Snapper rest where they see best: visually mediated choice behaviour of Australasian snapper (Chrysophrys auratus)

The sensory physiology and behaviour of many fish species are strongly affected by light. This short study demonstrates that in the Australasian snapper (Chrysophrys auratus) absolute light intensity governs visual acuity and also guides preference behaviour, with fish choosing to ‘rest where they see best’. Use of the optomotor response to test visual acuity at four light intensities (0.01, 0.05, 1 and 3 μmol s^?1 m²), showed that visual acuity (measured as directional bias) was best at a light intensity of 0.05 μmol s^?1 m² (84.9% directional bias), but weakened at the highest and lowest light intensity (41.1 and 35.3%). When provided with a choice of the same four light environments fish also spent most time in the 0.05 μmol s^?1 m² light environment, while the highest and lowest intensity light environments were usually avoided. Acclimated light intensity (that is daytime light intensity of the home aquarium) was also 0.05 μmol s^?1 m²_. By selecting an environment where visual function is optimised, C. auratus are also optimising the ability to search for prey and to detect predators. The results are discussed with reference to habitat utilisation and thoughts for trawl fishing practices.     

Depth selection and life history strategies as mutually exclusive responses to risk of fish predation in <Emphasis Type="Italic">Daphnia</Emphasis>

We tested if pelagic crustaceans of the genus Daphnia use different anti-predator defences in environmental conditions that do or do not offer deep refuge from planktivorous fish. We kept Daphnia catawba in two series of 9-m deep enclosures with and without caged cyprinid fish Phoxinus eos. In one series of enclosures, Daphnia could select its depth of residence and hide in deep dark water layers to avoid anticipated fish predation, while in another series of enclosures, a plankton net barrier fixed at 2-m depth forced them to stay in subsurface zone exposed to fish kairomones. We compared depth residence and migratory behaviour strategies with life history strategies (body size and size at first reproduction, diapause induction) in Daphnia exposed or not to fish kairomones with or without deep refuge. In deep enclosures with fish, Daphnia spent daytime hours in deep dark layers while at night, they resided closer to the water surface. Yet, no change in life history parameters of migrating individuals was observed compared to the fish-free conditions. In enclosures with fish, where the net barrier forced Daphnia to reside in subsurface zone, they produced smaller offspring, matured at smaller size and achieved lower maximum body length compared to the fish-free conditions. However, they did not produce diapausing eggs. Our experimental study supports the hypothesis that diel vertical migration behaviour with daytime residence in deep, dark water are the preferred antipredator strategy chosen by Daphnia facing anticipated fish predation over life history changes such as reduced size and low growth rate which are used when dark deep refuge is not present or accessible.     

Response of diapausing eggs hatching to changes in temperature and the presence of fish kairomones

It has been hypothesized that the production of diapausing eggs in Daphnia can be induced by fish kairomones. A population of Daphnia could survive severe predation using this predator avoidance strategy. However, in changing environments, diapausing eggs experience various temperature conditions, and hatchlings at emergence may be exposed to the same predation risks as their mothers. Therefore, staying in diapause or an immediate response upon hatching to available environmental information could be important for hatchling survival. For this study, we investigated the impact of water temperature (10, 15, 20, and 25°C) in the presence and absence of fish kairomones (Lepomis macrochirus) on the hatching success of resting eggs (D. galeata). Results show that no diapausing eggs hatched at the lowest temperature (10°C), and the highest hatch percentage occurred at 15°C. Although higher water temperatures reduced hatching success, diapausing eggs hatched more quickly. The number of hatchlings was significantly higher after exposure to fish kairomones, and this was more noticeable at higher temperatures (20 and 25°C). The present results suggest that the diapausing eggs were produced as a predator avoidance strategy in Daphnia; however, the presence of fish works as a positive signal to increase hatchlings when the diapausing stage is terminated.     

Predator-induced diapause in Daphnia magna may require two chemical cues

The production of diapausing eggs by Daphnia magna stimulated by fish exudates can be explained as an anti-predator defence ensuring genome protection in periods of high risk from fish predation. The combined effects on the induction of D. magna diapause of an “alarm” chemical originating from injured conspecific prey and fish kairomones were tested. The results of the experiment showed that the cues when present together promote both the production of ephippial eggs and male formation, indicating their role in the synchronization of the entire mode of Daphnia sexual reproduction. Ephippial eggs were only produced in the presence of both fish kairomone and conspecific alarm chemicals, while male offspring occurred in the treatments where both, one or none of the cues were present. However, production of males was the highest when both cues were provided. D. magna responded similarly to the tested cues whether or not the hypothetical alarm substance associated with predator odour came from Daphnia specimens actually eaten by fish or from crushed conspecific individuals. However, chemicals from crushed chironomid larvae combined with fish kairomones did not induce a similar response in D. magna. The relative advantage of utilization of alarm cues or predator kairomones in the induction of defence responses in prey organisms is discussed. Received: 8 June 1998 / Accepted: 11 January 1999     

The prevalence of diapause response to risk of size-selective predation in small- and large-bodied prey species

The prevalence of diapause response to the simulated threat of fish predation was compared in three species of planktonic crustaceans of the genus Daphnia (D. magna, D. pulicaria and D. longispina), which due to their different body size vary in vulnerability to fish predation pressure in natural conditions. Higher incidence of diapause response was presumed in the larger-bodied species, which due to their higher conspicuousness and higher energetic content experience the greatest size-selective pressure from visually foraging fish. Small-bodied species were expected to utilize less costly yet less effective active defences, e.g. lower conspicuousness achieved due to slower body growth when facing risk of size-selective predation. Proportions of females forming diapausing structures as well as females body size at the maturation period were compared in experimental beakers containing or not containing fish kairomones (chemical cues of fish predation) in a few clonal lineages of 3 species of Daphnia derived from a single lake inhabited by fish. The highest incidence of diapause response to fish kairomones was observed in D. magna (when measured both as proportion of sensitive individuals and as proportion of inductive clones), the largest of the tested Daphnia species. The lowest proportion of individuals and clones that employed diapause was reported in the smallest tested species, D. longispina. In addition, the large-bodied Daphnia (D. magna) showed a greater reduction in body size in response to fish kairomones than the small-bodied species (D. longispina). The results of the present study support the assumption of higher prevalence of diapause response to the threat of selective predation in larger and more vulnerable prey species.     

Enhanced growth at low population density in Daphnia: the absence of crowding effects or relief from visual predation?

1. It has been suggested that chemical information from crowded populations of an animal such as Daphnia carries a cue indicating imminent food limitation, and we suggest that in the presence of fish kairomones, it may also convey a hint of the need to enhance antipredation defences. 2. We performed two‐factorial experiments with Daphnia grown in flow‐through plankton chambers in medium containing high levels of Scenedesmus food plus chemical information on either low or high population density levels and in the presence or absence of fish chemical cues (kairomones) and recorded (i) the effects on Daphnia growth rate and reproduction, and (ii) the effects on Daphnia depth selection. Further depth‐selection experiments were performed to test the reaction of Daphnia to crowding information at different Daphnia concentrations and to test its effect on daytime and night‐time depth selection by different Daphnia instars in the presence of kairomones. 3. The effects of crowding information alone (in the absence of kairomones) were weak and were not significantly strengthened by the addition of kairomones. The effects of kairomones alone (in the absence of crowding information) were much stronger and were increased by the presence of crowding chemicals: Daphnia selected greater depths in daylight (the later the instar and the larger its body size, the greater the depth), their body growth was slower and daily reproductive investment reduced, compared with Daphnia grown in the absence of crowding information. This suggested that crowding chemicals carry a cue indicating the need to invest more into antipredation defences. 4. The adaptive significance of these effects was confirmed by the differential vulnerability to predation of the Daphnia when offered as prey to live roach after being grown for 6 days either in the presence (higher vulnerability) or in the absence (lower vulnerability) of information on high density. 5. The strong interaction between crowding information and fish kairomones may be explained either as the reaction to a cue indicating impending food stress or as the reaction to a signal of increased predation risk. While the former scenario is already known from crowding studies, the latter is a novel idea that stems from the old concept of ‘low‐density anti‐predation refuge’. The two scenarios are not mutually exclusive: each stems from the need to invest in survival rather than in growth and reproduction [Corrections were made to this paragraph after first online publication on 4 April 2012].     

The reaction distance of a planktivorous fish (<Emphasis Type="Italic">Scardinius erythrophthalmus</Emphasis>) and the evasiveness of its prey (<Emphasis Type="Italic">Daphnia pulex </Emphasis>×<Emphasis Type="Italic"> pulicaria</Emphasis>) under different artificial light spectra

Artificial light at night may affect mortality risk in prey from visually oriented predators because the effect of the artificial light spectrum may differ for a predator’s visual prey detection and for prey evasiveness. To test this, we conducted two types of experiment. First, we assessed the reaction distance and swimming speed of juvenile rudd (Scardinius erythrophthalmus) allowed to forage on juvenile Daphnia pulex?×?pulicaria under three artificial light sources: halogen, high pressure sodium (HPS), and metal halide bulbs, at the same light intensity. Second, we assessed the evasiveness of D. pulex?×?pulicaria under the same artificial light sources and in darkness (as a control), in the presence and absence of chemical information on predation risk (kairomones) of juvenile rudd. We found that while both reaction distance and swimming speed of fish was greater under halogen compared to HPS, and similar under metal halide light compared to halogen and HPS, the evasiveness of Daphnia was greater under halogen and HPS-generated light than under metal halide light. The results suggest a possible mismatch of Daphnia’s behavioural response under metal halide light to predicted predation risk, and thus a possible threat to predator–prey balance in a lake ecosystem.     

Impact of light quality on a native Louisiana Chlorella vulgaris/Leptolyngbya sp. co‐culture

Light effect on cultures of microalgae has been studied mainly on single species cultures. Cyanobacteria have photosynthetic pigments that can capture photons of wavelengths not available to chlorophylls. A native Louisiana microalgae (Chlorella vulgaris ) and cyanobacteria (Leptolyngbya sp.) co‐culture was used to study the effects of light quality (blue–467 nm, green–522 nm, red–640 nm and white–narrow peak at 450 nm and a broad range with a peak at 550 nm) at two irradiance levels (80 and 400 μmol m^?2 s^?1) on the growth, species composition, biomass productivity, lipid content and chlorophyll‐a production. The co‐culture shifted from a microalgae dominant culture to a cyanobacteria culture at 80 μmol m^?2 s^?1. The highest growth for the cyanobacteria was observed at 80 μmol μmol m^?2 s^?1 and for the microalgae at 400 μmol m^?2 s^?1. Red light at 400 μmol m^?2 s^?1 had the highest growth rate (0.41 d^?1), biomass (913 mg L^?1) and biomass productivity (95 mg L^?1 d^?1). Lipid content was similar between all light colors. Green light had the highest chlorophyll‐a content (1649 μg/L). These results can be used to control the species composition of mixed cultures while maintaining their productivity.     

Sulphate transport in the cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. leopoliensis) PCC 7942

Synechococcus R-2 (PCC 1942) actively accumulates sulphate in the light and dark. Intracellular sulphate was 1.35 ± 0.23 mol m^?3 (light) and 0.894 ± 0.152 mol m^?3 (dark) under control conditions (BG-11 media: pH_o, 7.5; [SO₄^2?]_o, 0.304 mol m^?3). The sulphate transporter is different from that found in higher plants: it appears to be an ATP-driven pump transporting one SO₄^2?/ATP [ΔμSO₄^2?_i,o=+ 27.7 ± 0.24 kJ mol^?1 (light) and + 24 ± 0.34 kj mol^?1 (dark)]. The rate of metabolism of SO₄^2?at pH_o, 7.5 was 150 ± 28 pmol m^?2 s^?1 (n = 185) in the light but only 12.8 ± 3.6 pmol m^?2 s^?1 (n = 61) in the dark. Light-driven sulphate uptake is partially inhibited by DCMU and chloramphenicol. Sulphate uptake is not linked to potassium, proton, sodium or chloride transport. The alga has a constitutive over-capacity for sulphate uptake [light (n= 105): K_m= 0.3 ± 0.1 mmol m^?3, V_max, = 1.8 ± 0.6 nmol m^?2 s^?1; dark (n= 56): K_m= 1.4 ± 0.4 mmol m^?3, V_max= 41 ± 22 pmol m^?2 s^?1]. Sulphite (SO₃^2?) was a competitive inhibitor of sulphate uptake. Selenate (SeO₄^2?) was an uncompetitive inhibitor.     

Photosynthesis of Marine Macroalgae from Antarctica: Light and Temperature Requirements

The photosynthetic performance of macroalgae isolated in Antarctica was studied in the laboratory. Species investigated were the brown algae Himantothallus grandifolius, Desmarestia anceps, Ascoseira mirabilis, the red algae Palmaria decipiens, Iridaea cordata, Gigartina skottsbergii, and the green algae Enteromorpha bulbosa, Acrosiphonia arcta, Ulothrix subflaccida and U. implexa. Unialgal cultures of the brown and red algae were maintained at 0°C, the green algae were cultivated at 10°C. I_K values were between 18 and 53 μmol m^?2 s^?1 characteristic or low light adapted algae. Only the two Ulothrix species showed higher I_K values between 70 and 74 μmol m^?2 s^?1. Photosynthesis compensated dark respiration at very low photon fluence rates between 1.6 and 10.6 μmol m^?2 s^?1. Values of α were high: between 0.4 and 1.1 μmol O₂ g^?1 FW h^?1 (μmol m^?2 s^?1)^?1 in the brown and red algae and between 2.1 and 4.9 μmol O₂ g^?1 FW h^?1 (μmol m^?2 s^?1)^?1 in the green algal species. At 0°C P_max values of the brown and red algae ranged from 6.8 to 19.1 μmol O₂ g^?1 FW h^?1 and were similarly high or higher than those of comparable Arctic-cold temperate species. Optimum temperatures for photosynthesis were 5 to 10°C in A. mirabilis, 10°C in H. grandifolius, 15°C in G. skottsbergii and 20°C or higher in D. anceps and I. cordata. P: R ratios strongly decreased in most brown and red algae with increasing temperatures due to different Q₁₀ values for photosynthesis (1.4 to 2.5) and dark respiration (2.5 to 4.1). These features indicate considerable physiological adaptation to the prevailing low light conditions and temperatures of Antarctic waters. In this respect the lower depth distribution limits and the northern distribution boundaries of these species partly depend on the physiological properties described here.     

Role of temperature in diapause response to fish kairomones in crustacean Daphnia

The effect of non-lethal thermal conditions on the diapause response to a simulated threat of fish predation was tested in the freshwater planktonic crustacean Daphnia magna.From an early developmental stage, female Daphnia were either exposed or not exposed to fish kairomones that notified of the threat of fish predation at the benign growth temperatures of 18, 22 or 26 °C. The proportion of females switching from the production of subitaneous to diapausing eggs and their rate of reproduction were recorded.Besides the faster development and more intense reproduction observed at higher temperatures, a smaller proportion of the females formed protective, diapausing eggs when exposed to fish kairomones than was the case in colder conditions. The production of diapausing eggs was not observed when the threat of fish predation was absent. These results indicate an interactive effect of fish kairomones and thermal conditions on diapause induction in D. magna.We interpret these findings in the context of strategies for the maximisation of reproductive success. The production of well protected diapausing eggs (which assures low yet stable gains under diverse thermal conditions) may be a more beneficial life history strategy at lower temperatures, where the chances of survival of active individuals until maturation and successful reproduction are low. Higher temperatures permit faster maturation and more intensive reproduction that may surpass numerical losses of active descendants due to predation, making diapause a less rewarding option.     

Influence of light on the foraging impact of an introduced predatory cladoceran,Bythotrephes longimanus

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Photon irradiance required to support optimal growth and interrelations between irradiance and pigment composition in the green alga Haematococcus pluvialis

The aim of the work was to find the optimal photon irradiance for the growth of green cells of Haematococcus pluvialis and to study the interrelations between changes in photochemical parameters and pigment composition in cells exposed to photon irradiances between 50 and 600?µmol?m^?2?s^?1 and a light:dark cycle of 12:12?h. Productivity of cultures increased with irradiance. However, the rate of increase was higher in the range 50–200?µmol?^?2?s^?1. The carotenoid content increased with increasing irradiance, while the chlorophyll content decreased. The maximum quantum yield of PSII (F_v/F_m) gradually declined from 0.76 at the lowest irradiance of 50?µmol?^?2?s^?1 to 0.66 at 600?µmol?^?2?s^?1. Photosynthetic activity showed a drop at the end of the light period, but recovered fully during the following dark phase. A steep increase in non-photochemical quenching was observed when cultures were grown at irradiances above 200?µmol?^?2?s^?1. A sharp increase in the content of secondary carotenoids also occurred above 200?µmol?m^?2?s^?1. According to our results, with H. pluvialis green cells grown in a 5-cm light path device, 200?µmol?^?2?s^?1 was optimal for growth, and represented a threshold above which important changes in both photochemical parameters and pigment composition occurred.     

Abstracts of Papers Read at the Winter Meeting at the University of Liverpool

Previous investigations with planktonic cyanobacteria have suggested that these organisms do not form new gas vesicles in the dark. This study, on Microcystis sp., confirmed that cells that had been preincubated at low photon irradiances (< 15 μmol m^-2 s^-1) formed negligible amounts of gas vesicles in the dark. Significant gas vesicle formation occurred, however, in cells preincubated continuously at higher irradiances, and particularly within the range 65 to 105 μmol m^-2 s^-1. The results suggest that gas vesicle formation in the dark is dependent on the prior accumulation of energy reserves. The amount of gas vesicles formed in continuous light was linearly related to irradiance over the range 0 to 20 μmol m^-2 s^-1, and reached a maximum at only 30 μmol m^-2 s^-1 that was over five times the amount formed at higher irradiances. This suggests that the rate of gas vesicle formation, regulated directly in response to irradiance, has a role in the light-mediated buoyancy regulation of this cyanobacterium.     

Finding the optimal vertical distribution: behavioural responses of Daphnia pulicaria to gradients of environmental factors and the presence of fish

1. The vertical distribution of zooplankton results from active habitat choice aiming to optimise fitness gain in a system of trade‐offs. 2. Using large, controlled indoor mesocosms (Plön Plankton Towers), we monitored the behavioural response of Daphnia pulicaria to vertical gradients of temperature, food, oxygen and light, in the presence and absence of fish predation. 3. In the absence of fish, Daphnia distributed as predicted by an ideal ‘free distribution with costs’. If the food was distributed homogeneously, they stayed in the warm epilimnion, while they balanced their time dwelling in epi‐ and hypolimnion if the food was concentrated in a deep‐water maximum. 4. However, oxygen depletion in the hypolimnion, representing an additional cost, prevented Daphnia from completely exploiting the hypolimnetic food maximum. Consequently, the proportion dwelling in the hypolimnion was larger if oxygen was not limiting. 5. Fish predation had an overwhelming effect, driving Daphnia into the hypolimnion under all experimental conditions. If permitted by oxygen availability, Daphnia used the whole hypolimnion, but oxygen depletion reduced their possible habitat to the upper hypolimnion with oxygen concentrations above c. 0.7 mg L^?1. As fish were less tolerant of low oxygen, the layer below the thermocline formed a predation refuge for Daphnia.     

Light and temperature regulation of greening in dark‐grown ginkgo (Ginkgo biloba)

The last steps of chlorophyll (Chl) biosynthesis were studied at different light intensities and temperatures in dark‐germinated ginkgo (Ginkgo biloba L.) seedlings. Pigment contents and 77 K fluorescence emission spectra were measured and the plastid ultrastructure was analysed. All dark‐grown organs contained protochlorophyllide (Pchlide) forms with similar spectral properties to those of dark‐grown angiosperm seedlings, but the ratios of these forms to each other were different. The short‐wavelength, monomeric Pchlide forms were always dominating. Etioplasts with small prolamellar bodies (PLBs) and few prothylakoids (PTs) differentiated in the dark‐grown stems. Upon illumination with high light intensities (800 μmol m^?2 s^?1 photon flux density, PFD), photo‐oxidation and bleaching occurred in the stems and the presence of ¹O₂ was detected. When Chl accumulated in plants illuminated with 15 μmol m^?2 s^?1 PFD it was significantly slower at 10°C than at 20°C. At room temperature, the transformation of etioplasts into young chloroplasts was observed at low light, while it was delayed at 10°C. Grana did not appear in the plastids even after 48 h of greening at 20°C. Reaccumulation of Pchlide forms and re‐formation of PLBs occurred when etiolated samples were illuminated with 200 μmol m^?2 s^?1 PFD at room temperature for 24 h and were then re‐etiolated for 5 days. The Pchlide forms appeared during re‐etiolation had similar spectral properties to those of etiolated seedlings. These results show that ginkgo seedlings are very sensitive to temperature and light conditions during their greening, a fact that should be considered for ginkgo cultivation.     

Photosynthetic performance of freshwater Rhodophyta in response to temperature, irradiance, pH and diurnal rhythm

Responses of net photosynthetic rates to temperature, irradiance, pH/inorganic carbon and diurnal rhythm were analyzed in 15 populations of eight freshwater red algal species in culture and natural conditions. Photosynthetic rates were determined by oxygen concentration using the light and dark bottles technique. Parameters derived from the photosynthesis–irradiance curves indicated adaptation to low irradiance for all freshwater red algae tested, confirming that they tend to occur under low light regimes. Some degree of photo‐inhibition (β= ‐0.33–0.01 mg O₂ g^?1 DW h^?1 (μmol photons m^?2 s^?1)^?1) was found for all species/populations analyzed, whereas light compensation points (Ic) were very low (≤ 2 μmol photons m‐ photons s^?1) for most algae tested. Saturation points were low for all algae tested (Ik = 6–54 μmol photons m^?2 s^?1; Is = 20–170 umol photons m^?2 s^?1). Rates of net photosynthesis and dark respiration responded to the variation in temperature. Optimum temperature values for net photosynthesis were variable among species and populations so that best performances were observed under distinct temperature conditions (10, 15, 20 or 25°C). Rates of dark respiration exhibited an increasing trend with temperature, with highest values under 20–25°C. Results from pH experiments showed best photosynthetic performances under pH 8.5 or 6.5 for all but one species, indicating higher affinity for inorganic carbon as bicarbonate or indistinct use of bicarbonate and free carbon dioxide. Diurnal changes in photosynthetic rates revealed a general pattern for all algae tested, which was characterized by two relatively clear peaks, with some variations around it: a first (higher) during the morning (07.00–11.00 hours.) and a second (lower) in the afternoon (14.00–18.00 hours). Comparative data between the ‘Chantransia’ stage and the respective gametophyte for one Batrachospermum population revealed higher values (ca 2‐times) in the latter, much lower than previously reported. The physiological role of the ‘Chantransia’ stage needs to be better analyzed.     

Evolution of predator avoidance in a Daphnia population: evidence from the egg bank

Dormant propagule pools (egg banks) of zooplankton populations that accumulate in sediments provide biological archives of past conditions and enable the investigation of evolutionary changes in populations over relatively long periods of time (many decades). This study examined the egg bank of a Daphnia pulicaria population in a lake that has been stocked annually with rainbow trout (a zooplanktivore) since 1961. Resting eggs from sediments from the 1920s to 2001 (determined by ²¹⁰Pb dating) were hatched and established as isofemale clonal lines. The phototactic behavior (a proxy for vertical migration behavior) of clones was assessed in the presence and absence of fish kairomones to evaluate the hypothesis that the heavy and consistent level of predation imposed by the stocked trout would have selected for clones that are more negatively phototactic. In addition, exposure to kairomones was expected to induce stronger negative phototaxis for all clones relative to control conditions. The behavior of clones derived from resting eggs from the trout era (after 1961) was significantly more negatively phototactic than it was for pre-trout era clones. Kairomone exposure induced a more negative phototactic response in clones from both eras, but the response was much greater for the pre-trout era clones. These results suggest that the consistently high level of predation by trout over several decades has selected for a conservative (less plastic) vertical migration strategy in which Daphnia maintain a deep daytime distribution with or without the presence of chemical cues from fish predators.     

Quantifying the dynamics of light tolerance in Arabidopsis plants during ontogenesis

The amount of light plants can tolerate during different phases of ontogenesis remains largely unknown. This was addressed here employing a novel methodology that uses the coefficient of photochemical quenching (qP) to assess the intactness of photosystem II reaction centres. Fluorescence quenching coefficients, total chlorophyll content and concentration of anthocyanins were determined weekly during the juvenile, adult, reproductive and senescent phases of plant ontogenesis. This enabled quantification of the protective effectiveness of non‐photochemical fluorescence quenching (NPQ) and determination of light tolerance. The light intensity that caused photoinhibition in 50% of leaf population increased from ～70 μmol m^?2 s^?1, for 1‐week‐old seedlings, to a maximum of 1385 μmol m^?2 s^?1 for 8‐week‐old plants. After 8 weeks, the tolerated light intensity started to gradually decline, becoming only 332 μmol m^?2 s^?1 for 13‐week‐old plants. The dependency of light tolerance on plant age was well‐related to the amplitude of protective NPQ (pNPQ) and the electron transport rates (ETRs). Light tolerance did not, however, show a similar trend to chlorophyll a/b ratios and content of anthocyanins. Our data suggest that pNPQ is crucial in defining the capability of high light tolerance by Arabidopsis plants during ontogenesis.     

Alterations in light-induced stomatal opening in a starch-deficient mutant of Arabidopsis thaliana L. deficient in chloroplast phosphoglucomutase activity

Stomatal responses to light of Arabidopsis thaliana wild-type plants and mutant plants deficient in starch (phosphoglucomutase deficient) were compared in gas exchange experiments. Stomatal density, size and ultrastructure were identical for the two phenotypes, but no starch was observed in guard cells of the mutant plants whatever the time of day. The overall extent of changes in stomatal conductance during 14 h light–10 h dark cycles was similar for the two phenotypes. However, the slow endogenous stomatal opening occurring in darkness in the wild type was not observed in the mutant plants. Stomata in the mutant plants responded much more slowly to blue light (70 μmol m^?2 s^?1) though the response to red light (250 μmol m^?2 s^?1) was similar to that of wild-type plants. In paradermal sections, stomatal responses to red light (300 μmol m^?2 s^?1) were weak for wild-type plants as well as for mutant plants. Stomatal opening was greater under low blue light (75 μmol m^?2 s^?1) than under red light for the two genotypes. However, in mutant plants, a high chloride concentration (50 mol m^?3) was necessary to achieve the same stomatal aperture as observed for the wild-type plants. These results suggest that starch metabolism, via the synthesis of a counter-ion to potassium (probably malate), is required for full stomatal response to blue light but is not involved in the stomatal response to red light.