Spectroscopic Characterization of the Excitation Energy Transfer in the Fucoxanthin–Chlorophyll Protein of Diatoms

We characterized the energy transfer pathways in the fucoxanthin–chlorophyll protein (FCP) complex of the diatom Cyclotella meneghiniana by conducting ultrafast transient absorption measurements. This light harvesting antenna has a distinct pigment composition and binds chlorophyll a (Chl-a), fucoxanthin and chlorophyll c (Chl-c) molecules in a 4:4:1 ratio. We find that upon excitation of fucoxanthin to its S₂ state, a significant amount of excitation energy is transferred rapidly to Chl-a. The ensuing dynamics illustrate the presence of a complex energy transfer network that also involves energy transfer from the unrelaxed or ‘hot’ intermediates. Chl-c to Chl-a energy transfer occurs on a timescale of a 100 fs. We observe no significant spectral evolution in the Chl-a region of the spectrum. We have applied global and target analysis to model the measured excited state dynamics and estimate the spectra of the states involved; the energy transfer network is discussed in relation to the pigment organization of the FCP complex.     

Chlorophyll a fluorescence induction: a personal perspective of the thermal phase, the J–I–P rise

The fast (up to 1?s) chlorophyll (Chl) a fluorescence induction (FI) curve, measured under saturating continuous light, has a photochemical phase, the O-J rise, related mainly to the reduction of Q(A), the primary electron acceptor plastoquinone of Photosystem II (PSII); here, the fluorescence rise depends strongly on the number of photons absorbed. This is followed by a thermal phase, the J-I-P rise, which disappears at subfreezing temperatures. According to the mainstream interpretation of the fast FI, the variable fluorescence originates from PSII antenna, and the oxidized Q(A) is the most important quencher influencing the O-J-I-P curve. As the reaction centers of PSII are gradually closed by the photochemical reduction of Q(A), Chl fluorescence, F, rises from the O level (the minimal level) to the P level (the peak); yet, the relationship between F and [Q(A) (-)] is not linear, due to the presence of other quenchers and modifiers. Several alternative theories have been proposed, which give different interpretations of the O-J-I-P transient. The main idea in these alternative theories is that in saturating light, Q(A) is almost completely reduced already at the end of the photochemical phase O-J, but the fluorescence yield is lower than its maximum value due to the presence of either a second quencher besides Q(A), or there is an another process quenching the fluorescence; in the second quencher hypothesis, this quencher is consumed (or the process of quenching the fluorescence is reversed) during the thermal phase J-I-P. In this review, we discuss these theories. Based on our critical examination, that includes pros and cons of each theory, as well mathematical modeling, we conclude that the mainstream interpretation of the O-J-I-P transient is the most credible one, as none of the alternative ideas provide adequate explanation or experimental proof for the almost complete reduction of Q(A) at the end of the O-J phase, and for the origin of the fluorescence rise during the thermal phase. However, we suggest that some of the factors influencing the fluorescence yield that have been proposed in these newer theories, as e.g., the membrane potential ΔΨ, as suggested by Vredenberg and his associates, can potentially contribute to modulate the O-J-I-P transient in parallel with the reduction of Q(A), through changes at the PSII antenna and/or at the reaction center, or, possibly, through the control of the oxidation-reduction of the PQ-pool, including proton transfer into the lumen, as suggested by Rubin and his associates. We present in this review our personal perspective mainly on our understanding of the thermal phase, the J-I-P rise during Chl a FI in plants and algae.     

Pigment–Protein Complexes and the Number of Reaction Centers of the Photosystems in Pea Chlorophyll Mutants

We studied fluorescent and absorption properties of the chloroplasts and pigment–protein complexes isolated by gel electrophoresis from the leaves of pea, the parent cultivar Torsdag and mutants chlorotica 2004 and 2014. Specific fluorescence peaks of chlorophyll forms in individual complexes have been determined from the absorption and fluorescence spectra of the chloroplast chlorophyll and their second derivatives at 23 and –196°C. The mutant chlorotica 2004 proved to have an increased intensity of a long-wave band of the light-harvesting complex I at both 23°C (745 nm) and –196°C (728 nm). At the same time, this mutant manifested a decreased accumulation of the chlorophyll forms making up the nearest-neighbor antenna of the PS I reaction center (at 690, 697, and 708 nm). No spectral differences have been revealed between chlorotica 2014 mutant and the parent cultivar. Gel electrophoresis revealed the synthesis of all chlorophyll–protein complexes in both mutants. At the same time, analysis of photochemical activity of PS I and PS II reaction centers and calculations of their number and the size of the light-harvesting antenna have shown that the number of reaction centers in the PS I of chlorotica 2004 mutant is reduced by a factor of 1.7 because its chlorophyll a–protein complex is disturbed by the mutation. The primary effect of chlorotica 2014 mutation remains unclear. The proportional changes in the content of photosystem complexes in this mutant suggest that they are secondary and result from a 50% decrease in chlorophyll content.     

Light-Induced Biogenesis of Chlorophyll–Protein Complexes in Developing Wheat Thylakoids

It is shown that the apoproteins of core complexes (CC) I and II, the - and -subunits of CF₁ ATP-synthase complexes, are present in seedlings grown under intermittent light (IML). The levels of light-harvesting complex (LHC) apoproteins in the 30- to 18-kD region increase rapidly upon exposure to continuous light (CL). The newly synthesized LHC apoproteins appear to be present predominantly in monomeric forms that later assemble into higher-order oligomeric forms. During the early stages of greening of wheat seedlings, polypeptides in the 20.5-19 and 17.5-15.5 kD regions (so-called early light-induced proteins (ELIP)) are observed, but they disappear fully after 6 h. As greening proceeds, the 727-nm band in low-temperature fluorescence spectra (77 K) gradually shifts to longer wavelength (740-742 nm), which clearly demonstrates the light-driven biogenesis of LHC I and its assembly with CC I.     

Chlorophyll Content as an Indicator of the Modern (2015–2016) Trophic State of Volga River Reservoirs

Data on the chlorophyll a (Chl a) content in water of the Volga River reservoirs sampled at 64–73 sampling stations in the summer period of 2015 and 2016, analyzed using the standard spectrophotometric method, are presented. Chl a content varied from a minimum of 0.9–3.0 to a maximum of 7.6–32.1 μg/L in June 2016, from 2.1–15.5 to 9.4–86 μg/L in August 2015, and from 1.1–6.7 to 15.1–62.7 μg/L in August 2016. A local increase in Chl a occurred in the near-dam areas of the reservoirs, as well as downstream of the confluence with tributaries. The highest Chl a concentrations were constantly recorded in the highly eutrophic Shoshinskii Reach in the Ivankovo Reservoir and at the Oka River mouth in the Cheboksary Reservoir. The average values of Chl a concentrations make it possible to classify the Ivankovo, Uglich, Rybinsk, and Cheboksary reservoirs as eutrophic; the Gorky and Kuibyshev reservoirs as moderately eutrophic; and the Saratov and Volgograd reservoirs as mesotrophic.     

Reconstitution of the Peridinin–chlorophyll a Protein (PCP): Evidence for Functional Flexibility in Chlorophyll Binding

The coding regions for the N-domain, and full length peridinin–chlorophyll a apoprotein (full length PCP), were expressed in Escherichia coli. The apoproteins formed inclusion bodies from which the peptides could be released by hot buffer. Both the above constructs were reconstituted by addition of a total pigment extract from native PCP. After purification by ion exchange chromatography, the absorbance, fluorescence excitation and CD spectra resembled those of the native PCP. Energy transfer from peridinin to Chl a was restored and a specific fluorescence activity calculated which was ~86% of that of native PCP. Size exclusion analysis and CD spectra showed that the N-domain PCP dimerized on reconstitution. Chl a could be replaced by Chl b, 3-acetyl Chl a, Chl d and Bchl using the N-domain apo protein. The specific fluorescence activity was the same for constructs with Chl a, 3-acetyl Chl a, and Chl d but significantly reduced for those made with Chl b. Reconstitutions with mixtures of chlorophylls were also made with eg Chl b and Chl d and energy transfer from the higher energy Qy band to the lower was demonstrated.     

Characterization of the Quenching of Chlorophyll a Fluorescence by β-Carotene Using the Non-Linear Analysis

Carotenoids (Car) regulate energy flow in photosynthesis by a specific Car-chlorophyll (Chl) interaction in the singlet-excited states, leading to a reduction in Chl fluorescence. We studied quenching of Chl a-fluorescence in benzene by trans--carotene. Non-linear analysis of the quenching process enables to explain the possible molecular mechanism leading to the de-excitation of Chl a. The fluorescence intensity was measured at 670 nm for excitation wavelengths of 380, 430, 640, and 650 nm. The -carotene concentrations ranged from 4×10^–5 M to 5×10^–3 M. When the samples were excited at 640 and 650 nm, the Stern-Volmer plots showed that the quenching process has high rate constants, hence -carotene is a very efficient quencher. Two different types of quenching process could take place.     

Chlorophyll–nutrient relationships of different lake types using a large European dataset

In Europe there is a renewed focus on relationships between chemical determinands and ecological impact as a result of the Water Framework Directive (WFD). In this paper we use regression analysis to examine the relationship of growing season mean chlorophyll a concentration with total phosphorus and total nitrogen using summary data from over 1,000 European lakes. For analysis, lakes were grouped into types with three categories of mean depth, alkalinity and humic content. The lakes were also divided into broad geographic regions covering Atlantic, Northern, Central/Baltic and for some types the Mediterranean areas of Europe. Chlorophyll a was found to be significantly related to both total phosphorus and total nitrogen, although total phosphorus was almost always found to be the best predictor of chlorophyll. Different nutrient chlorophyll relationships were found for lakes according to mean depth and alkalinity, although no significant effect of geographic region or humic content was found for the majority of lake types. We identified three groups of lakes with significantly different responses. Deep lakes had the lowest yield of chlorophyll per unit of nutrient, low and moderate alkalinity shallow lakes the highest and high alkalinity lakes were intermediate. We recommend that the regression models provided for these three lake groups should be used for lake management in Europe and discuss the limitations of such models.     

Glucose and δ-Aminolevulinic Acid Stimulate the Dark Chlorophyll Synthesis of Rice Seedlings

This research was to examine if rice (Oryza sativa L.), a monocotyledon of angiosperm, was able to synthesize chlorophyll (Chl) in complete darkness. Five-cm-tall etiolated seedlings of rice were used as starting materials and treated with or without various concentrations of glucose and/or δ-aminolevulinic acid (ALA) in the dark. Leaves harvested at the indicated time were determined for their contents of Chl, protoporphyrin Ⅸ(Proto), Mg-protoporphyrin Ⅸ(Mg-Proto) and protochlorophyllide (Pchlide). The mole percentage of porphyrin was calculated. The Chl content in the etiolated rice seedlings slightly increased from about 2.5 μg/g to 7.5 μg/g within 12 d in the dark, but the total Chl of dark-grown rice increased from 0.36 μg/g to 3.6 μg/g. While the mole percentages of Proto, Mg-Proto and Pchlide in the dark-grown seedlings without any treatment were about 65%, 27.5% and 7.5% at the beginning, respectively, those in the light-grown seedlings were about 42.5%, 35% and 22.5%, respectively. The mole percentage of porphyrin of etiolated seedlings resumed its normal ratio within 2 d after treatment with glucose. While the Chl content of etiolated seedlings grown in culture solution with 3% and 6% glucose increased 2.5 and 4.0 folds, respectively, those with 3% and 6% glucose and 1 mmol/L ALA increased 22 and 24 folds, respectively. It is concluded that angiosperm might be able to synthesize a small amount of Chl in complete darkness, that either glucose or ALA could stimulate dark Chl synthesis in angiosperm, and that a combination of glucose and ALA exhibited an additional effect. It is still unknown and remains to be further explored what is the mechanism of the effect of glucose and ALA on the Chl synthesis of rice in the dark. Key words: angiosperm; rice; dark chlorophyll synthesis; glucose; δ-aminolevulinic acid; protoporphyrin Ⅸ; Mg-protoporphyrin Ⅸ; protochlorophyllide     

Characterization of Chlorophyll–protein Complexes Isolated from Two Marine Green Algae, Bryopsis maxima and Ulva pertusa, Growing in the Intertidal Zone

Three Chl–protein complexes were isolated from thylakoid membranes of Bryopsis maxima and Ulva pertusa, marine green algae that inhabit the intertidal zone of the Pacific Ocean off the eastern coast of Japan by dodecyl-β-d-maltoside polyacrylamide gel electrophoresis. The slowest-moving fractions showed low Chl a/b and Chl/P-700 ratios, indicating that this fraction corresponds to complexes in PS I, which is large in both algae. The intermediate and fastest-moving fractions showed the traits of PS II complexes, with some associated Chl a/b–protein complexes and LHC II, respectively. The spectral properties of the separated Chl–proteins were also determined. The absorption spectra showed a shallow shoulder at 540 nm derived from siphonaxanthin in Bryopsis maxima, but not in Ulva pertusa. The 77 K emission spectra showed a single peak in Bryopsis maxima and two peaks in Ulva pertusa. Besides the excitation spectra indicated that the excitation energy transfer to the PS I complexes differed quite a lot higher plants. This suggested that the mechanisms of energy transfer in both of these algae differ from those of higher plants. Considering the light environment of this coastal area, the large size of the antennae of PS I complexes implies that the antennae are arranged so as to balance light absorption between the two photosystems. In addition, we discuss the relationships among the photosystem stoichiometry, the energy transfer, and the distribution between the two photosystems.     

Involvement of Pheophytinase in Ethylene-Mediated Chlorophyll Degradation in the Peel of Harvested ‘Yali’ Pear

It was recently proposed that pheophytinase (PPH) is a key protein that mediates chlorophyll (Chl) breakdown in leaves. To study the role and regulation of PPH on Chl breakdown of peel in harvested ‘Yali’ pear (Pyrus bretschneideri Rehd. cv. ‘Yali’) fruit, the partial sequence of PbPPH was obtained from the NCBI database, and the alignment results revealed that the amino acid sequence of PbPPH shared great similarity to PPHs of Chinese flowering cabbage (Brassica rapa var. parachinensis) and Arabidopsis (Arabidopsis thaliana), indicating that these proteins have similar functions. Ethephon treatment significantly increased ethylene production of pear fruit and accelerated the proceeding of Chl breakdown. Conversely, 1-methylcyclopropene (1-MCP) treatment decreased ethylene production and delayed Chl breakdown. PbPPH expression was closely related to the process of Chl breakdown and was correlated with the expression of Chl degradation-associated genes such as pheide a oxygenase and stay-green 1. The chlorophyllase 1 expression level was well maintained by 1-MCP treatment, whereas red Chl catabolite reductase expression was inhibited by 1-MCP. Further analysis indicated that the gene expression levels of four ethylene receptors were stimulated by ethephon and suppressed by 1-MCP treatment and that these changes were strongly correlated with Chl breakdown and similar to the expression pattern of PbPPH. These results suggest that PPH is one of the enzymes responsible for the ethylene-mediated Chl degradation pathway of peel in harvested ‘Yali’ pear.     

Are Bryophytes Shade Plants? Photosynthetic Light Responses and Proportions of Chlorophyll a,Chlorophyll b and Total Carotenoids

BACKGROUND AND AIMS: Data are presented from 39 species of mosses and 16 liverworts for ratios of chlorophylls and total carotenoids, and light saturation of photosynthetic electron flow or photosynthetic CO2 uptake, in relation to the postulate that bryophyte cells in general show shade-plant characteristics. METHODS: Pigment concentrations were measured by spectrophotometer in 80 % acetone extracts. Light-saturation curves were constructed by (modulated) chlorophyll florescence and for some species by infra-red gas analysis. KEY RESULTS: The pigment measurements were widely variable but broadly in line with the findings of previous authors. Median values (mosses/liverworts) were: total chlorophyll, 1.64/3.76 mg g(-1); chlorophyll a : b, 2.29/1.99; chlorophylls : carotenoids, 4.74/6.75). The PPFD values at 95 % saturation (estimated from fitted curves) also ranged widely, but were almost all <1000 micromol m(-2) s(-1); the median for mosses was 583 and for liverworts 214 micromol m(-2) s(-1). The two highest PPFD95% values were from Polytrichum species with lamella systems forming a ventilated photosynthetic tissue. Total chlorophyll, chlorophyll a : b and chlorophylls : carotenoids all correlated significantly with PPFD95%. CONCLUSIONS: Bryophytes include but are not inherently shade plants. Light-saturation levels for species of open sun-exposed habitats are lower than for vascular sun plants and are probably limited by CO2 diffusion into unistratose leaves; this limit can only be exceeded by bryophytes with ventilated photosynthetic tissues which provide increased area for CO2 uptake.     

Characterization of the Main Light-Harvesting Chlorophyll a／b-Protein Complex of Green Alga, Bryopsis corticulans

The main light-harvesting chlorophyll a/b -protein complex (LHC Ⅱ) has been isolated directly from thylakoid membranes of shiphonous 8Teen alga, Bryopsis corticulans Setch. by using two consecutive runs of anion exchange and gel-filtration chromatography. Monomeric and trimeric subcomplexes of LHC Ⅱ were obtained by using sucrose gradient ultracentrifugation. Pigment analysis by reversed-phase high performance liquid chromatography showed that chlorophyll a (Chl a), chlorophyll b (Chl b), neoxanthin, violaxanthin and siphonaxanthin were involved in LHC Ⅱ from B. corticulans. The properties of electronictransition of monomeric LHC Ⅱ showed similarities to those of trimeric LHC Ⅱ. Circular dichroism spectroscopy showed that strong intramolecular interaction of excitonic dipoles between Chl a and between Chl b exist in one LHC Ⅱ apoprotein, while the intermolecular interaction of these dipoles can be intensified in the trimeric structure. The monomer has high efficient energy transfer from Chl b and siphonaxanthin to Chl a similarly to that of the trimer. Our results suggest that in B. corticulans, LHC Ⅱ monomer has high ordered pigment organization that play effective physiological function as the trimer, and thus it might be also a functional organization existing in thylakoid membrane of B.corticulans.     

A Circular Dichroism Spectroscopic Study Revealing the Cause of the Changes of Chlorophyll a Fluorescence Induction of Photosystem Ⅱ During Heat Treatment

Chlorophyll a fluorescence and circular dichroism (CD) spectra of photosystem Ⅱ (PSⅡ) membrane were measured after heat treatment. The chlorophyll fluorescence parameter Fo' remained stable after treatment at the temperatures from 30 ℃ to 40 ℃ and then reached a maximum after treatment at 55 ℃. In PSⅡ membranes and LHCⅡ (light-harvesting chlorophyll a/b binding complex)-enriched complexes, anomalous CD signals with extremely large amplitudes occurred during the heat treatment. The temperature corresponding to the maximum anomalous CD intensity peaking at 677 nm was 40 ℃. The results indicate that the aggregation state of the LHCⅡ in PSⅡ is related to the anomalous CD signal, and can be an important factor influencing Fo' in the heat treatment of PSⅡ membrane.     

High-Temperature Induced Chlorophyll Fluorescence Rise in Plants at 40–50 °C: Experimental and Theoretical Approach

We studied the temperature dependence of chlorophyll fluorescence intensity in barley leaves under weak and actinic light excitation during linear heating from room temperature to 50 degrees C. The heat-induced fluorescence rise usually appearing at around 40-50 degrees C under weak light excitation was also found in leaves treated with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) or hydroxylamine (NH(2)OH). However, simultaneous treatment with both these compounds caused a disappearance of the fluorescence rise. We have suggested that the mechanism of the heat-induced fluorescence rise in DCMU-treated leaves is different than that in untreated or NH(2)OH-treated leaves. In DCMU-treated leaves, the heat-induced fluorescence rise reflects an accumulation of Q(A) (-) even under weak light excitation due to the thermal inhibition of the S(2)Q(A) (-) recombination as was further documented by a decrease in the intensity of the thermoluminescence Q band. Mathematical model simulating this experimental data also supports our interpretation. In the case of DCMU-untreated leaves, our model simulations suggest that the heat-induced fluorescence rise is caused by both the light-induced reduction of Q(A) and enhanced back electron transfer from Q(B) to Q(A). The simulations also revealed the importance of other processes occurring during the heat-induced fluorescence rise, which are discussed with respect to experimental data.     

Phosphoregulation of the Na–K–2Cl and K–Cl cotransporters by the WNK kinases

Precise regulation of the intracellular concentration of chloride [Cl?]i is necessary for proper cell volume regulation, transepithelial transport, and GABA neurotransmission. The Na–K–2Cl (NKCCs) and K–Cl (KCCs) cotransporters, related SLC12A transporters mediating cellular chloride influx and efflux, respectively, are key determinants of [Cl?]i in numerous cell types, including red blood cells, epithelial cells, and neurons. A common “chloride/volume-sensitive kinase”, or related system of kinases, has long been hypothesized to mediate the reciprocal but coordinated phosphoregulation of the NKCCs and the KCCs, but the identity of these kinase(s) has remained unknown. Recent evidence suggests that the WNK (with no lysine = K) serine–threonine kinases directly or indirectly via the downstream Ste20-type kinases SPAK/OSR1, are critical components of this signaling pathway. Hypertonic stress (cell shrinkage), and possibly decreased [Cl?]i, triggers the phosphorylation and activation of specific WNKs, promoting NKCC activation and KCC inhibition via net transporter phosphorylation. Silencing WNK kinase activity can promote NKCC inhibition and KCC activation via net transporter dephosphorylation, revealing a dynamic ability of the WNKs to modulate [Cl?]. This pathway is essential for the defense of cell volume during osmotic perturbation, coordination of epithelial transport, and gating of sensory information in the peripheral system. Commiserate with their importance in serving these critical roles in humans, mutations in WNKs underlie two different Mendelian diseases, pseudohypoaldosteronism type II (an inherited form of salt-sensitive hypertension), and hereditary sensory and autonomic neuropathy type 2. WNKs also regulate ion transport in lower multicellular organisms, including Caenorhabditis elegans, suggesting that their functions are evolutionarily-conserved. An increased understanding of how the WNKs regulate the Na–K–2Cl and K–Cl cotransporters may provide novel opportunities for the selective modulation of these transporters, with ramifications for common human diseases like hypertension, sickle cell disease, neuropathic pain, and epilepsy.     

Changes of Photosystem Ⅱ Electron Transport in the Chlorophyll-deficient Oilseed Rape Mutant Studied by Chlorophyll Fluorescence and Thermoluminescence

The photosystem Ⅱ (PSII) complex of photosynthetic membranes comprises a number of chlorophyll-binding proteins that are important to the electron flow. Here we report that the chlorophyll b-deficient mutant has de creased the amount of light-harvesting complexes with an increased amount of some core polypeptides of PSII,including CP43 and CP47. By means of chlorophyll fluorescence and thermoluminescence, we found that the ratio of Fv/Fm, qP and electron transport rate in the chlorophyll b-deficient mutant was higher compared to the wild type.In the chlorophyll b-deficient mutant, the decay of the primary electron acceptor quinones (QA-) reoxidation was decreased, measured by the fluorescence. Furthermore, the thermolumlnescence studies in the chlorophyll b deficient mutant showed that the B band (S2/S3QB-) decreased slightly and shifted up towards higher temperatures.In the presence of dichlorophenyl-dimethylurea, which is inhibited in the electron flow to the second electron acceptor quinines (QB) at the PSII acceptor side, the maximum of the Q band (S2QA-) was decreased slightly and shifted down to lower temperatures, compared to the wild type. Thus, the electron flow within PSll of the chlorophyll b-deficient mutant was down-regulated and characterized by faster oxidation of the primary electron acceptor quinine QA- via forward electron flow and slower reduction of the oxidation S states.     

Putting the insect into the birch–insect interaction

Leaf maturation in mountain birch (Betula pubescens ssp. czerepanovii) is characterized by rapid shifts in the types of dominant phenolics: from carbon-economic flavonoids aglycons in flushing leaves, via hydrolysable tannins and flavonoid glycosides, to carbon-rich proanthocyanidins (condensed tannins) in mature foliage. This shift accords with the suggested trade-offs between carbon allocation to plant defense and growth, but may also relate to the simultaneous decline in nutritive leaf traits, such as water, proteins and sugars, which potentially limit insect growth. To elucidate how birch leaf quality translates into insect growth, I introduce a simple model that takes into account defensive compounds but also acknowledges insect demand for nutritive compounds. The effects of defensive compounds on insect growth depend strongly on background variation in nutritive leaf traits: compensatory feeding on low nutritive diets increases the intake of defensive compounds, and the availability of growth-limiting nutritive compounds may modify the effects of defenses. The ratio of consumption to larval growth (both in dry mass) increases very rapidly with leaf maturation: from 2.9 to 9.8 over 2 weeks in June-July, and to 15 by August. High concentrations in mature birch leaves of "quantitative" defenses, such as proanthocyanidins (15-20% of dry mass), presumably prevent further consumption. If the same compounds had also protected half-grown leaves (which supported the same larval growth with only one third of the dry matter consumption of older leaves), the same intake of proanthocyanidins would have demanded improbably high concentrations (close to 50%) in young leaves. The model thus suggests an adaptive explanation for the high levels of "quantitative" defenses, such as proanthocyanidins, in low-nutritive but not in high-nutritive leaves because of the behavioral responses of insect feeding to leaf nutritive levels.     

Three-dimensional Microorganization of the Soil–Root–Microbe System

Soils contain the greatest reservoir of biodiversity on Earth, and the functionality of the soil ecosystem sustains the rest of the terrestrial biosphere. This functionality results from complex interactions between biological and physical processes that are strongly modulated by the soil physical structure. Using a novel combination of biochemical and biophysical indicators and synchrotron microtomography, we have discovered that soil microbes and plant roots microengineer their habitats by changing the porosity and clustering properties (i.e., spatial correlation) of the soil pores. Our results indicate that biota act to significantly alter their habitat toward a more porous, ordered, and aggregated structure that has important consequences for functional properties, including transport processes. These observations support the hypothesis that the soil–plant–microbe complex is self-organized.