Chlorophyll fluorometry sheds light on the role of desiccation resistance for vegetative overland dispersal of aquatic plants

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Genetic analysis of in vitro shoot regeneration from cotyledonary petioles of<Emphasis Type="Italic"> Brassica oleracea</Emphasis>

Diallel analysis was used to investigate the genetic control of in vitro shoot regeneration in Brassica oleracea. Twelve doubled haploid (DH) lines, selected to include a range of genotypes with differing shoot regeneration potentials, were crossed reciprocally to produce 132 F₁ and 12 selfed, DH families. Cotyledonary petioles from 4-day-old seedlings, from all families, were excised and maintained on MS medium supplemented with 2 mg/l BAP. Explants were scored after 44 days for both the presence or absence of shoots and the number of regenerating shoots per explant. Diallel analysis showed both shoot regeneration and the production of multiple shoots to be controlled by additive and dominant gene effects, with additive effects being more important. Additive gene effects accounted for 71% and 77% of the genetic variation observed within the diallel for shoot regeneration and multiple shoot regeneration, respectively. By investigating the shoot regeneration potential of subsequent backcross and F₂ populations, the ability to introduce and increase shoot regeneration potential into otherwise recalcitrant lines was demonstrated.Communicated by G. Wenzel     

The geometry of light interception by shoots of Heteromeles arbutifolia: morphological and physiological consequences for individual leaves

The influence of leaf orientation and position within shoots on individual leaf light environments, carbon gain, and susceptibility to photoinhibition was studied in the California chaparral shrub Heteromeles arbutifolia with measurements of gas exchange and chlorophyll fluorescence, and by application of a three-dimensional canopy architecture model. Simulations of light absorption and photosynthesis revealed a complex pattern of leaf light environments and resulting leaf carbon gain within the shoots. Upper, south-facing leaves were potentially the most productive because they intercepted greater daily photon flux density (PFD) than leaves of any other orientation. North-facing leaves intercepted less PFD but of this, more was received on the abaxial surface because of the steep leaf angles. Leaves differed in their response to abaxial versus adaxial illumination depending on their orientation. While most had lower photosynthetic rates when illuminated on their abaxial as compared to adaxial surface, the photosynthetic rates of north-facing leaves were independent of the surface of illumination. Because of the increasing self-shading, there were strong decreases in absorbed PFD and daily carbon gain in the basipetal direction. Leaf nitrogen per unit mass also decreased in the basipetal direction but on a per unit area basis was nearly constant along the shoot. The decrease in leaf N per unit mass was accounted for by an increase in leaf mass per unit area (LMA) rather than by movement of N from older to younger leaves during shoot growth. The increased LMA of older lower leaves may have contributed directly to their lower photosynthetic capacities by increasing the limitations to diffusion of CO₂ within the leaf to the sites of carboxylation. There was no evidence for sun/shade acclimation along the shoot. Upper leaves and especially south-facing upper leaves had a potential risk for photoinhibition as demonstrated by the high PFDs received and the diurnal decreases in the fluorescence ratio F _v/F _m. Predawn F _v/F _m ratios remained high (>0.8) indicating that when in their normal orientations leaves sustained no photoinhibition. Reorientation of the leaves to horizontal induced a strong sustained decrease in F _v/F _m and CO₂ exchange that slowly recovered over the next 10–15?days. If leaves were also inverted so that the abaxial surface received the increased PFDs, then the reduction in F _v/F _m and CO₂ assimilation was much greater with no evidence for recovery. The heterogeneity of responses was due to a combination of differences between leaves of different orientation, differences between responses on their abaxial versus adaxial surfaces, and differences along the shoot due to leaf age and self-shading effects.     

Evaluating the Superoxide Dismutase Activity and Chlorophyll Fluorescence in Picea Abies Leaves Growing at Different Altitudes

Superoxide dismutase (SOD) activity and parameters of chlorophyll fluorescence, the ratio of maximal to variable fluorescence (F_v/F_m), maximal fluorescence (F_m), and minimal fluorescence (F₀) were determined on Picea abies growing at different altitudes. The decreases of F_v/F_m and F_m, in comparison to samples from the lower stands (control), were found on trees from the highest stands. The decrease of fluorescence parameters was reversible, at least partly, after keeping branches for some days in the laboratory. F_v/F_m measured in spring when trees were partially covered with snow revealed greater degree of photoinactivation in branches collected from above the snow in comparison to those from below the snow. In samples collected from above snow also slower recovery from stress was observed. Two main SOD isoforms were determined in needles of P. abies, and classified as CuZnSODs. The activity of both SOD isoforms was increasing with the altitude, thus indicating the highest level of oxidative stress at the timberline zone. This revised version was published online in August 2006 with corrections to the Cover Date.     

Recovery from winter depression of photosynthesis in pine and spruce

Summary Recovery from winter depression of photosynthesis was studied in Pinus sylvestris, Pinus conforta and Picea abies by means of chlorophyll fluorescence and gas exchange measurements. During the winter 1986–1987 the fluorescence yield was low and no variable fluorescence was detectable before the end of March. In the field recovery of variable fluorescence/maximum fluorescence (F_v/F_m) during spring was slow for all three species studied. The temperature dependence of recovery was confirmed from measurements of the potential rate of recovery of F_v/F_m at different temperatures in the laboratory. At 20° C, F_v/F_m increased from 0.1 to 0.8 within 3 days. Recovery of F_v/F_m was paralleled by an increase in apparent photon yield. No significant differences could be demonstrated between the studied tree species in potential rate of recovery in the laboratory or in actual recovery in the field.     

The Effect of Photoautotrophy on Photosynthesis and Photoinhibition of Gardenia Plantlets during Micropropagation

We studied the relationships between the degree of photoautotrophy, photosynthetic capacity, and extent of photoinhibition of Gardenia jasminoides Ellis plantlets in vitro. Two successive micropropagation stages (shoot multiplication and root induction), and three culture conditions [tube cap closure, photosynthetic photon flux density (PPFD), and sucrose concentration] which may influence the development of photoautotrophy in vitro were assayed. The ratios of variable chlorophyll fluorescence to either maximal (F_v/F_m) or ground (F_v/F₀) values were low, irrespective of the culture stage or growing conditions. Incomplete development of the photosynthetic apparatus and permanent photoinhibition may be involved. However, F_v/F_m and F_v/F₀ increased from shoot multiplication to root induction owing to a decrease in F₀ and an increase in F_m. This suggests that photoinhibition decreases later during micropropagation, when the photoautotrophy of plantlets is more advanced. The low sucrose content and high PPFD increased the photoinhibition of plantlets, whereas growth in tubes with permeable caps showed the opposite effect. The only culture factor with a significant (positive) effect on maximum photosynthetic rate (P _max) was PPFD. At shoot multiplication net photosynthetic rate (P _N) was positively correlated with the half time of the increase from F₀ to F_m (t_1/2). Such association may be mainly due to a common response of both traits to higher PPFD in culture. Within each culture stage, no relationship was observed between P _N and the degree of photoautotrophy, which was positively correlated with F_v/F_m and F_v/F₀ during root induction. During shoot multiplication, these correlations were not significant, or were even negative. Hence during the last stage of micropropagation, plantlets with a higher degree of photoautotrophy are less photoinhibited, whereas they do not follow this pattern at the earlier stage.     

Development of photoautotrophy and photoinhibition of Gardenia jasminoides plantlets during micropropagation

This paper reports on the fast fluorescence responses of Gardenia jasminoides Ellis plantlets, at two successive stages (shoot multiplication and root induction) of culture in vitro. We test whether plantlets in vitro suffer photoinhibition during culture and whether the degree of photoautotrophy of these mixotrophic plantlets has any effect on the extent of photoinhibitory impairment. In this regard the effects of different sucrose levels in the medium and PPFD during growth on the development of photoautotrophy and the extent of photoinhibition were evaluated. Plantlets were grown under low, intermediate, and high (50, 100, and 300 mol m^-2 s^-1) PPFD, and at 3 different sucrose concentrations (0.5, 1.5, and 3.0%, w/v) in the medium, during shoot multiplication. During root induction the same growth conditions were assayed except for the high PPFD. The development of photoautotrophy was assessed via the difference between the stable carbon isotope composition of sucrose used as heterotrophic carbon source and that of leaflets grown in vitro. Plantlets from root induction showed more developed photoautotrophy than those from shoot multiplication. For both stages the low-sucrose medium stimulated the photoautotrophy of plantlets in vitro. In addition, intermediate PPFD induced photoautotrophy during shoot multiplication. For plantlets of both culture stages at the lowest PPFD no photoinhibition occurred irrespective of the sucrose concentration in media. However, during the shoot multiplication stage chlorophyll fluorescence measurements showed a decrease in F _v /F _m and in t _1/2 as growing PPFD increased, indicating photoinhibitory damage. The decline of F _v /F _m was caused mostly by an increase in F _o , indicating the inactivation of PSII reaction centers. However plantlets growing under low sucrose showed reduced susceptibility to photoinhibition. During root induction, only plantlets cultured with high sucrose showed a decrease in F _v /F _m as PPFD increased, although t _1/2 remained unchanged. In this case, the decline of F _v /F _m was mostly due to a decrease in F _m , which indicates increased photoprotection rather than occurrence of photodamage. Therefore, growth in low-sucrose media had a protective effect on the resistance of PSII to light stress. In addition, plantlets were more resistant to photoinhibition during root induction than during shoot multiplication. Results suggest that increased photoautotrophy of plantlets reduces susceptibility to photoinhibition during gardenia culture in vitro.Abbreviations AP apparent photosynthesis - Chl total chlorophyll content - Chl a/b chlorophyll a-to-b ratio - Chl/Car total chlorophyll-to-carotenoids ratio - ¹³C ratio of ¹³C/¹²C relative to PeeDee belemnite standard - F _m maximum chlorophyll fluorescence - F _o fluorescence emission when all reaction centres are open and the photochemical quenching is minimal - F _v variable chlorophyll fluorescence (F _m -F _o ) - F _v /F _m the ratio of variable to maximum chlorophyll fluorescence, indicator photochemical efficiency of PSII - MS medium Murashige and Skoog (1962) medium - PPFD photosynthetic photon flux density - Rd dark respiration, t _1/2 the half-time of the increase from F _o to F _m - IAA indole butyric acid     

Performance index in assessing the physiological state of trees in urban ecosystems

Based on PAM and PEA measurements of fluorescence of bark chloroplasts, we have compared the information capacity of the methods for assessing the physiological state of Tilia cordata Mill. by the maximal quantum efficiency of PS II photochemistry (F _v/F _m) and by the performance index (PI). The measurements were made on annual shoots of linden trees growing in different environs. It was shown that the chlorophyll content in the bark of shoots growing near a busy urban street was twice less than in controls growing out of town. For the trees in the unfavorable environment, a small decrease in (F _v/F _m) was registered, and there was a significant statistical scatter in these values as compared with controls. The PI and its constituent parameters calculated from fluorescence induction curves (PEA method) are more informative and allow recognizing changes in the primary energy conversion processes in PS II when they are still small. Thus, PI can be used as a sensitive, robust, and rapid test to evaluate the physiological state of trees and other plant objects even under minor environmental changes.     

Winter photoinhibition in needles of <Emphasis Type="Italic">Taxus baccata</Emphasis> seedlings acclimated to different light levels

Seasonal variability of maximum quantum yield of PSII photochemistry (F_v/F_m) was studied in needles of Taxus baccata seedlings acclimated to full light (HL, 100% solar irradiance), medium light (ML, 18% irradiance) or low light (LL, 5% irradiance). In HL plants, F_v/F_m was below 0.8 (i.e. state of photoinhibition) throughout the whole experimental period from November to May, with the greatest decline in January and February (when F_v/F_m value reached 0.37). In ML seedlings, significant declines of F_v/F_m occurred in January (with the lowest level at 0.666), whereas the decline in LL seedlings (down to 0.750) was not significant. Full recovery of F_v/F_m in HL seedlings was delayed until the end of May, in contrast to ML and LL seedlings. F_v/F_m was significantly correlated with daily mean (T _mean), maximal (T _max) and minimal (T _min) temperature and T _min was consistently the best predictor of F_v/F_m in HL and ML needles. Temperature averages obtained over 3 or 5 days prior to measurement were better predictors of F_v/F_m than 1- or 30-day averages. Thus our results indicate a strong light-dependent seasonal photoinhibition in needles of T. baccata as well as suggest a coupling of F_v/F_m to cumulative temperature from several preceding days. The dependence of sustained winter photoinhibition on light level to which the plants are acclimated was further demonstrated when plants from the three light environments were exposed to full daylight over single days in December, February and April and F_v/F_m was followed throughout the day to determine residual sensitivity of electron transport to ambient irradiance. In February, the treatment revealed a considerable midday increase in photoinhibition in ML plants, much less in HL (already downregulated) and none in LL plants. This suggested a greater capacity for photosynthetic utilization of electrons in LL plants and a readiness for rapid induction of photoinhibition in ML plants. Further differences between plants acclimated to contrasting light regimes were revealed during springtime de-acclimation, when short term regeneration dynamics of F_v/F_m and the relaxation of nonphotochemical quenching (NPQ) indicated a stronger persistent thermal mechanism for energy dissipation in HL plants. The ability of Taxus baccata to sustain winter photoinhibition from autumn until late spring can be beneficial for protection against an excessive light occurring together with frosts but may also restrict photosynthetic carbon gain by this shade-tolerant species when growing in well illuminated sites.     

Chlorophyll fluorescence and leaf chlorophyll content of bean leaves injured by spider mites (Acari: Tetranychidae)

The use of chlorophyll fluorescence as a method for detecting and monitoring plant stress arising from Tetranychus urticae (Koch) feeding injury was investigated. The effect of mite density (1–32 mites per 1.5 cm² of leaf) and the duration of the feeding period (1–5 days) on the chlorophyll fluorescence parameters of bean (Phaseolus vulgaris) leaves were examined. Changes in chlorophyll fluorescence parameters were dependent both on mite density and duration of feeding. Decreases in F _o, the initial fluorescence and F _m, the maximum fluorescence led to a decrease in the ratio of variable to maximum fluorescence, F _v/F _m. The decrease in F _v/F _m is typical of the response of many plants to a wide range of environmental stresses and indicates a reduced efficiency of photosystem II (PSII) photochemistry. T _1/2, which is proportional to the pool size of electron acceptors on the reducing side of PSII, was also reduced in response to mite-feeding injury. The leaf chlorophyll content decreased with increasing mite density and duration of feeding but did not appear to contribute to the decrease in F _v/F _m. Chlorophyll fluorescence is an effective method for detecting and monitoring stress in T. urticae-injured bean leaves.     

Photoinhibition of colonial and unicellular <Emphasis Type="Italic">Microcystis</Emphasis> cells in a summer bloom in Lake Taihu

To evaluate the photoinhibition of colonial and unicellular cells of Microcystis aeruginosa under natural conditions, the maximum and effective quantum yields of photosystem II were measured from variable chlorophyll a fluorescence in samples from Lake Taihu during a summer bloom from June 19 to 21, 2006. Diurnal changes in the photoinhibition of Microcystis cells incubated immediately below the surface in clear bottles for 30 min and in situ samples under natural conditions were measured. At solar noon during the three days, the mean values of maximum quantum yield (F _v/F _m) and effective quantum yield (ΔF/F _m′) for unicellular cells (F _v/F _m = 0.15, ΔF/F _m′ = 0.10) were lower than those for colonial cells (F _v/F _m = 0.25, ΔF/F _m′ = 0.15). For in situ samples, the values of F _v/F _m and ΔF/F _m′ for colonial cells at solar noon on the three days (F _v/F _m 0.30, 0.25, 0.29; ΔF/F _m′ 0.24, 0.21, 0.22) were also higher than those of unicellular cells (F _v/F _m 0.26, 0.18, 0.25; ΔF/F _m′ 0.15, 0.11, 0.14). The results indicate that colony formation has a protective effect on Microcystis cells by reducing the occurrence of photoinhibition under high light intensities.     

Age-related differences in frost sensitivity of the photosynthetic apparatus of two Plagiomnium species

Shoots of two species of moss, Plagiomnium undulatum (Hedw.) Kop. and Plagiomnium affine (Funck) Kop., were subjected to freezing at various temperatures. After thawing, the activities of different photosynthetic reactions were determined in relation to the ages of the leaves. Analysis of the fast kinetics of chlorophyll-a fluorescence of individual leaves showed that young and old tissues were considerably less frost tolerant than mature ones. In principle, the pattern of freeze inactivation of photosynthetic reactions resembles that observed in higher plants. The decreases in the amplitude of F_v (variable fluorescence) and the ratio of F_v to F_m (maximum fluorescence) with increasing freezing stress reflect a progressive inactivation of photosystem II (PSII)-mediated electron transport, i.e. inhibition of photoreaction to photochemistry and-or electron donation to the photochemical reaction, and thus a decline in the potential photochemical efficiency of PSII. The insignificant change in the F₀ (constant fluorescence) level during progressive decline of F_v indicates that the excitation-energy transfer between antenna pigments and from those to reaction centres of PSII was little impaired by lethal freezing stress. Sugar analyses of various stem sections showed that ontogenetic variation in the frost tolerance of leaves cannot be attributed to differences in the cellular levels of sucrose, glucose and fructose.Abbreviations and Symbols DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_m maximum fluorescence - F₀ constant (initial) fluorescence - F_v variable fluorescence     

Effect of drought stress on chlorophyll a fluorescence and electrical admittance of shoots in Norway spruce seedlings

Effects of mild and severe soil drought on the water status of needles, chlorophyll a fluorescence, shoot electrical admittance, and concentrations of photosynthetic pigments in needles of seedlings of Picea abies (L.) Karst. were examined under controlled greenhouse conditions. Drought stress reduced shoot admittance linearly with a decrease in shoot water potential (_w) and increase in water deficit (WD) and led to a decrease in concentrations of chlorophyll a, b and carotenoids. Severe water stress (shoot _w=–2.4 MPa) had a negative effect on chlorophyll a fluorescence parameters including PSII activity (F_v/F_m), and the vitality index (R_fd). Variations in these parameters suggest an inhibition of the photosynthetic electron transport in spruce needles. Water stress led to a decrease in the mobility of electrolytes in tissues, which was reflected by decreased shoot electrical admittance. After re-watering for 21 days the WD in needles decreased and the shoot water potential increased. In the re-watered plants, the chloroplast function was restored and chlorophyll a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in the seedlings triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. We conclude that the shoot electrical admittance and photosynthetic electron transport in leaves are closely linked to changes in water status and their decrease is among the initial responses of seedlings to water stress.     

Photoinhibition of the CAM succulent Opuntia basilaris growing in Death Valley: evidence from 77K fluorescence and quantum yield

Summary Diurnal measurements of low temperature (77K) fluorescence at 690 nm (PS II) from north, south, east, and west facing cladode surfaces of Opuntia basilaris in Death Valley, California were made on six occasions during 1985. The absolute levels of F _o(instantaneous fluorescence) and F _m(maximum fluorescence), as well as the ratio F _v/F _m(variable fluorescence, F _m-F _o, over maximum fluorescence), were greater in the north face relative to the other faces. Diurnal decreases in F _o, F _mand F _v/F _mwere found concomitant with increases in incident photon flux area density (PFD). F _v/F _mwas fairly low throughout the year, indicative of photoinhibition, but became somewhat elevated after a spring rain. In early fall the quantum yield of the south face was considerably depressed relative to that of the north face, and corresponding differences were observed in F _v/F _m. A decrease in PFD during growth of glasshouse plants led to an increase in chlorophyll concentration, F _oand F _m, but not F _v/F _m. Although there was some variability in the quantum yield of well watered glasshouse cladodes, a correlation was found between quantum yield and the light and CO₂ saturated rate of photosynthesis. When O. basilaris was water stressed under glasshouse conditions, reductions in quantum yield, F _m, and F _v/F _mwere observed. Reductions in F _v/F _malways indicated a reduced quantum yield, although the converse was not necessarily so in well watered glasshouse plants. The results of this study indicate that O. basilaris is likely to experience photoinhibition throughout much of its life in Death Valley.Abbreviations CAM crassulacean acid metabolism - MPa megapascal - PFD photon flux area density - PS II photosystem II - vater potential - F _o instantaneous fluorescence - F _m maximum fluoescence - F _o variable fluorescence     

Physiological ecology of photosynthesis in Prasiola stipitata (Trebouxiophyceae) from the Bay of Fundy,Canada

The physiological ecology of Prasiola stipitata was examined in situ from two supralittoral sites in the Bay of Fundy (Nova Scotian, Canada) during November 2011, when the population was undergoing major expansion. Photosynthetic parameters (effective quantum yield, Φ_PSII, maximum quantum yield, F_v/F_m, and relative electron transport rate, rETR) were evaluated using chlorophyll fluorescence of PSII. A largely shaded and continuously moist population showed no change in Φ_PSII from one hour after sunrise to sunset in which natural irradiance varied between 3 and 300 μmol photons m^?2 s^?1. High irradiance (up to 1800 μmol photons m^?2 s^?1) had no apparent negative impacts on either quantum yield or rETR, but high desiccation in the field reduced quantum yield to almost zero. When thalli were brought into the laboratory, no change in F_v/F_m was observed up to 60% dehydration; however, there was a steep decline in F_v/F_m between 60% and 85% dehydration. Thalli showed complete recovery of F_v/F_m within one hour of reimmersion in seawater after 2 days of desiccation. After 15 days of desiccation full recovery required 24 h and after 30 days of desiccation thalli showed only partial recovery. These observations confirm the adaptation to photosynthesis in high irradiances and the rapid recovery following extreme desiccation observed in other Prasiola species.     

In vitro regeneration via caulogenesis and brassin-induced shoot conversion of dormant buds from plumular explants of peanut (Arachis hypogaea L. cv `Okrun')

Shoot buds were induced from plumular explants of peanut (Arachis hypogaea L., cv `Okrun') preconditioned on medium containing 2,4-dichlorophenoxyacetic acid and kinetin and then transferred to regeneration medium containing benzylaminopurine and β-naphthoxyacetic acid. Buds differentiated 25 days following transfer to regeneration medium. Each explant produced 30 to 40 buds, but only 4 shoots. The remaining buds were dormant and did not produce shoots when maintained on regeneration medium. Shoots were regenerated continuously, however, when explants were subsequently transferred to shoot conversion medium containing 1 μM brassin, benzylaminopurine and β-naphthoxyacetic acid, respectively. Approximately 5 shoots were harvested every 30 days after transfer to shoot conversion medium for up to 7 months. No further shoot production was observed from explants maintained on regeneration medium without brassin. Regenerated shoots could be rooted and produced viable seeds. This procedure provides an efficient and reliable system for regeneration and transformation studies using cv `Okrun'. Received: 9 April 1997 / Revision received: 27 August 1997 / Accepted: 20 September 1997     

Photosynthetic Properties of Photosystem Ⅱ in Arabidopsis thaliana Ipa1 Mutant

In a previous study, we characterized a high chlorophyll fluorescence Ipal mutant of Arabidopsis thallana, in which approximately 20% photosystem （PS） Ⅱ protein is accumulated. In the present study, analysis of fluorescence decay kinetics and thermoluminescence profiles demonstrated that the electron transfer reaction on either the donor or acceptor side of PSII remained largely unaffected in the Ipa1 mutant. In the mutant, maximal photochemical efficiency （Fv/Fm, where Fm is the maximum fluorescence yield and Fv is variable fluorescence） decreased with increasing light intensity and remained almost unchanged in wildtype plants under different light conditions. The Fv/Fm values also increased when mutant plants were transferred from standard growth light to low light conditions. Analysis of PSll protein accumulation further confirmed that the amount of PSll reaction center protein is correlated with changes in Fv/Fm in Ipal plants. Thus, the assembled PSll in the mutant was functional and also showed increased photosensitivity compared with wild-type plants.     

FLUORESCENCE-BASED MAXIMAL QUANTUM YIELD FOR PSII AS A DIAGNOSTIC OF NUTRIENT STRESS

In biological oceanography, it has been widely accepted that the maximum quantum yield of photosynthesis is influenced by nutrient stress. A closely related parameter, the maximum quantum yield for stable charge separation of PSII, (φ _PSII )_m, can be estimated by measuring the increase in fluorescence yield from dark-adapted minimal fluorescence (F_o) to maximal fluorescence (F_m) associated with the closing of photosynthetic reaction centers with saturating light or with a photosynthetic inhibitor such as 3′-(3,4-dichlorophenyl)-1′,1′-dimethyl urea (DCMU). The ratio F_v/F_m (= (F_m− F_o)/F_m) is thus used as a diagnostic of nutrient stress. Published results indicate that F_v/F_m is depressed for nutrient-stressed phytoplankton, both during nutrient starvation (unbalanced growth) and acclimated nutrient limitation (steady-state or balanced growth). In contrast to published results, fluorescence measurements from our laboratory indicate that F_v/F_m is high and insensitive to nutrient limitation for cultures in steady state under a wide range of relative growth rates and irradiance levels. This discrepancy between results could be attributed to differences in measurement systems or to differences in growth conditions. To resolve the uncertainty about F_v/F_m as a diagnostic of nutrient stress, we grew the neritic diatom Thalassiosira pseudonana (Hustedt) Hasle et Heimdal under nutrient-replete and nutrient-stressed conditions, using replicate semicontinuous, batch, and continuous cultures. F_v/F_m was determined using a conventional fluorometer and DCMU and with a pulse amplitude modulated (PAM) fluorometer. Reduction of excitation irradiance in the conventional fluorometer eliminated overestimation of F_o in the DCMU methodology for cultures grown at lower light levels, and for a large range of growth conditions there was a strong correlation between the measurements of F_v/F_m with DCMU and PAM (r² = 0.77, n = 460). Consistent with the literature, nutrient-replete cultures showed consistently high F_v/F_m (∼0.65), independent of growth irradiance. Under nutrient-starved (batch culture and perturbed steady state) conditions, F_v/F_m was significantly correlated to time without the limiting nutrient and to nutrient-limited growth rate before starvation. In contrast to published results, our continuous culture experiments showed that F_v/F_m was not a good measure of nutrient limitation under balanced growth conditions and remained constant (∼0.65) and independent of nutrient-limited growth rate under different irradiance levels. Because variable fluorescence can only be used as a diagnostic for nutrient-starved unbalanced growth conditions, a robust measure of nutrient stressed oceanic waters is still required.     

Photosynthetic activity of poikilochlorophyllous desiccation tolerant plant <Emphasis Type="Italic">Reaumuria soongorica</Emphasis> during dehydration and re-hydration

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as Chl content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub during dehydration and rehydration. The net photosynthetic rate (P _N), maximum photochemical efficiency of PS2 (variable to maximum fluorescence ratio, F_v/F_m), quantum efficiency of non-cyclic electron transport of PS2, and Chl content decreased, but non-photochemical quenching of fluorescence and carotenoid content increased in stems with the increasing of drought stress. 6 d after re-hydration, new leaves budded from stems. In the re-watered plants, the chloroplast function was restored and Chl a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in plant triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. Thus R. soongorica plants are able to sustain drought stress through leaf abscission and keep part of Chl content in stems.