Inorganic tripton in the Finger Lakes of New York: importance to optical characteristics

Inorganic particles in the upper waters of the 11 Finger Lakes of New York are morphometrically and elementally characterized by individual particle analysis conducted with scanning electron microscopy interfaced with automated image and X-ray analyses (IPA/SAX). Coupled measurements of Secchi disk transparency (SD), the attenuation coefficient for downwelling irradiance (K_d), the beam attenuation coefficient at 660 nm, and turbidity (T_n) were made to support evaluation of the importance of non-living, inorganic particles (inorganic tripton) in regulating these optical features of water quality. Wide differences in levels of inorganic tripton, represented in terms of particle projected area per unit volume (PAV_in), and the optical measures are reported for these lakes. However, generally similar size distributions are observed for the inorganic tripton for the lakes. Terrigenous suspensoids, in the form of clay minerals, dominated the inorganic tripton particle assemblage of nine lakes, while CaCO₃, formed autochthonously, dominated in the other two and was a noteworthy contributor in four others. PAV_in is demonstrated to be an important regulator of the optical properties of these lakes, performing substantially better than chlorophyll in predicting SD, and T_n, and interlake differences in these optical measures.Contribution No. 226 of the Upstate Freshwater Institute     

Patterns and impacts of inorganic tripton in Cayuga Lake

Inorganic tripton in the upper waters of Cayuga Lake, NY, is characterized, quantified and contrasted for shallow (<6 m) and deep (>100 m) water regions based on analyses with scanning electron microscopy equipped with automated image analysis and X-ray energy spectroscopy (SAX) of samples collected in 1999 and 2000. The impacts of this material on water clarity and the phosphorus (P) pool are evaluated based on paired measurements of turbidity (T _n), Secchi disc transparency (SD), chlorophyll, and four fractions of particulate P (PP) determined according to selective extraction protocols. The major components of inorganic tripton in the lake are reported to be clay minerals and quartz, received from the watershed, and calcium carbonate (CaCO₃), that is produced internally. Clay minerals are the dominant component in a shallow region that receives tributary inflows. CaCO₃, precipitated during a mid-summer whiting event, is reported to be the dominant component of inorganic tripton for the deep water region for a low runoff year, while clays can become dominant in these areas after prolonged intervals of high runoff. It is demonstrated that inorganic tripton, rather than phytoplankton, is the primary regulator of T _n and SD, it represents most of the PP, and is primarily responsible for the higher T _n, lower SD, and higher TP in the shallow area compared to the deep water region. Efforts to improve clarity in this shallow region of the lake should consider the feasibility of controlling the input of terrigenous suspensoids rather than reductions in P loading.     

Effects of recent increases in salinity and nutrient concentrations on the microbialite community of Lake Clifton (Western Australia): are the thrombolites at risk?

There has been a strong research focus on optical properties in temperate estuaries but very much less in tropical estuaries. These properties comprise light and beam attenuation dominated by suspended particulate matter, Chl a, and CDOM. Spatially and temporally distributed data on optical properties in a tropical wet and dry estuary are compared and discussed in relation to those of temperate estuaries. Sampling in the Nha Phu estuary, Vietnam, consisted of five stations on a transect from head to mouth that was sampled four times during dry conditions and three times during wet conditions between May 2006 and April 2008. Methods comprised CTD, optical measurements, and water sampling for suspended matter, Chl a, and CDOM. Results showed high light attenuation—K _d(PAR)—in wet conditions and low in dry. K _d(PAR) was highest at the estuary head and lower in the outer part. Spatial and temporal variations in K _d(PAR) were in general dominated by variations in suspended particulate matter concentrations in both wet and dry conditions. Chl a concentrations were low and showed no strong variations between wet and dry conditions. CDOM absorption coefficients were higher in wet conditions with high values at the head and lower in the central part of the estuary. The depth of the photic zone was reduced by up to 50% during wet conditions. A residence time in the estuary of 5–6 days was derived from the rate of change of K _d(PAR) after a period of heavy rain and discharge of freshwater into the estuary. This complied with a residence time of four and a half days derived from a basic physical relation. Optical properties were in general comparable to temperate estuaries in dry conditions although Chl a concentrations were lower in Nha Phu. A second distinctive point, as compared to temperate estuaries, was the episodic character with days of strong rainfall followed by longer periods of dry weather. All sampling, both wet and dry, was carried out in the dry season which implies a less definitive perception of wet and dry seasons.     

Light-limited algal growth in Lake Loosdrecht: steady state studies in laboratory scale enclosures

Phytoplankton growth in the shallow, turbid Lake Loosdrecht (The Netherlands) is importantly influenced by light availability, and thus the concentrations of the various light-attenuating materials. The system is highly eutrophic and supports an algal biomass of ca. 160 mg Chl m^–3. A model is proposed here which predicts algal growth in the lake as a function of the light received and subsequent attenuation in the water column by phytoplankton, tripton and background colour. The model is based on an energy balance which relates growth rate to the true growth yield on light energy and the energy demand for cell maintenance. The coefficients for energy conversion (Y = 0.002 gDW kJ^–1) and cell maintenance (µ_e = 0.031 day^–1) were determined from steady state growth kinetics of Prochlorothrix hollandica in light-limited laboratory flow systems with the same depth as the lake and receiving summer average conditions of irradiance. Light attenuation by phytoplankton and tripton were quantified using specific attenuation coefficients: 0.011 m² mg^–1 Chl for the phytoplankton and 0.23 m² g^–1 DW for tripton.The growth studies demonstrated that Lake Loosdrecht can support a much higher algal biomass in the absence of non-algal particulate matter. The proposed model is used to predict chlorophyll a concentrations in dependence on growth rate and levels of tripton. Since approximately 75% of the sestonic dry weight in Lake Loosdrecht may be attributed to tripton, it is concluded that the algal biomass is markedly lowered by the abundance of tripton in the water column. A knowledge of the sources and fate of tripton in the lake is thus of fundamental importance in modelling phytoplankton dynamics.     

Spatial variations in xylem sap flux density in evergreen oak trees with radial-porous wood: comparisons with anatomical observations

To estimate whole-tree water use when employing sap flow measurements, integration of the sap flux density (F _d) over the sapwood area is needed. Accordingly, it is necessary to obtain information on the characteristics of stem water transportation such as spatial variations in F _d and the active xylem area in the stem cross-section. Although evergreen oak trees with radial-porous wood represent a major component of secondary forests in western Japan, detailed information on their stem water transportation characteristics remains unclear. In the present study, we used the heat dissipation method (Granier method) to conduct measurements of azimuthal and radial variations in the F _d of Quercus glauca Thunb. ex Murray, a representative evergreen broad-leaved tree in western Japan. Further, by analyzing the anatomy of the xylem structure, we examined why F _d varies spatially in the stem cross-section. By using a dye solution injected into a radial hole bored into the tree trunk, we confirmed that the entire stem is hydroactive. We also compared the spatial variations in F _d and water conductivity per xylem area (K _s) which were estimated by using the observed vessel diameters and their density over the stem cross-section and Hagen–Poiseuille’s law. Azimuthal and radial variations in F _d reached about 60 and 50% of the maximum values, respectively, and could be explained by spatial variation in K _s. As a result, we obtained statistical parameters describing the spatial variation in F _d in Q. glauca and determined that whole-tree water use estimated from measurements in one direction had at most ±20% potential errors for studied trees.     

Resolution of spatial patterns in three reservoirs with rapid profiling instrumentation

Rapid profiling instrumentation is used to resolve spatial patterns of temperature (T), specific conductance (SC), chlorophyll (Chl) and beam attenuation (c ₆₆₀), in three dimensional space for three water supply reservoirs (Pepacton, Rondout and Ashokan) located in New York State, U.S.A. Conspicuous patterns depict the operation of important processes that include the development of deep chlorophyll maxima, the entry of major tributary and tunnel inflows as interflows, and the development of benthic nepheloid layers. SC is demonstrated to be a valuable tracer of the interflow process in these reservoirs. Distinct longitudinal structures are documented for T in spring, SC, Chl and c ₆₆₀, along the major axes of Pepacton, Rondout, and one of the two (separated) basins of Ashokan. Substantial differences are demonstrated to prevail between the two basins of Ashokan; treatment as two basins in series is recommended for modeling purposes. Three-dimensional structures in SC and c ₆₆₀, apparently imparted from interflows, are documented for one of the basins of this impoundment. The needs of mechanistic model frameworks to accommodate the observed spatial patterns and processes are considered.     

The hydrochemical state of Lake Peipsi-Pihkva

The distribution and time dependence of total phosphorus (TP), dissolved inorganic phosphate (PO₄P), total nitrogen (TN), chlorophyll a (Chl), dichromate oxidizability (CODCr), permanganate oxidizability (CODMn), water colour (Col) and transparency (SD), pH, dissolved oxygen (O₂) and oxygen saturation (O₂%) in the surface water of Lake Peipsi-Pihkva are studied by using 65-parameter regression models with the help of the SAS system. The yearly means, polarity and seasonal dependence of each investigated parameter during 1985–1994 are estimated from fitted models. The bulk of data consists of 456 to 1149 measurements per parameter. L. Peipsi-Pihkva appears to be polar with respect to the majority of the studied parameters. The content of TP, PO₄P, TN, Chl, CODCr, CODMn, and Col decrease from south to north, while SD has an opposite trend. pH, O₂, and O₂% are quite uniform all over the lake. L. Peipsi is eutrophic, L. Pihkva is hypertrophic. The lake is influenced by significant yearly and seasonal changes. It is concluded that the Velikaja River is the main source of pollution for L. Peipsi-Pihkva.     

On the practical identifiability of microbial growth models incorporating Michaelis-Menten type nonlinearities

The reason for difficulties in obtaining unique estimates of the parameters μ_m and K_s of the Michaelis-Menten equation are analysed for a microbial batch growth process. With the aid of simulation studies in which the influences of different types of noise on the parameter estimates are compared, it is shown that, although theoretically identifiable in the deterministic case with ideal measurements, the parameters cannot in general be correctly determined from noisy measurements. The difficulties are further illuminated by estimation examples using real data. It certain situations, in which the value of the ratio K_s/s_o is high or in which only few and noisy measurements are available, the linear approximation of the Michaelis-Menten equation gives a better fit. The practical difficulties in obtaining correct values of the model parameters do not limit the applicability of the Michaelis-Menten model, which in most cases explains the bacterial growth behavior excellently. Rather, they underline the fact that care must be taken when utilizing parameter estimates for biological interpretations.     

Optical properties and optical constituents of the Faroe Islands shelf and off-shelf waters,North East Atlantic

The study comprises a data set of CTD, optical properties—K ₀(PAR), c _p, a(PAR), b(PAR)—and optical constituents—Chl a, SPM, CDOM—from 72 shelf and off-shelf stations in the Faroe Islands (62°N, 7°W) North East Atlantic, in early spring 2005. Results showed that shelf waters surrounding the islands were cold and low saline, whereas off-shelf waters were warmer (~1°C) and more saline (~0.05) PSU. A pronounced oceanographic front separated the two waters, and diffuse light attenuation K ₀(PAR), beam attenuation c _p, Chl a, absorption a(PAR), and scattering coefficient b(PAR) were all significantly higher on the shelf. Analyses showed that off-shelf light attenuation K ₀(PAR) was governed by Chl a, shown by a high (r ² = 0.64) Chl a–K ₀(PAR) correlation, whereas light attenuation on the shelf was governed by both Chl a, SPM, and CDOM in combination. A Chl a specific diffuse attenuation coefficient K₀^* ( \textPAR ) K_{0}^{*} \left( {\text{PAR}} \right) of 0.056 (m² mg⁻¹ Chl a) and a Chl a specific beam attenuation ( c_\textp^* c_{\text{p}}^{*} ) of 0.27 (m² mg⁻¹ Chl a) coefficients were derived for the off-shelf. It is pointed out that Chl a is the single variable that changes over time as no rivers with high SPM and CDOM enter the shelf area. Data were obtained in early spring, and Chl a concentrations were low ~0.5 mg Chl a m⁻³. Spring bloom Chl a are about 10 mg Chl a m⁻³ and estimations showed that shelf K ₀(PAR) will increase about 5 times and beam attenuation about 10 times. The Faroe Islands shelf–off-shelf waters is a clear example where physical conditions maintain some clear differences in optical properties and optical constituents. The complete data set is enclosed.     

Optically active components and light attenuation in an offshore station of Harima Sound, eastern Seto Inland Sea, Japan

In the eastern Seto Inland Sea, Japan, phytoplankton abundance in the surface water has gradually declined, whereas Secchi depth has risen in recent years, particularly in offshore areas. Therefore, it may be hypothesized that phytoplankton dominate light attenuation in the offshore area, and that other constituents are less important. To test this hypothesis, we examined the roles of seawater, colored dissolved organic matter (CDOM), non-algal particles (tripton), and phytoplankton in the light attenuation at an offshore station of Harima Sound in the eastern Sea. The magnitude of light attenuation was then determined from the attenuation coefficient of photosynthetically available radiation (PAR) through the water column (K _d). During a 13-month period, K _d ranged from 0.179 to 0.507 m^?1, with a mean of 0.262 m^?1. The mean relative contributions of seawater (15%) and CDOM (13%) to K _d were small, while the most dominant K _d constituent was tripton (45%). The mean contribution of phytoplankton to K _d (27%) was consequently less than that of tripton. However, 75% of the temporal variability in K _d was attributed to phytoplankton, measured as chlorophyll a. Our results emphasize that the main component of light attenuation does not always govern the temporal variation of light attenuation in coastal regions.     

Effects of water stress and high temperature on gas exchange and chlorophyll fluorescence in Triticum aestivum L.

Wheat plants grown in controlled growth chambers were exposed to drought stress (DS) and high temperature (HT) singly and in combination (DS+HT). The effects of these two stresses on net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (C _i), quantum efficiency of photosystem 2 (Φ_PS2), variable to maximum chlorophyll (Chl) fluorescence (F_v/F_m), photochemical (q_p) and non-photochemical (NPQ) Chl fluorescence, and yield were investigated. Grain yield was decreased by 21 % due to DS, while it was increased by 26 % due to HT. P _N, g _s, C _i, and Chl fluorescence were dramatically reduced to DS, HT, and their interaction, except NPQ which showed an increase due to HT.     

Diurnal and seasonal variations in stomatal conductance of rice at elevated atmospheric CO2 under fully open‐air conditions

Understanding of leaf stomatal responses to the atmospheric CO₂ concentration, [CO₂], is essential for accurate prediction of plant water use under future climates. However, limited information is available for the diurnal and seasonal changes in stomatal conductance (g_s) under elevated [CO₂]. We examined the factors responsible for variations in g_s under elevated [CO₂] with three rice cultivars grown in an open‐field environment under flooded conditions during two growing seasons (a total of 2140 individual measurements). Conductance of all cultivars was generally higher in the morning and around noon than in the afternoon, and elevated [CO₂] decreased g_s by up to 64% over the 2 years (significantly on 26 out of 38 measurement days), with a mean g_s decrease of 23%. We plotted the g_s variations against three parameters from the Ball‐Berry model and two revised versions of the model, and all parameters explained the g_s variations well at each [CO₂] in the morning and around noon (R² > 0.68), but could not explain these variations in the afternoon (R² < 0.33). The present results provide an important basis for modelling future water use in rice production.     

Chlorophyll fluorescence imaging of photosynthetic activity with the flash-lamp fluorescence imaging system

A flash-lamp chlorophyll (Chl) fluorescence imaging system (FL-FIS) is described that allows to screen and image the photosynthetic activity of several thousand leaf points (pixels) of intact leaves in a non-destructive way within a few seconds. This includes also the registration of several thousand leaf point images of the four natural fluorescence bands of plants in the blue (440 nm) and green (520 nm) regions as well as the red (near 690 nm) and far-red (near 740 nm) Chl fluorescence. The latest components of this Karlsruhe FL-FIS are presented as well as its advantage as compared to the classical single leaf point measurements where only the fluorescence information of one leaf point is sensed per each measurement. Moreover, using the conventional He-Ne-laser induced two-wavelengths Chl fluorometer LITWaF, we demonstrated that the photosynthetic activity of leaves can be determined measuring the Chl fluorescence decrease ratio, R_Fd (defined as Chl fluorescence decrease F_d from maximum to steady state fluorescence F_s:F_d/F_s), that is determined by the Chl fluorescence induction kinetics (Kautsky effect). The height of the values of the Chl fluorescence decrease ratio R_Fd is linearly correlated to the net photosynthetic CO₂ fixation rate P _N as is indicated here for sun and shade leaves of various trees that considerably differ in their P _N. Imaging the R_Fd-ratio of intact leaves permitted the detection of considerable gradients in photosynthetic capacity across the leaf area as well as the spatial heterogeneity and patchiness of photosynthetic quantum conversion within the control leaf and the stressed plants. The higher photosynthetic capacity of sun versus shade leaves was screened by Chl fluorescence imaging. Profile analysis of fluoresence signals (along a line across the leaf area) and histograms (the signal frequency distribution of the fluorescence information of all measured leaf pixels) of Chl fluorescence yield and Chl fluorescence ratios allow, with a high statistical significance, the quantification of the differences in photosynthetic activity between various areas of the leaf as well as between control leaves and water stressed leaves. The progressive uptake and transfer of the herbicide diuron via the petiole into the leaf of an intact plant and the concomitant loss of photosynthetic quantum conversion was followed with high precision by imaging the increase of the red Chl fluorescence F₆₉₀. Differences in the availability and absorption of soil nitrogen of crop plants can be documented via this flash-lamp fluorescence imaging technique by imaging the blue/red ratio image F₄₄₀/F₆₉₀, whereas differences in Chl content are detected by collecting images of the fluorescence ratio red/far-red, F₆₉₀/F₇₄₀.     

The role of carbon dioxide in the initiation and maintenance of blue-green dominance in lakes

1. In 1993 the south basin of Squaw Lake, Wisconsin, U.S.A., was artificially circulated and injected with CO₂ in an attempt to eliminate the massive quantities of blue-greens normally present during summer. The unmixed, uninjected north basin was the control. Despite a great difference in CO₂ concentration and in pH between the two basins, their blue-green maxima began simultaneously and eventually reached the same size. The predominant algae in both basins were Aphanizomenon flos-aquae and Anabaena flos-aquae. 2. From 13 May to 9 September, experiments were done to determine the responses of photosynthesis (oxygen production) of the phytoplankton communities in both basins to changes in pH over the range 7–9. Both the north and south basins underwent two distinct shifts in the slopes of their photosynthetic responses to pH. These shifts paralleled changes in the proportion of blue-greens in their phytoplankton. 3. Experiments with additions of KHCO₃ or NaHCO₃ showed that the responses to pH were really responses to CO₂ concentration. Therefore, the data obtained in the photosynthesis experiments were used to calculate the kinetic parameters K_s* and V_max*. The K_s* values varied in relation to the proportion of blue-greens, with lower values during the period when blue-greens predominated. The V_max*values showed no such systematic changes. 4. CO₂ compensation and zero photosynthesis concentrations confirm that when the blue-greens dominated, CO₂ was taken up much more efficiently than when the phytoplankton comprised mostly non-blue-greens. No evidence appeared suggesting direct use of bicarbonate even at pH values of 10.0. 5. Experiments with water collected seasonally from four other lakes show that Squaw Lake is not unique. In each case the response slopes (and therefore K_s* values) were high during spring when non-blue-greens were dominant, but decreased when the blue-green maximum began. 6. Experiments with individual species of algae confirm that blue-greens generally have better CO₂ kinetics than do greens. 7. It is concluded that initiation of the blue-green maximum does not depend upon conditions of low CO₂ concentration or high pH. However, once the blue-greens become abundant they ensure their dominance by reducing concentrations of CO₂ to levels available only to themselves.     

Leaf rolling and photosystem II efficiency in <Emphasis Type="Italic">Ctenanthe setosa</Emphasis> exposed to drought stress

Photochemical efficiency of PSII of Ctenanthe setosa was investigated to understand the photosynthetic adaptation mechanism under drought stress causing leaf rolling. Stomatal conductance (g _s), the levels of photosynthetic pigments and chlorophyll (Chl) fluorescence parameters were determined in leaves that had four different visual leaf rolling scores from 1 to 4, opened after re-watering and mechanically opened at score 4. g _s value gradually decreased in adaxial and abaxial surfaces in relation to scores of leaf rolling. Pigment contents decreased until score 3 but approached score 1 level at score 4. No significant variations in effective quantum yield of PSII (Φ_PSII), and photochemical quenching (q_p) were found until score 3, while they significantly decreased at score 4. Non-photochemical quenching (NPQ) increased at score 2 but then decreased. After re-watering, the Chl fluorescence and other physiological parameters reached to approximately score 1 value, again. As for mechanically opened leaves, g _s decreased during drought period. The decrease in adaxial surface was higher than that of the rolled leaves. NPQ was higher than that of the rolled leaves. Φ_PSII and q_p significantly declined and the decreases were more than those of the rolled leaves. In conclusion, the results indicate that leaf rolling protects PSII functionality from damage induced by drought stress.     

Gas Exchange Characteristics and Water Relations in Some Elite Okra Cultivars Under Water Deficit

Thirty-days-old plants of two cultivars of okra (Hibiscus esculentus L.), Sabzpari and Chinese-red, were subjected for 30 d to two water regimes (100 and 60 % field capacity). Leaf water potential and osmotic potential of both lines decreased significantly with the imposition of drought. Both the leaf pressure potential and osmotic adjustment were much lower in Chinese-red than those in Sabzpari. Chlorophyll (Chl) b content increased, whereas Chl a content remained unchanged and thus Chl a/b ratios were reduced in both lines. Drought stress also caused a significant reduction in net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), and water use efficiency (WUE) especially in cv. Sabzpari. The lines did not differ in intrinsic WUE (P _Ng_s) or intercellular/ambient CO₂ ratio. Overall, the growth of two okra cultivars was positively correlated with P _N, but not with g _s or P _N/E, and negatively correlated with osmotic adjustment.     

Photosynthesis and apparent affinity for dissolved inorganic carbon by cells and chloroplasts of Chlamydomonas reinhardtii grown at high and low CO2 concentrations

Chloroplasts with high rates of photosynthetic O₂ evolution (up to 120 mol O₂· (mg Chl)^-1·h^-1 compared with 130 mol O₂· (mg Chl)^-1·h^-1 of whole cells) were isolated from Chlamydomonas reinhardtii cells grown in high and low CO₂ concentrations using autolysine-digitonin treatment. At 25° C and pH=7.8, no O₂ uptake could be observed in the dark by high- and low-CO₂ adapted chloroplasts. Light saturation of photosynthetic net oxygen evolution was reached at 800 mol photons·m^-2·s^-1 for high- and low-CO₂ adapted chloroplasts, a value which was almost identical to that observed for whole cells. Dissolved inorganic carbon (DIC) saturation of photosynthesis was reached between 200–300 M for low-CO₂ adapted chloroplasts, whereas high-CO₂ adapted chloroplasts were not saturated even at 700 M DIC. The concentrations of DIC required to reach half-saturated rates of net O₂ evolution (K_m(DIC)) was 31.1 and 156 M DIC for low- and high-CO₂ adapted chloroplasts, respectively. These results demonstrate that the CO₂ concentration provided during growth influenced the photosynthetic characteristics at the whole cell as well as at the chloroplast level.Abbreviations Chl chlorophyll - DIC dissolved inorganic carbon - K_m(DIC) coneentration of dissolved inorganic carbon required for the rate of half maximal net O₂ evolution - PFR photon fluence rate - SPGM silicasol-PVP-gradient medium     

Coding sequence divergence between two closely related plant species: Arabidopsis thaliana and Brassica rapa ssp. pekinensis

To characterize the coding-sequence divergence of closely related genomes, we compared DNA sequence divergence between sequences from a Brassica rapa ssp. pekinensis EST library isolated from flower buds and genomic sequences from Arabidopsis thaliana. The specific objectives were (i) to determine the distribution of and relationship between K _a and K _s, (ii) to identify genes with the lowest and highest K _a:K _s values, and (iii) to evaluate how codon usage has diverged between two closely related species. We found that the distribution of K _a:K _s was unimodal, and that substitution rates were more variable at nonsynonymous than synonymous sites, and detected no evidence that K _a and K _s were positively correlated. Several genes had K _a:K _s values equal to or near zero, as expected for genes that have evolved under strong selective constraint. In contrast, there were no genes with K _a:K _s >1 and thus we found no strong evidence that any of the 218 sequences we analyzed have evolved in response to positive selection. We detected a stronger codon bias but a lower frequency of GC at synonymous sites in A. thaliana than B. rapa. Moreover, there has been a shift in the profile of most commonly used synonymous codons since these two species diverged from one another. This shift in codon usage may have been caused by stronger selection acting on codon usage or by a shift in the direction of mutational bias in the B. rapa phylogenetic lineage.     

Enhancement of susceptivity to photoinhibition and photooxidation in rice chlorophyll <Emphasis Type="Italic">b</Emphasis>-less mutants

Two rice chlorophyll (Chl) b-less mutants (VG28-1, VG30-5) and the respective wild type (WT) plant (cv. Zhonghua No. 11) were analyzed for the changes in Chl fluorescence parameters, xanthophyll cycle pool, and its de-epoxidation state under exposure to strong irradiance, SI (1 700 μmol m⁻² s⁻¹). We also examined alterations in the chloroplast ultrastructure of the mutants induced by methyl viologen (MV) photooxidation. During HI (0–3.5 h), the photoinactivation of photosystem 2 (PS2) appeared earlier and more severely in Chl b-less mutants than in the WT. The decreases in maximal photochemical efficiency of PS2 in the dark (F_v/F_m), quantum efficiency of PS2 electron transport (Φ_PS2), photochemical quenching (q_P), as well as rate of photochemistry (P_rate), and the increases in de-epoxidation state (DES) and rate of thermal dissipation of excitation energy (D_rate) were significantly greater in Chl b-mutants compared with the WT plant. A relatively larger xanthophyll pool and 78–83 % conversion of violaxanthin into antheraxanthin and zeaxanthin in the mutants after 3.5 h of HI was accompanied with a high ratio of inactive/total PS2 (0.55–0.73) and high 1–q_P (0.57–0.68) which showed that the activities of the xanthophyll cycle were probably insufficient to protect the photosynthetic apparatus against photoinhibition. No apparent difference of chloroplast ultrastructure was found between Chl b-less mutants and WT plants grown under low, LI (180 μmol m⁻² s⁻¹) and high, HI (700 μmol m⁻² s⁻¹) irradiance. However, swollen chloroplasts and slight dilation of thylakoids occurred in both mutants and the WT grown under LI followed by MV treatment. These typical symptoms of photooxidative damage were aggravated as plants were exposed to HI. Distorted and loose scattered thylakoids were observed in particular in the Chl b-less mutants. A greater extent of photoinhibition and photooxidation in these mutants indicated that the susceptibility to HI and oxidative stresses was enhanced in the photosynthetic apparatus without Chl b most likely as a consequence of a smaller antenna size.     

Analysis of Qualitative Contribution of Assimilatory and Non-Assimilatory De-Excitation Processes to Adaptation of Photosynthetic Apparatus of Barley Plants to High Irradiance

The adaptation of barley (Hordeum vulgare L. cv. Akcent) plants to low (LI, 50 µmol m^–2 s^–1) and high (HI, 1000 µmol m^–2 s^–1) growth irradiances was studied using the simultaneous measurements of the photosynthetic oxygen evolution and chlorophyll a (Chl a) fluorescence at room temperature. If measured under ambient CO₂ concentration, neither increase of the oxygen evolution rate (P) nor enhancement of non-radiative dissipation of the absorbed excitation energy within photosystem 2 (PS2) (determined as non-photochemical quenching of Chl a fluorescence, NPQ) were observed for HI plants compared with LI plants. Nevertheless, the HI plants exhibited a significantly higher proportion of Q_A in oxidised state (estimated from photochemical quenching of Chl a fluorescence, q_P), by 49–102 % at irradiances above 200 µmol m^–2 s^–1 and an about 1.5 fold increase of irradiance-saturated PS2 electron transport rate (ETR) as compared to LI plants. At high CO₂ concentration the degree of P stimulation was approximately three times higher for HI than for LI plants, and the irradiance-saturated P values at irradiances of 2 440 and 2 900 µmol m^–2 s^–1 were by 130 and 150 % higher for HI plants than for LI plants. We suggest that non-assimilatory electron transport dominates in the adaptation of the photosynthetic apparatus of barley grown at high irradiances under ambient CO₂ rather than an increased NPQ or an enhancement of irradiance-saturated photosynthesis.