Tillering in grain sorghum over a wiide range of population densities: modelling dynamics of tiller fertility

The prediction of tillering is poor or absent in existing sorghum crop models even though fertile tillers contribute significantly to grain yield. The objective of this study was to identify general quantitative relationships underpinning tiller dynamics of sorghum for a broad range of assimilate availabilities. Emergence, phenology, leaf area development and fertility of individual main culms and tillers were quantified weekly in plants grown at one of four plant densities ranging from two to 16 plants m(-2). On any given day, a tiller was considered potentially fertile (a posteriori) if its number of leaves continued to increase thereafter. The dynamics of potentially fertile tiller number per plant varied greatly with plant density, but could generally be described by three determinants, stable across plant densities: tiller emergence rate aligned with leaf ligule appearance rate; cessation of tiller emergence occurred at a stable leaf area index; and rate of decrease in potentially fertile tillers was linearly related to the ratio of realized to potential leaf area growth. Realized leaf area growth is the measured increase in leaf area, whereas potential leaf area growth is the estimated increase in leaf area if all potentially fertile tillers were to continue to develop. Procedures to predict this ratio, by estimating realized leaf area per plant from intercepted radiation and potential leaf area per plant from the number and type of developing axes, are presented. While it is suitable for modelling tiller dynamics in grain sorghum, this general framework needs to be validated by testing it in different environments and for other cultivars.     

Effects of elevated ultraviolet-B radiation on native and cultivated plants of southern Africa

Seventeen herb, shrub and tree species of commercial and ecological importance in southern Africa were exposed at one location to ultraviolet-B (UV-B, 280-315 nm) radiation approx. 35 % above clear-sky background (control). The aims were to assess how UV-B affects canopy area, dry mass, and some biochemical and morphological properties of leaves, and to investigate whether differences between species are related to growth form of the plants. There was no pattern of response to UV-B related to growth form. Leaves of trees had altered chlorophyll a and b, carotenoid and flavonoid concentrations, but those of shrubs or herbs did not. Non-structural carbohydrates were unaffected. Smaller canopy areas and dry masses were observed under enhanced UV-B, but these were not statistically different among growth forms. There was a general insensitivity of species to elevated UV-B. Only five species had significantly altered leaf biochemical and morphological properties, canopy area and dry mass, the changes differing in magnitude. There was no consistent pattern of change in leaf thickness or biochemical composition with increased UV-B. Correlation analyses did not support the view that growth is less negatively affected in species with thick leaves or in those where leaf thickness increases, or in species with naturally high leaf flavonoid contents or that are able to synthesize additional flavonoids in response to UV-B enhancement. The analyses did not support the hypothesis that growth was inhibited by starch accumulation in leaves under elevated UV-B. However, changes in leaf shape did correlate with canopy area and dry mass, showing the importance of photomorphogenetic changes caused by UV-B which affect species' performance. We conclude that generalizations on plant sensitivity to UV-B based on growth form and functional type could be misleading, and that the great majority of economically important species of the region are likely to be insensitive to future UV-B increases. Notable exceptions include the Colophospermum mopane tree ecotypes chota and leslie and the arable annual Vigna unguiculata, both of which are traditional sources of livelihood to rural African populations and of importance to African industry and agriculture.     

Origins of protein denatured state compactness and hydrophobic clustering in aqueous urea: inferences from nonpolar potentials of mean force

Free energies of pairwise hydrophobic association are simulated in aqueous solutions of urea at concentrations ranging from 0-8 M. Consistent with the expectation that hydrophobic interactions are weakened by urea, the association of relatively large nonpolar solutes is destabilized by urea. However, the association of two small methane-sized nonpolar solutes in water has the opposite tendency of being slightly strengthened by the addition of urea. Such size effects and the dependence of urea-induced stability changes on the configuration of nonpolar solutes are not predicted by solvent accessible surface area approaches based on energetic parameters derived from bulk-phase solubilities of model compounds. Thus, to understand hydrophobic interactions in proteins, it is not sufficient to rely solely on transfer experiment data that effectively characterize a single nonpolar solute in an aqueous environment but not the solvent-mediated interactions among two or more nonpolar solutes. We find that the m-values for the rate of change of two-methane association free energy with respect to urea concentration is a dramatically nonmonotonic function of the spatial separation between the two methanes, with a distance-dependent profile similar to the corresponding two-methane heat capacity of association in pure water. Our results rationalize the persistence of residual hydrophobic contacts in some proteins at high urea concentrations and explain why the heat capacity signature (DeltaC(P)) of a compact denatured state can be similar to DeltaC(P) values calculated by assuming an open random-coil-like unfolded state.     

Changes in Stem and Leaf Hydraulics Preceding Leaf Shedding in Castanea Sativa L.

This paper describes changes in leaf water status and in stem, petiole and leaf blade hydraulics preceding leaf senescence and shedding in Castanea sativa L. (chestnut). Measurements of maximum diurnal leaf conductance to water vapour (g_L), minimum water potential (_L), hydraulic conductance per unit leaf surface area of stems (K_SL), petioles (K_PL) and leaf blades (K_LL) and number of functional conduits and inside diameter distribution were performed in June, September and October 1999. In September, still green leaves had undergone some dehydration as indicated by decreased g_L (by 75 %) and _L with respect to June. In the same time, K_SL decreased by 88 %, while K_PL and K_LL decreased by 50 % and 20 % of the conduits of stems and 10 % of the petioles (all belonging to the widest diameter range) were no longer functioning, causing a decrease in the theoretical flow by 82 % in stems and 27 % in petioles. Stem xylem blockage was apparently due to tyloses growing into conduits. We advance the hypothesis that the entire process of leaf shedding and winter rest may be initiated by extensive stem embolism occurring during the summer.     

Intrinsic qualities of primate bones as predictors of skeletal element representation in modern and fossil carnivore feeding assemblages

Plio-Pleistocene faunal assemblages from Swartkrans Cave (South Africa) preserve large numbers of primate remains. Brain, C.K., 1981. The Hunters or the Hunted? An Introduction to African Cave Taphonomy. University of Chicago Press, Chicago suggested that these primate subassemblages might have resulted from a focus by carnivores on primate predation and bone accumulation. Brain's hypothesis prompted us to investigate, in a previous study, this taphonomic issue as it relates to density-mediated destruction of primate bones (J. Archaeol. Sci. 29, 2002, 883). Here we extend our investigation of Brain's hypothesis by examining additional intrinsic qualities of baboon bones and their role as mediators of skeletal element representation in carnivore-created assemblages. Using three modern adult baboon skeletons, we collected data on four intrinsic bone qualities (bulk bone mineral density, maximum length, volume, and cross-sectional area) for approximately 81 bones per baboon skeleton. We investigated the relationship between these intrinsic bone qualities and a measure of skeletal part representation (the percentage minimum animal unit) for baboon bones in carnivore refuse and scat assemblages. Refuse assemblages consist of baboon bones not ingested during ten separate experimental feeding episodes in which individual baboon carcasses were fed to individual captive leopards and a spotted hyena. Scat assemblages consist of those baboon bones recovered in carnivore regurgitations and feces resulting from the feeding episodes. In refuse assemblages, volume (i.e., size) was consistently the best predictor of element representation, while cross-sectional area was the poorest predictor in the leopard refuse assemblage and bulk bone mineral density (i.e., a measure of the proportion of cortical to trabecular bone) was the poorest predictor in the hyena refuse assemblage. In light of previous documentation of carnivore-induced density-mediated destruction to bone assemblages, we interpret the current findings as suggestive of the secondary importance of bulk bone mineral density to other intrinsic qualities of skeletal elements (e.g., size, maximum dimension, and average cross-sectional area). It is only when skeletal elements are too large for consumption (e.g., many long bones) that they are fragmented following intra-element patterns of density-mediated carnivore destruction. There appears to be a size threshold beneath which bulk bone mineral density contributes little to mediating carnivore destruction of carcasses. Thus, depending on body size of the predator, body size of the prey, and specific size of the element, bulk bone mineral density may play little or no role of primary importance in mediating the destruction of skeletal elements. We compare patterns in modern comparative assemblages to patterns in primate fossil assemblages from Swartkrans. One of the fossil assemblages, Swartkrans Member 1, Hanging Remnant, most closely approximates a hyena (possibly refuse) assemblage pattern, while the Swartkrans Member 2 assemblage most closely approximates a leopard (possibly scat) assemblage pattern. The Swartkrans Member 1, Lower Bank, assemblage does not closely approximate any of our modern comparative assemblage patterns.     

On the progressive enrichment of the oxygen isotopic composition of water along a leaf

A model has been derived for the enrichment of heavy isotopes of water in leaves, including progressive enrichment along the leaf. In the model, lighter water is preferentially transpired leaving heavier water to diffuse back into the xylem and be carried further along the leaf. For this pattern to be pronounced, the ratio of advection to diffusion (Péclet number) has to be large in the longitudinal direction, and small in the radial direction. The progressive enrichment along the xylem is less than that occurring at the sites of evaporation in the mesophyll, depending on the isolation afforded by the radial Péclet number. There is an upper bound on enrichment, and effects of ground tissue associated with major veins are included. When transpiration rate is spatially nonuniform, averaging of enrichment occurs more naturally with transpiration weighting than with area-based weighting. This gives zero average enrichment of transpired water, the modified Craig-Gordon equation for average enrichment at the sites of evaporation and the Farquhar and Lloyd (In Stable Isotopes and Plant Carbon-Water Relations, pp. 47-70. Academic Press, New York, USA, 1993) prediction for mesophyll water. Earlier results on the isotopic composition of evolved oxygen and of retro-diffused carbon dioxide are preserved if these processes vary in parallel with transpiration rate. Parallel variation should be indicated approximately by uniform carbon isotope discrimination across the leaf.     

Age-dependent response of maize leaf segments to cadmium treatment: effect on chlorophyll fluorescence and phytochelatin accumulation

The relationship between the age of leaf tissue and response of the photosynthetic apparatus and phytochelatin accumulation to Cd treatment was studied. Studies were carried out with seedlings of Zea mays L. cv. Hidosil grown in the presence of 100-200 mumol/L Cd for 14 days under low light conditions. The third leaf was divided into 3 segments of equal length differing in the stage of tissue maturity and used for measurements of chlorophyll content, chlorophyll fluorescence, glutathione and phytochelatin content and Cd accumulation. A close relationship between the age of leaf tissue and response of the photosynthetic apparatus to Cd was shown. Cadmium (200 mumol/L) reduced photochemical processes more in older than younger leaf segments as seen in the Chl fluorescence parameters Fv/F0, and t1/2, while the chlorophyll fluorescence decrease ratio (Rfd) was inhibited more strongly in younger ones. Fv/Fm was slightly affected. Cd-induced enhancement of GSH content was correlated with higher phytochelatin accumulation to a greater extent in younger than in older leaf segments. Phytochelatin level corresponded to changes of photochemical processes in older leaves. The peptide thiol:Cd molar ratio for the phytochelatins varied depending on Cd concentration and age of leaf segments. The protective role of phytochelatins for the photosynthetic apparatus is discussed.     

Isocortical Hyperemia and Allocortical Inflammation and Atrophy Following Generalized Convulsive Seizures of Thalamic Origin in the Rat

1. Generalized convulsive seizures can be elicited by a single unilateral microinjection of the cholinergic muscarinic agonist, carbachol, into the specific sites of the thalamus including ventral posterolateral and the reticular thalamic nuclei. The implication of the thalamic specific and reticular neurons is reviewed and discussed.2. On the basis of the c-fos regional expression and well-known efferent and afferent pathways linking these regions, a neuronal network relating the limbic, thalamo–striatal–cortical, and central autonomic systems, was constructed.3. The pattern of Fos immunoreactivity associated with long-lasting isocortical vasodilatation elicited by generalized convulsive seizures in anesthetized rat following cholinergic stimulation of the thalamus can be attributed to both the electrocortical activity and the long-lasting increase in cortical blood flow. We propose that the sustained cerebral cortical blood flow response during convulsive epileptic seizures may implicate intracerebral vasodilatory and vasoconstrictory neural mechanisms. Double-labeled NADPH-d and Fos-positive neurons implicated in maintaining the sustained isocortical vasodilatory response were found in the anterior lateral hypothalamic area. Inhibition of these neurons prevented the increase in cortical blood flow despite an increased metabolic demand manifested by the ictal electrocortical activity.4. Medial temporal lobe atrophy, including hippocampus, amygdala, and parahippocampal gyrus (piriform and entorhinal cortices) are the most common pathology in man. However the origin of medial lobe atrophy remain uncertain. Our results provide evidence that the allocortical microvascular inflammation may be in origin of the neurovascular degenerative processes leading to atrophy.     

Comparison of Aerial and Submerged Leaves in Two Amphibious Species,Myosotis scorpioides and Ranunculus trichophyllus

Both amphibious species, Myosotis scorpioides and Ranunculus trichophyllus, thrive in a stressful environment (alternated flooding and drying), which is variable regarding water and radiation regimes. Plants from the field and plants grown under controlled water table maintained at 40 cm were analysed for content of chlorophyll (Chl) and UV-B screening compounds, and the efficiencies of PS2 and electron transport systems. We detected no significant differences in contents of Chl and UV-B screening compounds between submerged and aerial leaves. The measurements of respiratory potential and photochemical efficiency revealed the presence of permanent stress in M. sporpioides in the natural environment. Differences in physiological responses of submerged and aerial leaves indicated that the terrestrial environment was more favourable for M. scorpioides than for R. trichophyllus. Characteristics of both species suggested that R. trichophyllus might be a phylogenetically older aquatic plant than M. scorpioides.