Partial floral evocation by exogenous cytokinin in the long-day plant Sinapis alba

A single application of benzyladenine (BA) at 4.3 × 10⁻⁵ M to the shoot apex of vegetative plants of Sinapis alba L. causes an increase of the import of [¹⁴C]-assimilates in the apical bud and of the starch content. BA also produces the breakdown of the vacuoles into smaller vacuolar units and an increase of the mitotic index leading to nearsynchronization of the meristem cell population in G₁. All these changes normally occur in the meristem of Sinapis plants during the transition to flowering (full evocation). Other events normally associated with full evocation are not caused by BA which thus produces only partial evocation. The part of evocation produced by BA is different from that caused by high irradiance.     

Cytokinin Fluxes during Floral Induction in the Long Day Plant Sinapis alba L

Sinapis alba is a long-day (LD) plant that can be induced to flower by a single LD. A number of changes normally occurring in the meristem of plants subjected to the LD can be produced in short day by a single application of a cytokinin to the apical bud. However, flower buds are not produced indicating that evocation by the cytokinin is only partial. In this work, the cytokinin content of root exudate, obtained under vacuum, and of leaf exudate, obtained by the EDTA-method, has been analyzed comparatively in vegetative and induced plants, using reversed-phase HPLC coupled to the Amaranthus bioassay. The results show that, as early as 16 hours after the start of the LD, there is an increase of cytokinin activity in both the root and leaf exudates of induced plants. These observations fit nicely with previous results obtained on Sinapis, and they indicate that cytokinins are part of the floral stimulus in this species.     

Carbon Assimilation and Leaf Water Status in Sugar Beet Leaves during a Simulated Natural Light Regimen

Carbon assimilation and leaf water status were studied in sugar beet (Beta vulgaris L., Klein E-type multigerm) leaves during a light period in which illumination either increased rapidly to full irradiance or changed gradually in a sinusoidal manner as generally occurs during a natural day. A light regimen that simulated the light of a natural day was produced by adjusting irradiance with a neutral-density filter under the control of a computer. Under this light regimen, photosynthesis, transpiration, and stomatal conductance followed the irradiance pattern very closely and ribulose bisphosphate carboxylase was nearly fully activated. When illumination was increased rapidly at the beginning of a light period, transpiration also increased quickly, causing leaves to wilt to some extent. The activation state of ribulose bisphosphate carboxylase increased to only 52%, but ribulose bisphosphate level was nearly twice as high as during the simulated natural day. In spite of the differences in activation state and ribulose bisphosphate levels, photosynthesis rates were very similar under both regimens. Nevertheless, differences in parameters between leaves under the two irradiance regimens can affect how a plant responds to internal or external factors, and therefore, the rate at which irradiance increases at the beginning of a light period is an important consideration when interpreting data.     

RNA Synthesis in the Apex of Sinapis alba in Transition to Flowering

RNA synthesis in the 1.5 mm apex during floral evocation ofSinapis alba (a long-day plant) was measured using a double-labellingtechnique to compare precursor incorporation in evoked and vegetativeapices, followed by analysis by gel electrophoresis and oligo(dT)-cellulosechromatography. In plants induced to flower by exposure to asingle long day, higher levels of RNA synthesis showed a two-phasepattern, the first increase starting very early in evocation,from 10 h after the start of the long day. This early extrasynthesis is of rRNA, and then also sRNA.² Subsequently higherlevels of RNA synthesis, especially rRNA and 5S RNA, are shownfrom 36 h. At no time was such additional synthesis of the RNAfraction retained on oligo(dT)-cellulose detected. The experimentwas repeated with plants subjected to a single displaced shortday, a treatment which also induces flowering, and similar higherRNA synthesis found in evoked apices. These results were comparedwith those obtained in two non-inductive treatments which resultin some features of floral evocation: a single short day athigh intensity light, and a single treatment with benzyladenine.Neither gave rise to additional RNA synthesis, and thus theobserved high levels of synthesis of rRNA and sRNA seem to beassociated with other features of floral evocation.     

Mesophyll conductance to CO₂, assessed from online TDL-AS records of ¹³CO₂ discrimination, displays small but significant short-term responses to CO₂ and irradiance in Eucalyptus seedlings

Mesophyll conductance (g(m)) is now recognized as an important limiting process for photosynthesis, as it results in a significant decrease of CO(2) diffusion from substomatal cavities where water evaporation occurs, to chloroplast stroma. Over the past decade, an increasing number of studies proposed that g(m) can vary in the short term (e.g. minutes), but these variations are still controversial, especially those potentially induced by changing CO(2) and irradiance. In this study, g(m) data estimated with online (13)C discrimination recorded with a tunable diode laser absorption spectrometer (TDL-AS) during leaf gas exchange measurements, and based on the single point method, are presented. The data were obtained with three Eucalyptus species. A 50% decrease in g(m) was observed when the CO(2) mole fraction was increased from 300 μmol mol(-1) to 900 μmol mol(-1), and a 60% increase when irradiance was increased from 200 μmol mol(-1) to 1100 μmol mol(-1) photosynthetic photon flux density (PPFD). The relative contribution of respiration and photorespiration to overall (13)C discrimination was also estimated. Not taking this contribution into account may lead to a 50% underestimation of g(m) but had little effect on the CO(2)- and irradiance-induced changes. In conclusion, (i) the observed responses of g(m) to CO(2) and irradiance were not artefactual; (ii) the respiratory term is important to assess absolute values of g(m) but has no impact on the responses to CO(2) and PPFD; and (iii) increasing irradiance and reducing the CO(2) mole fraction results in rapid increases in g(m) in Eucalyptus seedlings.     

The dependence of leaf hydraulic conductance on irradiance during HPFM measurements: any role for stomatal response?

This paper examines the dependence of whole leaf hydraulic conductance to liquid water (K(L)) on irradiance when measured with a high pressure flowmeter (HPFM). During HPFM measurements, water is perfused into leaves faster than it evaporates hence water infiltrates leaf air spaces and must pass through stomates in the liquid state. Since stomates open and close under high versus low irradiance, respectively, the possibility exists that K(L) might change with irradiance if stomates close tightly enough to restrict water movement. However, the dependence of K(L) on irradiance could be due to a direct effect of irradiance on the hydraulic properties of other tissues in the leaf. In the present study, K(L) increased with irradiance for 6 of the 11 species tested. Whole leaf conductance to water vapour, g(L), was used as a proxy for stomatal aperture and the time-course of changes in K(L) and g(L) was studied during the transition from low to high irradiance and from high to low irradiance. Experiments showed that in some species K(L) changes were not paralleled by g(L) changes. Measurements were also done after perfusion of leaves with ABA which inhibited the g(L) response to irradiance. These leaves showed the same K(L) response to irradiance as control leaves. These experimental results and theoretical calculations suggest that the irradiance dependence of K(L) is more consistent with an effect on extravascular (and/or vascular) tissues rather than stomatal aperture. Irradiance-mediated stimulation of aquaporins or hydrogel effects in leaf tracheids may be involved.     

Mannitol metabolism in cultured plant cells

Non-structural storage carbohydrates were measured in 9-day-old barley ( Hordeum vulgare L. cv. Brant) primary leaves. Accumulation rates of starch, sucrose and total non-structural carbohydrates (TNC) were approximately linear when measured between 2- and 12-h of light. Progressively higher TNC accumulation rates were observed at higher irradiance levels (i.e., comparing 250, 550 and 1050 ·mol m⁻² s⁻¹). Synthesis of a low-molecular-weight fructan also was enhanced by high irradiances. Low irradiance treatments decreased leaf sucrose levels and there was a corresponding increase in the lag period preceding starch synthesis in the light. Increased starch accumulation rates were usually observed when sucrose concentrations were high. These and other results suggested that cytosolic sucrose concentrations affected starch metabolism in the chloroplast. However, sucrose accumulation rates increased and starch storage decreased when barley seedlings were transferred from 20 to 10°C during the light period. Lowering the night temperature from 20 to 10°C for a single dark period 8-days after planting increased the TNC content of barley primary leaves at the beginning of day nine. In this experiment, TNC accumulation rates of treated and untreated leaves were similar. Changes in the accumulation rate of TNC were usually observed within 2- to 4-h after barley seedlings were exposed to altered environmental conditions. Monitoring rapid changes in leaf carbohydrate levels is a sensitive method for assessing the effects of environmental treatments on photosynthetic metabolism.     

The thalamic intergeniculate leaflet mediates locomotor activity-induced reversal of phenotype in photoperiod nonresponsive Siberian hamsters

The role of the intergeniculate leaflet of the thalamus (IGL) in photoperiod responsiveness was examined in a laboratory-selected line of photoperiod nonresponsive (NR) Siberian hamsters. NR hamsters fail to exhibit typical winter-type responses (i.e., gonadal regression and development of winter-type pelage) when exposed to short day lengths (e.g., 10 h of light/day). Earlier studies revealed that NR hamsters will exhibit winter-type responses when exposed to short photoperiod if they are given free access to a running wheel. The present study tested the hypothesis that this locomotor activity-induced reversal of phenotype is dependent on the IGL. Male NR hamsters underwent destruction of the IGL prior to being housed in short day lengths in cages equipped with running wheels. Activity rhythms were monitored for 8 weeks, after which time pelage response and paired testes weights were obtained. In contrast to sham-operated NR animals given access to running wheels, IGL-ablated animals showed no increase in the duration of nocturnal running wheel activity and became active later in the night than sham-lesioned animals. Lesioned animals also failed to exhibit the typical short photoperiod-induced gonadal regression and pelage molt. The results implicate the IGL in the mechanism by which running wheel activity can influence photoperiodic responses.     

Exploiting opportunistic observations to estimate changes in seasonal site use: An example with wetland birds

Nonsystematically collected, a.k.a. opportunistic, species observations are accumulating at a high rate in biodiversity databases. Occupancy models have arisen as the main tool to reduce effects of limited knowledge about effort in analyses of opportunistic data. These models are generally using long closure periods (e.g., breeding season) for the estimation of probability of detection and occurrence. Here, we use the fact that multiple opportunistic observations in biodiversity databases may be available even within days (e.g., at popular birding localities) to reduce the closure period to 1 day in order to estimate daily occupancies within the breeding season. We use a hierarchical dynamic occupancy model for daily visits to analyze opportunistic observations of 71 species from nine wetlands during 10 years. Our model derives measures of seasonal site use within seasons from estimates of daily occupancy. Comparing results from our “seasonal site use model” to results from a traditional annual occupancy model (using a closure criterion of 2 months or more) showed that our model provides more detailed biologically relevant information. For example, when the aim is to analyze occurrences of breeding species, an annual occupancy model will over‐estimate site use of species with temporary occurrences (e.g., migrants passing by, single itinerary prospecting individuals) as even a single observation during the closure period will be viewed as an occupancy. Alternatively, our model produces estimates of the extent to which sites are actually used. Model validation based on simulated data confirmed that our model is robust to changes and variability in sampling effort and species detectability. We conclude that more information can be gained from opportunistic data with multiple replicates (e.g., several reports per day almost every day) by reducing the time window of the closure criterion to acquire estimates of occupancies within seasons.     

Peripheral feedback mechanisms acting on the central pattern generators for locomotion in fish and cat

The feedback mechanisms taking part in the control of locomotion in cat and fish are reviewed, particularly with regard to position- and movement-related feedback. It is shown that in both fish and cat there is a powerful position-dependent negative feedback which will act only in the position range where the muscle activity normally changes, e.g., from extensor to flexor activity. In addition, there is positive feedback in the middle of the movement range which will act in certain conditions, e.g., to promote and maintain flexor activity during the flexion of the hind limb.     

Differential responses of Nostoc sphaeroides and Arthrospira platensis to solar ultraviolet radiation exposure

During October to December 2003 we carried out experiments to assess the impact of high solar radiation levels (as normally occurring in a tropical region of Southern China) on the cyanobacteria Nostoc sphaeroides and Arthrospira (Spirulina) platensis. Two types of experiments were done: a) Short-term (i.e., 20 min) oxygen production of samples exposed to two radiation treatments (i.e., PAR+UVR—280–700 nm, and PAR only—400–700 nm, PAB and P treatments, respectively), and b) Long-term (i.e., 12 days) evaluation of photosynthetic quantum yield (Y) of samples exposed to three radiation treatments (i.e., PAB; PA (PAR+UV-A, 320–700 nm) and P treatments, respectively). N. sphaeroides was resistant to UVR, with no significant differences (P>0.05) in oxygen production within 20 min of exposure, but with a slight inhibition of Y within hours. A fast recovery of Y was observed after one day even in samples exposed to full solar radiation. A. platensis, on the other hand, was very sensitive to solar radiation (mainly to UV-B), as determined by oxygen production and Y measurements. A. platensis had a circadian rhythm of photosynthetic inhibition, and during the first six days of exposure to solar radiation, it varied between 80 and 100% at local noon, but cells recovered significantly during afternoon hours. There was a significant decrease in photosynthetic inhibition after the first week of exposure with values less than 50% at local noon in samples receiving full solar radiation. Samples exposed to PA and P treatments recovered much faster (within 2–3 days), and there were no significant differences in Y between the three radiation treatments when irradiance was low (late afternoon to early morning). Long-term acclimation seems to be important in A. platensis to cope with high UVR levels however, it is not attained through the synthesis of UV-absorbing compounds but it seems to be mostly related to adaptive morphological changes.     

On the problem of ecological evolution in Volvox

A concept of evolution of ontogeny based on the original data on the comparative biology of volvox development and published paleoclimatic data is presented. Previously, we have demonstrated that evolutionary reorganizations of asexual development in Volvox are related to the changes in the rate, diel rhythms, and light/dark control of cell divisions. Here, we propose that such rearrangements could take place during much of Cenozoic time (e.g., in Eocene and Miocene) as adaptations to short and warm winter day in high latitudes. This proposal is confirmed by experimental data on culturing volvox species with different types of development under short photoperiod.     

Photoinhibition and the xanthophyll cycle are not enhanced in the salt-acclimated halophyte Artimisia anethifolia

The effects of high salinity (up to 400 m M NaCl) on photosystem II (PSII) photochemistry, photoinhibition and the xanthophyll cycle were investigated in the halophyte Artimisia anethifolia grown under outdoor conditions. In order to examine the changes in PSII photochemistry, photoinhibition, thermal dissipation associated with the xanthophyll cycle in salt-acclimated plants, the experiments were conducted at midday on a clear day (maximal irradiance 1500 μmol m⁻¹ s⁻¹) and on a cloudy day (maximal irradiance 700 μmol m⁻¹ s⁻¹), respectively. With increasing salt concentration, the accumulation of sodium and chloride in leaves increased considerably while the relative growth rate and CO₂ assimilation rate decreased significantly. Salinity induced no effects on PSII photochemistry, thermal energy dissipation, and the contents of the xanthophyll cycle pigments either on a clear day or on a cloudy day. However, when compared with those on a cloudy day, PSII photochemistry decreased and thermal energy dissipation increased significantly in both control and salt-acclimated plants on a clear day. The levels of zeaxanthin and antheraxanthin at the expense of violaxanthin were higher on a clear day than on a cloudy day. The results suggest that photoinhibition and the xanthophyll cycle were not induced by high salinity but by high light only in A. anethifolia plants. The results also suggest that A. anethifolia showed high resistance not only to high salinity, but also to photoinhibition even when it was treated with high salinity and exposed to full sunlight.     

Photosynthetic characteristics of leaves developed at different irradiances and temperatures: an extension of the current hypothesis

Photosynthetic characteristics at high measurement irradiance were analyzed for single leaves of two C₃ and one C₄ species grown under twenty one combinations of irradiance level, irradiance duration, and air temperature in order to test the idea that photosynthetic characteristies developed by leaves in different environments are controlled by the daily amount of photosynthesis. Photosynthetic rates per unit area and mesophyll conductances at 25°C and air levels of CO₂ and O₂, and parameters for two photosynthesis models were used to characterize the photosynthetic properties of the leaves. Leaves with highest values of the photosynthetic parameters for each species were often developed in environments with irradiance levels below saturation for photosynthesis, and with only 12 hours of irradiance per day. Lower air temperature during growth increased the photosynthetic characteristics for a given irradiance regime. Photosynthetic characteristics had higher correlation coefficients with daily photosynthesis of mature leaves divided by 24-hour leaf elongation rates of young leaves, than with daily photosynthesis alone, indicating that photosynthetic characteristics may be related to a balance between photosynthesis and leaf expansion.     

An adaptive hierarchical test procedure for selecting safe and efficient treatments

We consider the situation where during a multiple treatment (dose) control comparison high doses are truncated because of lack of safety and low doses are truncated because of lack of efficacy, e.g., by decisions of a data safety monitoring committee in multiple interim looks. We investigate the properties of a hierarchical test procedure for the efficacy outcome in the set of doses carried on until the end of the trial, starting with the highest selected dose group to be compared with the placebo at the full level alpha. Left truncation, i.e., dropping doses in a sequence starting with the lowest dose, does not inflate the type I error rate. It is shown that right truncation does not inflate the type I error if efficacy and toxicity are positively related and dose selection is based on monotone functions of the safety data. A positive relation is given e.g. in the case where the efficacy and toxicity data are normally distributed with a positive pairwise correlation. A positive relation also applies if the probability for an adverse event is increasing with a normally distributed efficacy outcome. The properties of such truncation procedures are investigated by simulations. There is a conflict between achieving a small number of unsafely treated patients and a high power to detect safe and efficient doses. We also investigated a procedure to increase power where a reallocation of the sample size to the truncated treatments and the control remaining at the following stages is performed.     

Internalization of seasonal time

Orientation of physiology and behavior in time is a major adaptation common to many organisms and represents a significant challenge. In the face of predictable seasonal changes in climatic factors, and as a result of natural selection, many mammals now restrict their reproductive efforts to the fraction of the year when conditions of temperature, food, and water are most favorable for successful weaning of offspring. Changes in day length figure prominently in the synchronization of mammalian seasonal reproductive cycles. In summer breeders (e.g., several hamster, vole, and mouse species), decreasing summer day lengths induce reproductive involution. During this interval, neuroendocrine mechanisms analogous to a simple reference memory permit discrimination of stimulatory from inhibitory photoperiods. The non-reproductive phenotype is sustained for several months thereafter by the inhibitory short days of late summer, autumn, and early winter. Mid-winter reactivation of the reproductive system is triggered after 20-25 weeks of exposure to decreasing or short days by an interval timer that renders the reproductive neuroendocrine system refractory to short days. Recent work that has explored formal and physiological properties of this photorefractoriness interval timer has identified sex differences, neural substrates, and changes in hypothalamic gene expression that may participate in the measurement of seasonal time.     

Photosynthetic characteristics of leaves developed at different irradiances and temperatures: an extension of the current hypothesis.

Photosynthetic characteristics at high measurement irradiance were analyzed for single leaves of two C₃ and one C₄ species grown under twenty one combinations of irradiance level, irradiance duration, and air temperature in order to test the idea that photosynthetic characteristics developed by leaves in different environments are controlled by the daily amount of photosynthesis. Photosynthetic rates per unit area and mesophyll conductances at 25°C and air levels of CO₂ and O₂, and parameters for two photosynthesis models were used to characterize the photosynthetic properties of the leaves. Leaves with highest values of the photosynthetic parameters for each species were often developed in environments with irradiance levels below saturation for photosynthesis, and with only 12 hours of iradiance per day. Lower air temperature during growth increased the photosynthetic characteristics for a given irradiance regime. Photosynthetic characteristics had higher correlation coefficients with daily photosynthesis of mature leaves divided by 24-hour leaf elongation rates of young leaves, than with daily photosynthesis alone, indicating that photosynthetic characteristics may be related to a balance between photosynthesis and leaf expansion.     

Down regulation of hypertrophied follicular cell volume in thyroid hyperplastic gland

In the present study, changes in thyroid follicular cell volume and its regulation have been investigated during the early involution of a hyperplastic goitre. Male Wistar rats were administered an iodine deficient diet for 6 months with propylthiouracil (PTU, 0.15%) during the last two months. At the end of iodine deficiency (day 0), some rats were killed and the others received a normal iodine diet. These rats were killed after different periods of iodine refeeding. Thyroid follicular cell volume was very high in hyperplastic gland whereas thyroid protein concentration was low. Thyroid follicular cell volume quickly decreased when rats were normally iodine refed, whereas thyroid protein concentration increased. Electron microscopal observations showed that thyroid follicular cells retained their endocrine aspect in hyperplastic state and throughout the iodine refeeding period. Using concomitant stereological and biochemical techniques, it is shown that the amount of cellular iodide and an unknown iodinated compound strongly increased during the early iodine refeeding. Plasma TSH was high on day 0 and remained at this level until day 8 whereas plasma T3 and T4 were low on day 0 and remained at this low level until day 4. The present data show that the involution of thyroid follicular cell volume is induced by iodide and mediated by an iodinated compound at least in the initial phase, and is independent of plasma TSH, T3, T4, so indicating the involvement of a thyroid autoregulatory mechanism. These changes in cell volume may be of importance in ion transport, i.e. in the metabolism of thyroid follicular cell during the early involution of the hyperplastic goitre.     

Physiological behaviour of Arthrospira (Spirulina) maxima during acclimation to changes in irradiance

The physiological behaviour of Arthrospira (Spirulina) maxima during acclimation to sudden changes in irradiance from high (HL) to low light (LL) and vice versa was studied by following parameters concerning growth rate, pigment, carbohydrate and protein cell contents. Applying first order kinetics, the specific acclimation rates for the parameters considered were calculated. During HL to LL shift, pigments increased to compensate for a reduction in growth irradiance in order to maintain relatively high growth rates, whereas carbohydrates decreased at the highest rate. The synthesis of phycobiliproteins proceeded at a rate similar or little higher than that of chlorophyll a, indicating their importance in the light harvesting at low irradiance. During LL to HL shift, carbohydrate biosynthesis was increased, whereas pigment and protein cell contents decreased. The kinetic analysis suggested that the pigment decrease could be accounted for both by dilution through growth and in vivo degradation. During this transition, the initially high cell pigment content gave rise to a very heavy carbohydrate synthesis, which for a short time, after the shift to HL conditions, overshot the final steady-state. In the same period the specific growth rate also increased notably, overshooting the μmax. The acclimation rates of the measured parameters were faster during LL to HL transition then during the reverse. The physiological response of A. maxima during the acclimation to sudden irradiance shifts points out the ability of this cyanobacterium to alter light harvesting and highlights again the key role of carbohydrates when the cells underwent an energy crisis during down-shift. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of irradiance upon the population of Pseudomonas coronafaciens in leaves and symptom expression of halo blight of rye.

The toxin-induced chlorosis caused by Pseudomonas coronafaciens is influenced by irradiance. Three levels of irradiance caused differences in symptom expression but did not affect the rate of increase or final population of viable cells of P. coronafaciens in rye leaves. Distinct and faint halo blight symptoms appeared in 3--4 days in full light (1425 microW cm-2), and 58% shade (598 microW cm-2) respectively. No symptoms or only faint symptoms appeared after 7 days at 86% shade (202 microW cm-2). When plants kept in 58 and 86% shade were moved to full light 5 days after inoculation, lesion size and chlorosis increased rapidly during the next 2 days. On the 7th day after inoculation, the size of lesions from the 58 and 86% shade treatments exceeded those in full light by 2.5 and 5 times, respectively. A chlorosis index based on lesion size and chlorophyll loss within lesions also reflected this trend although chlorophyll loss was greater in lesions in full light for 7 days. Conditions of low irradiance such as that caused by overcast weather and (or) a dense leaf canopy followed by bright sunshine can cause greater losses from halo blight than a continuous period of high irradiance. Sympton expression may be masked by low irradiance but increase of inoculum is not impaired. Although increased light enhances chlorosis, toxin diffusion or production may be reduced.