Showing papers on "Photoinhibition published in 1987"

Photon yield of O2 evolution and chlorophyll fluorescence characteristics at 77 K among vascular plants of diverse origins.

Abstract: Photon yields of oxygen evolution at saturating CO2 were determined for 44 species of vascular plants, representing widely diverse taxa, habitats, life forms and growth conditions. The photonyield values on the basis of absorbed light (φa) were remarkably constant among plants possessing the same pathway of photosynthetic CO2 fixation, provided the plants had not been subjected to environmental stress. The mean φa value ±SE for 37 C3 species was 0.106±0.001 O2·photon-1. The five C4 species exhibited lower photon yields and greater variation than the C3 species (φa=0.0692±0.004). The φa values for the two Crassulaceanacid-metabolism species were similar to those of C3 species. Leaf chlorophyll content had little influence on φa over the range found in normal, healthy leaves. Chlorophyll fluorescence characteristics at 77 K were determined for the same leaves as used for the photon-yield measurements. Considerable variation in fluorescence emission both at 692 nm and at 734 nm, was found 1) among the different species; 2) between the upper and lower surfaces of the same leaves; and 3) between sun and shade leaves of the same species. By contrast, the ratio of variable to maximum fluorescence emission at 692 nm (Fv/FM, 692) remained remarkably constant (The mean value for the C3 species was 0.832±0.004). High-light treatments of shade leaves resulted in a reduction in both φa and the Fv/FM, 692 ratio. The extent of the reductions increased with time of exposure to bright light. A linear relationship was obtained when φa was plotted against Fv/FM, 692. The results show that determinations of the photon yield of O2 evolution and the Fv/FM, 692 ratio can serve as excellent quantitative measures of photoinhibition of overall photosynthetic energy-conversion system and of photochemistry of photosystem II, respectively. This is especially valuable in field work where it is often impossible to obtain appropriate controls.

Comparison of the effect of excessive light on chlorophyll fluorescence (77K) and photon yield of O2 evolution in leaves of higher plants.

Abstract: High-light treatments (1750–2000 μmol photons m−2 · s−1) of leaves from a number of higher-plant species invariably resulted in quenching of the maximum 77K chlorophyll fluorescence at both 692 and 734 nm (FM, 692 and FM, 734). The response of instantaneous fluorescence at 692 nm (FO, 692) was complex. In leaves of some species FO, 692 increased dramatically in others it was quenched, and in others yet it showed no marked, consistent change. Regardless of the response of FO, 692 an apparently linear relationship was obtained between the ratio of variable to maximum fluorescence (FV/FM, 692) and the photon yield of O2 evolution, indicating that photoinhibition affects these two variables to approximately the same extent. Treatment of leaves in a CO2−free gas stream containing 2% O2 and 98% N2 under weak light (100 μmol · m−2 · s−1) resulted in a general and fully reversible quenching of 77K fluorescence at 692 and 734 nm. In this case both FO, 692 and FM, 692 were invariably quenched, indicating that the quenching was caused by an increased non-radiative energy dissipation in the pigment bed. We propose that high-light treatments can have at least two different, concurrent effects on 77K fluorescence in leaves. One results from damage to the photosystem II (PSII) reaction-center complex and leads to a rise in FO, 692; the other results from an increased non-radiative energy dissipation and leads to quenching of both FO, 692 and FM, 692 This general quenching had a much longer relaxation time than reported for ΔpH-dependent quenching in algae and chloroplasts. Sun leaves, whose FV/FM, 692 ratios were little affected by high-light exposure in normal air, suffered pronounced photoinhibition when the exposure was made under conditions that prevent photosynthetic gas exchange (2% O2, 0% CO2). However, they were still less susceptible than shade leaves, indicating that the higher capacity for energy dissipation via photosynthesis is not the only cause of their lower susceptibility. The rate constant for recovery from photoinhibition was much higher in mature sun leaves than in mature shade leaves, indicating that differences in the capacity for continuous repair may in part account for the difference in their susceptibility to photoinhibition.

Photoinhibition and zeaxanthin formation in intact leaves. A possible role of the xanthophyll cycle in the dissipation of excess light energy. [Populus balsamifera; Hedera; helix; Monstrosa deliciosa]

Abstract: Comparative studies of chlorophyll a fluorescence, measured with a pulse amplitude modulated fluorometer, and of the pigment composition of leaves, suggest a specific role of zeaxanthin, a carotenoid formed in the xanthophyll cycle, in protecting the photosynthetic apparatus against the adverse effects of excessive light. This conclusion is based on the following findings: (a) exposure of leaves of Populus balsamifera, Hedera helix, and Monstera deliciosa to excess excitation energy (high light, air; weak light, 2% O/sub 2/, 0% CO/sub 2/) led to massive formation of zeaxanthin and a decrease in violaxanthin. (b) When exposed to photoinhibitory light levels in air, shade leaves of H. helix had a higher capacity for zeaxanthin formation, at the expense of ..beta..-carotene, than shade leaves of M. deliciosa. Changes in fluorescence characteristics suggested that, in H. helix, the predominant response to high light was an increase in the rate of nonradiative energy dissipation, whereas, in M. deliciosa, photoinhibitory damage to photosystem II reaction centers was the prevailing effect. (c) Exposure of a sun leaf of P. balsamifera to increasing photon flux densities in 2% O/sub 2/ and 0% CO/sub 2/ resulted initially in increasing levels of zeaxanthin (matched by decreases in violaxanthin) and was accompaniedmore » by fluorescence changes indicative of increased nonradiative energy dissipation. Above the light level at which no further increase in zeaxanthin content was observed, fluorescence characteristics indicated photoinhibitory damage. (d) A linear relationship was obtained between the ratio of variable to maximum fluorescence, F/sub V/F/sub M/, determined with the modulated fluorescence technique at room temperature, and the photon yield of O/sub 2/ evolution.« less

Photoinhibition and zeaxanthin formation in intact leaves : a possible role of the xanthophyll cycle in the dissipation of excess light energy.

Abstract: Comparative studies of chlorophyll a fluorescence, measured with a pulse amplitude modulated fluorometer, and of the pigment composition of leaves, suggest a specific role of zeaxanthin, a carotenoid formed in the xanthophyll cycle, in protecting the photosynthetic apparatus against the adverse effects of excessive light. This conclusion is based on the following findings: (a) exposure of leaves of Populus balsamifera, Hedera helix, and Monstera deliciosa to excess excitation energy (high light, air; weak light, 2% O2, 0% CO2) led to massive formation of zeaxanthin and a decrease in violaxanthin. Over a wide range of conditions, there was a linear relationship between either variable, Fv, or maximum fluorescence, Fm, and the zeaxanthin content of leaves. (b) When exposed to photoinhibitory light levels in air, shade leaves of H. helix had a higher capacity for zeaxanthin formation, at the expense of β-carotene, than shade leaves of M. deliciosa. Changes in fluorescence characteristics suggested that, in H. helix, the predominant response to high light was an increase in the rate of nonradiative energy dissipation, whereas, in M. deliciosa, photoinhibitory damage to photosystem II reaction centers was the prevailing effect. (c) Exposure of a sun leaf of P. balsamifera to increasing photon flux densities in 2% O2 and 0% CO2 resulted initially in increasing levels of zeaxanthin (matched by decreases in violaxanthin) and was accompanied by fluorescence changes indicative of increased nonradiative energy dissipation. Above the light level at which no further increase in zeaxanthin content was observed, fluorescence characteristics indicated photoinhibitory damage. (d) A linear relationship was obtained between the ratio of variable to maximum fluorescence, Fv/Fm, determined with the modulated fluorescence technique at room temperature, and the photon yield of O2 evolution, similar to previous findings (O Bjorkman, B Demmig 1987 Planta 170: 489-504) on chlorophyll fluorescence characteristics at 77 K and the photon yield of photosynthesis.

Effects of water deficit on photosynthetic capacity

Abstract: Under drought, CO2 assimilation rates decrease already at small leaf water deficits. At least part of the inhibition is attributed to non-stomata1 effects at the chloroplast level, with electron transport and phosphorylation being main targets of inhibition. These findings are questioned by direct measurements of photosynthetic capacity with systems that are not Limited by stomata, e.g. leaf slices in solution or leaves at ex-ternal CO2 concentrations exceeding 5%. Here, photosynthesis was rather insensitive to dehydration down to 50–70% relative water content, and different plant species re-sponded in a very similar way. More severe dehydration affected not only pboto-synthesis, but also dark CO2 fixation and presumably also photorespiration. Rever-sible and unspecific inhibition is thought to be mediated mainly by increased concen-trations of solutes in dehydrated cells. Inhibition of photorespiration might favour photoinhibition when long-term water stress is coupled with full sunlight. Photo-inhibition, together with general senescence phenomena might be involved in long-term effects of water stress under natural drought conditions. This offers an explanation for the conflicting results of short-term water stress experiments and studies carried out under field conditions.

Effect of irradiances up to 2000 μE m−2 s−1 on marine Synechococcus WH7803—I. Growth, pigmentation, and cell composition

Abstract: We grew Synechococcus WH7803 at rates exceeding 1.4 d −1 at irradiances from 200 to 2000 μE m −2 s −1 under continuous light in nutrient replete media with no evidence of photoinhibition. Concentrations of the photosynthetic pigments phycoerythrin, phycocyanin, and chlorophyll a , were inversely related to growth irradiance. Phycoerythrin exhibited the greatest plasticity with the concentration in cells adapted to 30 μE m −2 s −1 being ca . 20 times greater than that in cells adapted to 700 μE m −2 s −1 . Changes in the phycoerythrin: phycocyanin ratio as well as their respective concentrations indicate that phycobilisomes underwent changes in size at irradiances which saturated or nearly saturated growth and underwent changes in number at irradiances which limited growth. Phycoerythrin in high light adapted cells contained 20% in light limited cells. Results support the notion that nutrient replete Synechococcus have the capacity to grow at maximal growth rates in brightly lit oceanic surface mixed layers.

Environmental Effects on Photosynthesis, Nitrogen-Use Efficiency, and Metabolite Pools in Leaves of Sun and Shade Plants

Abstract: Effects of varying light intensity and nitrogen nutrition on photosynthetic physiology and biochemistry were examined in the sun plant Phaseolus vulgaris (common bean) and in the shade plant Alocasia macrorrhiza (Australian rainforest floor species). In both Phaseolus and Alocasia, the differing growth regimes produced large changes in photosynthetic capacity and composition of the photosynthetic apparatus. CO2-saturated rates of photosynthesis were linearly related to leaf nitrogen (N) content in both species but photosynthesis per unit leaf N was markedly higher for Phaseolus than for Alocasia. Photosynthetic capacity was also higher in Phaseolus per unit ribulose 1,5-bisphosphate (RuBP) carboxylase (RuBPCase) protein. The leaf content of RuBPCase was linearly dependent on leaf N content in the two species. However, the proportion of leaf N which was RuBPCase was greater in Phaseolus than in Alocasia and was more sensitive to growth conditions, ranging from 6% of leaf N at low light to 20% at high light. In Alocasia, this range was much less, 6 to 11%. However, chlorophyll content was much more sensitive to light intensity in Alocasia. Thus, the RuBPCase/chlorophyll ratio was quite responsive to N availability and light intensity in both species (but for different reasons), ranging from 6 grams per gram for Phaseolus and 2 grams per gram for Alocasia at high leaf N and 1.5 gram per gram for Phaseolus and 0.5 gram per gram for Alocasia at low leaf N. These large changes in the proportions of components of the photosynthetic apparatus had marked effects on the sensitivity of these species to photoinhibition. These environmental effects also caused changes in the absolute levels of metabolites of the photosynthetic carbon reduction cycle. Concentrations of RuBP and P-glycerate were approximately 2-fold higher in high light-grown than low light-grown Phaseolus and Alocasia when expressed on a leaf area basis. However, if metabolite pool sizes are expressed on the basis of the RuBPCase catalytic site concentration, then they were little affected by the marked changes in leaf makeup. There appears to be fundamental differences between these species in the mechanism of sun-shade adaptation and N partitioning in the photosynthetic apparatus that result in significant differences in the N-use efficiency of photosynthesis between Phaseolus and Alocasia but similar RuBPCase:substrate:product ratios despite these differences.

Effects of drought on primary photosynthetic processes of cotton leaves.

Abstract: The effects of drought on Photosystem II (PSII) fluorescence and photosynthetic electron transport activities were analyzed in cotton. Water stress did not modify the amplitude of leaf variable fluorescence at room temperature in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) nor at 77 K. It is therefore concluded that photon collection, their distribution between the two photosystems, and PSII photochemistry are unaffected by the stress. In droughted leaves at room temperature under low exciting light, the transitory maximum (Fp) and steady state (Ft) fluorescence levels are increased; under high exciting light, Fp level and the rise time from the initial level (Fo) to Fp are unchanged, whereas Fp to Ft decay time is increased. These results infer that the drought slows the rate of plastoquinone reoxidation. This conclusion agrees with a larger proportion of reduced primary PSII electron acceptor QA measured at the steady state under low light. In thylakoids isolated from droughted leaves, PSII mediated electron flow was the same as in thylakoids from control leaves, whereas PSI mediated electron transport was inhibited. It is shown that water stress does not induce sensitization to photoinhibition in cotton.

Effect of irradiances up to 2000 μE m−2 s−1 on marine Synechococcus WH7803—II. Photosynthetic responses and mechanisms

Abstract: We investigated the photosynthetic behavior of Synechococcus WH7803 when grown over the irradiance range of 30–2000 μE m −2 s −1 in nutrient-replete, continuous light, pre-adapted batch cultures. For each of 8 growth irradiances investigated, we found a unique photosynthesis vs irradiance ( P vs I ) relationship. Cell- and carbon-specific photosynthetic light harvesting efficiencies (α (cell) and α (C) ) decreased 10-fold from the lowest to highest growth irradiances. Chlorophyll and phycocyanin-specific efficiencies also decreased but to a lesser extent. Phycoerythrin-specific efficiencies increased ca . 2-fold with increasing irradiances. Photosynthetic capacity ( P max ) approximated the in situ rate of photosynthesis ( P i ) only at growth irradiances which saturated growth rate; at light limiting irradiances, P max increased to 2.9 times P i . Photoinhibition of photosynthesis was only observed in light-limited cells. The mechanism of photoadaptation which resulted in the observed growth rate vs irradiance (μ vs I ) response involved regulation of α (cell) by changes in phycobilisome size over saturating irradiances and phycobilisome numbers over limiting irradiances. Our results are consistent with previously reported field observations of α (Chl) , α (cell) and P max increasing with depth, and of no photoinhibition of near-surface collected assemblages, suggesting that high irradiances in near-surface waters can support maximal growth rates of Synechococcus . These results raise important questions regarding the interpretation of parameters of P vs I measurement from field studies. Our results do not conform to the previously reported hypothesis that a significant percentage of phycoerythrin serves as a non-photosynthetic nitrogen storage compound in these cells.

The Susceptibility of Photosynthesis to Photoinhibition and the Capacity of Recovery in High and Low Light Grown Cyanobacteria, Anacystis nidulans

Abstract: The susceptibility of photosynthesis to photoinhibition and the rate of its recovery were studied in the cyanobacterium Anacystis nidulans grown at a low (10 micromoles per square meter per second) and a high (120 micromoles per square meter per second) photosynthetically active radiation. The rate of light limited photosynthetic O2 evolution was measured to determine levels of photoinhibition and rates of recovery. Studies of photoinhibition and recovery with and without the translation inhibitor streptomycin demonstrated the importance of a recovery process for the susceptibility of photosynthesis to photoinhibition. We concluded that the approximately 3 times lower susceptibility to photoinhibition of high light than of low light grown cells, significantly depended on high light grown cells having an approximately 3 times higher recovery capacity than low light grown cells. It is suggested that these differences in susceptibility to photoinhibition and recovery depends on high light grown cells having a higher turnover rate of photosystem II protein(s) that is(are) the primary site(s) of photodamage, than have low light grown cells. Furthermore, we demonstrated that photoinhibition of A. nidulans may occur under physiological light conditions without visible harm to the growth of the cell culture. The results give support for the hypotheses that the net photoinhibitory damage of photosystem II results from the balance between the photoinhibitory process and the operation of a recovery process; the capacity of the latter determining significant differences in the susceptibility of photosynthesis to photoinhibition of high and low light grown A. nidulans.

High-Irradiance Stress in Higher Plants and Interaction with other Stress Factors

Abstract: Exposure of leaves to light levels in excess of what can be utilized in photosynthesis often results in a decline in photosynthetic activity (1). This high-light effect is especially evident after return of the leaf to a low light level as a reduction in the photon yield of photosynthetic O2 evolution or CO2 uptake. In common usage the term photoinhibition includes any sustained reduction in photosynthetic activity induced by excessive light, irrespective of mechanistic considerations, but does not include transient reductions that are rapidly reversible and likely to reflect short-term regulation. In my talk today I will attempt to distinguish between two kinds of high-light-induced reduction in the efficiency of photochemistry of PSII: 1) an increase in the rate constant for dissipation of excitation energy in the antenna and 2) a decrease in the rate constant for the photochemistry of PSII which is likely to be caused by damage to the PSII reaction centers.

Light-dependent damage to photosynthesis in olive leaves during chilling and high temperature stress

Abstract: The leaves of olive are long lived and likely to experience both chilling and high temperature stress during their life. Changes in photosynthetic CO2 assimilation resulting from chilling and high temperature stress, in both dim and high light, are investigated. The quantum yield (φ) of photosynthesis at limiting light levels was reduced following chilling (at 5°C for 12 h), in dim light by approximately 10%, and in high light by 75%; the difference being attributed to photoinhibition. Similar reductions were observed in the light-saturated rate of CO2 uptake (Amax). Decrease in Amax correlated with a halving of the leaf internal CO2 concentration (ci), suggesting an increased limitation by stomata following photoinhibition. Leaves were apparently more susceptible to photoinhibitory damage if the whole plant, rather than the leaf alone, was chilled. On return to 26 °C, I he photosynthetic capacity recovered to pre-stress levels within a few hours if leaves had been chilled in high light for 8 h or less, but did not fully recover from longer periods of chilling when loss of chlorophyll occurred. Leaves which were recovering from chilling in high light showed far more damage on being chilled a second time in high light. Three hours in high light at 38 °C reduced φ by 80%, but φ recovered within 4h of return to 26 °C. Although leaves of Olive are apparently less susceptible to photoinhibitory damage during chilling stress than the short-lived leaves of chilling-sensitive annual? crops, the results nevertheless show that photoinhibition during temperature stress is potentially a major factor influencing the photosynthetic productivity of Olive in the field.

Light‐dependence of photosynthesis and growth in cyanobacteria: Implications for their dominance in eutrophic lakes

Abstract: In eutrophic lakes cyanobacteria are favoured relative to other phytoplankton, both under stratified and mixed conditions. During stratification, gas vacuole formation allows the accumulation of dense surface scums which attain the highest possible area‐specific photosynthetic rates in aquatic environments owing to high irradiances, near‐complete harvesting of impinging light, and minimal light inhibition and photo‐oxidation. During moderate mixing, high yields of biomass can be achieved by effective light harvesting for photosynthesis (aided by phycobilin pigments) and low maintenance energy requirements at low mean irradiances. Howevrr, nitrogen fixation competes for energy and reductant with photosynthesis, and leads to a decline of light‐saturated maximum growth rates. Wind‐driven vertical mixing and lateral advection are the main causes for the instability of cyanobacterial blooms in hyper‐eutrophic lake ecosystems.

Effects of low night temperature and light on chlorophyll fluorescence of field-grown seedlings of Scots pine (Pinus sylvestris L.).

Abstract: In vivo chlorophyll fluorescence kinetics was observed in naturally regenerated and planted Scots pine (Pinus sylvestris L.) seedlings growing at two adjacent sites in northern Sweden. Some seedlings were shaded from direct solar radiation and some were protected from frost at night. Air temperature and solar radiation were monitored. In vivo chlorophyll fluorescence kinetics of current-year needles was measured at both room temperature and 77 K. On clear days during August, variable fluorescence of photosystem II of needles from exposed, naturally regenerated seedlings was reduced by night frosts, whereas frost caused little change in fluorescence of needles from shaded seedlings. Overnight frost-protection reduced the inhibition of photosystem II by direct sunlight. Recovery from photoinhibitory damage occurred on cloudy days. In September, the decline of variable fluorescence of photosystem II was more pronounced in unshaded than in shaded seedlings, and coincided with frosts at night. In addition, the reduction in variable fluorescence was larger for planted seedlings than for naturally regenerated seedlings. It is suggested that the inhibition of photosystem II activity was caused by an interaction between low temperatures and light. Natural night frosts are proposed to increase the susceptibility to photoinhibition in the following day(s).

Photosynthesis and cell division by Antarctic microalgae: comparison of benthic, planktonic and ice algae

Abstract: Irradiance-dependent rates of photosynthesis and cell division of six species of microalgae isolated from the benthos, plankton and sea ice microbial community in McMurdo Sound, Antarctica were compared. Microalgae isolated from different photic environments had distinct photosynthetic and growth characteristics. For benthic and ice algae, photosynthesis saturated at 6 to 20 μE.m−2.s−1 and was photoinhibited at 10 to 80 μE.m−2.s−1 while for the planktonic algae, saturation irradiances were up to 13 times higher and photoinhibition was not detected. The slope of the light-limited portion of the P-I relationship was up to 50 times greater for the benthic algae than for either the ice or planktonic algae suggesting that benthic algae used the low irradiances more efficiently for carbon uptake. Cell division was dependent on the incubation irradiance for all but one microalga examined. The dependence of division rates on irradiance was however much smaller than for carbon uptake, suggesting that cell division buffers the influence of short term variations of irradiance on cellular metabolism.

Responses of Two CAM Species to Different Irradiances during Growth and Susceptibility to Photoinhibition by High Light

Abstract: Two CAM species, Kalanchoe daigremontiana Hamet et Perrier and Hoya carnosa (L.) R. Br., were grown under a range of five photon flux area densitites (PFD) and then characterized. Significant acclimation to shade was indicated by progressive decreases in leaf thickness, rates of respiratory O(2) uptake, light compensation point, maximum rates of photosynthetic O(2) evolution, nocturnal acid accumulation, and delta(13)C values, and increases in chlorophyll concentration and absolute levels of room temperature (25 degrees C) and 77K fluorescence. Quantum yields (as measured by O(2) exchange) and the ratio of variable 77K fluorescence over the maximum yield (F(v)/F(m)) were relatively constant across the treatments. The only significant deviation from the above characteristics was in H. carnosa grown under full glasshouse PFD, where it apparently experienced photoinhibition. Following a photoinhibitory treatment, K. daigremontiana exhibited increases in the light compensation point and progressively greater reductions in the quantum yield, maximum photosynthetic rate, F(v)/F(m), and the variable component of room temperature fluorescence with increasing shade during growth. Thus although Crassulacean acid metabolism plants can adjust to shaded conditions, they are susceptible to photoinhibition when exposed to higher PFD than that experienced during growth.

Diurnal Courses of Photosynthesis, Transpiration, and Diffusive Conductance in the Single-leaf of the Rice Plants Grown in the Paddy Field under Submerged Condition

Abstract: It had been reported by the authors that leaf stomatal aperture in the rice plant under submerged condition decreased in fine midday with high evaporative demand and that close relation was found between diffusive conductance and photosynthetic rate in rice leaves. From these results it had been predicted that photosynthesis would reach the maximum early in the morning and then decrease gradually toward the afternoon with increase in light intensity and vapour pressure deficit. This study was conducted to ascertain this prediction by measuring the diurnal courses of photosynthetic rate (P), transpiration rate (T), P/T ratio (water use efficiency) and diffusive conductance in the single-leaf of the rice plant in paddy field under submerged condition on fine and cloudy days and to clarify the factors determining the diurnal courses of photosynthesis in a fine day using the simultaneous measurement system of photosynthesis and transpiration developed by KOCH, SCHULTZ and LANGE (Siemens Co. Ltd.). The photosynthetic rate on a fine day increased with increase in light intensity, reached the maximum early in the morning and then gradually decreased down to 75% of the maximum rate toward afternoon even under sufficient light intensity and this was accompanied by a decrease of diffusive conductance (Figs. 2A and 4). The photosynthetic rate in the morning was higher than that in the afternoon under the same light intensity more than 600 μE/m2/sec (Fig. 3A). Transpiration rate increased toward the midday with the increase of light intensity and vapor pressure deficit, and reached the maximum at about 13:00 PM. Water use efficiency was higher early in the morning and late in the evening and lower in the midday (Fig. 2A). The photosynthetic rate, transpiration rate and diffusive conductance on a cloudy day changed according to the change in light intensity (Figs. 2B and 3B). Therefore, light intensity was the dominant factor determining diurnal courses of photosynthetic rate and transpiration rate on a cloudy day. It was found that the time of the daily maximum of diffusive conductance was different from the time of that of photosynthetic rate in their diurnal changes, that is, diffusive conductance had already started to decrease even before the photosynthetic rate reached the maximum early in the morning. Under sufficient light intensity with artificially reflected sunlight by the mirror when diffusive conductance reached the maximum early in the morning, the time when both photosynthetic rate and diffusive conductance reached the maximum coincided. The maximum rater of the photosynthesis was higher than that of the day without the reflected light (Fig. 5). This result suggested that leaf photosynthetic capacity was not always fully realized in diurnal courses of photosynthesis on a fine day. To examine the factor reducing the photosynthetic rate on a fine day, diurnal courses of photosynthetic rate was measured under lower vapor pressure deficit by humidifying air in the chamber. The photosynthetic rate was much higher under lower vapor pressure deficit at least up to noon compared with that under the same vapor pressure deficit as outside (Fig. 6). This fact showed that water stress related to high vapor pressure deficit was the main factor for the decrease of photosynthesis in the midday even though the effects of photosynthate accumulation in the leaf blade and photoinhibition could not be neglected. From these results it was clarified that leaf photosynthetic rate in the rice plants under submerged condition decreased to some extent in the midday on fine days and they could not utilize high solar energy fully for dry matter production. [the rest omitted]

Diurnal photosynthetic responses to light by macroalgae

Abstract: The diurnal variability of photosynthesis vs. irradiance (P-I) curves was evaluated for the six most dominant species of macroalgae that occur annually in North Inlet Estuary, South Carolina. Three existing models best simulated the observed data: (1) the Hyperbolic Tangent Model of Jassby and Platt (1976), which was applied to data that showed no photoinhibition; (2) the Exponential Inhibition Model of Parker (1974); and (3) the Photoinhibition Model of Platt et al. (1980), which were employed when photo-inhibition was measured. Photoinhibition was observed in about 75% of the experiments, and in some cases at irradiance levels as low as 500 μE.m−2. s−1. Most of the resulting P-I curves did not differ significantly when measured at various times of the day. As a consequence of these results, we question conclusions expressed by others that it is not possible to accurately determine diurnal photosynthesis of macroalgae from light saturation curves that are measured over short time periods.

Damage to Maize Photosynthesis in the Field During Periods When Chilling is Combined With High Photon Fluxes

Abstract: Laboratory studies using controlled environments have shown that during exposure to chilling temperatures, the photosynthetic apparatus of chilling sensitive crops can be damaged by high light (1). Zea mays (cv.LG11) leaves subjected to 5°C and a photon flux density of 1.5 mmol m-2S-1 for 6h show a significant reduction in their quantum yield of CO2 assimilation (o) and characteristic changes in the induction of chlorophyll fluorescence (2, 3). These effects are not observed when chilling occurs in the dark. The present study sought to assess whether such photoinhibition of photosynthesis can occur under field conditions and if so, the effect on crop growth.

Water stress in Eucalyptus pauciflora: comparison of effects on stomatal conductance with effects on the mesophyll capacity for photosynthesis, and investigation of a possible involvement of photoinhibition.

Abstract: Seedlings of Eucalyptus pauciflora Sieb. ex Spreng., grown in 4-1 pots, were stressed by withholding water while relationships between net assimilation rate (A) and intercellular partial pressure of CO2 (pi) in selected leaves were obtained repeatedly throughout the stress cycle. Water stress at first caused stomatal closure without any decline in the A(pi) relationship. As stress became more severe, the A(pi) relationship was affected as well. This always affected assimilation rate at both high and low intercellular partial pressures of CO2. It was then tested whether water-stressed leaves were more prone to photoinhibition than unstressed ones. Plants were water-stressed while at the same time subjected to strong photon flux area density (2000 μmol quanta·m(-2)·s(-1)). A possible light-induced inhibition was assessed by comparing quantum yields of photosynthesis with light directed onto one or the other surface of the leaf. A decline in quantum yield was observed, and the decline on the previously irradiated side was more pronounced than on the previously shaded side, but the effect was small and disappeared entirely within 1 d of rewatering the plants. It is concluded that photoinhibition can play a role, but not an important one, in the effect of water stress on the A(pi) relationship in leaves of E. pauciflora.

Induction kinetics of heat emission before and after photoinhibition in cotyledons of Raphanus sativus

Abstract: Heat emitted during non-radiative de-excitation was determined in vivo by the photoacoustic method. The dependence of the photoacoustic signal on the length of the pulses (modulation frequency) of the excitation light and the effect of continuous light, which saturates photosynthesis but does not directly contribute to the signal, are described. The induction kinetic of heat emission measured with intact leaves differed only slightly from the induction kinetic of fluorescence (Kautsky effect) detected in parallel. The photoacoustic signal at high modulation frequencies (279 Hz), which represents the signal of heat emission, and the photoacoustic signal at low modulation frequencies (17 Hz), interpreted as a signal of pulsed oxygen evolution superimposed on the heat emission, were measured with leaves before and after photoinhibition. It was demonstrated that after photoinhibition the decrease in fluorescence yield and in photosynthetic activity (here detected as photoacoustic signal at 17 Hz) are paralleled by an increase in the yield of non-radiative deexcitation (photoacoustic signal at 279 Hz). The increase of heat emission, which has been hypothized for photoinhibited leaves, could now be proved by measuring the induction kinetics of the photoacoustic signal.

Photoinhibition of Photosynthesis and its Recovery in the Green Alga Chlamydomonas reinhardii

Abstract: PHOTOINHIBITION OF PHOTOSYNTHESIS AND ITS RECOVERY IN THE GREEN-ALGA CHLAMYDOMONAS-REINHARDII