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.     

Metabolic and performance responses during endurance exercise after high-fat and high-carbohydrate meals

We studied the effects of preexercise mealcomposition on metabolic and performance-related variables duringendurance exercise. Eight well-trained cyclists (maximal oxygen uptake65.0 to 83.5 ml · kg¹ · min¹)were studied on three occasions after an overnight fast. They weregiven isoenergetic meals containing carbohydrate (CHO), protein (P),and fat (F) in the following amounts (g/70 kg body wt):high-carbohydrate meal, 215 CHO, 26 P, 3 F; high-fat meal, 50 CHO, 14 P, 80 F. On the third occasion subjects were studied after an overnightfast. Four hours after consumption of the meal, subjects startedexercise for 90 min at 70% of their maximal oxygen uptake, followed by a 10-km time trial. The high-carbohydrate meal compared with the high-fat meal resulted in significant decreases(P < 0.05) in blood glucose, plasmanonesterified fatty acids, plasma glycerol, plasmachylomicron-triacylglycerol, and plasma 3-hydroxybutyrate concentrations during exercise. This was accompanied by anincrease in plasma insulin (P < 0.01 vs. no meal), plasma epinephrine, and plasma growth hormoneconcentrations (each P < 0.05 vs.either of the other conditions) during exercise. Despite these large differences in substrate and hormone concentrations in plasma, substrate oxidation during the 90-min exercise period was similar inthe three trials, and there were no differences in performance on thetime trial. These results suggest that, although the availability offatty acids and other substrates in plasma can be markedly altered bydietary means, the pattern of substrate oxidation during enduranceexercise is remarkably resistant to alteration.

     

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.     

The abundance of an invasive freshwater snail Tarebia granifera (Lamarck, 1822) in the Nseleni River,South Africa

The invasive freshwater snail Tarebia granifera (Lamarck, 1822) was first reported in South Africa in 1999 and it has become widespread across the country, with some evidence to suggest that it reduces benthic macroinvertebrate biodiversity. The current study aimed to identify the primary abiotic drivers behind abundance patterns of T. granifera, by comparing the current abundance of the snail in three different regions, and at three depths, of the highly modified Nseleni River in KwaZulu-Natal, South Africa. Tarebia granifera was well established throughout the Nseleni River system, with an overall preference for shallow waters and seasonal temporal patterns of abundance. Although it is uncertain what the ecological impacts of the snail in this system are, its high abundances suggest that it should be controlled where possible and prevented from invading other systems in the region.     

Characterizing the normal proteome of human ciliary body

Background

The ciliary body is the circumferential muscular tissue located just behind the iris in the anterior chamber of the eye. It plays a pivotal role in the production of aqueous humor, maintenance of the lens zonules and accommodation by changing the shape of the crystalline lens. The ciliary body is the major target of drugs against glaucoma as its inhibition leads to a drop in intraocular pressure. A molecular study of the ciliary body could provide a better understanding about the pathophysiological processes that occur in glaucoma. Thus far, no large-scale proteomic investigation has been reported for the human ciliary body.

Results

In this study, we have carried out an in-depth LC-MS/MS-based proteomic analysis of normal human ciliary body and have identified 2,815 proteins. We identified a number of proteins that were previously not described in the ciliary body including importin 5 (IPO5), atlastin-2 (ATL2), B-cell receptor associated protein 29 (BCAP29), basigin (BSG), calpain-1 (CAPN1), copine 6 (CPNE6), fibulin 1 (FBLN1) and galectin 1 (LGALS1). We compared the plasma proteome with the ciliary body proteome and found that the large majority of proteins in the ciliary body were also detectable in the plasma while 896 proteins were unique to the ciliary body. We also classified proteins using pathway enrichment analysis and found most of proteins associated with ubiquitin pathway, EIF2 signaling, glycolysis and gluconeogenesis.

Conclusions

More than 95% of the identified proteins have not been previously described in the ciliary body proteome. This is the largest catalogue of proteins reported thus far in the ciliary body that should provide new insights into our understanding of the factors involved in maintaining the secretion of aqueous humor. The identification of these proteins will aid in understanding various eye diseases of the anterior segment such as glaucoma and presbyopia.     

Apps can help bridge restoration science and restoration practice

Scientists need to find innovative ways to communicate their findings with restoration practitioners in an era of global change. Apps are a promising bridge between restoration science and practice because they apply broad scientific concepts to specific situations. For example, habitat connectivity promotes ecological function, but practitioners lack ways to incorporate connectivity into decision‐making. We created an app where users calculate how habitat restoration or loss affects connectivity. By providing our app as an example and discussing the benefits and challenges in creating apps for practitioners, we encourage other restoration ecologists to similarly create apps that bridge science with practice.     

Pharmacological activity of the Samoan ethnopharmacopoeia

The Samoan ethnopharmacopoeia was surveyed for pharmacological activity using broad in vitro and in vivo screens. Residues of 74 different plant species were tested for activity in a Hippocratic screen and in a guinea pig ileum test. Over 86% of the plant species exhibited pharmacological activity. This high percentage of active species strongly supports the belief that ethnobotanical analyses of indigenous floras are more likely than random screens to efficiently identify plants likely to yield new drugs. However, collaboration of ethnobotanists and pharmacognosists is necessary to adequately move promising plants from the realm of indigenous knowledge systems to the laboratories of Western pharmacology.     

Differential mechanisms for regulation of the stress response across latitudinal gradients

We examined plasticity of the stress response among three populations of the white-crowned sparrow (Zonotrichia leucophrys). These populations breed at different elevations and latitudes and thus have breeding seasons that differ markedly in length. We hypothesize that in populations where birds raise only one or rarely two broods in a season, the fitness costs of abandoning a nest are substantially larger than in closely related populations that raise up to three broods per season. Thus individuals with short breeding seasons should be less responsive to stressors and therefore less likely to abandon their young. In our study, baseline and handling-induced corticosterone levels were similar among populations, but corticosteroid-binding globulins differed, leading to a direct relationship between stress-induced free corticosteroid levels and length of breeding season. There were also population-specific differences in intracellular low-affinity (glucocorticoid-like) receptors in both liver and brain tissue. Although investigations of population-based differences in glucocorticoid secretion are common, this is the first study to demonstrate population-level differences in binding globulins. These differences could lead to dramatically different physiological and behavioral responses to stress.     

Wood anatomy constrains stomatal responses to atmospheric vapor pressure deficit in irrigated, urban trees

Plant transpiration is strongly constrained by hydraulic architecture, which determines the critical threshold for cavitation. Because species vary greatly in vulnerability to cavitation, hydraulic limits to transpiration and stomatal conductance have not generally been incorporated into ecological and climate models. We measured sap flow, leaf transpiration, and vulnerability to cavitation of a variety of tree species in a well-irrigated but semi-arid urban environment in order to evaluate the generality of stomatal responses to high atmospheric vapor pressure deficit (D). We found evidence of broad patterns of stomatal responses to humidity based on systematic differences in vulnerability to cavitation. Ring-porous taxa consistently had vulnerable xylem and showed strong regulation of transpiration in response to D, while diffuse-porous taxa were less vulnerable and transpiration increased nearly linearly with D. These results correspond well to patterns in the distribution of the taxa, such as the prevalence of diffuse-porous species in riparian ecosystems, and also provide a means of representing maximum transpiration rates at varying D in broad categories of trees.     

Plant hydraulics as a central hub integrating plant and ecosystem function: meeting report for ‘Emerging Frontiers in Plant Hydraulics’ (Washington,DC, May 2015)

Water plays a central role in plant biology and the efficiency of water transport throughout the plant affects both photosynthetic rate and growth, an influence that scales up deterministically to the productivity of terrestrial ecosystems. Moreover, hydraulic traits mediate the ways in which plants interact with their abiotic and biotic environment. At landscape to global scale, plant hydraulic traits are important in describing the function of ecological communities and ecosystems. Plant hydraulics is increasingly recognized as a central hub within a network by which plant biology is connected to palaeobiology, agronomy, climatology, forestry, community and ecosystem ecology and earth‐system science. Such grand challenges as anticipating and mitigating the impacts of climate change, and improving the security and sustainability of our food supply rely on our fundamental knowledge of how water behaves in the cells, tissues, organs, bodies and diverse communities of plants. A workshop, ‘Emerging Frontiers in Plant Hydraulics’ supported by the National Science Foundation, was held in Washington DC, 2015 to promote open discussion of new ideas, controversies regarding measurements and analyses, and especially, the potential for expansion of up‐scaled and down‐scaled inter‐disciplinary research, and the strengthening of connections between plant hydraulic research, allied fields and global modelling efforts.