Growth and development of cucumber Cucumis sativus L. in the prereproductive period under long photoperiods

When plants are grown in a greenhouse or in controlled environment growth rooms, prolonging the photoperiod, including towards continuous light, is one of the ways to increase plant productivity and energy savings. However, exposing some plant species to long photoperiods causes leaf injuries and growth reductions. We studied the effect of the photoperiod (8, 12, 16, 20, and 24 h) and photosynthetic photon flux density (60, 120, and 160 μmol/m² s PAR) on cucumber plants Cucumis sativus L. in a prereproductive period. It was shown that the response of the cucumber plants to a photoperiod over 20 h, including continuous light, depending on the plant age and light intensity, may include leaf movement or paraheliotropism, non-photochemical energy dissipation, and/or reversible photoinhibition of a reaction center of photosystem II, development of reversible chlorosis, reduction of a light-harvesting complex, and increase in the content of carotenoids. Reaction of immature and virginile plants to long photoperiods was different, which high-lights the need for experimental separation of the prereproductive period in terms of age states and consideration of this when developing crop production plan.     

Photoperiodic regulation of CS-ACS2, CS-ACS4 and CS-ERS gene expression contributes to the femaleness of cucumber flowers through diurnal ethylene production under short-day conditions

Photoperiod and the plant hormone, ethylene, modify sex expression of flowers in cucumber (Cucumis sativus L.). In the present study, femaleness of cucumber occurred under short‐day (8 h photoperiod) conditions compared to that under long‐day (16 h photoperiod) conditions, although the effect of photoperiod was more pronounced in a monoecious than in an andromonoecious cucumber. Application of ethylene had a greater effect than photoperiod on the production of female and bisexual flowers in monoecious and andromonoecious cucumbers, respectively. Ethylene evolution and the expression of CS‐ACS2, CS‐ACS4 and CS‐ERS genes in the shoot apices of both monoecious and andromonoecious cucumber plants had a diurnal rhythm with a peak in the middle of an 8 h or a 16 h light period. Peak ethylene evolution and expression of CS‐ACS2 was greater under short‐day conditions than under long‐day conditions in a monoecious cucumber but not in an andromonoecious one. Expression of CS‐ACS4 in monoecious and andromonoecious cucumber plants did not differ, but the level was higher under short‐day conditions compared with that under long‐day conditions. Thus, CS‐ACS2 and CS‐ACS4 might be involved in the basic diurnal rhythm of ethylene evolution in cucumber. Because exogenous ethylene increased the expression of CS‐ACS2 and CS‐ERS in monoecious cucumber possessing the M locus, but not in andromonoecious cucumber in which the function of the M locus was lost (Yamasaki et al. Plant and Cell Physiology 42, 608–619, 2001), the CS‐ACS2 gene might also be involved in ethylene production by positive feedback via regulation of M locus under short‐day conditions.     

Role of phytochrome B in the development of cold tolerance in cucumber plants under light and in darkness

Cold tolerance of cucumber (Cucumis sativus L.) seedlings was investigated using wild-type plants and the phytochrome B-deficient mutant (lh-mutant). Plants were subjected for 6 days to intermittent short-term cooling (12°C for 2 h per day) and to continuous chilling under conditions of 16-h photoperiod (day/night = 16/8 h) and permanent illumination. “Dehardening” process was initiated by the transfer of plants to either light or dark conditions at 23°C. It was concluded that phytochrome B participates in the development of cold tolerance in cucumber plants under stress conditions, i.e., under short-term intermittent chilling at nights and during dehardening in continuous darkness.     

Role of phytochrome B in organ formation processes in Cucumis sativus L.

The role of phytochrome B in the organogenesis process in the apical and axillary shoot meristems during early ontogenesis stages in cucumber Cucumis sativus L. at photoperiods (day/night) 10/14, 16/8 h, and continuous light in comparison with wild type plants and phytochrome B-deficient mutant (lh-mutant) was investigated. In mutant phytochrome B, deficiency caused inhibition of initiation of leaves both in the main shoot and lateral shoots and increased the number of flower buds (IV stage of organogenesis). With continuous light, the number of lateral shoots and flowers during stage IV of organogenesis in wild-type plants increased twofold in comparison with the mutant. Short-term temperature drops did not induce floral ontogenesis in mutants but increased the number of off-shoots in both experimental variants during a long photoperiod and continuous lighting. We propose that phytochrome B, by increasing the compactness of chromatin, may facilitate coordination of ontogenesis processes with changing environmental conditions.     

Influence of Temperature and Light Conditions on Germination,Growth and Conidiation of Oidium neolycopersici

The effect of temperature and light conditions (spectral quality, intensity and photoperiod) on germination, development and conidiation of tomato powdery mildew (Oidium neolycopersici) on the highly susceptible tomato cv. Amateur were studied. Conidia germinated across the whole range of tested temperatures (10–35°C); however, at the end‐point temperatures, germination was strongly limited. At temperatures slightly lower than optimum (20–25°C), mycelial development and time of appearance of the first conidiophores was delayed. Conidiation occurred within the range of 15–25°C, however was most intense between 20–25°C. Pathogen development was also markedly influenced by the light conditions. Conidiation and mycelium development was greatest at light intensities of approximately 60 μmol/m² per second. At lower intensities, pathogen development was delayed, and in the dark, conidiation was completely inhibited. A dark period of 24 h after inoculation had no stimulatory effect on later mycelium development. However, 12 h of light after inoculation, followed by continuous dark, resulted in delayed mycelium development and total restriction of pathogen conidiation (evaluated 8 days postinoculation). When a longer dark period (4 days) was followed by normal photoperiod (12 h/12 h light/dark), mycelium development accelerated and the pathogen sporulated normally. When only inoculated leaf was covered with aluminium foil while whole plant was placed in photoperiod 12 h/12 h, the intensive mycelium development and slight subsequent sporulation on covered leaf was recorded.     

Light strongly promotes gene transfer from <Emphasis Type="Italic">Agrobacterium tumefaciens</Emphasis> to plant cells

Light conditions during Agrobacterium-based plant transformation, the most routinely used method in plant genetic engineering, differ widely and, to our knowledge, have not been studied systematically in relation to transformation efficiency. Here, light effects were examined in two already optimized transformation procedures: coculture of Agrobacterium tumefaciens with callus from two genotypes of the crop plant Phaseolus acutifolius (tepary bean) and coculture of root segments from two ecotypes of Arabidopsis thaliana. Except for the light conditions during coculture, all steps followed established procedures. Coculture was done either under continuous darkness, under a commonly used photoperiod of 16 h light/8 h darkness or under continuous light. beta-glucuronidase (GUS) production due to the transient expression of an intron-containing uidA gene in the binary vector was used to evaluate T-DNA transfer. In all situations, uidA expression correlated highly and positively with the light period used during coculture; it was inhibited severely by darkness and enhanced more under continuous light than under a 16 h light/8 h dark photoperiod. The promotive effect of light was observed with Agrobacterium strains harboring either a nopaline-, an octopine- or an agropine/succinamopine-type non-oncogenic helper Ti plasmid. The observed positive effect of light has obvious implications for developing and improving transient and stable transformation protocols, specifically those involving dark coculture conditions.     

Interaction of photoperiod and temperature in the development of conchocelis of <Emphasis Type="Italic">Porphyra purpurea</Emphasis> (Rhodophyta: Bangiales)

The indoor cultivation of the free-living conchocelis of a Porphyra purpurea (Roth) C. Ag. strain, isolated from Long Island Sound, was established, and the effects of both photoperiod and cultural temperature on conchosporangia development were studied. Statistical analysis revealed that temperatures between 10°C and 15°C and light phases between 12 and 16 h per day comprised an ideal growth “window” for both the vegetative growth and reproductive development of conchocelis. For vegetative growth, there was a significant interaction between temperature and photoperiod. Conchospores were released from mature conchosporangia under both neutral (12/12 h) and long (16/8 h) day lengths. Different seawater supplements, such as full- and half-strength Von Stosch enrichment, showed no significantly different effects on growth or development. This work provides a guideline for maintaining conchocelis cultures of P. purpurea, which is a type of the Porphyra genus.     

Effects of different photoperiods and wavelengths of light on the life-history traits of an aphidophagous ladybird, Coelophora saucia (Mulsant)

Abstract:  The influence of three photoperiods, viz. short day [8 : 16 h; light : dark (L : D)], long day (16 : 8 h; L : D) and continuous light (24 : 0 h; L : D) and four wavelengths of light, viz. white (control), yellow, blue and red on pre-imaginal development and reproductive performance of a ladybird, Coelophora (= Lemnia ) saucia (Mulsant) (Col., Coccinellidae) have been investigated under laboratory conditions. Long-day photoperiod and white light resulted in low immature stage mortality. Total larval period was longest under short-day and red-light conditions. Females kept at long-day photoperiod and under white light showed better reproductive performance than those placed under other photoperiods and wavelengths.     

Influence of photoperiod on growth changes in juvenile Atlantic salmon, Salmo salar L.

During their first 6 months sibling Atlantic salmon parr, Salmo salar L., grew larger under constant light than under natural photoperiod or simulated natural photoperiod (control). When rate of change of photoperiod was accelerated after midsummer, ×2, ×3 and ×4, there were no growth differences between the three groups, but all were smaller than the control population. Under constant autumn photoperiod of 8 h light: 16 h dark growth was less than under all other experimental photoperiod conditions. Mean length was directly correlated with total hours of daylight experienced, excluding those fish kept under constant light. Fish reared from first feeding under photoperiod regimes delayed 6 and 9 months out of phase with the natural light cycle were smaller than the controls, whereas those under a regime 3 months out of phase did not differ from the controls. The clear segregation of modal length groups within the 3, 6 and 9 months out-of-phase populations occurred 1, 4 and 4 months, respectively, after the segregation in the control group. Under constant light, and under constant 12 h light: 12 h dark (12 LD), the segregation was delayed 3 and 4 months, respectively. The proportion of the population which maintained growth (upper modal group) was significantly less in the 9 and 6 months out-of-phase and 12 LD groups (39, 40 and 42%, respectively) than in the other three groups (82.5-85%). Acceleration of photoperiod change also resulted in decreased growth. The results support a model of salmon development in which, 2-3 months after first feeding, growth is maintained if feeding opportunities at that time are above a threshold level, and in which this critical timing is influenced by photoperiod. It is suggested that the delays reflect a synchronizing effect of photoperiod on an endogenous rhythm of appetite and growth. The differences in upper modal group proportions observed in the present experiments, reflect the relative feeding opportunities available at the critical period in July-August.     

Long day photoperiods and temperature of 20 degrees C induce spermatogenesis in blinded and non-blinded marbled newts during the period of testicular quiescence

Adult male marbled newts (Triturus marmoratus) were collected at the end of the spermatogenesis period and exposed to different photoperiods (natural-daylength-simulated photoperiod, total darkness, 8L:16D, 12L:12D, 16L:8D, and continuous light) for 3 mo. Temperature was maintained at 20 degrees C. Two additional groups of newts were blinded and exposed to either the natural-simulated photoperiod and to 16 h of light per day respectively. Quantitative histologic studies on testicular development and germ cell volume per testis were performed. The newts captured in the field at the beginning (initial controls) or at the end of the experiments (final controls) were in the period of testicular quiescence. Newts kept in total darkness or exposed to a short photoperiod (8L:16D) showed germ cell development up to primary spermatocytes, whereas germ cell development in the newts exposed to long photoperiods (12L:12D or 16L:8D) progressed to elongated spermatids. The newts exposed either to intermediate photoperiods (natural-simulated photoperiod) or to constant light showed an intermediate degree of germ cell development (up to round spermatids). No significant differences between non-blinded and blinded animals were found. These results suggest that (1) mild temperature initiates testicular development in the period of testicular quiescence, (2) long photoperiods associated with mild temperatures produce spermatogenesis in this period, (3) complete darkness or constant light are less effective than some intermediate photoperiod, and (4) the effect of photoperiod on testicular function in newts is not related to ocular photoreception.     

光周期对穗花狐尾藻生长、开花与种子形成的影响

通过人工补光和遮光处理,初步研究了光周期对狐尾藻生长和开花的影响。研究发现:8h短日照条件不利于狐尾藻的生长和花序形成,导致狐尾藻不能正常开花和结实;与自然日照长度(平均日照14h)条件下相比,16h长光照条件可以增加植株的高度和分枝数,形成更大的生物量;但24h全光照条件对生长有抑制作用。长光照条件下,狐尾藻花序形成时间和开花时间均比自然条件下延迟,形成花序的数目也显著较自然条件下的少,但长光照条件下形成的种子比自然条件下形成的种子具有更高的萌发率。     

Vindoline formation in shoot cultures of Catharanthus roseus is synchronously activated with morphogenesis through the last biosynthetic step

Background and Aims

The Madagascar periwinkle (Catharanthus roseus) produces the monoterpenoid alkaloid vindoline, which requires a specialized cell organization present only in the aerial tissues. Vindoline content can be affected by photoperiod and this effect seems to be associated with the morphogenetic capacity of branches; this association formed the basis of the study reported here.

Methods

Vindoline-producing in vitro shoot cultures were exposed either to continuous light or a 16-h photoperiod regime. New plantlet formation and alkaloid biosynthesis were analysed throughout a culture cycle.

Key Results

In cultures under the photoperiod, the formation of new plantlets occurred in a more synchronized fashion as compared to those under continuous light. The accumulation of vindoline in cultures under the photoperiod occurred in co-ordination with plantlet formation, in constrast to cultures under continuous light, and coincided with a peak of activity of deacetylvindoline acetyl CoA acetyltransferase (DAT), the enzyme that catalyses the last step in vindoline biosynthesis. When new plantlet formation was blocked in cultures under the photoperiod by treatment with phytoregulators, vindoline synthesis was also reduced via an effect on DAT activity. No association between plantlet formation and other biosynthetic enzymes, such as tryptophan decarboxylase (TDC) and deacetoxyvindoline 4-hydroxylase (D4H), was found. Effects of light treatment on vindoline synthesis were not mediated by ORCA-3 proteins (which are involved in the induction of alkaloid synthesis in response to elicitation), suggesting that the presence of a different set of regulatory proteins.

Conclusions

The data suggest that vindoline biosynthesis is associated with morphogenesis in shoot cultures of C. roseus.Key words: Catharanthus roseus, deacetylvindoline acetyl CoA acetyltransferase, DAT, in vitro shoot cultures, morphogenesis, vindoline     

Effects of maintaining solstice light and temperature on reproductive activity, coat growth, plasma prolactin and melatonin in goats.

Interactive effects of light and temperature on aspects of seasonality were studied in female British Saanen dairy goats. Four groups of adult non-pregnant non-lactating goats (n = 5) were housed under the following conditions: controls (July-June): natural photoperiod and temperature; group 1 (July-December): long days (16 h light: 8 h dark) and natural temperature; group 2 (July-December): long days and average summer temperature (17.6 degrees C); group 3 (December-June): short days (8 h light: 16 h dark) and winter temperature (8.4 degrees C). Plasma prolactin and progesterone were measured once a week, circadian changes in prolactin and melatonin were determined in December and May, and coat development was assessed. Seasonal variation in prolactin was influenced by manipulation of both daylength and temperature. In group 1, prolactin concentrations decreased as the environmental temperature decreased, despite maintenance of long days. When light and temperature were maintained under summer (group 2) and winter (group 3) conditions, prolactin remained relatively constant, although at different high and low set points, respectively, but with indications of a seasonal rhythm. An asymptotic relationship between prolactin and temperature was maintained under all daylengths. The circadian pattern of melatonin was related to daylength and was not influenced significantly by temperature. Onset of oestrus was unaltered. In group 3 (maintained winter solstice light and temperature), anoestrus was delayed (P < 0.05) from a median control date of 17 March to a median date of 28 April. Winter coat development was delayed in group 1; group 2 showed premature moulting of the winter coat; and in group 3, development of the summer coat was delayed. The results imply that temperature modifies the influence of daylength on prolactin secretion and hair follicle growth by mechanisms that do not involve melatonin.     

Melatonin and 6-methoxy-2-benzoxazolinone (6-MBOA) alter the response of the male Syrian hamster to natural photoperiod

Adult male hamsters bearing either a blank beeswax, 6-methoxy-2-benzoxazolinone (6-MBOA), or melatonin pellet were exposed to 8 weeks (Oct. 6–Dec. 6) of natural autumn decreasing photoperiod (<11 h light) and temperature conditions (mean 10°C for last 4 weeks) or to a 14 h light/10 h dark (14L10D) photoperiod and controlled temperature (20°C). Melatonin but not 6-MBOA pellets partially prevented the combined effects of short photoperiod and cold temperatures on the testes and accessory organs. However, both 6-MBOA-and melatonin-treated hamsters maintained outdoors had significantly higher pituitary follicle stimulating hormone (FSH) values compared to their respective indoor-treated controls or to the animals kept outdoors and treated with a blank beeswax pellet. When one compares the various effects of 6-MBOA and melatonin (2 mg/month) on the reproductive system of the male hamster, 6-MBOA is not as effective as melatonin in altering reproductive responses to short photoperiod and cool temperatures at the dose administered.     

Effects of temperature on the development of overwintering immature stages of the near‐threatened butterfly Leptalina unicolor (Bremer & Grey) (Lepidoptera: Hesperiidae)

Overwintering larvae of multivoltine and univoltine populations of Leptalina unicolor were reared under various constant and fluctuating temperatures superimposed on a photoperiod of either 12 h of light and 12 h of darkness (LD 12:12) or LD 15:9. Diapause of the larvae terminated in midwinter (by early February). All the larvae of both populations pupated after two molts without feeding and the head capsule width of the final instar larvae was smaller than that of the penultimate instar ones. The photoperiod did not significantly affect larval development, but long‐day conditions (LD 15:9) hastened pupal development. The thermoperiod had a significant effect on the development of the multivoltine population. When multivoltine population larvae were kept under a low fluctuating temperature regime (cryophase/thermophase = 14/20°C), the period until adult eclosion was shorter than that under a constant temperature of 17°C. On the contrary, when larvae were kept under a high fluctuating temperature regime (24/30°C), the period until adult eclosion was longer than that under a constant temperature of 27°C. However, the univoltine population did not show such a reaction to the fluctuating temperature. The durations of final instar larva and pupa of the multivoltine population were shorter than those of the univoltine population. The developmental zeros of penultimate and final instar larvae and pupae of the univoltine population were lower than those of the multivoltine population. The head capsule width of penultimate instar larvae and the forewing length of adults of the univoltine population were larger than those of the multivoltine population for both sexes.     

Photoperiod and temperature interaction in the determination of reproduction of the edible snail, Helix pomatia

Snails were kept in self-cleaning housing chambers in an artificially controlled environment. Mating was frequent under long days (18 h light) and rare under short days (8 h light) regardless of whether the snails were kept at 15 degrees C or 20 degrees C. An interaction between photoperiod and temperature was observed for egg laying. The number of eggs laid (45-50/snail) and the frequency of egg laying (90-130%) were greater in long than in short days (16-35/snail and 27-77%) but a temperature of 20 degrees C redressed, to some extent, the inhibitory effect of short days. At both temperatures only long photoperiods brought about cyclic reproduction over a period of 16 weeks, confirming the synchronizing role of photoperiod on the neuroendocrine control of egg laying in this species of snail.     

Influence of long photoperiods on plant development and expression of Crassulacean acid metabolism in Mesembryanthemum crystallinum

Abstract. In the natural habitat plants of Mesembryanthemum crystallinum are induced to perform Crassulacean acid metabolism (CAM) after 3 months, and reproductive growth begins after 5 months (Winter, Liittge & Winter, 1978, Oecologia (Berlin), 34, 225-237). The life cycle of M. crystallinum and the extent of growth required prior to induction of enzymes of Crassulacean acid metabolism (CAM) are dramatically shortened by growing seedlings with a long photoperiod (3=16h/8h light/dark). Reproductive growth begins as soon as five weeks after germination when plants are grown in continuous light (under 600μmol quanta m⁻² s⁻¹, 30°C). In plants grown under well-watered conditions, the activities of PEP carboxylase and NADP-malic enzyme begin increasing markedly 2 weeks after germination, with plants grown under longer photoperiods having higher enzyme activities. After 3 weeks of growth, leaves accumulated a large amount of malate, but the microequivalents of malate present were up to nine times greater than the total titratable acidities. Interestingly, plants from a 24h/0h or a 20h/4h photo-period showed no diurnal fluctuation of malate, but did produce malate in the light as a major photosynthetic end product. That is, under these environmental conditions, principal enzymes of CAM can be induced without the plants performing CAM. However, plants grown in a 16h/8h photoperiod did exhibit nocturnal accumulation of malate after 3 weeks of growth. In plants of all three growth conditions, the activities of NADP-malic enzyme and PEP carboxylase were further increased two- to live-fold by irrigating 3-week-old-plants with 350mol m⁻³ NaCl. Such early enhancement of these enzymes by salt and the shortened life cycle may be due to an accelerated development under the long photoperiods.     

Rhizome transition to storage organ is under phytochrome control in lotus <Emphasis Type="Italic">(Nelumbo nucifera)</Emphasis>

We examined photoperiodic response of lotus (Nelumbo nucifera) rhizome morphogenesis (its transition to a storage organ) by using seed-derived plants. Rhizome enlargement (increase in girth) was brought about under 8, 10 and 12 h photoperiods, whereas the rhizomes elongated under 13 and 14 h photoperiods. Rhizomes elongated under 14 h light regimes supplied as 8 h of natural light plus 6 h supplemental hours of white, yellow or red light, but similar treatments with supplemental blue, green or far red light, caused enlargement in girth of the rhizomes. A 2 h interruption of the night with white, yellow or red light, in plants entrained to 8 h photoperiod brought rhizome elongation, whereas 2 h-blue, green or far red light night breaks still resulted in rhizome increase in girth. The inhibitory effect of a red (R) light night break on rhizome increase in girth was reversed by a far-red (FR) light given immediately afterwards. Irradiation with R/FR/R inhibited the rhizome increase in girth. FR light irradiation following R/FR/R irradiation cancelled the effect of the last R light irradiation. It was demonstrated that the critical photoperiod for rhizome transition to storage organ is between 12 and 13 h photoperiod. It was also evident that the optimal light quality range for interruption of dark period (night break) is between yellow and red light and that a R/FR reversible reaction is observed. From these results, we propose that phytochrome plays an important role in photoperiodic response of rhizome increase in girth in lotus. This is the first report on phytochrome-dependent morphogenesis of storage organs in rhizomous plants.     

Optimal temperature and photoperiod for the cultivation of Agardhiella subulata microplantlets in a bubble-column photobioreactor

The optimal temperature and illumination photoperiod requirements for the phototrophic growth of a novel microplantlet suspension culture derived from the macrophytic marine red alga Agardhiella subulata were determined. The optimal growth temperature was 24 degrees C. The effects of illumination light-dark (LD) photoperiod (hour of light:hours of darkness within a 24 h cycle) on biomass production was studied within a bubble-column photobioreactor. The 4.5 cm diameter photobioreactor was maintained at near-saturation conditions with respect to light flux (38 mciromol photons m(-2) s(-1)), nutrient medium delivery (20% nutrient replacement per day), and CO(2) delivery (0.35 mmol CO(2) L(-1) h(-1)) so that the cumulative effects of photodamage on the cell density versus time curve at photoperiods approaching continuous light could be observed. Biomass production was maximized at 16:8 LD, where biomass densities exceeding 3.6 g dry cell mass L(-1) were achieved after 60 days in culture. Biomass production was proportional to photoperiod at low fractional photoperiods (< or =10:14 LD), but high fractional photoperiods approaching continuous light (> or = 20:4 LD) shut down biomass production. Biomass production versus time profiles under resource-saturated cultivation conditions were adequately described by a cumulative photodamage growth model, which coupled reversible photodamage processes to the specific growth rate. Under light-saturated growth conditions, the rate constant for photodamage was kd = 1.17 +/- 0.28 day(-1) (+/-1.0 SE), and the rate constant for photodamage repair was kr = 5.12 +/- 0.95 day(-1) (+/-1.0 SE) at 24 degrees C.     

Reversible phytochrome regulation influenced the severity of ozone-induced visible foliar injuries in Trifolium subterraneum L.

In Trifolium subterraneum, oxidative stress caused by ozone has been shown to result in more severe visible foliar injuries when plants were kept in dim broadband white light during the night (i.e. a long photoperiod) compared to darkness during the night (a short photoperiod). As phytochrome signalling is involved in photoperiod sensing, the effect of night-time red and far-red illumination on the ozone-induced response was studied. T. subterraneum plants were treated with ozone enriched air (70?ppb) for either 1?h for a single day or 6?h for three consecutive days. After the first ozone exposure, plants were separated into six night-time light regimes during the two subsequent nights (10?h?day, 14?h night): (1) darkness, (2) far-red light (FR), (3) a short night-break of red followed by far-red light during an otherwise dark night (R FR), (4) a short night-break of red, far-red and finally red light during an otherwise dark night (R FR R), (5) dim white light (L) and (6) red light (R). The treatments L and R resulted in significantly more severe ozone-induced visible foliar injuries relative to D and FR treatments, indicating a phytochrome-mediated response. The night-breaks resulted in a photoreversible and significantly different ozone response depending on the light quality of the last light interval (R FR or R FR R), supporting a photoreversible (between P_r and P_fr) phytochrome signalling response. Thus, in T. subterraneum, the outcome of oxidative stress due to ozone appears to depend on the photoperiod mediated by the night-time conformation of phytochrome.