PERIODIC ZOOSPORE PRODUCTION BY PROTOSIPHON BOTRYOIDES UNDER ALTERNATING LIGHT-DARK PERIODS

The effect of light-dark periods, of 24 hr or longer complete cycle, on time of zoospore production by Protosiphon botryoides Klebs was investigated in unstirred flask cultures and in stirred cultures supplied 1% CO₂ in air. Synchronized zoospore production was observed in both types of cultures. In stirred cultures supplied 1% CO₂ a light-dark cycle of 36–12 hr gave better synchrony than did a cycle of 12–12 or 60–12 hr. Illumination by cool-white fluorescent bulbs inhibited zoospore formation strongly, possibly by inhibiting cytoplasmic cleavage in parent cells. Darkness, in comparison to illumination, promoted the formation of zoospores, and their synchronous production under alternating light-dark cycles occurred as a consequence.     

Studies on the vegetative life cycle of Chlamydomonas reinhardi Dangeard in synchronous culture I. Some characteristics of the cell cycle

Cells of Chlamydomonas reinhardi Dangeard were grown synchronouslyunder a 12 hr light-12 hr dark regime. Time courses of nucleardivision, chloroplast division, "apparent cytokinesis" and zoosporeliberation were followed during the vegetative cell cycle inthe synchronous culture. Liberation of zoospores occurred atabout 23–24 hr after the beginning of the light periodat 25°C. Four zoospores were produced per mother cell underthe conditions used. At lower temperatures, the process of zoosporeliberation as well as length of the cell cycle was markedlyprolonged, but the number of zoospores produced per mother cellwas approximately the same. At different light intensities,lengths of the cell cycle were virtually the same, while thenumber of zoospores liberated was larger at higher rather thanat lower light intensities. During the dark period, nuclear division, chloroplast divisionand apparent cytokinesis took place, in diis order, and proceededless synchronously than did the process of zoospore liberation.When the 12 hr dark period was replaced with a 12 hr light periodduring one cycle, the time of initiation as well as the durationof zoospore liberation was litde affected in most cases, whereasnuclear division, chloroplast division and apparent cytokinesiswere considerably accelerated by extended illumination. Whenalgal cells which had been exposed to light for 24 hr were furtherincubated in the light, zoospore liberation started much earlierand proceeded far less synchronously, compared with that under12 hr light-12 hr dark alternation. (Received October 12, 1970; )     

Production of Conidia by Botrytis fabae grown in vitro

Conidiation in Botrytis fabae was stimulated by irradiating 1 to 3 day old, but not 4 to 5 day old mycelium. Three cycles of 12 h irradiation + 12 h darkness stimulated the production of about twice as many spores compared with only 12 h irradiation. At 18°C all the spores had been produced within 3 days but not within 2 days from the start of irradiation. Near-u.v. irradiation at wavelengths of 375–400 nm induced most sporulation. Red light at 600–650 nm also stimulated conidiation but irradiation at other wavelengths from 300 to 700 nm was ineffective. Fewer conidia were produced when the fungus was kept in darkness at 4°C between periods of irradiaton at 18°C compared with continuous 18°C. The optimum osmotic potential of the culture, medium for conidiation was about-27 bar although more mycelium grew at even lower osmotic potentials. Abundant spore production occurred when the fungus was grown in media with a wide range of pH values.     

Evidence against the involvement of phytochrome in UVB-induced inhibition of stem growth in green tomato plants

The effects of UVB on the kinetics of stem elongation of wild type (WT) and photomorphogenic mutants of tomato were studied by using linear voltage transducers connected to a computer. Twenty-one or twenty-six-day-old plants, grown in 12 h white light (150 μmol m⁻² s⁻¹ PAR)/12 h dark cycles, were first transferred to 200 μmol m⁻² s⁻¹ monochromatic yellow light for 12 h, then irradiated with 0.1 or 4.5 μmol m⁻² s⁻¹ UVB for 12 h and finally kept in darkness for another 24 h. The measurements of the kinetics of stem elongation started after 4 h under yellow light. Significant differences in stem growth during the irradiation with yellow light, as well as during the dark period, were found between the genotypes. In darkness, the magnitude of stem growth followed the order: tri > AC = fri > MMau > hp1. Two factors determined the large differences of growth in darkness: 1) the different stem elongation rate (SER) and 2) the different duration of the growing phase among the genotypes. In darkness the stem growth of au and hp1 mutants lasted for about 18 h, whereas it continued for the whole experimental period (36 h) in the other genotypes. UVB irradiation substantially reduced elongation growth of all genotypes (4.5 μmol m⁻² s⁻¹ being more effective than 0.1 μmol m⁻² s⁻¹). Both fluence rates of UVB induced a detectable reduction of SER already after 15 min of irradiation. Red light inhibited, while far red light promoted stem growth of all the genotypes tested. fri (phyA null), tri (phyB1 null), hp1 (exhibiting exaggerated phytochrome responses) mutants and WT tomato showed similar levels of UVB–induced inhibition of growth, while the aurea mutant showed the largest growth inhibition during the 12 h of irradiation. These results indicate that phytochrome is not directly involved in UVB control of stem elongation. The results of dichromatic irradiations UVB + red or UVB + far red indicate the presence of distinct and additive action of UVB photoreceptor and of the phytochrome system in the photoregulation of stem growth. This revised version was published online in June 2006 with corrections to the Cover Date.     

Translocation in colored light

The translocation of ¹⁴C-photosynthate in detached blades of sugarcane was studied under illumination from red, green, blue, and cool-white fluorescent lamps; under far-red illumination from the sun, and from incandescent lamps; and in total darkness.

The percentage of basipetal translocation and the accumulation against the concentration gradient were stimulated by light from the red or blue lamps more than by green or cool-white fluorescent illumination.

Basipetal translocation took place equally well in red light lacking blue irradiation and in blue light. Since the action spectrum for light-induced change in viscosity is a typical blue-type spectrum, the effect of light upon translocation is not due merely to changes in the physicochemical properties of protoplasm.

Basipetal translocation took place in red light lacking blue irradiation better than in cool-white fluorescent light, which may suggest a red stimulation of translocation.

Illumination in the far-red region of the spectrum did not support basipetal translocation but acted like total darkness.

Because of the wide emission characteristics of the fluorescent lamps employed, it is impossible to decide whether a chlorophyll-like system or some other pigment is involved in the light stimulation of phototranslocation.

Whatever the activating wavelength and whatever the pigment system involved, these results show that the phototranslocation of sucrose in the phloem is influenced by the quality of illumination.

     

Light-induced changes in the period of the circadian rhythm of carbon dioxide output in Bryophyllum leaves

Summary The period of the rhythm of carbon dioxide output from leaves of Bryophyllum fedtschenkoi R. Hamet et Perrier at 15°C was shorter in continuous white light than in darkness. The period was monitored in leaves exposed to narrow spectral bands of monochromatic radiation at an incident quantum flux density of 4.7×10^-11 einsteins cm^-2s^-1. Bands centred on 660, 600, 730 and 530 nm significantly shortened the period, the greatest effect being achieved at 660 nm and the smallest at 530 nm; those centred on 760 and 450 nm were without effect. None of the bands tested significantly lengthened the period. The period of the rhythm in leaves exposed continuously to monochromatic radiation at 660 nm decreased with increasing quantum flux density. The extent to which a quantum flux density of 4.7×10^-11 einsteins cm^-2s^-1 at wavelength 660 nm shortened the period depended on the ambient temperature. At 15°C a significant reduction of 4.4 h occurred as compared with the dark control, while at 30°C no significant reduction was observed. The transient (the time from the initiation of the rhythm to the first peak) showed a greater dependence on temperature than did the steady-state period. No such difference could be detected in relation to the intensity or quality of irradiation. The reduction of the transient by the various irradiation treatments was, in general, proportional to the reduction of the period.     

THE EFFECTS OF LIGHT ON THE DEVELOPMENT OF THE CELLULAR SLIME MOLD ACRASIS ROSEA

No fruiting of the NC-18 isolate of Acrasis rosea occurs in cultures maintained in continuous light or in continuous dark. The use of different food organisms does not alter the aforementioned behavior. The time at which fruiting occurs in this isolate can be regulated by administering stimulatory light followed by a dark period. Mature sorocarps are formed approximately 14 hr after the termination of light and the start of darkness. Within this 14-hr interval aggregation and sorocarp development occur. After about 6 hr of dark incubation, NC-18 amebae, previously stimulated by light, form a few weak aggregation centers. After 8 hr of dark incubation there are numerous aggregation areas, large in size and deep rose in color. By 10 hr the aggregations are quite compact and firm in appearance, and between 12 and 14 hr late aggregations, sorogens and, finally, mature sorocarps are formed. The minimum dark period, i.e., the minimum time that is required in darkness (for cultures previously stimulated with light) to obtain at least some fruiting within the 14-hr developmental period, is 7–8 hr for NC-18 and 5–6 hr for Tu-26. Maximum numbers of sorocarps form when cultures are given 10–11 hr of uninterrupted dark. Light-stimulated cultures of NC-18 placed in darkness and interrupted by a 10- or 30-min exposure to wide-spectrum blue or cool white fluorescent light an hour prior to the minimal dark period exhibit a 4–5 hr-delay in fruiting when returned to darkness and inspected at intervals following the second irradiation. Growth and fruiting of NC-18 occurred with purified food sources of each of five different species of Chlorella and with the alga Stichococcus bacillaris. This is apparently the first report of the utilization of algae as food sources by a cellular slime mold. Fruiting of NC-18 was readily arrested by lowering the relative humidity to 40–45%. This change in the moisture content of the surrounding air induced microcyst formation. Growth on buffered medium occurred in the entire pH range tested, 3.5–7.6, but fruiting occurred only between pH 5.0 and 6.6.     

THE RHYTHMICAL CHANGE IN SENSITIVITY OF A LONG-DAY DUCKWEED, LEMNA GIBBA G3, TO DARK-BREAK

1. Effect of varied lengths of darkness given before continuousillumination, and that of dark-break of continuous light asa function of the time of its application, on the flower formationin a long-day duckweed, Lemna gibba G3, were studied. The results obtained suggested a rhythmic change in sensitivityto darkness, i.e., a cycle of 36 hr-period consisting of 12hr of sensitivity and the following 24 hr of insensitivity.The inhibition by darkness (12–36 hr) given before thestart of, or by dark-break (12, 24 hr) inserted in, the inductionperiod involved an extension of the induction period, but nota slow-down of the rate of flower formation. The dark-breakgiven after the induction period, however, suppressed the rateof flower production in proportion to the length of the darkness. 2. The inhibition of flowering by darkness given in the darksensitivephase was cancelled by a relatively brief light period insertedin the darkness. 3. Relation between the rhythm and the length of induction periodwas discussed. (Received August 13, 1965; )     

Eine fördernde Wirkung von Blaulicht auf die Carotinoidbildung einer gelben Chlorella-Mutante

Summary In growing as well as in resting cells of a chlorophyll-free yellow mutant of Chlorella vulgaris (211-11h/20) synthesis of carotenoids is enhanced by blue light. Permanent irradiation is necessary to maintain the effect (Fig. 2). At wavelengths around 454 nm the additional carotenoid production is half-saturated at about 1000 erg cm^-2s^-1 and saturated at about 4000 erg cm^-2s^-1 (Fig. 3). An action spectrum exhibits highest efficiency of wavelengths around 465 and 370 nm, a minimum near 400 nm and inefficacy of yellow, red and far-red light (Fig. 4). This wavelength dependence resembles those of light enhanced O₂-uptake, carbohydrate consumption and gain of organically bound nitrogen by the organism.     

The inhibition of potato sprout growth by light.

When potato seed tubers (Solanum tuberosum cv. Pentland Javelin) were stored in darkness or diffuse daylight at 9°C and transferred at intervals to conditions suitable for sprouting, their capacity for sprout growth was unaffected by the presence or absence of light during previous storage. When similar tubers were stored at 10°C, 18°C or 25°C, sprout growth commenced earliest at 25°C, but the date was unaffected by fluorescent light. It was concluded that light did not affect the length of the dormant period, but only the rate of sprout elongation after that period had ceased. When tubers with growing sprouts at 10°C or 18°C were transferred from darkness into fluorescent light, sprout growth virtually ceased. Transfer from light into darkness resulted in immediate sprout growth, at a rate comparable with tubers stored continuously in the dark. Tubers of three Peruvian cultivars, stored in farm-scale diffuse-daylight stores, grew progressively shorter sprouts with increasing daily exposure to light from 30 min to 12 h. Storage of cv. Wilja under 21 Wm^-2 (total) of white fluorescent light for 10 h per day maintained the sprouts at the same length as ten times this light intensity for 1 h per day. In a subsequent experiment with cv. Bintje the 10 h, low-intensity light regime gave slightly shorter sprouts. It appeared that the total light energy falling on the tubers was the dominant factor controlling sprout growth.     

RELATIONSHIP OF TEMPERATURE TO ZOOSPORE PRODUCTION IN TETRASPORA GELATINOSA

A strain of Tetraspora gelatinosa was isolated from a field collection and a technique for measuring quantitatively the production of zoospores was devised. The method employed compound colonies grown on agar from zoospore suspensions. The effect of 3 temperatures on zoospore production was tested: no zoospores were produced by the colonies at 3 C; zoospores were produced at 11 and 23 C and the final yield of zoospores at the 2 temperatures was not significantly different. However, at 11 C the maximum density of zoospores produced, measured by cell count, occurred after IS hr, while the maximum density at 23 C occurred after 21 hr.     

Egg release in gametophytes of Laminaria saccharina: Induction by darkness and inhibition by blue light and u.v.

Gametophytes of Laminaria saccharina cultivated from zoospores in a light-dark regime (16:8), release eggs exclusively during the dark cycles, 8–10 days after seeding of the zoospores, and mainly during the first 30 min of darkness. The inhibiting effects of light during the light cycle of 16 h per day is also apparent in gametophytes which have experienced only two dark cycles prior to day 8, when egg release begins. Egg release can be shifted to any time during the light cycle by prolonging the irradiation with white fluorescent light and by subsequent darkening for 1 h. In gametophytes cultivated in continuous white fluorescent light eggs are also released from day 8–10, so in this case no inhibiting activity of light is apparent. Egg release is inhibited by blue light and u.v., with peak wavelengths for inhibition at 372, 413, 438 and 481 nm. No inhibition occurs at wavelengths above 513 nm. The light requirement for inhibition is very low. A photon fluence rate of 1·4 μE m^-2s^-1, given for 45 min at 449 nm, inhibits egg release in 50% of the mature gametophytes. There is some evidence that a circadian rhythm is involved, primarily since in gametophytes which are transferred at the beginning of day 8 from 16:8 to constant conditions (darkness, continuous red or green light) the diel rhythm of egg release persists until day 11.     

Rapid, Blue-Light Induced Transpiration in Avena

The transpiration responses of primary Avena leaves to blue-light pulses were investigated. Only light with wave length shorter than 524 nm can produce the rapid transpiration response. The action spectrum has a maximum around 450 nm. The rapid transpiration response induced by blue-light pulses successively disappeared in long-term experiments if the plant was kept in darkness between the pulses. However, if visible light was given to the plant between the pulses, the rapid response was restored. The magnitude of the rapid transpiration response was investigated under different conditions of background illumination and blue-light exposure. Saturation of the response was obtained with an irradiation level of 1.5–2 mW.cm^?2 (5 min pulses) and with a pulse duration of 4 min (pulse irradiance 2 mW.cm^?2). A pulse duration of 3 s was sufficient to produce a significant rapid response at an irradiation level of 2 mW.cm^?2.     

THE DISTRIBUTION OF PHOTOSYNTHATE IN ASCOPHYLLUM NODOSUM AS IT RELATES TO EPIPHYTIC POLYSIPHONIA LANOSA12

The brown alga Ascophyllum nodosum is not a major source of organic carbon for its epiphytic red alga Polysiphonia lanosa. Plants pulse-labeled for 24–25 hr with NaH¹⁴CO₃ were examined for exudation and translocation. The maximum amount of radioactive carbon compounds lost from A. nodosum during this experimental period was less than 0.3% of the total ¹⁴C fixed by the alga, and of this amount, only 5% could, have moved through the frond. The remaining fraction of the ¹⁴C lost from the thallus was released into the water. The total exudate from A. nodosum was collected for 1 week in a series of flasks of filtered seawater changed at 12-hr intervals corresponding with the beginning of the light and dark periods, respectively. During 7 days at 15 C only 1.5% of the total ¹⁴C originally fixed had been released as radioactive organic carbon, whereas at 5 C, 0.6% of the total ¹⁴C fixed was found in the medium. No significant difference in the rate of exudation of organic ¹⁴C was observed in light or darkness. After fractionation of the exudate it was found that only 10% of the radioactivity in these exudates was composed of organic acids or amino acids. P. lanosa, on the other hand, is perfectly capable of fixing its own carbon. The photosynthetic rates measured by H¹⁴CO₃ uptake confirm the observations of Bidwell: 3.96 mg CO₂/g/hr (0.09 m mole/g/hr).     

LIGHT,TEMPERATURE AND PHOTOPERIOD AS FACTORS CONTROLLING REPRODUCTION IN ULOTHRIX ZONATA (ULVOPHYCEAE)1

In many lakes in the northern United States and Canada the filamentous green alga Ulothrix zonata (Weber and Mohr) Kütz grows abundantly in early spring in shallow waters. Asexual reproduction occurs by formation of quadriflagellate zoospores which disrupt, the integrity of the cells upon release causing the filament to disintegrate. Study of the effects of 100 different combinations of irradiance, temperature and photoperiod revealed that zoospore formation is favored by high temperatures near 20°C, high light levels of 520 μE·m^?2·s^?1 and photoperiods of either short day (8:16 h light-dark) or long day cycles (16:8 h light-dark). Zoospore formation is minimal under conditions of low temperature (5°C), low irradiance (32.5 μE·m^?2·s^?1) and neutral day-lengths (12:12 h light-dark). These observations explain the decline in U. zonata biomass when water temperatures rise above 10° C. The combined effect of rising water temperatures and increasing daylengths causes progressively more filaments to switch from vegetable growth to zoospore production resulting in an increasing loss of biomass.     

Differential patterns of mitochondrial,chloroplastic and nuclear DNA synthesis in the synchronous cell cycle of Chlamydomonas reinhardtii

Nuclear DNA (ncDNA) synthesis in Chlamydomonas reinhardtii was measured by both ³²P[or-thophosphoric acid] (³²P) and [¹⁴C]adenine incorporation and found to be highly synchronous. Ca. 85% of incorporation was confined to the first 6 h of the dark period of a synchronized regime consisting of an alternating light-dark period of 12 h each. In contrast, no such synchronous incorporation pattern was found for chloroplast (cp) and mitochondrial (mt) DNAs in the same cell population. These two organellar DNAs also exhibited different ³²P-incorporation patterns in the cell cycle. Considerable amounts of ³²P were incorporated into cpDNA throughout the light-dark synchronous cycle under both mixo- and phototrophic growth conditions, although the second 6-h light period under phototrophy showed an increase not apparent under mixotrophy. This change in growth conditions did not affect ³²P incorporation into mtDNA, which was found throughout the cell cycle, with a modest peak in the first 6-h of the dark period. The pattern of [³H]thymidine incorporation into cpDNA was also determined. Under synchronous phototrophic conditions, this pattern was quite different from that obtained with ³²P. Most [³H]thymidine incorporation occurred during the light period of the synchronous cycle; this period had been shown previously by density transfer experiments to be the time of cpDNA duplication. Such preferential [³H]thymidine incorporation into cpDNA in the light period was not observed under mixotrophic synchronous growth conditions; in these, [³H]thymidine incorporation was detected throughout the cell cycle. This lack of coincidence between the patterns of ³²P- and of [³H]thymidine incorporation into cpDNA during the synchronous cell cycle indicates that in addition to replication, the considerably reiterated organelle-DNA molecules may also regularly undergo an extensive repair process during each cell cycle.     

Action Spectra for Conversions of Phycochrome c from Nostoc muscorum

The reversibly photochromic pigment, phycochrome c, was extracted from the blue-green alga Nostoc muscorum strain A. Action spectra were determined for in vitro conversions of the pigment from the short wavelength to the long wavelength form and vice versa. The action peak for the absorbance decrease at 650 nm is at 630 nm. During this decrease there is only a slight increase of the absorbance in the green region. Green and yellow light (maximum efficiency at 580 nm) completely restores absorbance at 650 nm. The observations are explained by the existence of three spectrally different forms of phycochrome c: P^c₆₃₀ and P^c₆₅₀ which equilibrate in darkness and P^c₅₈₀ which is reversibly photoconvertible to P^c₆₃₀. We have also measured the absorbance changes brought about by saturating irradiations with light of various wavelengths (“photostationary state spectrum”). Extreme photostationary states were obtained with about 650 nm and 500 nm light.     

ENDOGENOUS LEVELS OF INDOLE-3-ACETIC ACID IN SYNCHRONOUS CULTURES OF CHLORELLA PYRENOIDOSA12

Endogenous levels of indole-3-acetic acid were mesaured in synchronous cultures of Chlorella pyrenoidosa (TX-7-11-05). The cultures were synchronized by alternating light:dark periods of 15:9 hr at a temperature of 40 ± 1 C. After 2 synchronous cycles the cultures were exposed to a low light treatment of 350 ± 100 ft-c. The time to incipient cell division under these conditions was 6 hr and 15 min. Samples were taken at 3 sampling periods during the low light treatment period:low light 0 hr (LL0); low light 3 hr (LL3); and low light 6:15 hr (LL6:15). The algal extracts were analyzed by a fluorometric procedure which measured the indole-α-pyrone product formed by the action of the trifluoracetic acid-acetic anhydride reagent with IAA. The IAA levels increased gradually from the autospore stage (5.19 μg × 10^?4/mg dry wt) to the adolescent stage (7.13 μg × 10^?4/mg dry wt) and more rapidly when approaching the ripened adult stage (14.55 μg × 10^?4/mg dry wt). The mean percentage increase from autospore to adolescent was 36.9%, and from adolescent to ripened adult 104.6%. The total percentage increase from autospore to adult was 180.3%. Levels of IAA were 2 times higher just prior to division than in the autospore stage.     

DIEL CHANGES IN PHASED-DIVIDING CULTURES OF CERATIUM FURCA (DINOPHYCEAE): NUCLEOTIDE TRIPHOSPHATES,ADENYLATE ENERGY CHARGE,CELL CARBON,AND PATTERNS OF VERTICAL MIGRATION1

The diel pattern of cell division, cell carbon, adenine nucleotides and vertical migration was determined for laboratory cultures of the photosynthetic marine dinoflagellate, Ceratium furca (Ehr.) Clap. & Lachm., entrained on an alternating 12:12 LD schedule at 20 C. Cell division was initiated during the latter portion of the dark period with ca. 30% of the population undergoing division. Cell C increased during the light period and exhibited a linear decrease with a loss of 33% during the dark period. ATP · cell^?1 increased during the light period and decreased by ca. 40–50% during the dark period. The diel patterns of cell C and ATP tended to “buffer” the magnitude of the change in C:ATP ratios around an overall mean value of 89. There was no obvious trend in the concentration of [GTP + UTP] · cell^?1 over the cell cycle. The cellular adenylate energy charge was maintained at values between 0.8 to 0.9 throughout the 24 h LD cycle, despite a ca. 40% decrease in total adenylates (A_T= ATP + ADP + AMP) during the dark period on 12:12 LD, and over a 68% decrease in ATP during 42 h of continuous darkness. These data lend experimental support to the theory of cellular metabolic control by the adenine nucleotides. With lateral illumination on 12:12 LD cycles, the cells began to concentrate at the surface of the experimental tubes shortly before the lights were turned on, and at the bottom of the tubes shortly before the lights were extinguished. This pattern continued for 6 days in continuous darkness, suggesting that the vertical migration pattern is independent of a phototactic response and may be under the control of an endogenous rhythm.     

Photoinhibition of Photosynthetic Oxygen Production and its Recovery in the Subtidal Red Alga Polyneura hilliae

In the marine red alga Polyneura hilliae photoinhibition of photosynthesis was investigated by measuring the photosynthetic oxygen production. The extent of photoinhibition in this alga depends on the fluence rate as well as on the time of exposure. Strongly photoinhibited algae recover slowly. Full recovery is reached only in weak light after 3 days. Two phases of recovery can be distinguished: a first relatively fast phase of recovery that is independent of light, and a second slow phase which begins after about 6 hours and requires weak light. Consequently, even in complete darkness partial recovery is observed. An action spectrum of photoinhibition and its comparison with the in vivo absorption spectrum of Polyneura demonstrates that photoinhibition is mainly caused by light which is absorbed by phycobiliproteins, chlorophyll a and carotenoids. This fact and the ineffectiveness of light above 686 nm clearly indicate that the main photoinhibition site in this alga is PS II. No photosynthetic oxygen production is detectable after 3 h irradiation at 402 nm, 150 μmol quanta m^?2s^?1. As photosynthetic activity recovers slowly and to a limited extent, the suppression of oxygen production is apparently only in part the result of photoinhibition sensu strictu and in part due to permanent photodamage.