Bibliothek

Notizen: Phyllotaxis, the arrangement of leaves around a stem, shows in the vast majority of cases a regularity in the divergence angle of subsequent leaves which divide the whole circle into regular fractions. These are in most cases rational fractions of two Fibonacci numbers in an alternating series, converging towards the irrational limit of the golden section, corresponding to the golden divergence angle of 137.5  . . . degrees. This peculiarity was a long-standing mystery. Here, it is related to the evolutionary pressure of optimal light capture for maximal photosynthetic activity. A model is established which relates minimal shadowing for the lower leaves to the divergence angle. Numerical results of this model agree well with semi-empirical data on the dependence of light capture from the divergence angle. The basic shadow function of the model is also related with the demand of minimal shadowing for the angular separation of leaves and obtain, using elementary number theory, as solution the golden section. Further numerical studies show that the rational approach to the golden section (Schimper–Braun series) is related to the leaf width and the number of leaves of the plant.

Notizen: Time series of net CO2 exchange (JCO2) and leaf conductivity for water vapour (gH2O) were measured and subsequently analysed mathematically in the Crassulacean acid metabolism (CAM) plant Kalanchoe daigremontiana (Hamet et Perrier de la Bâthie) under constant environmental conditions and under imposed external rhythms of lower and higher light intensity. The time series were analysed by Fourier methods and a correlation analysis considering the first time derivatives of JCO2, gH2Oand photosynthetically active photon flux density (PPFD). The ratio of internal to external CO2 (ci/ca) was also considered in the analysis, leading to a discussion of the interaction of stomata and carbon assimilation under periodic stimulation. It is suggested that for stimulation with frequencies close to the endogeneous circadian period, stomatal conductance and carbon assimilation oscillate synchronously, guard-cell movements trailing behind changes in internal CO2 with a delay of 10–15 min. For stimulation frequencies far shorter than the endogeneous period, this synchrony can be disturbed due to independent responses of stomata and assimilation to light pulses, leading to an arrhythmic gas exchange pattern. These results are discussed in the context of understanding circadian oscillations as the output of a multioscillator, multisignalling pathway system on the organismic and metabolic level.

Notizen: Miniaturized pulse-amplitude modulated photosynthesis yield analysers are primarily designed for measuring effective quantum yield (ΔF/Fm′) of photosystem II under momentary ambient light conditions in the field. Although this provides important ecophysiological information, it is often necessary to learn more about the potential intrinsic capacities of leaves by measuring light-response curves. Thus, instruments provide light-curve programmes, where light intensities are increased in short intervals and instant light-response curves are recorded within a few minutes. This method can be criticized because photosynthesis will most likely not be in steady state. This technical report shows that with the appropriate precautions instant light curves can nevertheless provide reliable information about cardinal points of photosynthesis. First, the geometry of the light source of the instrument in relation to the quantum sensor must be considered and quantum sensor readings must be corrected. Second, the measurements of the light-response curves must be compared with readings of effective quantum yield of photosystem II under ambient light conditions where photosynthesis is in steady state. This may show that in the critical range of the light curves either both measurements perfectly coincide or are offset against each other by a constant value (examples are given here). In the first case results of light curves can be taken at face values, and in the second case a simple correction can be applied. With these precautions and careful interpretations instant light-response curves can be an enormous advantage in ecophysiological field work.

Notizen: The genus Ananas has its centre of origin in northern South America. In this area, several varieties of Ananas comosus are widely cultivated, and a number of wild species are found growing under variable conditions of light intensity, soil fertility and water availability. Here we report detailed daily courses of titratable acidity, and malate, citrate and free-sugars content of several cultivated varieties of A. comosus and of A. ananassoides, a closely related species growing on granitic rock-outcrops in southern Venezuela. Day-night oscillations of both malate and citrate were detected in plants growing under full sun, but malate was by far the most important organic anion associated with CAM performance in ail populations sampled. Fructose was the dominant compound in the neutral fraction, but only sucrose showed a consistent inverse relation with the cycle of titratable acidity. The diel oscillations of free sugars measured were not always enough to account for the amount of organic anions accumulated during the night. Plants cultivated under shady conditions always showed a lower night-time increase in titratable acidity and organic acids, and also smaller oscillations in the amount of free sugars than sun exposed plants. In all populations growing under full sun, osmolality increased during the night, but it was not always possible to explain these changes on the basis of variations in molar concentrations of organic acids and sugars. Besides, no diel variations in the cations K+, Ca2+ and Mg2+ were detected. K+ was always the dominant cation (K/Ca ratios ∼ 19), while Mg2+ was always higher than Ca2+ (Mg/Ca ∼ 2).

Notizen: Night-time citrate accumulation has been proposed as a response to stress in CAM plants. To address this hypothesis, gas exchange patterns and nocturnal acid accumulation in three species of Clusia were investigated under controlled conditions with regard to water stress and responses to low and high photosynthetic photon flux density (PPFD). Under high PPFD, leaves of Clusia nocturnally accumulated large amounts of both malic and citric acids. Under low PPFD and well-watered conditions, substantial night-time citrate accumulation persisted, whereas malate accumulation was close to zero. Malate accumulation and night-time CO2 uptake from the atmosphere declined in all three species during prolonged drought periods, whereas citrate accumulation remained similar or increased. Recycling of respiratory CO2 was substantial for both well-watered and water-stressed plants. The suggestion that citrate accumulation is energetically more favourable than malate accumulation is not supported if the source of CO2 for the formation of malate is respiratory CO2. However, the breakdown of citric acid to pyruvate in the light period releases three molecules of CO2, while the breakdown of malic acid releases only one CO2 per pyruvate formed. Thus, citric acid should be more effective than malic acid as a mechanism to increase CO2 concentration in the mesophyll and may help to prevent photoinhibition. Organic acid accumulation also affected the vacuolar pH, which reached values of 2·6–3·0 at dawn. At these pH values, the transport of 2H+/ATP is still feasible, suggesting that it is the divalent form of citrate which is being transported in the vacuoles. Since citrate is a well-known buffer, and Clusia spp. show the largest day-night changes in organic acid levels measured in any CAM plant, it is possible that citrate increases the buffer capacity of the vacuoles. Indeed, malate and titratable acidity levels are positively related to citrate levels. Moreover, Clusia species that show the highest nocturnal accumulation of organic acids are also the ones that show the greatest changes in citric acid levels.

Notizen: Abstract. Kalanchoe pinnata (Lam.) Pers., a plant having crassulacean acid metabolism (CAM), was grown in high light (16–23 mol photons m−2 d−1) and in the shade (0.8–2.1 mol photons m−2 d−1), respectively. Plants were stressed in three ways, i.e. by transfer from high light to shade or vice versa just before measurements, and by withholding nitrogen and/or water. During the day-night cycle of CAM, K. pinnata showed day-night changes of citrate levels (Δ citrate) in addition to malate changes (Δ malate). Changes of leaf-cell sap osmotic pressure. Δπ, were linearly correlated with these changes of organic-acid anion levels with a relation of Δπ/(Δ citrate +Δ malate) = 1/1. The environmental stressor, i.e. limited N-nutrition, drought and higher or lower irradiance than experienced during growth, affected the absolute and relative contributions made by Δ citrate and Δ malate to total nocturnal organic-acid accumulation. In the high-light-grown plants transferred to the shade, changes of citrate levels were much less affected than changes of malate levels by the generally decreased metabolic activity and inhibition of CO2 uptake. In the shade-grown plants, Δ citrate increased in response to stress imposed by interactive effects of the three stressors.

Notizen: We propose a simple oscillatory model of crassulacean acid metabolism (CAM) describing the CO2 uptake and nocturnal acidification of CAM plants by a system of coupled non-linear differential equations. Large differences in the content of metabolite pools are treated using a pseudo-steady-state approach. For the first time, simulations of the CAM cycle investigate its dependence on all three major control parameters simultaneously: temperature, photon flux density and external CO2 concentration. Under stationary conditions in time the model shows either endogenous rhythmicity or two distinct steady states. Stability boundaries are calculated in parameter space.

Notizen: Abstract This article deals with the physiological ecology of the Bromeliaceae, a large neotropical family containing both terrestrial and epiphytic forms, as well as many species with crassulacean acid metabolism (CAM).The article is in two parts. In the first, we review what is known of the occurrence of CAM and C3 species in the Bromeliaceae. The photosynthetic pathways are discussed in the context of the major taxonomic divisions within the family and the great diversity of bromeliad life-forms. Of the three subfamilies, the Pitcairnioideae contain both C3 and CAM species and are essentially all terrestrial. In contrast, the Tillandsioideae are entirely epiphytic or saxicolous, with CAM species being restricted to the genus Tillandsia, And in the Bromelioideae all species show CAM, but terrestrial and epiphytic forms are found in about equal numbers. The evidence suggests that both CAM and the epiphytic habit arose more than once in the family's evolutionary history.In the second part we consider the photosynthetic ecology of the various bromeliad life-forms in more detail using the specific example of Trinidad (West Indies). CAM bromeliads tend to be centred on the drier regions of the island and C3 forms on the wetter areas. However, at any one site there is a marked vertical stratification of species within the forest profile. Based on the known habitat preferences of the bromeliads, six contrasting sites were selected for field studies in Trinidad. These ranged from arid coastal scrub to montane rain forest, the vegetational and climatic characteristics of which are described here. The constancy of δ13C values (carbon-isotope ratios) for individual CAM species in these markedly different habitats emphasized the need for ecophysiological studies to characterize environmental effects on CO2 assimilation and transpiration. The following papers in this series present the results of a comparative investigation of gas exchange and leaf water relations of CAM and C3 bromeliads in situ at the various sites.

Notizen: Abstract Field measurements of the gas exchange of epiphytic bromeliads were made during the dry season in Trinidad in order to compare carbon assimilation with water use in CAM and C3 photosynthesis.The expression of CAM was found to be directly influenced by habitat and microclimate. The timing of nocturnal CO2 uptake was restricted to the end of the dark period in plants found at drier habitats, and stomatal conductance in two CAM species was found to respond directly to humidity or temperature. Total night-time CO2 uptake, when compared with malic-acid formation (measured as the dawn-dusk difference in acidity, ΔH+), could only account for 10–40% of the total ΔH+ accumulated. The remaining malic acid must have been derived from the refixation of respired CO2 (recycling). Within the genus Aechmea (12 samples from four species), recycling was significantly correlated with night temperature at the six sample sites. Recycling was lowest in A. fendleri (54% of ΔH+ derived from respired CO2), a CAM bromeliad with little water-storage parenchyma that is restricted to wetter, cooler regions of Trinidad.Gas-exchange rates of C3 bromeliads were found to be similar to those of the CAM bromeliads, with CO2 uptake from 1 to 3 μmol m−2 s−1 and stomatal conductances generally up to 100 mmol m−2 s−1. The midday depression of photosynthesis occurred in exposed habitats, although photosynthetically active radiation (PAR) limited photosynthesis in shaded habitats. CO2 uptake of the C3 bromeliad Guzmania lingulata was saturated at around 500 μmol m−2 s−1 PAR, suggesting that epiphytic plants found in the shaded forest understorey are shade-tolerant rather than shade-demanding.Transpiration ratios (TR) during CO2 fixation in CAM (Phase I and IV) and C3 bromeliads were compared at different sites in order to assess the efficiency of water utilization. For the epiphytes displaying marked uptake of CO2, TR were found to be lower than many previously published values. In addition, the average TR values were very similar for dark CO2 uptake in CAM (42 ± 41, n= 12), Phase IV of CAM (69 ± 36, n= 3) and for C3 photosynthesis (99 ± 73, n= 4) in these plants. It appears that recycling of respired CO2 by CAM bromeliads and efficient use of water in all phases of CO2 uptake are physiological adaptations of bromeliads to arid microclimates in the humid tropics.

Notizen: Abstract. Lipophilic cations inhibit nocturnal malic acid accumulation in leaf cells of the Crassulacean Acid Metabolism plant Kalanchoë tubiflora. perhaps by interacting directly or indirectly with active malic acid transport into the vacuoles. Lipophilic cations do not affect passive efflux of malic acid from the vacuoles. Membrane potentials are depolarized, oxygen uptake is stimulated by lipophilic cations and there may also be stomatal responses. Thus it is striking that lipophilic cations do not alter the stoichiometry of 2 titratable H : 1 enzymatically-determined malate2− during diurnal malic acid oscillations of Crassulacean Acid Metabolism in Kalanchoë. This suggests that coupling between protons and malate during transport into the vacuole must be tight. Transport as undissociated acid is unlikely because the dissociation equilibrium in the cytoplasm is largely on the side of malate2−. These results appear to suggest an intimate molecular interaction between a proton pump and a presumed malate2− translocator at the tonoplast of leaf cells with Crassulacean Acid Metabolism.