Do Woody Plants Operate Near the Point of Catastrophic Xylem Dysfunction Caused by Dynamic Water Stress? : Answers from a Model

We discuss the relationship between the dynamically changing tension gradients required to move water rapidly through the xylem conduits of plants and the proportion of conduits lost through embolism as a result of water tension. We consider the implications of this relationship to the water relations of trees. We have compiled quantitative data on the water relations, hydraulic architecture and vulnerability of embolism of four widely different species: Rhizophora mangle, Cassipourea elliptica, Acer saccharum, and Thuja occidentalis. Using these data, we modeled the dynamics of water flow and xylem blockage for these species. The model is specifically focused on the conditions required to generate `runaway embolism,' whereby the blockage of xylem conduits through embolism leads to reduced hydraulic conductance causing increased tension in the remaining vessels and generating more tension in a vicious circle. The model predicted that all species operate near the point of catastrophic xylem failure due to dynamic water stress. The model supports Zimmermann's plant segmentation hypothesis. Zimmermann suggested that plants are designed hydraulically to sacrifice highly vulnerable minor branches and thus improve the water balance of remaining parts. The model results are discussed in terms of the morphology, hydraulic architecture, eco-physiology, and evolution of woody plants.     

A method for measuring xylem hydraulic conductance and embolism in entire root and shoot systems

Current methods for determining the influence of xylem cavitationon hydraulic conductance are limited to unbranched stem or rootsegments with hydraulic conductances above c. 2 mmol s^–1MPa^–1. Lower conductances and/or highly branched systemsare encountered in seedlings, arid-land shrubs, herbs, and distalportions of shoot and root systems of trees. In order to quantifythe hydraulic impact of cavitation in such systems, existingtechniques have been modified. Branched shoot or root systemswere prepared for measurement by removal of leaves, or roottips, respectively. The shoot or root system was enclosed ina vacuum chamber with the proximal end protruding and suppliedwith perfusing solution. Flow through the xylem was inducedby chamber vacuum. Hydraulic conductance was determined fromthe slope of the flow rate versus pressure relationship. Xylemembolism was quantified from the increase in hydraulic conductancefollowing high pressure (100 kPa) perfusion of solution throughthe plant. Examples are provided of the application of the methodto cavitation studies in the cold desert shrub Artemisia tridentata. Key words: Hydraulic conductance, xylem cavitation, embolism, whole root/shoot system     

Freezing-induced xylem cavitation and the northern limit of Larrea tridentata

We investigated the occurrence of freezing-induced cavitation in the evergreen desert shrub Larrea tridentata and compared it to co-occurring, winter-deciduous Prosopis velutina. Field measurements indicated that xylem sap in L. tridentata froze at temperatures below c. –5°C, and that this caused no measurable cavitation for minimum temperatures above –7°C. During the same period P. velutina cavitated almost completely. In the laboratory, we cooled stems of L. tridentata to temperatures ranging from –5 to –20°C, held them at temperature for 1 or 12 h, thawed the stems at a constant rate and measured cavitation by the decrease in hydraulic conductivity of stem segments. As observed in the field, freezing exotherms occurred at temperatures between –6.5 and –9°C and as long as temperatures were held above –11°C there was no change in hydraulic conductivity after thawing. However, when stems were cooled to between –11°C and –20°C, stem hydraulic conductivity decreased linearly with minimum temperature. Minimum temperatures between –16 and –20°C were sufficient to completely eliminate hydraulic conductance. Record (>20 year) minimum isotherms in this same range of temperatures corresponded closely with the northern limit of L. tridentata in the Mojave and Sonoran deserts.     

Kinesin-related proteins in the mammalian testes: candidate motors for meiosis and morphogenesis.

The kinesin superfamily of molecular motors comprises proteins that participate in a wide variety of motile events within the cell. Members of this family share a highly homologous head domain responsible for force generation attached to a divergent tail domain thought to couple the motor domain to its target cargo. Many kinesin-related proteins (KRPs) participate in spindle morphogenesis and chromosome movement in cell division. Genetic analysis of mitotic KRPs in yeast and Drosophila, as well as biochemical experiments in other species, have suggested models for the function of KRPs in cell division, including both mitosis and meiosis. Although many mitotic KRPs have been identified, the relationship between mitotic motors and meiotic function is not clearly understood. We have used sequence similarity between mitotic KRPs to identify candidates for meiotic and/or mitotic motors in a vertebrate. We have identified a group of kinesin-related proteins from rat testes (termed here testes KRP1 through KRP6) that includes new members of the bimC and KIF2 subfamilies as well as proteins that may define new kinesin subfamilies. Five of the six testes KRPs identified are expressed primarily in testes. Three of these are expressed in a region of the seminiferous epithelia (SE) rich in meiotically active cells. Further characterization of one of these KRPs, KRP2, showed it to be a promising candidate for a motor in meiosis: it is localized to a meiotically active region of the SE and is homologous to motor proteins associated with the mitotic apparatus. Testes-specific genes provide the necessary probes to investigate whether the motor proteins that function in mammalian meiosis overlap with those of mitosis and whether motor proteins exist with functions unique to meiosis. Our search for meiotic motors in a vertebrate testes has successfully identified proteins with properties consistent with those of meiotic motors in addition to uncovering proteins that may function in other unique motile events of the SE.     

Erythritol catabolism by Brucella abortus.

Cell extracts of Brucella abortus (British 19) catabolized erythritol through a series of phosphorylated intermediates to dihydroxyacetonephosphate and CO-2. Cell extracts required adenosine 5'-triphosphate (ATP), nicotinamide adenine dinucleotide (NAD), Mg2+, inorganic orthophosphate, and reduced glutathione for activity. The first reaction in the pathway was the phosphorylation of mesoerythritol with an ATP-dependent kinase which formed d-erythritol 1-phosphate (d-erythro-tetritol 1-phosphate). d-Erythritol 1-phosphate was oxidized by an NAD-dependent dehydrogenase to d-erythrulose 1-phosphate (d-glycero-2-tetrulose 1-phosphate). B. abortus (US-19) was found to lack the succeeding enzyme in the pathway and was used to prepare substrate amounts of d-erythrulose 1-phosphate. d-Erythritol 1-phosphate dehydrogenase (d-erythro-tetritol 1-phosphage: NAD 2-oxidoreductase) is probably membrane bound. d-Erythrulose 1-phosphate was oxidized by an NAD-dependent dehydrogenase to 3-keto-l-erythrose 4-phosphate (l-glycero-3-tetrosulose 4-phosphate) which was further oxidized at C-1 by a membrane-bound dehydrogenase coupled to the electron transport system. Either oxygen or nitrate had to be present as a terminal electron acceptor for the oxidation of 3-keto-l-erythrose 4-phosphate to 3-keto-l-erythronate 4-phosphate (l-glycero-3-tetrulosonic acid 4-phosphate). The beta-keto acid was decarboxylated by a soluble decarboxylase to dihydroxyacetonephosphate and CO-2. Dihydroxyacetonephosphate was converted to pyruvic acid by the final enzymes of glycolysis. The apparent dependence on the electron transport system of erythritol catabolism appears to be unique in Brucella and may play an important role in coupling metabolism to active transport and generation of ATP.     

Cytochrome spectrum of an obligate anaerobe, Eubacterium lentum.

An obligately anaerobic bacterium, Eubacterium lentum, was shown to contain cytochromes a, b, and c and a carbon monoxide-binding pigment. Extracts of cells grown with hemin gave a typical absorption spectrum for cytochrome c with maxima at 424, 525, and 553 nm. Extracts from cells grown in the absence of hemin also had an absorption peak corresponding to cytochrome b (562 nm) in their reduced versus oxidized spectrum. Extraction of hemes and formation of pyridine hemochromes allowed quantitation of protoheme IX and heme c. Large amounts of cytochrome c masked the presence of cytochrome b in cells grown in medium containing hemin. When cells were grown in the presence of 50 mM nitrate, cytochrome A (606 nm) was detected. In anaerobic extracts of cells grown either with or without nitrate, cytochromes b and c were reduced by formate and oxidized by NO3. Cytochrome a appeared to be partially oxidized by NO3 and completely oxidized by air.     

Risk factors of peripheral arterial disease: a case control study in Sri Lanka

Background

Peripheral artery disease (PAD) is an important global health problem and contributes to notable proportion of morbidity and mortality. This particular manifestation of systemic atherosclerosis is largely under diagnosed and undertreated. For sustainable preventive strategies in a country, it is mandatory to identify country-specific risk factors. We intended to assess the risk factors of PAD among adults aged 40–74 years.

Methods

This case control study was conducted in 2012–2013 in Sri Lanka. Seventy-nine cases and 158 controls in the age group of 40–74 years were selected for the study in order to have case to control ratio 1:2. The criterion for selecting cases and control was based on Ankle brachial pressure index (ABPI). Cases were selected from those who had ABPI 0.85 or less (ABPI ≤0.85) in either lower limb. Controls were selected from those ABPI score between 1.18 and 1.28 in both lower limbs. Only newly identified individuals with PAD were selected as cases. Controls were selected from the same geographical location and within the 5 year age group as cases.

Results

The history of diabetes mellitus more than 10 years (OR 5.8, 95% CI 2.2–14.2), history of dyslipidemia for more than 10 years (OR 4.9, 95% CI 2.1–16.2), history of hypertension for more than 10 years (OR 3.8, 95% CI 1.8–12.7) and smoking (OR 2.9, 95% CI 1.2–6.9), elevated HsCRP (OR 3.7, 95% CI 1.2–12.0) and hyperhomocysteinemia (OR 3.0, 95% CI 1.1–8.1) were revealed as country specific significant risk factor of PAD.

Conclusions

Diabetes mellitus, hypertension, dyslipidemia, smoking as well as elevated homocysteine and HsCRP found as risk factors of PAD. Longer the duration or higher level exposure to these risk factors has increased the risk of PAD. These findings emphasis the need for routine screening of PAD among patients with the identified risk factors.

     

The role of fish-poultry integration on fish growth performance,yields and economic benefits among smallholder farmers in sub-Saharan Africa,Tanzania

Aquaculture practices from sub-Saharan Africa are characterised by low production, owing to improper technology. Production can be increased through integrating fish farming with other existing on-farm activities, particularly livestock husbandry. We assessed the role of fish-poultry integration on all male Nile tilapia, Oreochromis niloticus growth performance, yields and economic benefits among smallholder farmers in sub-Saharan Africa, Tanzania. The study also compared phytoplankton species composition, abundance and biomass between the fish-poultry integration and non-integrated system. After 180 days of the experiment, all male O. niloticus cultured under fish-poultry integration exhibited significantly higher growth rates than those in the non-integrated system (p < 0.05). Gross fish yield (GFY), net fish yield (NFY) and net annual yields (NAY) obtained from fish-poultry integration were significantly higher than those from non-integrated system (p < 0.05). Partial enterprise budget analysis revealed that fish-poultry integration was more profitable than the non-integrated system. Moreover, fish-poultry integrated system produced significantly higher phytoplankton abundance and biomass than those from the non-integrated system. Results demonstrate that rural smallholder farmers can achieve higher growth rate, farm net yields and income by integrating all male O. niloticus with other on-farm activities than practising a stand-alone fish culture system.     

Leveraging plant hydraulics to yield predictive and dynamic plant leaf allocation in vegetation models with climate change

Plant functional traits provide a link in process‐based vegetation models between plant‐level physiology and ecosystem‐level responses. Recent advances in physiological understanding and computational efficiency have allowed for the incorporation of plant hydraulic processes in large‐scale vegetation models. However, a more mechanistic representation of water limitation that determines ecosystem responses to plant water stress necessitates a re‐evaluation of trait‐based constraints for plant carbon allocation, particularly allocation to leaf area. In this review, we examine model representations of plant allocation to leaves, which is often empirically set by plant functional type‐specific allometric relationships. We analyze the evolution of the representation of leaf allocation in models of different scales and complexities. We show the impacts of leaf allocation strategy on plant carbon uptake in the context of recent advancements in modeling hydraulic processes. Finally, we posit that deriving allometry from first principles using mechanistic hydraulic processes is possible and should become standard practice, rather than using prescribed allometries. The representation of allocation as an emergent property of scarce resource constraints is likely to be critical to representing how global change processes impact future ecosystem dynamics and carbon fluxes and may reduce the number of poorly constrained parameters in vegetation models.