Experimental and computational comparison of freeze-thaw-induced pressure generation in red and sugar maple

Sap exudation is the process whereby trees such as sugar (Acer saccharum Marsh.) and red maple (Acer rubrum L.) generate unusually high positive stem pressure in response to repeated cycles of freeze and thaw. This elevated xylem pressure permits the sap to be harvested over a period of several weeks and hence is a major factor in the viability of the maple syrup industry. The extensive literature on sap exudation documents competing hypotheses regarding the physical and biological mechanisms that drive positive pressure generation in maple, but to date, relatively little effort has been expended on devising mathematical models for the exudation process. In this paper, we utilize an existing model of Graf et al. (J Roy Soc Interface 12:20150665, 2015) that describes heat and mass transport within the multiphase gas-liquid-ice mixture in the porous xylem tissue. The model captures the inherent multiscale nature of xylem transport by including phase change and osmotic transport in wood cells on the microscale, which is coupled to heat transport through the tree stem on the macroscale. A parametric study based on simulations with synthetic temperature data identifies the model parameters that have greatest impact on stem pressure build-up. Measured daily temperature fluctuations are then used as model inputs and the resulting simulated pressures are compared directly with experimental measurements taken from mature red and sugar maple stems during the sap harvest season. The results demonstrate that our multiscale freeze-thaw model reproduces realistic exudation behavior, thereby providing novel insights into the specific physical mechanisms that dominate positive pressure generation in maple trees.

Perceptions of U.S. and Canadian maple syrup producers toward climate change, its impacts, and potential adaptation measures

The production of maple syrup is an important cultural and economic activity directly related to the climate of northeastern North America. As a result, there are signs that climate change could have negative impacts on maple syrup production in the next decades, particularly for regions located at the southern margins of the sugar maple (Acer saccharum Marsh.) range. The purpose of this survey study is to present the beliefs and opinions of maple syrup producers of Canada (N = 241) and the U.S. (N = 113) on climate change in general, its impacts on sugar maple health and maple syrup production, and potential adaptation measures. Using conditional inference classification trees, we examined how the socio-economic profile of respondents and the geographic location and size of respondents' sugar bushes shaped the responses of survey participants. While a majority (75%) of respondents are confident that the average temperature on Earth is increasing, less than half (46%) believe that climate change will have negative impacts on maple syrup yield in the next 30 years. Political view was a significant predictor of these results, with respondents at the right right and center-right of the political spectrum being less likely to believe in climate change and less likely to anticipate negative effects of climate change on maple syrup production. In addition, 77% of the participants indicated an interest in adopting adaptation strategies if those could increase maple syrup production. This interest was greater for respondents using vacuum tubing for sap collection than other collection methods. However, for many respondents (particularly in Canada), lack of information was identified as a constraint limiting adaptation to climate change.

Anthocyanin influence on light absorption within juvenile and senescing sugar maple leaves - do anthocyanins function as photoprotective visible light screens?

Foliar anthocyanins are hypothesised to function as photoprotective visible light screens, preventing over-excitation of the photosynthetic system, and decreasing the likelihood of photo-oxidative stress by absorbing green light and reducing the amount of light available to be absorbed by chloroplasts in deeper tissue layers. Chlorophyll fluorescence imaging was used to test the hypothesis that anthocyanins in the palisade mesophyll of juvenile and senescing sugar maple (Acer saccharum Marsh.) leaves function as visible light screens by assessing their influence on light absorption profiles within leaves. We hypothesised that an effective anthocyanic light screen should reduce light absorption, particularly of green wavelengths, by chloroplasts in the spongy mesophyll. Both anthocyanic juvenile and senescing leaves absorbed greater amounts of green light than corresponding nonanthocyanic leaves. However, profiles of green light absorption by chlorophyll within anthocyanic leaves were not shifted to reflect reduced absorption of green light by spongy mesophyll chloroplasts. Further, the spongy mesophyll of both anthocyanic juvenile and senescing leaves absorbed proportions of green light equal to or greater than the spongy mesophyll of corresponding nonanthocyanic leaves. These results indicate that though they may provide a general source of photoprotection by reducing the total quantity of light available to be absorbed by chlorophyll, the anthocyanins in juvenile and senescing sugar maple leaves do not attenuate light in a manner consistent with that expected for an anthocyanic screen in the palisade mesophyll.

Maple syrup-production, composition, chemistry, and sensory characteristics

Maple syrup is made from sap exuded from stems of the genus Acer during the springtime. Sap is a dilute solution of primarily water and sucrose, with varying amounts of amino and organic acids and phenolic substances. When concentrated, usually by heating, a series of complex reactions produce a wide variety of flavor compounds that vary due to processing and other management factors, seasonal changes in sap chemistry, and microbial contamination. Color also forms during thermal evaporation. Flavor and color together are the primary factors determining maple syrup grade, and syrup can range from very light-colored and delicate-flavored to very dark-colored and strong-flavored.

Contribution of anthocyanins to the antioxidant capacity of juvenile and senescing sugar maple (Acer saccharum) leaves

Foliar anthocyanins are hypothesised to provide an additional source of photoprotection from photooxidative stress to the leaves in which they occur through their ability to scavenge excess free radical species. Although demonstrated to significantly enhance the antioxidant status of red morphs of fully expanded leaves of some species, the contribution of anthocyanins to the antioxidant capacity of the juvenile and senescing leaves in which they frequently occur has not been examined. Antioxidant activity of extracts from anthocyanic and non-anthocyanic juvenile and senescing sugar maple (Acer saccharum Marsh.) leaves from similar light environments was assessed using the stable free radical 1,1-diphenyl-2-picryl hydrazyl (DPPH). Anthocyanin content was significantly correlated with antioxidant activity in extracts of anthocyanic juvenile leaves but only weakly correlated in extracts of anthocyanic senescing leaves. In addition, the antioxidant activity of anthocyanic and non-anthocyanic leaves was equal in both juvenile and senescing leaves. Thus, although anthocyanins may contribute to the antioxidant capacity of anthocyanic juvenile and senescing sugar maple leaves, these results are not consistent with the hypothesis that anthocyanins provide an enhancement to the photoprotection available in either leaf type through free radical scavenging. The results suggest anthocyanins may be part of alternative strategies employed by anthocyanic juvenile and senescing maple leaves to achieve similar levels of antioxidant capacity as their non-anthocyanic counterparts to cope with the same set of environmental challenges.

Chlorophyll content monitoring in sugar maple (Acer saccharum)

We conducted two experiments to determine the usefulness of a chlorophyll content meter (CCM) for the measurement of foliar chlorophyll concentration in sugar maple (Acer saccharum Marsh.) in the fall color period. In Experiment 1, four sugar maple trees were visually assigned to each of four fall foliage color categories in October 1998. On four dates in the fall of 1999, leaves were taken from the trees and analyzed for chlorophyll concentration by absorbance of pigment extracts and by determination of the chlorophyll content index (CCI) with a CCM. The two measures of chlorophyll concentration were strongly correlated (P < 0.001, r2 = 0.72). In Experiment 2, the CCI of leaves from sugar maple trees subjected to one of four fertilization treatments (lime, lime + manure, lime + 10:10:10 N,P,K fertilizer and an untreated control) were determined with a CCM. Treatment effects were distinguishable between all pairwise comparisons (P < 0.001), except for the lime versus lime + NPK fertilizer treatments.

Development of a Pharmacophore Model for Histamine H3Receptor Antagonists, Using the Newly Developed Molecular Modeling Program SLATE

New molecular modeling tools were developed to construct a qualitative pharmacophore model for histamine H3 receptor antagonists. The program SLATE superposes ligands assuming optimum hydrogen bond geometry. One or two ligands are allowed to flex in the procedure, thereby enabling the determination of the bioactive conformation of flexible H3 antagonists. In the derived model, four hydrogen-bonding site points and two hydrophobic pockets available for binding antagonists are revealed. The model results in a better understanding of the structure-activity relationships of H3 antagonists. To validate the model, a series of new antagonists was synthesized. The compounds were designed to interact with all four hydrogen-bonding site points and the two hydrophobic pockets simultaneously. These ligands have high H3 receptor affinity, thereby illustrating how the model can be used in the design of new classes of H3 antagonists.

SLATE: a method for the superposition of flexible ligands

A novel program for the superposition of flexible molecules, SLATE, is presented. It uses simulated annealing to minimise the difference between the distance matrices calculated from the hydrogen-bonding and aromaticring properties of two ligands. A method for generating a molecular stack using multiple pairwise matches is illustrated. These stacks are used by the program DOH to predict the relative positions of receptor atoms that could form hydrogen bonds to two or more ligands in the dataset. The methodology has been applied to ligands binding to dihydrofolate reductase, thermolysin. H3 histamine receptors, alpha2 adrenoceptors and 5-HT1D receptors. When there are sufficient numbers and diversity of molecules in the dataset, the prediction of receptor-atom positions is applicable to compound design.

An automated method for predicting the positions of hydrogen-bonding atoms in binding sites

Hydrogen bonds are the most specific, and therefore predictable of the intermolecular interactions involved in ligand-protein binding. Given the structure of a molecule, it is possible to estimate the positions at which complementary hydrogen-bonding atoms could be found. Crystal-survey data are used in the design of a program, HBMAP, that generates a hydrogen-bond map for any given ligand, which contains all the feasible positions at which a complementary atom could be found. On superposition of ligands, the overlapping regions of their maps represent positions of receptor atoms to which each molecule can bind. The certainty of these positions is increased by the incorporation of a larger number and diversity of molecules. In this work, superposition is achieved using the program HBMATCH, which uses simulated annealing to generate the correspondence between points from the hydrogen-bonding maps of the two molecules. Equivalent matches are distinguished on the basis of their steric similarity. The strategy is tested on a number of ligands for which ligand-protein complexes have been solved crystallographically, which allows validation of the techniques. The receptor atom positions of thermolysin are successfully predicted when the correct superposition is obtained.

Molecular surface-volume and property matching to superpose flexible dissimilar molecules

Steric complementarity is a prerequisite for ligand-receptor recognition; this implies that drugs with a common receptor binding site should possess sterically similar binding surfaces. This principle is used as the basis for an automatic and unbiased method that superposes molecules. One molecule is rotated and translated to maximize the overlap between the two molecular surface volumes. A fast grid-based method is used to determine the extent of this overlap, and this is optimized using simulated annealing. Matches with high steric similarity scores are then sorted on the basis of both hydrogen-bond and electrostatic similarity between the matched molecules. Flexible molecules are treated as a set of rigid representative conformers. The algorithm has correctly predicted superpositions between a number of paris of molecules, according to crystallographic data from ligands that have been co-crystallized at common enzyme binding sites.

Rapid freezing induces winter injury symptomatology in red spruce foliage

Red spruce (Picea rubens Sarg.) suffers frequent and extensive injury to current-year foliage during the winter. Experimental freezing of red spruce foliage at cooling rates > 10 degrees C min(-1) induced visible symptomatology similar to natural winter injury at the branch, needle and cellular levels. Such damage was associated with a low-temperature exotherm near -10 to -12 degrees C, a loss in needle fluorescence, massive cellular disruption, foliar discoloration, and low needle survival. Susceptibility of individual trees to rapid freezing injury was associated with historical winter injury patterns and alterations in foliar nutrition. We conclude that anthropogenic deposition may alter the sensitivity of trees to winter injury caused by rapid temperature changes.

Tibiocalcaneal arthrodesis for nonbraceable neuropathic ankle deformity

Seven patients with nonbraceable, neuropathic ankle joints have been successfully treated by tibiocalcaneal arthrodesis utilizing an adolescent condylar blade plate, large cannulated AO screws, and a special cancellous allograft mixture. All patients had fragmentation and partial resorption of the talus. This procedure was considered as an alternative to below-knee amputation. Goals were limb salvage and limited community ambulation. Criteria for proceeding with the fusion were a commitment by the patient to 6 to 8 months of nonweightbearing ambulation, a biopsy and culture of the talus revealing no evidence of infection, and a nonbraceable deformity of the foot and ankle that would otherwise require amputation. A toe-level Doppler index or a transcutaneous oxygen index of greater than 0.45 was required. All patients were treated initially in a total contact cast or bivalved total contact ankle-foot orthosis (AFO) until wounds and swelling were controlled and there was no erythema. Presence of an ulcer did not preclude surgery. The arthrodesis used a combination of 7.0-mm AO cannulated screws and an adolescent condylar blade plate. A special preparation of fresh-frozen, irradiated, cancellous allograft mixed with tobramycin and vancomycin powder was used. All ankles fused solidly in an average of 5.2 months. No infectious complications were encountered. Two patients developed a stress fracture of the tibia at the proximal aspect of the blade plate before use of a bivalved AFO. These healed with nonoperative treatment in 6 weeks. All patients were satisfied with their result at their latest follow-up (average 26.9 months).(ABSTRACT TRUNCATED AT 250 WORDS)

Cold tolerance and water content of current-year red spruce foliage over two winter seasons

Red spruce (Picea rubens Sarg.) in high elevation forests of northeastern North America suffers from frequent and severe winter injury, leading to apical dieback, decreased growth, and high mortality. To examine the role of winter desiccation and freezing injury in winter damage, weekly assessments of cold tolerance and water content were made on current-year foliage collected from native red spruce trees at a high elevation site over two winter seasons. In both years, foliage maintained high water contents and adequate cold tolerance; nonetheless, slight to moderate injury was observed each year on some trees. Despite brief thaw periods each winter, no mid-winter dehardening sufficient to put foliage at risk of freezing injury was evident. These findings suggest that, at least in some years, winter injury to current-year red spruce foliage is produced by a mechanism other than desiccation or absolute low temperatures.

An exploration of a novel strategy for superposing several flexible molecules

This paper describes a computational strategy for the superposition of a set of flexible molecules. The combinatorial problems of searching conformational space and molecular matching are reduced drastically by the combined use of simulated annealing methods and cluster analysis. For each molecule, the global minimum of the conformational energy is determined by annealing and the search trajectory is retained in a history file. All the significantly different low-energy conformations are extracted by cluster analysis of data in the history file. Each pair of molecules, in each of their significantly different conformations, is then matched by simulated annealing, using the difference-distance matrix as the objective function. A set of match statistics is then obtained, from which the best match taken from all different conformations can be found. The molecules are then superposed either by reference to a base molecule or by a consensus method. This strategy ensures that as wide a range of conformations as possible is considered, but at the same time that the smallest number of significantly different conformations is used. The method has been tested on a set of six angiotensin II antagonists with between 7-11 rotatable bonds.

RAMBLE: a conformational search program

A program, RAMBLE, is described which searches the conformational space of looped or cyclic molecules by random assignment of internal coordinates. Conformations produced are screened for energetically unfavorable intramolecular contacts and are subject to user-supplied constraints in terms of both bonded and nonbonded distances, bond angles, and torsion angles. The methodology employed is discussed in relation to alternative search strategies, and a description is given of its successful application in modeling the structures of cyclohexane, reverse-turn tetrapeptides, and a bacterial siderophore complex.

Proposed solution structure of endothelin

A model is proposed for the 3-dimensional structure of endothelin, a potent vasoconstrictor and pressor peptide from vascular endothelium. The model is derived through protein structure prediction and circular dichroism studies, and is based on the atomic coordinates for the bee-venom peptide apamin. The model derived shows the same turn-helix motif as observed for apamin and mast-cell degranulating peptide. On the basis of this model we suggest possible strategies for endothelin antagonist design, and note that this motif may be common in a number of peptides acting on channel proteins.