Plant growth modelling without integrating mechanisms

The integration of root and shoot activity for resource acquisition and allocation is a central problem both in experimental physiology and in mathematical growth modelling. A key feature in all proposed solutions is the mechanism by which roots and shoot communicate their resource status, their needs or their demands. In experimental studies, chemical messengers (hormones or nutrients) have been central. In mathematical models, partitioning or productivity functions of one form or another have been incorporated to fill a similar role. In this paper, the central hypothesis is that growth and resource allocation can be based solely on local, internal, non-structural resource concentrations and local kinetic rules, without the need for additional mechanisms to integrate activities at the organis-mal level. The specific objective was to develop a formal model using the classical two-compartment, two-resource framework which did not include direct intercompartmental coordinating or communicating mechanisms or resource partitioning functions. The formalization effort was successful based on two criteria. First, the ‘plants’ grow at steady and balanced rates when ‘growth conditions’ are constant; and second, challenged with experimental manipulations such as pruning or limitations in resource acquisition, their growth responses mimic those of real plants. Analysis of the results indicates that the complex integrated growth patterns of the modelled plants are emergent properties – having no direct or indirect mechanistic basis – of the simple system. They imply that similar ‘mechanism-less’ integration is possible in real plants as an emergent property of strictly local, cellular-level activities.     

Photosynthesis and photoprotection in mangroves under field conditions

Net CO₂ exchange and in vivo chlorophyll fluorescence were studied in mangrove (Rhizophora stylosa) leaves at a field site in Western Australia, and leaf samples were collected for the analysis of enzymes and substrates potentially involved in anti-oxidant photoprotection. Photosynthesis saturated at 900 μmol quanta m^?2 s^?1 and at no more than 7.5 μmol CO₂ m^?2 s^?1. However, fluorescence analysis indicated no chronic photoinhibition: F_v:F_m was 0.8 shortly after sunset, and quantum efficiencies of PSII were high up to 500 μmol quanta m^?2 s^?1. Electron flow through PSII was more than 3 times higher than electron consumption through Calvin cycle activity, however, even with photorespiration and temperature-dependent Rubisco specificities taken into account. Acknowledging the growing body of literature attributing a role to antioxidant systems in photoprotection, we also assayed the activities of superoxide dismutase (SOD) and several enzymes potentially involved in H₂O₂ metabolism. Their levels of maximal potential activity were compared with those in greenhouse-grown mangroves (R. mangle), and growth chamber-grown peas. Monodehydroascorbate reductase activities were similar in all species, and glutathione reductase was lower, and ascorbate peroxidase ～40% higher, in the mangroves. SOD activities in field-grown mangroves were more than 40 times those in peas. Our results support the hypothesis that O₂ may be a significant sink for photochemically derived electrons under field conditions, and suggest an important role for O₂^? scavenging in photoprotection. However, when relative patterns are compared between species, imbalances between SOD and the other enzymes in the mangroves suggest that more components of the system (e.g. phenolics or peroxidases) are yet to be identified.     

PATCHY: simulating and visualizing the effects of stomatal patchiness on photosynthetic CO2 exchange studies

Abstract. In this study, computer modelling has been used to simulate and analyse the effects which non-homogeneous stomatal conductance (patchiness) might have on the appearance of the macroscopic relationship between photosynthetic CO₂ assimilation (A) and intercellular CO₂ (Q) in leaves. The problem was formalized using the assumptions that (1) the biochemical model of Farquhar, von Caemmerer & Berry applies [ Planta , 149 , 78–90 (1980)]; (2) that the parameters for the model are fixed for the period required to determine the relationship; (3) that the distribution of conductances in any leaf area is normal (but restricted to positive values); and (4) that the leaf is perfectly heterobaric. The model is interactive, allowing the user to explore–well beyond the conditions for which data are presented here–the effects of carboxylation capacity, photosynthetic electron transport rate and photorespiration over a range of possible conductances and degrees of patchiness. Regardless of the parameters used in the model, the results fail to predict the change in appearance of the A -C₁ curve which has been attributed to patchiness in other reports: even when conductance varies with a standard deviation of twice its mean value, the effects on the curve are minor. The need for reconsideration of the methods currently used to interpret gas exchange studies is indicated.     

THE DIFFERENTIATION OF SPECIES OF LACTOBACILLI AND STREPTOCOCCI BY MEANS OF PAPER PARTITION CHROMATOGRAPHY

SUMMARY: Washed cells of micro-organisms were extracted with 10% acetic acid and the extracts subjected to two-dimensional paper partition chromatography. With closely related strains within species already defined serologically and physiologically, the patterns of amino acids and peptides revealed by ninhydrin were relatively constant; but with different species variations were noted, even between species hitherto not easy to differentiate.     

Cell Potentials and Turgor Pressures in Epidermal Cells of Tradescantia and Commelina

Cell membrane potentials have been measured both in epidermalstrips and intact leaf sections of Tradescantia virginiana andCommelina communis, and in epidermal cells over green and overalbino mesophyll cells of T. albiflora var. albovittata. Membranepotentials (_cell) in strips were considerably lower than thosein intact sections and were insensitive to light and to theabsence or presence of calcium. Their response to external cationlevels was indifferent to ionic species. However, in intactleaf sections incubated with calcium present, membrane potentialsresponded to K⁺ levels but not to Na⁺. were more negative thancells in epidermal strips, and responded to changes in illumination. Long-term recordings of _cell and vacuolar K⁺ levels in T. virginianaduring stomatal closure suggest that the fluctuations of _cellwere unrelated to K⁺ movement (which we could not detect) andthus probably to stomatal movement as well. Turgor pressures measured in epidermal cells of intact leafsections of T. virginiana were found to be of the same magnitudeas those previously reported for epidermal strips. It is concludedthat epidermal cells maintain their solute contents during strippingwithout the involvement of an electrophysiological transportsystem. With the possible exception of lateral subsidiary cells,there was no evidence suggesting that ordinary epidermal cellsare capable of osmotic adjustment even when additional KCI wassupplied in the osmoticum. Absolute turgor levels in intactleaf sections kept at constant external KCI were unrelated tosteady state _cell.     

Biochemical and ecological characterization of two peroxidase isoenzymes from the mangrove, Rhizophora mangle

This study examines phenolic peroxidase (POX) in Rhizophora mangle L. leaves in order to assess its role in phenolic manipulation and H₂O₂ scavenging. Sun-exposed and understorey leaves experiencing varying degrees of nutrient stress were analysed from an oligotrophic cay off the coast of Belize. POX activity was unaffected by growth environment, but increased throughout leaf development and persisted through senescence and after abscission. Histochemical analyses indicated POX activity throughout leaf tissues, especially in the apoplast. Phenolics were similarly broadly distributed. Two isoenzymes of POX were partially characterized with pIs of 4.1 and 6.3 and masses of 65.5 and 54.3 kDa, respectively. The larger, more acidic isoenzyme showed especially high heat stability, showing no reduced activity after 24 h at 60 °C. Rhizophora mangle POX oxidized quercetin preferentially, and, to a lesser extent, coniferyl alcohol, caffeic acid, chlorogenic acid, and p-coumaric acid. It did not oxidize ascorbate, but ascorbate could act as a secondary electron donor in the presence of a phenolic substrate and H₂O₂. However, because quercetin and other aglycones were not present in R. mangle leaves, and because POX showed no activity with the most abundant leaf flavonoid, rutin, it was concluded that detoxification of H₂O₂ is secondary to the other roles of POX in manipulation of phenolics.     

Comparison of dispersal and other movements in two Badger (Meles meles) populations

The dispersal and other movement patterns in two Badger populations, one in rural Gloucestershire and the other in suburban Bristol, are described and categorized into nine different types of movement. Movements were less common in the high density Badger population in Gloucestershire, but disturbance increased the likelihood of movements occurring. In the lower density Bristol population, where the social structure was more fluid, movements were more common. However, in both populations truly itinerant Badgers appeared to be rare. More male than female Badgers moved, but for each type of movement there was no difference in the distance moved by males and females. Movements were rare in animals less than a year old; most movements occurred in sexually mature animals, i.e. those more than 2 years old.     

The effects of external NaCl on thylakoid stacking in lettuce plants

The average degree of thylakoid stacking was determined for loose-leaf lettuce plants which were grown in complete nutrient solutions containing either 10 or 100mol m^?3 NaCl. Digitonin fractionation and differential centrifugation were used to assay the level of thylakoid stacking. Based on a comparison between 10mol m^?3 NaCl-grown and 100mol m^?3 NaCl-grown lettuce plants of equal ages, digitonin assays indicated that significantly less stacking occurred in 100mol m^?3 NaCl-grown plants. Isolated thylakoid membranes from 100mol m^?3 NaCl-grown plants were also characterized by a greater capacity to absorb divalent cations and by a higher chlorophyll a/b ratio. Since plants from both growth salinities were capable of a marked increase in thylakoid stacking upon a transition from high to low irradiance, the observed differences in thylakoid stacking were not attributed to a salinity-related impairment of stacking mechanisms. Instead, the salinity-induced differences in thylakoid stacking appear to represent a process of controlled adjustment.