Focus on Water: The Competition between Liquid and Vapor Transport in Transpiring Leaves

In leaves, the transpirational flux of water exits the veins as liquid and travels toward the stomata in both the vapor and liquid phases before exiting the leaf as vapor. Yet, whether most of the evaporation occurs from the vascular bundles (perivascular), from the photosynthetic mesophyll cells, or within the vicinity of the stomatal pore (peristomatal) remains in dispute. Here, a one-dimensional model of the competition between liquid and vapor transport is developed from the perspective of nonisothermal coupled heat and water molecule transport in a composite medium of airspace and cells. An analytical solution to the model is found in terms of the energy and transpirational fluxes from the leaf surfaces and the absorbed solar energy load, leading to mathematical expressions for the proportions of evaporation accounted for by the vascular, mesophyll, and epidermal regions. The distribution of evaporation in a given leaf is predicted to be variable, changing with the local environment, and to range from dominantly perivascular to dominantly peristomatal depending on internal leaf architecture, with mesophyll evaporation a subordinate component. Using mature red oak (Quercus rubra) trees, we show that the model can be solved for a specific instance of a transpiring leaf by combining gas-exchange data, anatomical measurements, and hydraulic experiments. We also investigate the effect of radiation load on the control of transpiration, the potential for condensation on the inside of an epidermis, and the impact of vapor transport on the hydraulic efficiency of leaf tissue outside the xylem.During steady-state transpiration, the evaporative flux from the cell surfaces lining a leaf’s intercellular airspaces balances the flux of water vapor exiting the stomatal pores. The question of how the phase change from liquid to vapor is distributed within a leaf pertains to many aspects of leaf function, including isotopic enrichment of leaf water (Farquhar et al., 1993; Gillon and Yakir, 2000; Cernusak and Kahmen, 2013), the hydromechanics of stomatal control (Buckley, 2005; Franks and Farquhar, 2007; Peak and Mott, 2011), and hydraulic constraints on maximum transpiration rates (Brodribb et al., 2007; Boyce et al., 2009; Brodribb et al., 2010). Experimental work with apoplastic tracers (Tanton and Crowdy, 1972; Byott and Sheriff, 1976), physical analogs (Meidner, 1976), and mathematical modeling (Tyree and Yianoulis, 1980; Yianoulis and Tyree, 1984) has challenged the idea that evaporation occurs more or less uniformly from the mesophyll, converging on the view that evaporation will be heavily skewed toward the internal wetted surfaces closest to the stomata (hereafter, peristomatal evaporation; Buckley and Mott, 2013). However, the interpretation of tracer accumulation as indicative of a local evaporative flux has been questioned (Yianoulis and Tyree, 1984), and calculation of the expected pressure drop for flow across the bundle sheath cells alone suggests a short liquid flow path, with evaporation from the vascular bundle directly to the airspace (hereafter, perivascular evaporation; Boyer, 1985).At the same time, a variety of experimental approaches for characterizing the hydraulic efficiency with which leaves replace the water lost to transpiration have been developed, of which evaporative flux measurement (EFM) of transpiring leaves is considered to be the most naturalistic (Sack et al., 2002). The location and water potential of the sites of evaporation in a leaf are unknown in EFM. Instead, a whole-leaf hydraulic conductance (K_leaf) is defined as the transpirational flux (E) divided by the difference between the water potential of a source at the petiole (e.g. the main stem, or a reservoir in the laboratory) and leaf water potential, as typically estimated from a pressure chamber measurement (xylem sap osmolality is assumed to be negligible; Sack et al., 2002). As a result, K_leaf is not physically well defined and bears an ambiguous relationship to the real hydraulic properties of leaf xylem and tissues (Rockwell et al., 2014b). Hydraulic studies that have sought to correlate K_leaf with internal leaf structure have further assumed peristomatal evaporation and neglected the possibility that the phase change for some portion of the flux occurs deeper inside the leaf (Brodribb et al., 2007; Brodribb et al., 2010; Buckley et al., 2011).Recently however, the possibility that internal vapor transport dominates liquid phase transport from the vascular bundles to a transpiring epidermis has received renewed attention, particularly in relation to stomatal behavior (Pieruschka et al., 2010; Peak and Mott, 2011). As saturated vapor pressure has a strong temperature dependence, modeling internal vapor transport requires an accounting of internal energy fluxes. In energy terms, the competition between liquid and vapor transport within a leaf can be viewed as a competition between thermal conduction and latent heat transport, as for peristomatal evaporation to occur both liquid water and thermal energy must be conducted to the transpiring epidermis. Taking a modified leaf energy balance approach, Pieruschka et al. (2010) interpreted observed stomatal opening in response to increased radiation load as evidence that latent heat transport serves as the dominant mode of energy transport from the mesophyll to the epidermis. In this model, internal vapor transport typically exceeds transpiration, with water condensing on the epidermis (peristomatal condensation) and flowing back toward the sites of short-wave energy absorption and evaporation in the mesophyll. Increasing the energy load is thought to increase condensation on the epidermis, allowing stomata to open hydropassively.Here, we provide a model of the competition between internal liquid and vapor transport that treats the leaf mesophyll as a homogenous composite effective medium of air and cells (Rockwell et al., 2014a) and the epidermis (including stomata) as a boundary characterized by temperature, water potential, and conductance to water vapor (Fig. 1). Like Pieruschka et al. (2010), we do not attempt to model stomatal mechanics, although we will assume at some points in the analysis of steady-state transpiration a phenomenological linkage between epidermal water potential and stomatal conductance (g_s). The most important difference between our approach and prior work is that our model allows the competition between the vapor and liquid phase transport of water, and the associated competition between latent and sensible transport of heat energy, to emerge from general conservation laws and the constraint of local thermal and chemical equilibrium between phases in mesophyll airspaces. By contrast, Pieruschka et al. (2010) fix the balance of internal heat transport between latent transfer and heat conduction in air based on a result for steady evaporation into saturated air (equilibrium evaporation). This result says that the proportion of an absorbed solar short-wave energy load dissipated as latent heat transport assumes a characteristic value that depends only on the temperature sensitivity of saturated vapor pressure, the latent heat of vaporization, and the heat capacity of air (Raupach, 2001). Another way of expressing this result is to say that, in the absence of lateral convection (i.e. wind), the Bowen ratio (sensible heat flux/latent heat flux) for an evaporating surface assumes a value that depends only on the physical properties of water and air and not on the properties of the evaporating body itself (Bowen, 1926; Lambers et al., 1998; Raupach, 2001). It is important to note that this equilibrium evaporation result applies to a one-dimensional (1D) system of transport from water to air in series, rather than in parallel, as in the cells and airspaces of leaf mesophyll tissue.Open in a separate windowFigure 1.Model overview: conservation of heat and molecules at transpiring and nontranspiring surfaces and in a representative volume element of mesophyll (air and cells). At steady state, global molecular conservation requires that the number of molecules entering in the liquid phase, J_x, equals the number leaving in the gas phase, E_b, such that the difference in the enthalpies of the two phases leads to the net consumption of energy as latent heat equivalent to λE_b. Thermal energy conservation requires that the total absorbed short-wave radiation load SR balances the total net surface latent and sensible (long-wave radiative and conductive) fluxes.As noted by Pieruschka et al. (2010), assuming the equilibrium evaporation result to describe energy fluxes inside a leaf affords an enormous simplification of the physical detail that must be represented in their model, yet it comes at the cost of neglecting the possibility of heat conduction in the liquid phase. As liquid phase thermal conductivity may be expected to be 1 order of magnitude larger than that of the gas phase (Tyree and Yianoulis, 1980), and as there exists continuous cell-to-cell contact through the leaf thickness in parallel with the intercellular airspaces, the actual balance of latent and sensible heat within a leaf may be quite different from the equilibrium evaporation case. Our goal, then, is to understand how the balance of vapor and liquid transport, as well as latent and sensible heat transport, depends on leaf and physical properties after relaxing the assumption that the liquid and vapor phases are arrayed in series. To do so, we account for liquid phase thermal conduction in the context of the simplest possible transport model that remains consistent with the principles of nonisothermal transport phenomena (Bird et al., 1960; Deen, 1998).In a general way, our approach has been anticipated by effective media formulations of liquid and vapor transport in unsaturated soils, most clearly in terms of the assumption of local equilibrium between wet surfaces and the vapor in the gas-filled pores, as well as the decomposition of the mole fraction gradients driving vapor transport into gradients in temperature and water status (De Vries, 1958; Philip and De Vries, 1957; Whitaker, 1977). Our own analysis adds elements important for leaves: a volumetric energy source representing short-wave solar energy absorbed in the cells, liquid (cell) and air fractions continuously connected through the leaf thickness, a source of water at the vascular plane (Rockwell et al., 2014a, 2014b), and bounding epidermal surfaces either with or without stomata. With this approach, the combination of local equilibrium between phases and energy and mass conservation for a composite medium leads to a linkage between water potential and temperature gradients. This linkage allows us to separate the coupled equations describing heat and molecular fluxes and find analytic solutions for the temperature and water potential fields inside a leaf. We then analyze these solutions to describe the effects of leaf structural properties and surface fluxes on the distribution of evaporation between perivascular, peristomatal, and mesophyll compartments. However, the lack of a general model for the dependence of stomatal aperture on the local temperature and water potential (as well as chemical signaling) prevents us from arriving at a completely general model with which to study environmental effects. Instead, we are restricted to exploring a few cases where the g_s realized for a particular set of environmental conditions is known. In the course of these analyses, we reanalyze the control of transpiration by radiation (Pieruschka et al., 2010). In exploring environmentally driven shifts in the distribution of evaporation within a particular leaf, we also clarify the relationship between K_leaf and the actual hydraulic conductivity of leaf tissue and show that this relationship is sensitive to the energy regime experienced by a leaf. Finally, we consider the possible functional significance of the distribution of evaporation in a leaf in terms of its impact on hydraulic efficiency, the response of transpiration to environmental forcings, stomatal control, and minimum leaf water potentials.     

Spontaneous Coagulopathy in Inbred WAG/RijYcb Rats

Here we describe a series of cases of spontaneous coagulopathy in a colony of inbred WAG/RijYcb (WAG/RijY) rats. This strain previously had been bred at our institution without symptomatology for several decades. The index case was a 10-wk-old male rat that developed a large hematoma at a subcutaneous injection site. Clinicopathologic findings included a decreased RBC count, decreased hematocrit, decreased hemoglobin concentration, normal PT, and prolonged (50% to 70%) aPTT (52 s; reference, 15 to 33 s). Examination of additional WAG/RijY rats that died unexpectedly or had clinical signs of bleeding in the absence of experimental manipulation also revealed normal PT and prolonged aPTT. Histologic examinations of tissues from all rats were unremarkable except for severe acute focally extensive hemorrhage corresponding to the macroscopic findings of acute hemorrhage. Furthermore the aPTT in 8 clinically normal adult rats and 8 clinically normal 4-wk-old WAG/RijY littermates of both sexes was prolonged. We conclude that these WAG/RijY rats have an inherited defect in the intrinsic coagulation pathway.A breeding colony of WAG/RijY rats was established at Yale University with breeding stock brought from the Netherlands in 1970.^4,5 Over the past 4 decades, these rats have been used extensively in cancer research testing the responses of transplanted tumors and normal tissues to potential new treatment agents, novel combined-modality regimens, and physiologic modulations that might alter the response of tumors and tissues to irradiation.^4,9,11,16 In addition, WAG/RijY rats have been used as a model for absence epilepsy^2,12 and in the development of physiologic imaging techniques.¹⁶ Review of the colony records indicated sporadic cases of unexplained postpartum maternal deaths in this breeding colony as early as June 2004. However, clinical signs of bleeding problems were not noted in these rats until the index case in late July 2007. This index case was followed by investigators’ reports of a series of unexpected injuries. Here we describe the results of clinicopathologic studies indicating that the bleeding diathesis in the WAG/RijY rats is caused by a defect in the intrinsic coagulation pathway. The noted in vivo and in vitro signs are unique to WAG/RijY rats and compatible with a de novo mutation that has become fixed in this colony.     

Femtosecond photodynamics of the red/green cyanobacteriochrome NpR6012g4 from Nostoc punctiforme. 1. Forward dynamics

Phytochromes are well-known red/far-red photosensory proteins that utilize the photoisomerization of a linear tetrapyrrole (bilin) chromophore to detect the ratio of red to far-red light. Cyanobacteriochromes (CBCRs) are related photosensory proteins with a bilin-binding GAF domain, but much more diverse spectral sensitivity, with five recognized subfamilies of CBCRs described to date. The mechanisms that underlie this spectral diversity have not yet been fully elucidated. One of the main CBCR subfamilies photoconverts between a red-absorbing ground state, like the familiar P(r) state of phytochromes, and a green-absorbing photoproduct (P(g)). Here, we examine the ultrafast forward photodynamics of the red/green CBCR NpR6012g4 from the NpR6012 locus of the nitrogen-fixing cyanobacterium Nostoc punctiforme. Using transient absorption spectroscopy with broadband detection and multicomponent global analysis, we observed multiphasic excited-state dynamics that induces the forward reaction (red-absorbing to green-absorbing), which we interpret as arising from ground-state heterogeneity. Excited-state decays with lifetimes of 55 and 345 ps generate the primary photoproduct (Lumi-R), and the fastest decay (5 ps) did not produce Lumi-R. Although the photoinduced kinetics of Npr6012g4 is comparable with that of the Cph1 phytochrome isolated from Synechocystis cyanobacteria, NpR6012g4 exhibits a ≥2-3-fold higher photochemical quantum yield. Understanding the structural basis of this enhanced quantum yield may prove to be useful in increasing the photochemical efficiency of other bilin-based photosensors.     

Femtosecond photodynamics of the red/green cyanobacteriochrome NpR6012g4 from Nostoc punctiforme. 2. reverse dynamics

Phytochromes are red/far-red photosensory proteins that utilize photoisomerization of a linear tetrapyrrole (bilin) chromophore to photoconvert reversibly between red- and far-red-absorbing forms (P(r) and P(fr), respectively). Cyanobacteriochromes (CBCRs) are related photosensory proteins with more diverse spectral sensitivity. The mechanisms that underlie this spectral diversity have not yet been fully elucidated. One of the main CBCR subfamilies photoconverts between a red-absorbing 15Z ground state, like the familiar P(r) state of phytochromes, and a green-absorbing photoproduct ((15E)P(g)). We have previously used the red/green CBCR NpR6012g4 from the cyanobacterium Nostoc punctiforme to examine ultrafast photodynamics of the forward photoreaction. Here, we examine the reverse reaction. Using excitation-interleaved transient absorption spectroscopy with broadband detection and multicomponent global analysis, we observed multiphasic excited-state dynamics. Interleaved excitation allowed us to identify wavelength-dependent shifts in the ground-state bleach that equilibrated on a 200 ps time scale, indicating ground-state heterogeneity. Compared to the previously studied forward reaction, the reverse reaction has much faster excited-state decay time constants and significantly higher photoproduct yield. This work thus demonstrates striking differences between the forward and reverse reactions of NpR6012g4 and provides clear evidence of ground-state heterogeneity in the phytochrome superfamily.     

Guidelines for including bamboos in tropical ecosystem monitoring

Bamboos are a diverse and ecologically important group of plants that have the potential to modulate the structure, composition, and function of forests. With the aim of increasing the visibility and representation of bamboo in forest surveys, and to standardize techniques across ecosystems, we present a protocol for bamboo monitoring in permanent research plots. A bamboo protocol is necessary because measurements and sampling schemes that are well-suited to trees are inadequate for monitoring most bamboo species and populations. Our protocol suggests counting all bamboo culms (stems) in the study plot and determining bamboo dimensions based on two different approaches: (a) measuring a random subset of 60 culms and calculating the average dimensions or (b) measuring all culms. With data from 1-ha plots in the Peruvian Andes, we show that both approaches provide very similar estimates of bamboo basal area. We suggest including all mature culms rooted inside change the to each plot from all woody bamboo species with maximum diameters ≥1 cm. We also present recommendations on how to collect vouchers of bamboo species for identification. Data collected according to our proposed protocols will increase our understanding of regional and global patterns in bamboo diversity and the role of bamboo in forest dynamics.     

HistoChoice® as an alternative to formalin fixation of undecalcified bone specimens

We compared histochemical and immunohistochemical staining as well as fluorochrome labeling in murine bone specimens that were fixed with 10% neutral buffered formalin to those fixed with HistoChoice®. We showed that sections from undecalcified tibiae fixed for 4 h in HistoChoice® resulted in enhanced toluidine blue and Von Kossa histochemical staining compared to formalin fixation. HistoChoice® produced comparable or improved staining for alkaline phosphatase. Acid phosphatase localization was better in formalin fixed specimens, but osteoclasts were visuralized more easily in HistoChoice® fixed specimens. As expected, immunohistochemical labeling was antibody dependent; some antibodies labeled better in HistoChoice® fixed specimens while others were better in formalin fixed specimens. Toluidine blue, Von Kossa, and alkaline phosphatase staining of sections fixed for 12 h produced sections that were similar to 4 h fixed sections. Fixation for 12 h preserved acid phosphatase activity better. Increasing fixation to 12 h affected immunolocalization differentially. Bone sialoprotein labeling in HistoChoice® fixed specimens was comparable to formalin fixed samples. On the other hand, after 12 h formalin fixation, osteocalcin labeling was comparable to HistoChoice®. For most histochemical applications, fixing murine bone specimens for 4 h with HistoChoice® yielded superior staining compared to formalin fixation. If immunohistochemical localization is desired, however, individual antibodies must be tested to determine which fixation process retains antigenicity better. In addition, there was no detectable difference in the intensity of fluorochrome labeling using either fixative. Finally, fixation duration did not alter the intensity of labeling.     

Trophic matches and mismatches: can polar bears reduce the abundance of nesting snow geese in western Hudson Bay?

Climate change driven advances in the date of sea ice breakup will increasingly lead to a loss of spring polar bear foraging opportunities on ringed seal pups creating a phenological trophic ‘mismatch’. However, the same shift will lead to a new ‘match’ between polar bears and ground nesting birds. This new match will be especially prevalent along the Cape Churchill Peninsula of western Hudson Bay where both polar bears and nesting snow geese are abundant. Easily foraged goose eggs will provide at least some of the earlier arriving polar bears with compensation for the energy deficit accrued through lost seal hunting opportunities. We examine the potential impact of changes in the extent and pattern of polar bear egg predation on snow goose abundance using projection models that account not only for increases in the temporal overlap of the two species but also for autocorrelation and stochasticity in the processes underlying polar bear onshore arrival and snow goose incubation. Egg predation will reduce reproductive output of the nesting lesser snow geese and, under all but trivial rates, will lead to a reduction in the size of their nesting population on the Cape Churchill Peninsula. Stochasticity associated with the asymmetrical advances in polar bear onshore arrival and the snow goose incubation period will lead to periodic mismatches in their overlap. These, in turn, will allow snow goose abundance to increase periodically. Climate driven changes in trophic matches and mismatches may reduce snow goose numbers but will not eliminate this over‐abundant species that poses a threat to Arctic landscapes.     

Comparative whole genome sequencing reveals phenotypic tRNA gene duplication in spontaneous Schizosaccharomyces pombe La mutants

We used a genetic screen based on tRNA-mediated suppression (TMS) in a Schizosaccharomyces pombe La protein (Sla1p) mutant. Suppressor pre-tRNA(Ser)UCA-C47:6U with a debilitating substitution in its variable arm fails to produce tRNA in a sla1-rrm mutant deficient for RNA chaperone-like activity. The parent strain and spontaneous mutant were analyzed using Solexa sequencing. One synonymous single-nucleotide polymorphism (SNP), unrelated to the phenotype, was identified. Further sequence analyses found a duplication of the tRNA(Ser)UCA-C47:6U gene, which was shown to cause the phenotype. Ninety percent of 28 isolated mutants contain duplicated tRNA(Ser)UCA-C47:6U genes. The tRNA gene duplication led to a disproportionately large increase in tRNA(Ser)UCA-C47:6U levels in sla1-rrm but not sla1-null cells, consistent with non-specific low-affinity interactions contributing to the RNA chaperone-like activity of La, similar to other RNA chaperones. Our analysis also identified 24 SNPs between ours and S. pombe 972h- strain yFS101 that was recently sequenced using Solexa. By including mitochondrial (mt) DNA in our analysis, overall coverage increased from 52% to 96%. mtDNA from our strain and yFS101 shared 14 mtSNPs relative to a 'reference' mtDNA, providing the first identification of these S. pombe mtDNA discrepancies. Thus, strain-specific and spontaneous phenotypic mutations can be mapped in S. pombe by Solexa sequencing.     

What to eat now? Shifts in polar bear diet during the ice-free season in western Hudson Bay

Under current climate trends, spring ice breakup in Hudson Bay is advancing rapidly, leaving polar bears (Ursus maritimus) less time to hunt seals during the spring when they accumulate the majority of their annual fat reserves. For this reason, foods that polar bears consume during the ice‐free season may become increasingly important in alleviating nutritional stress from lost seal hunting opportunities. Defining how the terrestrial diet might have changed since the onset of rapid climate change is an important step in understanding how polar bears may be reacting to climate change. We characterized the current terrestrial diet of polar bears in western Hudson Bay by evaluating the contents of passively sampled scat and comparing it to a similar study conducted 40 years ago. While the two terrestrial diets broadly overlap, polar bears currently appear to be exploiting increasingly abundant resources such as caribou (Rangifer tarandus) and snow geese (Chen caerulescens caerulescens) and newly available resources such as eggs. This opportunistic shift is similar to the diet mixing strategy common among other Arctic predators and bear species. We discuss whether the observed diet shift is solely a response to a nutritional stress or is an expression of plastic foraging behavior.