Potential Quantum Yield Research Articles

Chemical Composition of Celandine (<i>Chelidonium majus</i> L.) Extract and its Effects on <i>Botrytis tulipae</i> (Lib.) Lind Fungus and the Tulip

In this study, the content of chelidonine and berberine alkaloids, and sterols and phenols in the Chelidonium majus plant extract were analyzed. Subsequently, the effects of the extract on the germination and growth of Botrytis tulipae fungus on nutritive medium were compared to the effects of fluconazole. The plant extract was used at the minimum inhibitory concentration on B. tulipae developed in tulip leaves and the in vivo effects were investigated. The influence of different concentrations of C. majus extract on the physiological processes of the tulip (gas exchange parameters, photosynthetic light use efficiency, and induced chlorophyll fluorescence) were also tested to assess the applicability of the extract for the protection of ornamental plants against fungal infection. Our results demonstrated that 2% celandine extract does not significantly change the gas exchange parameters (transpiration rate, carbon dioxide uptake, and stomatal conductivity) of leaves exposed for 2 h, and does not interfere with the photochemical processes in the leaves. However, in higher concentrations, it increases the transpiration rate and net carbon dioxide influx. At concentrations of 15% and 20%, the extract lowers the potential quantum yield efficiency of photosystem II and the vitality index of the photosynthetic apparatus. Therefore we recommend the use of lower concentrations (≤6%) of celandine extract for the biological protection of tulips against gray mold.

Effect of metal content on chlorophyll fluorescence and chlorophyll degradation in lichen Pyxine cocoes (Sw.) Nyl.: a case study from Uttar Pradesh, India

The major aim of the present study is to identify the relationship of physiological parameters of the photosynthetic system with the elemental content of the naturally growing lichen Pyxine cocoes. The epiphytic foliose lichen P. cocoes was used as biomonitoring indicator and effect of atmospheric pollutants on physiological integrity was examined. Potential quantum yield of Photosystem II (fluorescence ratio Fv/Fm), chlorophyll degradation ratio and quantitative estimation of Al, As, Cd, Cr, Cu, Pb, Fe and Zn contained in the lichen thallus were ascertained. Statistical analysis revealed significantly positive correlations between Fv/Fm and element contents (Al and Cr). The chlorophyll degradation as well as alteration in the pigment content was found to be the most sensitive parameters to assess the vitality of lichen thallus against polluted environment. The species accumulated higher amounts of elements (Al, As, Cu, Fe and Zn) in the polluted sites as compared to the non polluted sites. It was also evident from this study that vehicular emission played a significant role in the release of elements as pollutants in the surrounding environment. The effectiveness of this lichen could be further investigated by comparing this species with other biomonitors.

Potential Energy Surfaces and Quantum Yields for Photochromic Diarylethene Reactions

Photochromic diarylethenes (DAEs) are among the most promising molecular switching systems for future molecular electronics. Numerous derivatives have been synthesized recently, and experimental quantum yields (QYs) have been reported for two categories of them. Although the QY is one of the most important properties in various applications, it is also the most difficult property to predict before a molecule is actually synthesized. We have previously reported preliminary theoretical studies on what determines the QYs in both categories of DAE derivatives. Here, reflecting theoretical analyses of potential energy surfaces and recent experimental results, a rational explanation of the general guiding principle for QY design is presented for future molecular design.

DROUGHT TOLERANCE SCREENING OF A ROSA POPULATION

Drought stress severely impacts plant production worldwide, and is predicted to increasingly affect crop production due to climate change. One of the main strategies to cope with the changing environment is the development of new cultivars with an improved tolerance to drought stress. A study was initiated to compare mitotically induced tetraploid roses with their diploid counterparts for their drought stress tolerance. The first step in this research is the development of a screening method to evaluate segregation for drought tolerance in a diploid rose population. Therefore, drought tolerance was screened in 69 genotypes selected from the F1 population resulting from a R. 'Yesterday' x R. wichurana hybridisation. The plants were subjected to two different irrigation regimes, reducing the amount of water received by the drought stress group to less than 10% of an optimal amount given in the control group. Relative water content (RWC), leaf water potential (Psi) and effective photochemical quantum yield of photosystem II (Y(II)) were measured, next to stomatal size and density, and leaf morphological characteristics. RWC and Psi were good indicators for screening drought sensitivity of the genotypes. However, Y(II) did not prove to be a reliable screening method in this study. This approach allowed the selection of interesting rose genotypes for further characterisation and tetraploidisation, potentially forming the basis of future drought tolerant rose cultivars.

Chlorophyll fluorescence of desiccation-tolerant cyanobacterial crusts of sub-tropical inselberg rocks in southern China: 2. Rehydration at different light intensities and temperatures

The chlorophyll fluorescence response of desiccation-tolerant cyanobacterial crusts from shaded and non-shaded habitats on sub-tropical inselberg rocks, Guangdong province, China to rehydration under different light intensities and temperature was studied. It was shown that chlorophyll fluorescence change showed the same trend under different light-temperature conditions. After rehydration in the dark following a term of desiccation, the potential quantum yield of PSII (Fv/Fm) and the effective quantum yield of PSII (Yield) of each sample recovered quickly, following activation after 1 minute. Moreover, the two samples could adapt to a wide range of light intensity and temperature while 250 mmol m s light and 30 °C were fitted for rehydration. However, Lower light caused more inhibition in non-shaded habitats, but higher lights caused more inhibition in shaded habitats. Further, samples from non-shaded habitats suffered more damage under temperature treatments than did samples from shaded habitats. Although extreme light and temperature inhibited photosynthetic activity of the samples to some extent, cyanobacterial crusts were protected from damage under high light through photochemical fluorescence quenching (qP). In conclusion, cyanobacteria as pioneers on such bare rocks can recover photosynthesis capability after a term of desiccation, and can adapt well to different light intensities and temperatures.

Growth-Phase Dependent Variation in Photosynthetic Activity and Cellular Protein Expression Profile in the Harmful RaphidophyteChattonella antiqua

This study investigated temporal variations in the potential maximum quantum yield of photosystem II (F(v)/F(m) ratio) and growth-phase dependent cellular protein expressions of Chattonella antiqua under laboratory conditions. Despite the culture conditions, significant positive correlations between the F(v)/F(m) ratio and daily growth rate were observed. Threshold F(v)/F(m) ratios associated with positive cell growth were calculated to be >0.44, >0.44, and >0.37, and those associated with active cell growth (growth rate >0.5 div. d(-1)) were >0.58, >0.60, and >0.49 under control culture, low nutrient and intense light conditions, respectively. Proteome profiles obtained by two-dimensional gel electrophoresis (2-DE) indicated that 42 protein spots were differentially expressed at various growth phases of C. antiqua, which indicates changes in cellular physiological status throughout the growth cycle, and suggests that oxygen evolving enhancer 1 and 2-cysteine peroxiredoxin play roles in maintaining the positive growth of C. antiqua.

Salt stress change chlorophyll fluorescence in mango

This study evaluated the tolerance of mango cultivars 'Haden', 'Palmer', 'Tommy Atkins' and 'Uba' grafted on rootstock 'Imbú' to salt stress using chlorophyll fluorescence. Plants were grown in modified Hoagland solution containing 0, 15, 30, and 45 mmol L-1 NaCl. At 97 days the parameters of the chlorophyll fluorescence (F0, Fm, Fv, F0/Fm, Fv/Fm, Fv'/Fm', ΦPSII = [(Fm'-Fs)/(Fm')], D = (1- Fv'/Fm') and ETR = (ΦPSII×PPF×0,84×0,5) were determined. At 100 days, the leaf emission and leaf area, toxicity and leaf abscission indexes were determined. In all cultivars evaluated, in different degree, there were decreases in photochemical efficiency of photosystem II, enhanced concentrations from 15 mmol L-1 NaCl. The decreases in the potential quantum yield of photosystem II (Fv/Fm) were 27.9, 18.7, 20.5, and 27.4%, for cultivars 'Haden', 'Palmer', 'Tommy Atkins', and 'Uba', respectively, when grown in 45 mmol L-1 NaCl. It was found decreases in leaf emission and mean leaf area in all cultivars from 15 mmol L-1 NaCl. There were increases in leaf toxicity of 33.0, 67.5, 41.6 and 80.8% and in leaf abscission of 71.8, 29.2, 32.5, and 67.9% for the cultivars 'Haden', 'Palmer', 'Tommy Atkins', and 'Uba' respectively, when grown in 45 mmol L-1 NaCl. Leaf toxicity and leaf abscission were not observed in 15 mmol L-1 NaCl. The decrease in Fv/Fm ratio were accompanied by decreasing in leaf emission and increased leaf toxicity index, showing, therefore, the potential of chlorophyll fluorescence in the early detection of salt stress in mango tree.

Freezing of air-dried samples of the lichen Evernia prunastri (L.) Ach. ensures that thalli remain healthy for later physiological measurements

We tested whether freezing of air-dried lichen thalli alter physiological parameters commonly used as valuable stress markers in laboratory and field ecophysiological studies, namely integrity of cell membranes (measured as electrolyte leakage), assimilation pigments (chlorophyll a, chlorophyll b, chlorophyll a+b, total carotenoids content), chlorophyll integrity (OD435/OD415) and chlorophyll a fluorescence emission (expressed as F V/F M, the potential quantum yield of primary photochemistry and PIABS, a global indicator of the photosynthetic performance). Thalli of the lichen Evernia prunastri (L.) Ach. were air-dried, stored in the freezer at ca. −18°C under dark conditions and analysed after short-term storage (15, 30 and 90 days). These periods are compatible with the needs of a relatively rapid data evaluation in biomonitoring studies. After freezing, the investigated parameters showed that lichens remained healthy and suitable for later physiological measurements without biasing data quality.

Physiological and biochemical responses of Theobroma cacao L. genotypes to flooding

Flooding is common in lowlands and areas with high rainfall or excessive irrigation. A major effect of flooding is the deprivation of O2 in the root zone, which affects several biochemical and morphophysiological plant processes. The objective of this study was to elucidate biochemical and physiological characteristics associated with tolerance to O2 deficiency in two clonal cacao genotypes. The experiment was conducted in a greenhouse with two contrasting clones differing in flood tolerance: TSA-792 (tolerant) and TSH-774 (susceptible). Leaf gas exchange, chlorophyll (Chl) fluorescence, chemical composition and oxidative stress were assessed during 40 d for control and flooded plants. Flooding induced a decrease in net photosynthesis, stomatal conductance and transpiration of both genotypes. In flood conditions, the flood-susceptible clone showed changes in chlorophyll fluorescence, reductions in chlorophyll content and increased activity of peroxidase and polyphenol oxidase. Flooding also caused changes in macro- and micronutrients, total soluble sugars and starch concentrations in different plant organs of both genotypes. Response curves for the relationship between photosynthetically active radiation (PAR) and net photosynthetic rate (PN) for flooded plants were similar for both genotypes. In flood conditions, the flood-susceptible clone exhibited (1) nonstomatal limitations to photosynthesis since decreased in maximum potential quantum yield of PSII (Fv/Fm) values indicated possible damage to the PSII light-harvesting complex; (2) oxidative stress; (3) increased leaf chlorosis; and (4) a reduction in root carbohydrate levels. These stresses resulted in death of several plants after 30 d of flooding.

Effects of parasitic plant <I>Cistanche deserticola</I> on chlorophyll a fluorescence and nutrient accumula-tion of host plant <I>Haloxylon ammodendron</I> in the Taklimakan Desert

The parasitic plant Cistanche deserticola attaches to Haloxylon ammodendron, a perennial shrub with high tolerance to salinity and drought. However, little was known about the parasite-host relation between the two species. Effects of the parasite on chlorophyll a fluorescence and nutrient accumulation in the host plant (H. am- modendron) were investigated in the Taklimakan Desert. Some photosynthetic parameters of both host and non-host H. ammodendron plants were measured by in vivo chlorophyll a fluorescence technology in the field. The assimilating branches of host and non-host plants were collected and nutrient and inorganic ion contents were analyzed. The results from field experiments showed that the infection of C. deserticola reduced the non-photochemical quenching of the variable chlorophyll fluorescence (NPQ) and the potential maximum quantum yield for primary photochemistry (Fv/Fm) of the host. Compared with non-host plants, the host H. ammodendron had low nutrient, low inorganic ion contents (Na + and K + ) and low K + /Na + ratios in the assimilating branches. It suggested that C. deserticola infection reduced the nutrient acquisition and caused damage to the photoprotection through thermal dissipation of the energy of the photosystem II in the host, resulting in a decrease in the tolerance to salinity and high radiation. It was concluded that the attachment of the parasite plant (C. deserticola) had negative effects on the growth of its host.

Photosynthetic characteristics and enzymatic antioxidant capacity of leaves from wheat cultivars exposed to drought

Two durum (Triticum durum L.), Barakatli-95 and Garagylchyg-2; and two bread (Triticum aestivum L.) wheat cultivars, Azamatli-95 and Giymatli-2/17 with different sensitivities to drought were grown in the field on a wide area under normal irrigation and severe water deficit. Drought caused a more pronounced inhibition in photosynthetic parameters in the more sensitive cvs Garagylchyg-2 and Giymatli-2/17 compared with the tolerant cvs Barakatli-95 and Azamatli-95. Upon dehydration, a decline in total chlorophyll and relative water content was evident in all cultivars, especially in later periods of ontogenesis. Potential quantum yield of PS II (Fv/Fm ratio) in cv Azamatli-95 was maximal during stalk emergency stage at the beginning of drought. This parameter increased in cv Garagylchyg-2, while in tolerant cultivar Barakatli-95 significant changes were not observed. Contrary to other wheat genotypes in Giymatli-2/17 drought caused a decrease in PS II quantum yield. Drought-tolerant cultivars showed a significant increase in CAT activity as compared to control plants. In durum wheat cultivars maximal activity of CAT was observed at the milk ripeness and in bread wheat cultivars at the end of flowering. APX activity also increased in drought-treated leaves: in tolerant wheat genotypes maximal activity occurred at the end of flowering, in sensitive ones at the end of ear formation. GR activity increased in the tolerant cultivars under drought stress at all stages of ontogenesis. SOD activity significantly decreased in sensitive cultivars and remained at the control level or increased in resistant ones. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Functional properties of photosynthetic apparatus in three Fucus species inhabiting the White Sea: Effect of dehydration

Three species of Fucus inhabiting different zones at the littoral and sublittoral of the White Sea (F. vesiculosus L., F. serratus L., and F. distichus L.) were compared with regard to performance of the photosynthetic apparatus (PSA) and PSA changes upon prolonged desiccation of algae. The content of chlorophyll a and the total content of carotenoids were significantly higher in F. serratus than in F. vesiculosus. The potential quantum yield (F v/F m) of photosystem II (PSII) was 0.69–0.74 in all species. The highest effective quantum yield of PSII (Y II) was observed in F. vesiculosus plants inhabiting zones with the most intense insolation. The ratio of non-photochemical quenching to photochemical quenching (qN/qP) increased with the depth of algal habitats in the row from F. vesiculosus to F. serratus. The fluorescence parameters F. serratus deviated during desiccation from their normal values and did not recover upon the subsequent return to water. By contrast, the fluorescence parameters of F. vesiculosus and F. distichus recovered gradually after the return of algae from air to sea water. It is supposed that F. serratus plants, unlike other species, are characterized by comparatively low physiological plasticity.

Interspecific differences in cryoresistance of lichen symbiotic algae of genus Trebouxia assessed by cell viability and chlorophyll fluorescence

Unicellular algae of genus Trebouxia are the most frequent symbiotic photobionts found in lichen species adapted to extreme environments. When lichenised, they cope well with freezing temperature of polar regions, high-mountains environments and were successfully tested in open-space experiments. Trebouxia sp. is considered potential model species for exobiological experiments. The aim of this paper is to evaluate cryoresistence of Trebouxia sp. when isolated from lichen thalli and cultivated on media. In our study, six algal strains were exposed to repeated freezing/thawing cycles. The strains of Trebouxia sp. (freshly isolated from lichen Lasallia pustulata), Trebouxia erici, Trebouxia asymmetrica, Trebouxia glomerata, Trebouxia irregularis, and Trebouxia jamesii from culture collection were cooled from 25 to −40°C at two different rates. The strains were also shock frozen in liquid nitrogen. After repeated treatment, the strains were inoculated and cultivated on a BBM agar for 7days. Then, cell viability was assessed as relative share of living cells. Potential quantum yield of photochemical reactions in PS II (FV/FM), and effective quantum yield of photochemical reactions in PS II (ΦPSII) were measured. While the slow cooling rate (0.5°Cmin−1) did not cause any change in viability, FV/FM, and ΦPSII, the fast cooling rate (6.0°Cmin−1) caused species-specific decrease in all parameters. The most pronounced interspecific differences in cryoresistance were found after shock freezing and consequent cultivation. While T. asymmetrica and T. jamesii exhibited low viability of living cells (18.9% and 34.7%) and full suppression of photosynthetic processes, the other strains had viability over 60%, and unaffected values of FV/FM, and ΦPSII. This indicated a high degree of cryoresistance of T. glomerata, T. erici, T. irregularis and Trebouxia sp. strains. These strains could be used for detailed investigation of underlying physiological mechanisms and as models for astrobiological tests taken in the Earth facilities.

Photoinhibition of photosynthesis in Antarctic lichen Usnea antarctica. I. Light intensity- and light duration-dependent changes in functioning of photosystem II

The paper deals with the differences in sensitivity of Antarctic lichen to photoinhibition. Thalli of Usnea antarctica were collected at the James Ross Island, Antarctica (57°52´57´´W, 63°48´02´´S) and transferred in dry state to the Czech Republic. After rewetting in a laboratory, they were exposed to 2 high light treatments: short-term (30 min), and long-term (6 h). In short-term treatment, the sample were exposed to 1000 and 2000 µmol m-2 s-1 of photosynthetically active radiation (PAR). In long-term experiment, PAR of 300, 600, and 1000 µmol m-2 s-1 were used. Photosynthetic efficiency of U. antarctica thalli was monitored by chlorophyll fluorescence parameters, potential (FV/FM) and actual (FPSII) quantum yield of photochemical processes in photosystem II in particular. In short-term treatments, the F0, FV and FM signals, as well as the values of FV/FM, and FPSII showed light-induced decrease, however substantial recovery after consequent 30 min. in dark. Longer exposition (60 min) to high light led to more pronounced decrease in chlorophyll fluorescence than after 30 min treatment, however dark recovery was faster in the thalli treated before for longer time (60 min). Long-term treatment by high light caused gradual decrease in FV/FM and FPSII with the time of exposition. The extent of the decrease was found light dose-dependent. The time course was biphasic for FV/FM but not for FPSII. The study showed that wet thalli of Usnea antarctica had high capacity of photoprotective mechanisms to cope well either with short- or long-term high light stress. This might be of particular importance in the field at the James Ross Island, particularly at the begining of growing season when melting water is available and, simultaneously, high light stress may happen on fully sunny days.

Two bifunctional Ru II/Re I photocatalysts for CO 2 reduction: A spectroscopic, photocatalytic, and computational study

Two Ru II/Re I dimers, [Ru(dmb) 2( L1)Re(CO) 3Cl] ( 1) and [Ru(dmb) 2( L2)Re(CO) 3Cl] ( 2), were synthesized and characterized, and their electrochemical and spectroscopic properties together with their photocatalytic CO 2 reduction activities were evaluated (dmb = 4,4′-dimethyl-2,2′-bipyridine; L1 = 1,2-bis(4′-methyl-2,2′-bipyridyl-4-yl)ethane; L2 = 1,2-bis(4′-methyl-2,2′-bipyridyl-4-yl)ethene). The structures of 1 and 2 are identical except for the difference in the conjugation content of bridging ligands (–CH 2–CH 2– for 1 and –CH CH– for 2). Density functional theory (DFT) methods were employed to model the ground-state electronic transition and electrochemical properties of both catalysts. Electronic transitions were identified using UV–Vis spectroscopic techniques, aided by time-dependent density functional theory (TD-DFT) methods. The redox properties of two complexes under N 2 and CO 2 pressure have been studied by means of cyclic voltammetric measurements. When the cyclic voltammetry was performed in the presence of CO 2, a substantial current enhancement was observed for the reduction wave of 1 and 2. Additionally, significant higher turnover numbers of CO formation in the photocatalytic CO 2 reduction are observed for 1 than that for 2. Although complex 2 exhibited longer wavelength absorption, its photocatalytic activation for the CO 2 reduction was lower than that of 1, due to the effect of conjugated linkage on the reduction potential and low emission quantum yield.

Slow-release and organic fertilizers on early growth of Rangpur lime

Slow-release and organic fertilizers are promising alternatives to conventional fertilizers, as both reduce losses by leaching, volatilization and problems of toxicity and/or salinity to plants. The objective of this work was to evaluate the effect of different rates of the organic fertilizer Humato-Macota® compared with the slow-release fertilizer Osmocote® on the growth and nitrogen content in the dry matter of Rangpur lime. A field experiment was conducted in a factorial completely randomized design with an additional treatment (4 x 4 +1). The first factor consisted of four HumatoMacota® rates (0, 1, 2, and 3%) applied to the substrate; the second factor consisted of the same Humato-Macota® concentrations, but applied as fortnightly foliar sprays; the additional treatment consisted of application of 5 kgm-3 Osmocote® 18-05-09. Means of all growth characteristics (plant height, total dry matter, root/shoot ratio and leaf area) and the potential quantum yield of photosystem II (Fv/Fm) were higher when plants were fertilized with the slow-release fertilizer. The organic fertilizer applied alone did not meet the N requirement of Rangpur lime.

CHLOROPHYLL FLUORESCENCE AND FRUIT QUALITY DURING STORAGE OF MANGO FRUIT IN MODIFIED ATMOSPHERE PACKAGING

Chlorophyll fluorescence was investigated as a possible nondestructive tool to detect quality changes in mangoes stored in modified atmosphere packaging. Mangoes were packed in MTEC breathable film and stored at 15 or 25°C. The chlorophyll fluorescence parameters of minimal fluorescence (Fo), maximal fluorescence (Fm) and potential quantum yield (Fv/Fm) as well as fruit firmness, skin colour and soluble solids content were evaluated every 3 d during storage. CO 2 and O 2 concentrations in the package headspace were determined daily. Chlorophyll fluorescence parameters (Fv/Fm, Fv and Fm) declined over storage time whereas the firmness changed immediately at 25°C and after 3 d at 15°C. The fluorescence parameters did not change until about 12 d of storage, thus fluorescence did not follow the changes in firmness. However, fruit firmness also declined during the storage period. Chlorophyll fluorescence parameters (Fv/Fm, Fv and Fm) and fruit firmness of mangoes stored at 15°C were higher than those of mangoes stored at 25°C and these differences increased with storage time until 15 d of storage after that firmness of the fruit from 15°C was the same as that from 25°C. These results suggest that chlorophyll fluorescence may be useful to determine changes in mango fruit quality nondestructively.

Subtle biological responses to increased CO2 concentrations by Phaeocystis globosa Scherffel, a harmful algal bloom species

Recent investigations into the role of carbon dioxide on phytoplankton growth and composition have clearly shown differential effects among species and assemblages, suggesting that increases in oceanic CO2 may play a critical role in structuring lower trophic levels of marine systems in the future. Furthermore, alarming increases in the occurrence of harmful algal blooms (HABs) in coastal waters have been observed, and while not uniform among systems, appear in some manner to be linked to human impacts (eutrophication) on coastal systems. Models of HABs are in their infancy and do not at present include sophisticated biological effects or their environmental controls. Here we show that subtle biological responses occur in the HAB species Phaeocystis globosa Scherffel as a result of CO2 enrichment induced by gentle bubbling. The alga, which has a polymorphic life history involving the formation of both colonies and solitary cells, exhibited altered growth rates of colonial and solitary forms at [CO2] of 750 ppm, as well as increased colony formation. In addition, substantial modifications of elemental and photosynthetic constituents of the cells (C cell−1, N cell−1, potential quantum yield, chl a cell−1) occurred under elevated CO2 concentrations compared to those found at present CO2 levels. In contrast, other individual and population variables (e.g., colony diameter, total chlorophyll concentration, carbon/nitrogen ratio) were unaffected by increased CO2. Our results suggest that predictions of the future impacts of Phaeocystis blooms on coastal ecosystems and local biogeochemistry need to carefully examine the subtle biological responses of this alga in addition to community and ecosystem effects.

Resurrection kinetics of photosynthesis in desiccation‐tolerant terrestrial green algae (Chlorophyta) on tree bark

The rough bark of orchard trees (Malus) around Darmstadt is predominantly covered in red to purple-brown layers (biofilms) of epiphytic terrestrial alga of Trentepohlia umbrina. The smooth bark of forest trees (Fagus sylvatica L. and Acer sp.) in the same area is covered by bright green biofilms composed of the green algae Desmococcus, Apatococcus and Trebouxia, with a few cells of Coccomyxa and 'Chlorella' trebouxioides between them. These algae are desiccation tolerant. After samples of bark with the biofilms were kept in dry air in darkness for various periods of time, potential quantum yield of PSII, F(v)/F(m), recovered during rehydration upon rewetting. The kinetics and degree of recovery depended on the length of time that the algae were kept in dry air in the desiccated state. Recovery was better for green biofilm samples, i.e. quite good even after 80 days of desiccation (F(v)/F(m) = ca. 50% of initial value), than the red samples, where recovery was only adequate up to ca. 30-40 days of desiccation (F(v)/F(m) = ca. 20-55% of initial value). It is concluded that the different bark types constitute different ecophysiological niches that can be occupied by the algae and that can be distinguished by their capacity to recover from desiccation after different times in the dry state.

Leaf Wilting Movement Can Protect Water-Stressed Cotton (Gossypium hirsutum L.) Plants Against Photoinhibition of Photosynthesis and Maintain Carbon Assimilation in the Field

Under severe water stress, leaf wilting is quite general in higher plants. This passive movement can reduce the energy load on a leaf. This paper reports an experimental test of the hypothesis that leaf wilting movement has a protective function that mitigates against photoinhibition of photosynthesis in the field. The experiments exposed cotton (Gossypium hirsutum L.) to two water regimes: water-stressed and well-watered. Leaf wilting movement occurred in water-stressed plants as the water potential decreased to −4.1 MPa, reducing light interception but maintaining comparable quantum yields of photosystem II (PS II; Yield for short) and the proportion of total PS II centers that were open (qP). Predrawn F v/F m (potential quantum yield of PS II) as an indicator of overnight recovery of PS II from photoinhibition was higher than or similar to that in well-watered plants. Compared with water-stressed cotton leaves for which wilting movement was permitted, water-stressed cotton leaves restrained from such movement had significantly increased leaf temperature and instantaneous CO2 assimilation rates in the short term, but reduced Yield, qP, and F v/F m. In the long term, predrawn F v/F m and CO2 assimilation capacity were reduced in water-stressed leaves restrained from wilting movement. These results suggest that, under water stress, leaf wilting movement could reduce the incident light on leaves and their heat load, alleviate damage to the photosynthetic apparatus due to photoinhibition, and maintain considerable carbon assimilation capacity in the long term despite a partial loss of instantaneous carbon assimilation in the short term.