient water shedding is achieved if contact angle hysteresis is low. Retention of (isolated) large drops requires a high contact angle hysteresis and a static contact angle of 65.5°, while maximum retention by optimum spacing of drops necessitates a high contact angle hysteresis and a static contact angle of 111.6°. Maximum persistence with respect to evaporation is obtained if the static contact angle amounts to 77.5°, together with a high contact angle hysteresis. It is to be expected that knowledge of these parameters can contribute to the capacity of a forest to intercept water.

Heat transfer of rhyniophytic plant axes

Abstract Heat transfer is important for plants being sedentary organisms and exposed fully or partly to direct sunlight. It comprises three different mechanisms: (1) emission of IR (infra-red) radiation, (2) heat conduction and convection (sensible heat) and (3) evaporative cooling by transpiration (latent heat). Transpiration has been shown to act as an efficient cooling device in the case of extant land plants. The earliest known land plants consist of a simple branching axis system without leaves or roots. This paper addresses the question of how rhyniophytic plants dissipated heat as well as the significance of evaporative cooling for these organisms. This is particularly interesting in light of the fact that rhyniophytic land plants show a low stomatal density compared to extant plants. Using formulae (representing approximative approaches) for forced convection (heat is ‘carried away’ by wind movements), the results suggest that if wind velocity is high enough for this heat transfer mechanism, then transpiration does not play a role in heat dissipation. This is due to the fact that the slender habit of rhyniophytic plant axes lead to high boundary layer conductance and that the transpiration rate is too low to significantly contribute to heat transfer. During low wind velocities, the regime of mixed convection develops which leads to heat transfer both by forced convection and free convection (heat transfer by buoyancy plumes). Computer simulations were applied in order to study mixed convection for rhyniophytic plants due to the complexity of this heat transfer regime. Slight air movements significantly decrease the plant temperature due to the high boundary layer conductance. Although the transpiration may be significant for heat transfer during low wind velocities if the plant surface temperature is very high, convective heat transfer is expected to dominate heat dissipation. Further detailed investigations of the interactions between a rhyniophytic plant stand and its micrometeorological environment would be of great interest, because these plants differ from extant land plants in various properties which also affect microclimatic factors. Gaining new information about the ecophysiological behaviour of rhyniophytic plants and their interactions with the microclimate created by these plants also concern other organisms associated with rhyniophytes, such as fungi or arthropods.

Latitudinal variation in morphological traits of the genus Pinus and its relation to environmental and phylogenetic signals

Background: Broad-scale analysis of interspecific trait variation is a fundamental approach in comparative ecology to investigate general species–environment relationships, but inferences from environmental and phylogenetic signals are still controversially discussed. Aims: The aim of this study was to globally analyse the genus Pinus and the interspecific variation of morphological traits with latitude as a surrogate of broad-scale environmental changes, and to compare latitudinal trait correlations with biogeographic, environmental and phylogenetic signals in trait variation. Methods: Based on native range maps of 103 Pinus species, the latitudinal correlations of nine morphological traits, including needle characters, cone length and tree height were calculated. Principal component analysis was used to explore trait covariation. Variation partitioning was applied to disentangle environmental and phylogenetic signals in trait variation. Results: Strong latitudinal correlations were detected for traits, such as needle length:width ratio or needle longevity, with similar trends for different taxonomic species subsets and geographic regions. Strong latitudinal correlations were related to a decrease in the pure phylogenetic signal and to an increase in the phylogenetically structured environmental variation (PSEV), whereas the pure environmental signal was almost negligible. Conclusions: Besides a ranking of traits that differ in environmental and phylogenetic signals, our results showed a general relationship between increasing latitudinal trait correlation and an increase in PSEV, which indicates phylogenetic niche conservatism. Thus, for the investigated morphological traits of the genus Pinus both environmental and phylogenetic signals are directly linked by PSEV to broad-scale spatial patterns in trait variation.

The Significance of Pit Shape for Hydraulic Isolation of Embolized Conduits of Vascular Plants During Novel Refilling

During plant water transport, the water in the conducting tissue (xylem) is under tension. The system is then in a metastable state and prone to bubble development and subsequent embolism blocking further water transport. It has recently been demonstrated, that embolism can be repaired under tension (= novel refilling). A model (Pit Valve Mechanism = PVM) has also been suggested which is based on the development of a special meniscus in the pores (pits) between adjacent conduits. This meniscus is expected to be able to isolate embolized conduits from neighbouring conduits during embolism repair. In this contribution the stability of this isolating meniscus against perturbations is considered which inevitably occur in natural environments. It can be shown that pit shape affects the stability of PVM fundamentally in the case of perturbation. The results show that a concave pit shape significantly supports the stability of PVM. Concave pit shape should thus be of selective value for species practicing novel refilling.

Applying Methods from Differential Geometry to Devise Stable and Persistent Air Layers Attached to Objects Immersed in Water

We describe a few mathematical tools which allow to investigate whether air-water interfaces exist (under prescribed conditions) and are mechanically stable and temporally persistent. In terms of physics, air-water interfaces are governed by the Young-Laplace equation. Mathematically they are surfaces of constant mean curvature which represent solutions of a nonlinear elliptic partial differential equation. Although explicit solutions of this equation can be obtained only in very special cases, it is — under moderately special circumstances — possible to establish the existence of a solution without actually solving the differential equation. We also derive criteria for mechanical stability and temporal persistence of an air layer. Furthermore we calculate the lifetime of a non-persistent air layer. Finally, we apply these tools to two examples which exhibit the symmetries of 2D lattices. These examples can be viewed as abstractions of the biological model represented by the aquatic fern Salvinia.

Visualization of embolism formation in the xylem of liana stems using neutron radiography

BACKGROUND AND AIMS Cold neutron radiography was applied to directly observe embolism in conduits of liana stems with the aim to evaluate the suitability of this method for studying embolism formation and repair. Potential advantages of this method are a principally non-invasive imaging approach with low energy dose compared with synchrotron X-ray radiation, a good spatial and temporal resolution, and the possibility to observe the entire volume of stem portions with a length of several centimetres at one time. METHODS Complete and cut stems of Adenia lobata, Aristolochia macrophylla and Parthenocissus tricuspidata were radiographed at the neutron imaging facility CONRAD at the Helmholtz-Zentrum Berlin für Materialien und Energie, with each measurement cycle lasting several hours. Low attenuation gas spaces were separated from the high attenuation (water-containing) plant tissue using image processing. KEY RESULTS Severe cuts into the stem were necessary to induce embolism. The formation and temporal course of an embolism event could then be successfully observed in individual conduits. It was found that complete emptying of a vessel with a diameter of 100 µm required a time interval of 4 min. Furthermore, dehydration of the whole stem section could be monitored via decreasing attenuation of the neutrons. CONCLUSIONS The results suggest that cold neutron radiography represents a useful tool for studying water relations in plant stems that has the potential to complement other non-invasive methods.