低温光抑制恢复过程中黄瓜叶片PSⅡ活性及其电子传递对PSⅠ的影响

以“津春4号”黄瓜为试材,通过测定黄瓜叶片叶绿素荧光快速诱导动力学曲线和对820 nm光的吸收曲线,结合叶绿素荧光淬灭分析,研究低温光胁迫(4℃,200 μmol·m-2·s-1)6 h后,黄瓜叶片在常温(25℃)不同光强(0、15、200μmol·m-2·s-1)下PS Ⅰ和PS Ⅱ活性的恢复,以及恢复过程中PS Ⅰ与PS Ⅱ的相互作用.结果表明:低温光胁迫6h后,PS Ⅰ和PS Ⅱ发生不同程度的光抑制.在常温恢复阶段,PS Ⅱ活性快速恢复且对光强不敏感;PS Ⅰ活性在弱光下(15 μmol·m-2·s-1)快速恢复,在较强光(200 μmol·m-2·s-1)下恢复较慢.在低温光抑制恢复过程中,常温下PS Ⅱ活性恢复较快可能导致PS Ⅱ向PS Ⅰ的线性电子传递过快,进而抑制PS Ⅰ的活性恢复.因此,在进行黄瓜抗冷性育种时,不应该仅追求较高的PS Ⅱ抗性和较快的PS Ⅱ恢复速度,还应该注意两个光系统活性的协调.在生产中,应当在低温逆境发生及其之后较长一段时间内采取措施降低叶表面光照强度,以利于对植株光合机构的保护和光合活性的恢复.     

Effects of temperature and UV radiation increases on the photosynthetic efficiency in four scleractinian coral species

Experiments were performed on coral species containing clade A (Stylophora pistillata, Montipora aequituberculata) or clade C (Acropora sp., Pavona cactus) zooxanthellae. The photosynthetic efficiency (F(v)/F(m)) of the corals was first assessed during a short-term increase in temperature (from 27 degrees C to 29 degrees C, 32 degrees C, and 34 degrees C) and acute exposure to UV radiation (20.5 W m(-2) UVA and 1.2 W m(-2) UVB) alone or in combination. Increasing temperature to 34 degrees C significantly decreased the F(v)/F(m) in S. pistillata and M. aequituberculata. Increased UV radiation alone significantly decreased the F(v)/F(m) of all coral species, even at 27 degrees C. There was a combined effect of temperature and UV radiation, which reduced F(v)/F(m) in all corals by 25% to 40%. During a long-term exposure to UV radiation (17 days) the F(v)/F(m) was significantly reduced after 3 days' exposure in all species, which did not recover their initial values, even after 17 days. By this time, all corals had synthesized mycosporine-like amino acids (MAAs). The concentration and diversity of MAAs differed among species, being higher for corals containing clade A zooxanthellae. Prolonged exposure to UV radiation at the nonstressful temperature of 27 degrees C conferred protection against independent, thermally induced photoinhibition in all four species.     

Non-invasive monitoring of the light-induced cyclic photosynthetic electron flow during cold hardening in wheat leaves

The effect of irradiance during low temperature hardening was studied in a winter wheat variety. Ten-day-old winter wheat plants were cold-hardened at 5 degrees C for 11 days under light (250 micromol m(-2) S(-1)) or dark (20 micromol m(-2) s(-1)) conditions. The effectiveness of hardening was significantly lower in the dark, in spite of a slight decrease in the Fv/Fm chlorophyll fluorescence induction parameter, indicating the occurrence of photoinhibition during the hardening period in the light. Hardening in the light caused a downshift in the far-red induced AG (afterglow) thermoluminescence band. The faster dark re-reduction of P700+, monitored by 820-nm absorbance, could also be observed in these plants. These results suggest that the induction of cyclic photosynthetic electron flow may also contribute to the advantage of frost hardening under light conditions in wheat plants.     

Thermostability and photostability of photosystem II of the resurrection plant Haberlea rhodopensis studied by chlorophyll fluorescence

The stability of PSII in leaves of the resurrection plant Haberlea rhodopensis to high temperature and high light intensities was studied by means of chlorophyll fluorescence measurements. The photochemical efficiency of PSII in well-hydrated Haberlea leaves was not significantly influenced by temperatures up to 40 degrees C. Fo reached a maximum at 50 degrees C, which is connected with blocking of electron transport in reaction center II. The intrinsic efficiency of PSII photochemistry, monitored as Fv/Fm was less vulnerable to heat stress than the quantum yield of PSII electron transport under illumination (phiPSII). The reduction of phiPSII values was mainly due to a decrease in the proportion of open PSII centers (qP). Haberlea rhodopensis was very sensitive to photoinhibition. The light intensity of 120 micromol m(-2) s(-1) sharply decreased the quantum yield of PSII photochemistry and it was almost fully inhibited at 350 micromol m(-2) s(-1). As could be expected decreased photochemical efficiency of PSII was accompanied by increased proportion of thermal energy dissipation, which is considered as a protective effect regulating the light energy distribution in PSII. When differentiating between the three components of qN it was evident that the energy-dependent quenching, qE, was prevailing over photoinhibitory quenching, qI, and the quenching related to state 1-state 2 transitions, qT, at all light intensities at 25 degrees C. However, the qE values declined with increasing temperature and light intensities. The qI was higher than qE at 40 degrees C and it was the major part of qN at 45 degrees C, indicating a progressing photoinhibition of the photosynthetic apparatus.     

Chlorella sorokiniana UTEX 2805, a heat and intense, sunlight-tolerant microalga with potential for removing ammonium from wastewater

In the summer of 2003, a microalga strain was isolated from a massive green microalgae bloom in wastewater stabilization ponds at the treatment facility of La Paz, B.C.S., Mexico. Prevailing environmental conditions were air temperatures over 40 degrees C, water temperature of 37 degrees C, and insolation of up to 2400 micromol m2 s(-1) at midday for several hours at the water surface for four months. The microalga was identified as Chlorella sorokiniana Shih. et Krauss, based on sequencing its entire 18S rRNA gene. In a controlled photo-bioreactor, this strain can grow to high population densities in synthetic wastewater at temperatures of 40-42 degrees C and light intensity of 2500 micromol m2 s(-1) for 5h daily and efficiently remove ammonium from the wastewater under these conditions better than under normal lower temperature (28 degrees C) and lower light intensity (60 micromol m2 s(-1)). When co-immobilized with the bacterium Azospirillum brasilense that promotes growth of microalgae, the population of microalga grew faster and removed even more ammonium. Under exposure to extreme growth conditions, the quantity of four photosynthetic pigments increased in the co-immobilized cultures. This strain of microalga has potential as a wastewater treatment agent under extreme conditions of temperature and light intensity.     

Photobleaching of photosynthetic pigments in spinach thylakoid membranes. Effect of temperature, oxygen and DCMU

The time dependence of photobleaching of photosynthetic pigments under high light illumination of isolated spinach thylakoid membranes at 22 and 4 degrees C was investigated. At 22 degrees C, the bleaching at 678, 472 and 436 nm was prominent but lowering the temperature up to 4 degrees C during illumination prevented the pigments from bleaching almost completely. The accelerating effect on pigment photobleaching by the presence of 3-(3,4 dichlorophenyl)-1,1-dimethyl-urea)-(DCMU), a well-known inhibitor of the electron transport and known to prevent photosystem I (PSI) and photosystem II (PSII) against photoinhibitory damage, was also suppressed at low temperature. At 22 degrees C in the presence and absence of DCMU, the decrease of the absorption at 678 and 472 nm was accompanied by a shift to the shorter wavelengths. To check the involvement of reactive oxygen species in the process, pigment photobleaching was followed in anaerobiosis. The effects of the three different environmental factors--light, temperature and DCMU--on the dynamics of photobleaching are discussed in terms of different susceptibility of the main pigment-protein complexes to photoinhibition.     

低温弱光胁迫对日光温室栽培杏树光系统功能的影响

以温室栽培的金太阳杏为材料,测定了金太阳杏叶片光合速率(P_n)、光系统Ⅱ(PSⅡ)光下实际光化学效率(Φ_PSⅡ)、光化学猝灭系数(q_P)和开放的PSⅡ反应中心的激发能捕获效率(F_v^′/F_m^′), 探讨了低温弱光(7 ℃、200 μmol·m^-2·s^-1 PFD)对叶片光系统Ⅰ(PSⅠ)和PSⅡ的抑制作用.结果表明：温室栽培的金太阳杏叶光合作用的最适温度在25 ℃左右.光下7 ℃的低温可使叶片净光合速率(P_n)大幅下降,造成激发压(1-q_P)增大,进而引起光抑制.低温弱光条件使PSⅠ和PSⅡ功能受到破坏,与单纯低温胁迫(7 ℃,黑暗)处理相比,经低温、弱光(7 ℃, 200 μmol·m^-2·s^-1PFD)胁迫2 h后,PSⅠ活性下降了28.26%,而PSⅡ最大光化学效率（F_v/F_m）没有发生显著变化,表明低温弱光条件下PSⅠ比PSⅡ 更易发生光抑制.     

A computerized tank system for studying the effect of temperature on calcification of reef organisms

Mediated by algal symbionts, calcification in reef building corals is one of the important processes, which enable coral's growth. In the present study, we used a buoyant weighing technique to study calcification of two coralline species, Stylophora pistillata and the hydrocoral Millepora dichotoma. The colonies were grown in a tank system, in which light, nutrition and water motion were kept constant and temperature was elevated by means of a computerized controlled apparatus. An almost constant rate of calcification was observed in the two species at 22-28 degrees C. Elevation of the temperature above this range to 29-31 degrees C caused a slow down in calcification in both species. A grater number of S. pistillata colonies became bleached at temperatures of >or=29 degrees C, whereas M. dichotoma colonies suffered from bleaching only after three days at 31 degrees C. For both species, control groups, remained viable during the experimental period. The differences in responses to changes in temperature of the two species may be as a consequence of different adaptive mechanisms or to different susceptibilities of the corals to elevated temperatures. We have shown that elevating temperatures above annual maximal ranges have a significant effect on coral calcification. We also demonstrated that sessile calcified marine organisms having ecological and biomedical significance could be cultured and manipulated under laboratory conditions.     

Involvement of zeaxanthin and of the Cbr protein in the repair of photosystem II from photoinhibition in the green alga Dunaliella salina.

A light-sensitive and chlorophyll (Chl)-deficient mutant of the green alga Dunaliella salina (dcd1) showed an amplified response to irradiance stress compared to the wild-type. The mutant was yellow-green under low light (100 micromol photons m(-2) s(-1)) and yellow under high irradiance (2000 micromol photons m(-2) s(-1)). The mutant had lower levels of Chl, lower levels of light harvesting complex II, and a smaller Chl antenna size. The mutant contained proportionately greater amounts of photodamaged photosystem (PS) II reaction centers in its thylakoid membranes, suggesting a greater susceptibility to photoinhibition. This phenotype was more pronounced under high than low irradiance. The Cbr protein, known to accumulate when D. salina is exposed to irradiance stress, was pronouncedly expressed in the mutant even under low irradiance. This positively correlated with a higher zeaxanthin content in the mutant. Cbr protein accumulation, xanthophyll cycle de-epoxidation state, and fraction of photodamaged PSII reaction centers in the thylakoid membrane showed a linear dependence on the chloroplast 'photoinhibition index', suggesting a cause-and-effect relationship between photoinhibition, Cbr protein accumulation and xanthophyll cycle de-epoxidation state. These results raised the possibility of zeaxanthin and Cbr involvement in the PSII repair process through photoprotection of the partially disassembled, and presumably vulnerable, PSII core complexes from potentially irreversible photooxidative bleaching.     

Irreversible photoinhibition of photosystem II is caused by exposure of Synechocystis cells to strong light for a prolonged period

Irreversible photoinhibition of photosystem II (PSII) occurred when Synechocystis sp. PCC 6803 cells were exposed to very strong light for a prolonged period. When wild-type cells were illuminated at 20 degrees C for 2 h with light at an intensity of 2,500 micromol photons m(-2) s(-1), the oxygen-evolving activity of PSII was almost entirely and irreversibly lost, whereas the photochemical reaction center in PSII was inactivated only reversibly. The extent of irreversible photoinhibition was enhanced at lower temperatures and by the genetically engineered rigidification of membrane lipids. Western and Northern blotting demonstrated that, after cells had undergone irreversible photoinhibition, the precursor to D1 protein in PSII was synthesized but not processed properly. These observations may suggest that exposure of Synechocystis cells to strong light results in the irreversible photoinhibition of the oxygen-evolving activity of PSII via impairment of the processing of pre-D1 and that this effect of strong light is enhanced by the rigidification of membrane lipids.     

Kinetics of the plastoquinone pool oxidation following illumination Oxygen incorporation into photosynthetic electron transport chain

The oxidation of the PQ-pool after illumination with 50 or 500 micromol quantam(-2)s(-1) was measured in isolated thylakoids as the increase in DeltaA(263), i.e., as the appearance of PQ. While it was not observed under anaerobic conditions, under aerobic conditions it was biphasic. The first faster phase constituted 26% or 44% of total reappearance of PQ, after weak or strong light respectively. The dependence on oxygen presence as well as the correlation with the rate of oxygen consumption led to conclusion that this phase represents the appearance of PQ from PQ(*-) produced in the course of PQH(2) oxidation by superoxide accumulated in the light within the membrane.     

Leaf respiration of snow gum in the light and dark. Interactions between temperature and irradiance

We investigated the effect of temperature and irradiance on leaf respiration (R, non-photorespiratory mitochondrial CO(2) release) of snow gum (Eucalyptus pauciflora Sieb. ex Spreng). Seedlings were hydroponically grown under constant 20 degrees C, controlled-environment conditions. Measurements of R (using the Laisk method) and photosynthesis (at 37 Pa CO(2)) were made at several irradiances (0-2,000 micromol photons m(-2) s(-1)) and temperatures (6 degrees C-30 degrees C). At 15 degrees C to 30 degrees C, substantial inhibition of R occurred at 12 micromol photons m(-2) s(-1), with maximum inhibition occurring at 100 to 200 micromol photons m(-2) s(-1). Higher irradiance had little additional effect on R at these moderate temperatures. The irradiance necessary to maximally inhibit R at 6 degrees C to 10 degrees C was lower than that at 15 degrees C to 30 degrees C. Moreover, although R was inhibited by low irradiance at 6 degrees C to 10 degrees C, it recovered with progressive increases in irradiance. The temperature sensitivity of R was greater in darkness than under bright light. At 30 degrees C and high irradiance, light-inhibited rates of R represented 2% of gross CO(2) uptake (v(c)), whereas photorespiratory CO(2) release was approximately 20% of v(c). If light had not inhibited leaf respiration at 30 degrees C and high irradiance, R would have represented 11% of v(c). Variations in light inhibition of R can therefore have a substantial impact on the proportion of photosynthesis that is respired. We conclude that the rate of R in the light is highly variable, being dependent on irradiance and temperature.     

Inhibition of Calvin–Benson cycle suppresses the repair of photosystem II in Symbiodinium: implications for coral bleaching

Inhibition of Calvin–Benson cycle (CBC) activity by thermal stress has been hypothesized to cause photoinhibition of photosystem II (PSII) in zooxanthellae of reef-building corals and consequently lead to bleaching. This study tests whether the interruption of CBC by glycolaldehyde (GA) leads to photoinhibition and subsequent coral bleaching in Stylophora pistillata. When S. pistillata was incubated with GA, the O₂ evolution rate declined in a dose-dependent manner and the extent of photoinhibition, reflected by a decreased maximum quantum yield of PSII (F _v/F _m), was enhanced. The effect of GA on photoinhibition was similar to that of chloramphenicol (CAP), an inhibitor of protein synthesis in chloroplasts. When S. pistillata was incubated in weak light following a high-light-induced photoinhibitory treatment, the recovery of PSII from photoinhibition was suppressed in a similar manner to both GA- and CAP-treated samples. After incubation in moderate light at 26°C, S. pistillata showed a bleaching response only in presence of GA. These results suggest that coral bleaching-like responses are caused by interruption of the CBC activity in S. pistillata and are associated with accelerated photoinhibition through suppression of the protein synthesis-dependent repair of PSII but not to an increase in photodamage to PSII.     

持续低温弱光对黄瓜叶片气体交换、叶绿素荧光猝灭和吸收光能分配的影响

持续常温弱光(25℃/18℃,l00umol m-2 s-1)、低温弱光(12℃/12℃,100 umol m-2 s-1和7℃/7℃,l00μmolm-2s-1)均导致黄瓜生长减慢或停滞、叶绿素含量、气孔导度和净光合速率、光合电子传递速率下降以及胞间CO2浓度上升.常温弱光和12℃弱光处理对光系统II的最大光化学效率Fv/Fm无显著影响,而7℃弱光处理导致Fv/Fm的可逆性下降.常温弱光和7℃、12℃弱光处理均导致了光化学反应速率的降低以及天线热耗散和反应中心过剩能量的增加.在胁迫后,12℃弱光0比7℃弱光更有利于植株光合功能的恢复.     

Contribution of photosynthetic electron transport,heat dissipation,and recovery of photoinactivated photosystem II to photoprotection at different temperatures in Chenopodium album leaves

Temperature dependence of photoinhibition and photoprotective mechanisms (10-35 degrees C) was investigated for Chenopodium album leaves grown at 25 degrees C under 500 micro mol quanta m(-2) s(-1). The fraction of active photosystem II (PSII) was determined after photoinhibitory treatment at different temperatures in the presence and absence of lincomycin, an inhibitor of chloroplast-encoded protein synthesis. In the absence of lincomycin, leaves were more tolerant to photoinhibition at high (25-35 degrees C) than at low (11-15 degrees C) temperatures. In the presence of lincomycin, the variation in the tolerance to photoinactivation became relatively small. The rate constant of photoinactivation (k(pi)) was stable at 25-35 degrees C and increased by 50% with temperature decrease from 25 to 11 degrees C. The rate constant of recovery of inactivated PSII (k(rec)) was more sensitive to temperature; it was very low at 11 degrees C and increased by an order of magnitude at 35 degrees C. We conclude that the recovery of photoinactivated PSII plays an essential role in photoprotection at 11-35 degrees C. Partitioning of light energy to various photoprotective mechanisms was further analyzed to reveal the factor responsible for k(pi). The fraction of energy utilized in photochemistry was lower at lower temperatures. Although the fraction of heat dissipation increased with decreasing temperatures, the excess energy that is neither utilized by photochemistry nor dissipated by heat dissipation was found to be greater at lower temperatures. The k(pi) value was strongly correlated with the excess energy, suggesting that the excess energy determines the rate of photoinactivation.     

Responses of Rubisco and Rubisco activase in cucumber seedlings to low temperature and weak light

Using 'Jinyou 3' cucumber seedlings as test materials, this paper studied their photosynthetic rate (P(n)), Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and Rubisco activase (RCA) activities, and gene expression of Rubisco and RCA under optimal temperature and weak light (WL: 25 degrees C/18 degrees C, 100 micromol x m(-2) x s(-1)), suboptimal temperature and weak light (ST+WL: 18 degrees C/12 degrees C, 100 micromol x m(-2) x s(-1)), and low temperature and weak light (LT+WL: 10 degress C/5 degrees C, 100 micromol x m(-2) x s(-1)). Comparing with the control (25 degrees C/18 degrees C, 400 micromol x m(-2) x s(-1)), treatments WL, ST+WL, and LT+WL all led to a remarkable decrease in leaf area and dry matter mass. At initial stage, the P(n), Rubisco activity, rbcL and rbcS expression, RCA activity, and CsRCA expression in the three treatments declined by a big margin; 5-7 days later, these parameters tended to be less changed in treatment WL, ascended slowly in treatment ST+WL, and decreased continuously in treatment LT+WL. These results suggested that the photosynthetic apparatus of test cucumber seedlings could gradually adapt to weak light or suboptimal temperature and weak light. The Rubisco and RCA activities and the gene expression of Rubisco and RCA showed the similar responses to low temperature and weak light as the P(n), suggesting that the decline in Rubisco and RCA activities and gene expression in cucumber seedlings under low temperature and weak light could be the important reason leading to the decrease of P(n).     

Responses of chlorophyll fluorescence parameters of the facultative halophyte and C3-CAM intermediate species Mesembryanthemum crystallinum to salinity and high irradiance stress

Mesembryanthemum crystallinum L. (Aizoaceae) is a facultative annual halophyte and a C(3)-photosynthesis/crassulacean acid metabolism intermediate species currently used as a model plant in stress physiology. Both salinity and high light irradiance stress are known to induce CAM in this species. The present study was performed to provide a diagnosis of alterations at the photosystem II level during salinity and irradiance stress. Plants were subjected for up to 13 days to either 0.4M NaCl salinity or high irradiance of 1000 micromol m(-2)s(-1), as well as to both stress factors combined (LLSA=low light plus salt; HLCO=high light of 1000 micromol m(-2)s(-1), no salt; HLSA=high light plus salt). A control of LLCO=low light of 200 micromol m(-2)s(-1), no salt was used. Parameters of chlorophyll a fluorescence of photosystem II (PSII) were measured with a pulse amplitude modulated fluorometer. HLCO and LLSA conditions induced a weak degree of CAM with day/night changes of malate levels (Deltamalate) of approximately 12mM in the course of the experiment, while HLSA induced stronger CAM of Deltamalate approximately 20 mM. Effective quantum yield of PSII, DeltaF/F'(m), was only slightly affected by LLSA, somewhat reduced during the course of the experiment by HLCO and clearly reduced by HLSA. Potential quantum efficiency of PSII, F(v)/F(m), at predawn times was not affected by any of the conditions, always remaining at 0.8, showing that there was no acute photoinhibition. During the course of the days HL alone (HLCO) also did not elicit photoinhibition; salt alone (LLSA) caused acute photoinhibition which was amplified by the combination of the two stresses (HLSA). Non-photochemical, NPQ, quenching remained low (<0.5) under LLCO, LLSA and HLCO and increased during the course of the experiment under HLSA to 1-2. Maximum apparent photosynthetic electron transport rates, ETR(max), declined during the daily courses and were reduced by LLSA and to a similar extent by HLSA. It is concluded that M. crystallinum expresses effective stress tolerance mechanisms but photosynthetic capacity is reduced by the synergistic effects of salinity and light irradiance stress combined.     

Insights on the development, kinetics, and variation of photoinhibition using chlorophyll fluorescence imaging of a chilled, variegated leaf

The effect of chilling on photosystem II (PSII) efficiency was studied in the variegated leaves of Calathea makoyana, in order to gain insight into the causes of chilling-induced photoinhibition. Additionally, a relationship was revealed between (chilling) stress and variation in photosynthesis. Chilling treatments (5 degrees C and 10 degrees C) were performed for different durations (1-7 d) under a moderate irradiance (120 micromol m-2 s-1). The individual leaves were divided into a shaded zone and two illuminated, chilled zones. The leaf tip and sometimes the leaf base were not chilled. Measurements of the dark-adapted Fv/Fm were made on the different leaf zones at the end of the chilling treatment, and then for several days thereafter to monitor recovery. Chilling up to 7 d in the dark did not affect PSII efficiency and visual appearance, whereas chilling in the light caused severe photoinhibition, sometimes followed by leaf necrosis. Photoinhibition increased with the duration of the chilling period, whereas, remarkably, chilling temperature had no effect. In the unchilled leaf tip, photoinhibition also occurred, whereas in the unchilled leaf base it did not. Whatever the leaf zone, photoinhibition became permanent if the mean value dropped below 0.4, although chlorosis and necrosis were associated solely with chilled illuminated tissue. Starch accumulated in the unchilled leaf tip, in contrast to the adjacent chilled irradiated zone. This suggests that photoinhibition was due to a secondary effect in the unchilled leaf tip (sink limitation), whereas it was a direct effect of chilling and irradiance in the chilled illuminated zones. The PSII efficiency and its coefficient of variation showed a unique negative linearity across all leaf zones and different tissue types. The slope of this curve was steeper for chilled leaves than it was for healthy, non-stressed leaves, suggesting that the coefficient of variation may be an important tool for assessing stress in leaves.     

Photoinhibition induced alterations in energy transfer process in phycobilisomes of PS II in the cyanobacterium, Spirulina platensis

Exposure of algae or plants to irradiance from above the light saturation point of photosynthesis is known as high light stress. This high light stress induces various responses including photoinhibition of the photosynthetic apparatus. The degree of photoinhibition could be clearly determined by measuring the parameters such as absorption and fluorescence of chromoproteins. In cyanobacteria and red algae, most of the photosystem (PS) II associated light harvesting is performed by a membrane attached complex called the phycobilisome (PBS). The effects of high intensity light (1000-4000 micromol photons m(-2) s(-1)) on excitation energy transfer from PBSs to PS II in a cyanobacterium Spirulina platensis were studied by measuring room temperature PC fluorescence emission spectra. High light (3000 micromol photons m(-2) s(-1)) stress had a significant effect on PC fluorescence emission spectra. On the other hand, light stress induced an increase in the ratio of PC fluorescence intensity of PBS indicating that light stress inhibits excitation energy transfer from PBS to PS II. The high light treatment to 3000 micromol photons m(-2) s(-1) caused disappearance of 31.5 kDa linker polypeptide which is known to link PC discs together. In addition we observed the similar decrease in the other polypeptide contents. Our data concludes that the Spirulina cells upon light treatment causes alterations in the phycobiliproteins (PBPs) and affects the energy transfer process within the PBSs.