Allometric neoteny and the evolution of succulence in cacti

With the objective of analysing the role of heterochrony in the evolution of succulence in the cactus family, a comparative study of xylem development in six species with contrasting morphologies was carried out. Two woody leaf-bearing cacti and four succulent cactus species belong to different subdivisions within the family were analysed. In each species and for different ages, vessel-element length was measured, vessel-element lateral wall-pitting described and the percentage of xylem and parenchyma in the stem quantified. In the succulent species it was found that vessel element length did not change between juvenile and adult wood, that wall-pitting in adult plants was similar to that of seedlings, and that the woody tissue in adult plants was organized in vascular bundles as in the primary tissue of seedlingS. Leaf-bearing cacti, in contrast, changed in both vessel element length and wall-pitting when secondary wood was produced, and the secondary woody tissue of adult plants was organized in a continuous cambial cylinder as in most dicotyledonS. An allometric analysis suggests that a retardation in the developmental rate of woody tissues (allometric neoteny) is the main mechanism in the development of succulence in cacti.     

Spiny invaders – Patterns and determinants of cacti invasion in Europe

In this paper, we analyse the patterns and determinants of cacti invasion in 22 European countries. We compiled a checklist for each country. Cacti were classified for each country according to their invasion status as casuals, locally established (1–5 localities of small population size) and widely established (>5 localities of considerable population size).We used generalised linear models (GLM) from the Poisson family with a log-link function and a set of seven country-specific explanatory variables to account for geographical, climatic, habitat-related and economic determinants to test which features of the recipient area determine invasion success and if distribution patterns of species at different invasion stages are governed by the same interplay of explanatory variables. Separate models were fitted with the same predictor variables for casual, locally established, widely established and all cacti. Further, we analysed the temporal invasion trend, and tested if niche breadth (expressed as the number of habitat types colonized) is influenced by the range size (measured as the number of countries invaded). Finally, we reviewed the consequences of cacti invasion for nature conservation.In total, 26 cacti species have been recorded in Europe. Ten species are more widespread and occur in at least three countries, Opuntia humifusa (six countries) being the most widespread species. The country with most cacti is Spain (21 species), whereas in 13 countries no cactus species have been recorded. By far the most important genus is Opuntia with 20 alien species. The temporal invasion pattern shows an exponential increase of the cumulative number of invasion events, increasing from three (1801–50) to nine (1951–2000) invasion events over a 50-year period.Regardless of the invasion stage, the factor explaining most of the variance in the models is the presence of the Mediterranean biogeographic region, and a significant positive effect of the country size on species numbers was identified.Considering the invasion stage, some interesting deviations in the models can be observed. Invasion of casual cacti is only influenced by the presence of the Mediterranean biogeographic region. For locally established cacti, precipitation is negatively correlated with the invasion rate, and the presence of the Alpine biogeographic region is positively correlated; the latter is due the local occurrence of few hardy cacti (Opuntia phaeacantha, O. humifusa) in low-lying valleys of the Alps. As all widely established cactus species are restricted to the Mediterranean region, only this factor was included in the model.All cacti are confined to dry, open habitats on acid siliceous bedrock. Thus, the predominant habitats invaded are rock vegetation, dry grassland, open Mediterranean scrub and dry ruderal habitats. The niche breadth of cacti increases with the numbers of countries colonized. Further, the niche breadth of cacti exhibits a geographic gradient towards the Mediterranean region.Until the 19th century, the dominant pathway of invasion was agriculture, as some cactus species had been introduced for the production of forage and fruits. However, in the last decades horticulture and deliberate planting in the wild have become the dominant pathways.The invasion of cacti in natural and semi-natural habitats in the Mediterranean region changes habitat structure and species composition. However, dense and extensive stands of cacti are restricted to few species (e.g. Opuntia ficus-indica).     

Xylem dysfunction during winter and recovery of hydraulic conductivity in diffuse-porous and ring-porous trees

Xylem embolism in winter and spring as well as the occurrence of positive xylem pressure were monitored in several diffuse-porous and one ring-porous tree species (Fraxinus excelsior). In Acer pseudoplatanus and Betula pendula embolism reversal was associated with positive (above-atmospheric) xylem pressures that frequently occurred during a 2-month period prior to leaf expansion. In Acer high stem pressures were occasionally triggered on sunny days after a night frost. The other species investigated showed no positive xylem pressure during the monitoring period in 1995. Populus balsamifera exhibited a complete embolism reversal in 1994, but, like Fagus sylvatica, recovery was slow and incomplete in 1995. Fraxinus did not refill embolized vessels, but relied entirely on the production of new earlywood conduits in May. Populus × canadensis Moench robusta did not recover from embolism during the monitoring period. Under a simulated root pressure of 20 kPa however, excised branches of Populus × canadensis restored maximum hydraulic conductance within 2 days, illustrating the great influence of even small positive pressures on cnductivity recovery in spring. In the absence of positive pressure there was no substantial refilling of embolized vessels within a rehydration period of 9 days.     

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.     

Effects of a 20-day-long dry period on cambial and apical meristem growth in Abies balsamea seedlings

There is general agreement that in many regions additional precipitation with climate change will not be able to balance the increased evaporation rate induced by higher air temperatures, causing periods of intense drought. Although seedlings of Abies balsamea growing in the boreal forest are known for their resistance to harsh environmental conditions, the impact of water stress on their growth still remains largely unexamined. The aim of this study was to investigate growth responses of this species during and after a dry period by monitoring cambial and apical meristem activity at short time scale. Meristem growth was studied from May to October 2005 on seedlings of A. balsamea submitted to a 20-day-long dry period in June–July. Lower rates of shoot lengthening were observed in non-irrigated seedlings only in the first part of the growing season. Irrigated and non-irrigated trees showed the same trend of xylem formation and timings of cell differentiation. Cell production during cambial activity was estimated at about one xylem cell per day thus achieving in 100 days 108 tracheids in the tree ring and a width of 2 mm, with thinner tree rings observed in non-irrigated plants. A reduction of up to 50% in lumen area and cell diameter was observed for the cells produced during the dry period. Response of A. balsamea seedlings to a 20-day-long dry period consisted of good resistance of the cambial meristems during and after water stress, high sensitivity and rapid recovery of cell sizes during water depletion and slow but effective recovery of shoot growth after treatment.     

Plasticity of stomatal distribution pattern and stem tracheid dimensions in Podocarpus lambertii: an ecological study

Background and Aims

Leaf and wood plasticity are key elements in the survival of widely distributed plant species. Little is known, however, about variation in stomatal distribution in the leaf epidermis and its correlation with the dimensions of conducting cells in wood. This study aimed at testing the hypothesis that Podocarpus lambertii, a conifer tree, possesses a well-defined pattern of stomatal distribution, and that this pattern can vary together with the dimensions of stem tracheids as a possible strategy to survive in climatically different sites.

Methods

Leaves and wood were sampled from trees growing in a cold, wet site in south-eastern Brazil and in a warm, dry site in north-eastern Brazil. Stomata were thoroughly mapped in leaves from each study site to determine a spatial sampling strategy. Stomatal density, stomatal index and guard cell length were then sampled in three regions of the leaf: near the midrib, near the leaf margin and in between the two. This sampling strategy was used to test for a pattern and its possible variation between study sites. Wood and stomata data were analysed together via principal component analysis.

Key Results

The following distribution pattern was found in the south-eastern leaves: the stomatal index was up to 25 % higher in the central leaf region, between the midrib and the leaf margin, than in the adjacent regions. The inverse pattern was found in the north-eastern leaves, in which the stomatal index was 10 % higher near the midrib and the leaf margin. This change in pattern was accompanied by smaller tracheid lumen diameter and length.

Conclusions

Podocarpus lambertii individuals in sites with higher temperature and lower water availability jointly regulate stomatal distribution in leaves and tracheid dimensions in wood. The observed stomatal distribution pattern and variation appear to be closely related to the placement of conducting tissue in the mesophyll.     

Structural and functional implications of an unusual foraminiferal beta-tubulin

We have obtained sequence data for beta-tubulin genes from eight species of Foraminifera (forams) and alpha-tubulin sequences from four species, sampling major taxonomic groups from a wide range of environments. Analysis of the beta-tubulin sequences demonstrates that foram beta-tubulins possess the highest degree of divergence of any tubulin gene sequenced to date and represent a novel form of the protein. In contrast, foram alpha-tubulin genes resemble the conventional alpha-tubulins seen in other organisms. Partition homogeneity analysis shows that the foraminiferal beta-tubulin gene has followed an evolutionary path that is distinct from that of all other organisms. Our findings indicate that positive selective pressure occurred on the beta-tubulin subunit in ancestral forams prior to their diversification. The specific substitutions observed have implications for microtubule (MT) assembly dynamics. The regions most strongly affected are implicated in lateral contacts between protofilaments and in taxol binding. We predict that these changes strengthen lateral contacts between adjacent dimers in a manner similar to that induced by taxol binding, thus allowing the formation of the tubulin helical filaments observed in forams by electron microscopy. Our results also indicate that substantial changes to these portions of the beta-tubulin molecule can be made without sacrificing essential MT functions.     

Evolution of extreme xeromorphic characters in wood: a study of nine evolutionary lines in Cactaceae

Vessels of xeric-adapted woods have been predicted to be narrower than those of mesic-adapted woods, to occur at higher densities, to occur in larger clusters, and to have a greater percentage of them in clusters. These predictions were tested by comparing wood structure of several evolutionary lines of xeric-adapted cacti to that of mesic-adapted Pereskia, which probably resembles the ancestral cacti. Although derived cacti occur in habitats with water stress ranging from mild (rain forests) to severe (open deserts with little vegetation other than cacti), as long as plants retain wood with an ordinary fibrous matrix, wood characters are remarkably uniform and not correlated with habitat aridity. However, in several evolutionary lines, novel wood types occur with characters that fulfill the predictions for xeric-adapted woods listed above. However, conductive area (fraction of wood transverse-sectional area occupied by conduits) and estimated specific conductance (conductance per square millimetre) are correlated with shoot height (the need for mechanical support from xylary fibers) rather than with habitat aridity: tall plants transport water through relatively few, wide vessels, permitting much of the wood volume to consist of fibers. Small plants with little wood use large numbers of narrow vessels rather than small numbers of wide ones, thereby achieving conductive safety.     

Pollination biology of two columnar cacti (Neobuxbaumia mezcalaensis and Neobuxbaumia macrocephala) in the Tehuacan Valley,central Mexico

We document the pollination biology and mating systems of Neobuxbaumia mezcalaensis and Neobuxbaumia macrocephala, two Mexican giant columnar cacti. These two species form mixed forests in the western Tehuacan Valley, Mexico. The flowers of both N. mezcalaensis and N. macrocephala exhibit nocturnal anthesis, are self-incompatible, and are pollinated primarily by three species of nectar-feeding bats (Choeronycteris mexicana, Leptonycteris curasoae, and Leptonycteris nivalis). Neobuxbaumia mezcalaensis is androdioecious, a breeding system that appears to be uncommon among Cactaceae. Neobuxbaumia macrocephala is hermaphroditic. We hypothesize that columnar cacti show a geographical dichotomy in floral biology specialization that probably can be related to predictability in pollinator abundance.     

Patterns of tree dieback in Queensland,Australia: the importance of drought stress and the role of resistance to cavitation

During the extreme 1992–1997 El Niño drought event, widespread stem mortality, or tree dieback, of both mature and juvenile eucalypts occurred within the tropical savannas of northeast Australia. Most of the dieback occurred in individuals of the ironbark species complex (Eucalyptus crebra – E. xanthoclada) while individuals of the bloodwood species Corymbia erythrophloia, exhibited significantly less stem mortality. Indicative of greater water stress, predawn and midday xylem water potentials of ironbark adults and saplings were significantly more negative than predawn values of bloodwoods. The very negative xylem water potentials in ironbarks suggest that stem mortality in both adult and juvenile ironbarks results from drought-induced embolism and that ironbarks perhaps have a shallower and less extensive root system than bloodwoods. Although predawn and midday water potentials for ironbark adults and saplings were similar, a census of mature and juvenile ironbark trees indicated that mortality was higher in adult trees. Cavitation vulnerability curves indicated that ironbark saplings may be better buffered against cavitation than adult trees. If they possess smaller root systems, saplings are more likely than adults to experience low xylem water potentials, even in non-drought years. Xylem conduits produced in adult trees during periods of normal rainfall, although perhaps more efficient in water conduction, may be more vulnerable to cavitation during infrequent severe droughts.     

Stress and Coping in the Waking and Dreaming States During an Examination Period

Dream diaries were kept by 35 female undergraduates for two ten-day periods, one of preparation for midterm or final examinations, and an exam-free one. Research questions were whether the stress and negative emotions induced by preparing for exams were reflected in dreams and what types of coping were used both in the waking and dreaming states. There was no consistent impact of the stressful situation on dreams, in terms of incorporation and negative emotions. However, 22 dreamers had incorporation dreams. They reported significantly less active problem-solving strategies in waking than the ones who had no such dreams. While a significant negative correlation was found between harm/threat emotions in waking and negative emotions in dream imagery, a positive correlation was found between positive reappraisal in waking and active problem-solving in dreams. Findings are discussed from the perspective of Lazarus and Folkman's theory of adaptation to stress in waking life.     

The effect of an intermittent water-table gradient on soil and xylem nitrate in cotton

Summary Cotton (Gossypium hirsutum cv Deltapine 61) was grown in a sloping plot of soil in the field to examine the effect of a gradient of water-table depth on soil nitrate availability and plant uptake during two periods of the growing season. Before the water-table was imposed NO₃ was less concentrated at the lower end of the sloping plot. This was attributed to slow denitrification at microsites within the soil at the lower end which was wetter than further up the plot. At flooding NO₃ disappeared only slowly due to a carbon substrate limitation to denitrification in the soil. This loss occurred primarily in areas where the water-table was high and oxygen concentration in the soil solution was low. Plant NO₃ uptake, assessed by measuring the concentration in the xylem, parallelled the distribution of NO₃ in the soil solution. Under high water-tables xylem NO₃ levels fell but it was not possible to say whether this was due to impaired root function or to the reduced concentration of NO₃ observed in the soil solution. At intermediate water-table depths where soil NO₃ availability remained high xylem NO₃ concentration fell relative to the well drained control plants, suggesting that flooding had damaged the root system.Manufactured by Merck. Mention of commerical names does not imply endorsement by either CSIRO or USDA.     

Influence of leaf water status on stomatal response to humidity,hydraulic conductance,and soil drought in Betula occidentalis

Whole-canopy measurements of water flux were used to calculate stomatal conductance (g _s ) and transpiration (E) for seedlings of western water birch (Betula occidentalis Hook.) under various soil-plant hydraulic conductances (k), evaporative driving forces (ΔN; difference in leaf-to-air molar fraction of water vapor), and soil water potentials (Ψ_s). As expected, g _s dropped in response to decreased k or Ψ_S, or increased ΔN(> 0.025). Field data showed a decrease in mid-day g _s with decreasing k from soil-to-petiole, with sapling and adult plants having lower values of both parameters than juveniles. Stomatal closure prevented E and Ψ from inducing xylem cavitation except during extreme soil drought when cavitation occurred in the main stem and probably roots as well. Although all decreases in g _s were associated with approximately constant bulk leaf water potential (ψ_l), this does not logically exclude a feedback response between Ψ_L and g _s . To test the influence of leaf versus root water status on g _s , we manipulated water status of the leaf independently of the root by using a pressure chamber enclosing the seedling root system; pressurizing the chamber alters cell turgor and volume only in the shoot cells outside the chamber. Stomatal closure in response to increased ΔN, decreased k, and decreased Ψ_S was fully or partially reversed within 5 min of pressurizing the soil. Bulk Ψ_L remained constant before and after soil pressurizing because of the increase in E associated with stomatal opening. When ΔN was low (i.e., < 0.025), pressurizing the soil either had no effect on g _s , or caused it to decline; and bulk Ψ_L increased. Increased Ψ_l may have caused stomatal closure via increased backpressure on the stomatal apparatus from elevated epidermal turgor. The stomatal response to soil pressurizing indicated a central role of leaf cells in sensing water stress caused by high ΔN, low k, and low Ψ_S. Invoking a prominent role for feedforward signalling in short-term stomatal control may be premature.     

Structural and Functional Evolution of Positively Selected Sites in Pine Glutathione S-Transferase Enzyme Family

Phylogenetic analyses have identified positive selection as an important driver of protein evolution, both structural and functional. However, the lack of appropriate combined functional and structural assays has generally hindered attempts to elucidate patterns of positively selected sites and their effects on enzyme activity and substrate specificity. In this study we investigated the evolutionary divergence of the glutathione S-transferase (GST) family in Pinus tabuliformis, a pine that is widely distributed from northern to central China, including cold temperate and drought-stressed regions. GSTs play important roles in plant stress tolerance and detoxification. We cloned 44 GST genes from P. tabuliformis and found that 26 of the 44 belong to the largest (Tau) class of GSTs and are differentially expressed across tissues and developmental stages. Substitution models identified five positively selected sites in the Tau GSTs. To examine the functional significance of these positively selected sites, we applied protein structural modeling and site-directed mutagenesis. We found that four of the five positively selected sites significantly affect the enzyme activity and specificity; thus their variation broadens the GST family substrate spectrum. In addition, positive selection has mainly acted on secondary substrate binding sites or sites close to (but not directly at) the primary substrate binding site; thus their variation enables the acquisition of new catalytic functions without compromising the protein primary biochemical properties. Our study sheds light on selective aspects of the functional and structural divergence of the GST family in pine and other organisms.     

A comparative study of the inducing effect of homoserine lactone and hexylresorcinol on phenotypic dissociation in bacteria

It has been shown that the phenotypic dissociation of Bacillus subtilis SK1 and Salmonella typhimurium TA100 is induced by hexylresorcinol, an exogenous non-species-specific autoregulator of pleiotropic action, which is genotoxic for both pro-and eukaryotes. Nongenotoxic homoserine lactone, a chemical analogue of cell-density-responsive species-specific regulators, does not induce bacterial dissociation. The phage resistance of the S-and R-type variants of S. typhimurium TA100 induced by hexylresorcinol has been found to be the same as that of the S-and R-type salmonella variants obtained by the routine subculturing method.     

Death and decay in the trees of Mango (Mangifera indica L.)

Death and decay of trees of Mango (Mangifera indica L.) var. Kesar due to fungal infection was studied histologically. Fungal infection in the trees was observed due to various reasons like mechanical injuries in the stem, pruning of the branches, through the inflorescences, attack of Ambrosia beetle and termites. In the initial stage, fungal spores get settled on the flowers due to presence of nectar, followed by their germination and entry of the hypha into peduncle, which gradually spreads into younger branches. The inflorescences were first attacked by Fusarium moniliformis followed by other fungi like Alternaria, Chetomium sp., Aspregillus ellipticus, Aspregillus niger, etc. Fungal mycelia gradually invade the xylem tissues from the top of the branches and spread basipetally ultimately causing death of the infected branches. During monsoon, the crevices on the surface of bark of the healthy plants supported the growth of fungi like Pleurotus, Auricularia, Xyleria, Daldinia sp., and Polyporous sp. The removal of bark from such infected trees revealed minute holes on the surface of the woody cylinder made by Ambrosia beetles. During wet season fungal mycelia makes an easy entry into the xylem through the wounded portion of the stem or pruned branches. Initial entry of the hyphae into xylem was seen through the ray cells. Then the hyphae enter into the lumen of axial elements lining the ray cells through pits and intracellular spaces. The vessel elements located in the xylem (transition zone) between healthy and infected portion were filled with tyloses while axial and ray parenchyma showed heavy accumulation of tannin contents. On the other hand, the infected xylem was also found devoid of reserve metabolites while in normal trees, axial and ray parenchyma showed heavy accumulation of starch grains.