Showing papers on "Photosynthesis published in 1986"

Strategy by which nitrogen-fixing unicellular cyanobacteria grow photoautotrophically

Abstract: Among nitrogen-fixing microorganisms, nitrogen-fixing cyanobacteria are unique in their ability to carry out oxygen-evolving photosynthesis and oxygen-labile nitrogen fixation within the same organisms1–3. These seemingly incompatible reactions take place in heterocystous cyanobacteria by the spatial separation of the site of nitrogen fixation (heterocysts) from the site of photosynthesis (vegetative cells)4,5. Several hypotheses have been proposed to explain these mechanisms in non-heterocystous cyanobacteria3,6–11. Using batch cultures of Gloeothece (Gloeocapsa) spp., Gallon and collaborators demonstrated the mechanism of temporal separation of photosynthesis and nitrogen fixation into the light and dark periods of growth, respectively9. However, the mechanisms by which these two incompatible reactions can occur under continuous light conditions still remained ambiguous. Using novel strains of aerobic nitrogen-fixing, unicellular marine cyanobacteria, Synechococcus spp., grown under synchronized conditions, we report here that nitrogen fixation and photosynthesis occur at different phases in the cell division cycle. Our data, obtained under both diurnal light/dark cycle and continuous illumination, indicate that the temporal separation of the two phases during the cell division cycle is the mechanism by which these unicells can grow photoautotrophically under nitrogen-fixing conditions.

Water vapor uptake and photosynthesis of lichens: performance differences in species with green and blue-green algae as phycobionts.

Abstract: Dry lichen thalli were enclosed in gas exchange chambers and treated with an air stream of high relative humidity (96.5 to near 100%) until water potential equilibrium was reached with the surrounding air (i.e., no further increase of weight through water vapor uptake). They were then sprayed with liquid water. The treatment took place in the dark and was interrupted by short periods of light. CO2 exchange during light and dark respiration was monitored continuously. With no exception water uptake in all of the lichen species with green algae as phycobionts lead to reactivation of the photosynthetic metabolism. Further-more, high rates of CO2 assimilation were attained without the application of liquid water. To date 73 species with different types of Chlorophyceae phycobionts have been tested in this and other studies. In contrast, hydration through high air humidity alone failed to stimulate positive net photosynthesis in any of the lichens with blue-green algae (Cyanobacteria). These required liquid water for CO2 assimilation. So far 33 species have been investigated, and all have behaved similarly. These have included gelatinous as well as heteromerous species, most with Nostoc phycobionts but in addition some with three other Cyanophyceae phycobionts. The same phycobiont performance differences existed even within the same genus (e.g. Lobaria, Peltigera) between species pairs containing green or blue-green phycobionts respectively. Free living algae also seem to behave in a similar manner. Carbon isotope ratios of the lichen thalli suggest that a definite ecological difference exists in water status-dependent photosynthesis of species with green and blue-green phycobionts. The underlying biochemical or biophysical mechanisms are not yet understood. Apparently, a fundamental difference in the structure of the two groups of algae is involved.

Carbonic anhydrase and CO2 concentrating mechanisms in microalgae and cyanobacteria

Abstract: Among the microbial phototrophs, those belonging to the cyanobacteria utilize CO2 and HCO−3 for photosynthesis. Some Chlorophyceae mainly take up CO2 in photosynthesis, and others, which have carbonic anhydrase (CA) on their cell surface can utilize HCO−3 as well as CO2. Kinetic studies revealed that most of the HCO−3 is utilized after this ion is converted to CO2 via CA located on the cell surface. Therefore, the actual molecular species which crosses the plasmalemma is mostly free CO2. There is apparent variation in the mode of utilization of dissolved inorganic carbon (DIC) for photosynthesis in microalgae in other classes. The apparent Km(CO2) values for photosynthesis in most microalgae grown in ordinary air (low-CO2 cells) are as low as in terrestrial C4 plants, although the algal cells fix CO2 via the C3 pathway. In contrast, the apparent Km(CO2) values in cells grown on CO2-enriched air (high-CO2 cells) are as high as those in the terrestrial C3 plants. Most low-CO2 cells show low photorespiration; a low CO2 compensation point, low rates of glycolate excretion and no or low O2 inhibition of photosynthesis. These results indicate that the efficiency of DIC utilization for photosynthesis in low-CO2 cells is very high. The activity of CA in low-CO2 cells is higher than that in high-CO2 cells, while no difference has been confirmed in the activities of other photosynthetic enzymes between low- and high-CO2 cells. In addition, low-CO2 cells can accumulate large amounts of DIC internally, indicating the existence of CO2-concentrating mechanisms in these cells. When CA activity or CO2 concentrating ability is reduced by inhibitors or by mutation, the apparent Km(CO2) values for photosynthesis and the rate of photorespiration increased notably. These results indicate that the high efficiency of DIC utilization in low-CO2 cells depends on both CA and a CO2-concentrating mechanism. It is concluded that CA facilitates the diffusion of DIC from outside the cells to the site(s) of the carboxylation reaction and the concentration of DIC is achieved via an active transporter.

Limitation of Photosynthesis by Carbon Metabolism: I. Evidence for Excess Electron Transport Capacity in Leaves Carrying Out Photosynthesis in Saturating Light and CO2

Abstract: It has been investigated how far electron transport or carbon metabolism limit the maximal rates of photosynthesis achieved by spinach leaves in saturating light and CO2. Leaf discs were illuminated with high light until a steady state rate of O2 evolution was attained, and then subjected to a 30 second interruption in low light, to generate an increased demand for the products of electron transport. Upon returning to high light there is a temporary enhancement of photosynthesis which lasts 15 to 30 seconds, and can be up to 50% above the steady state rate of O2 evolution. This temporary enhancement is only found when saturating light intensities are used for the steady state illumination, is increased when low light rather than darkness is used during the interruption, and is maximal following a 30 to 60 seconds interruption in low light. Decreasing the temperature over the 10 to 30°C range led to the transient enhancement becoming larger. The temporary enhancement is associated with an increased ATP/ADP ratio, a decreased level of 3-phosphoglycerate, and increased levels of triose phosphate and ribulose 1,5-bisphosphate. Since electron transport can occur at higher rates than in steady state conditions, and generate a higher energy status, it is concluded that leaves have a surplus electron transport capacity in saturating light and CO2. From the alterations of metabolites, it can be calculated that the enhanced O2 evolution must be accompanied by an increased rate of ribulose 1,5-bisphosphate regeneration and carboxylation. It is suggested that the capacity for sucrose synthesis ultimately limits the maximal rates of photosynthesis, by restricting the rate at which inorganic phosphate can be recycled to support electron transport and carbon fixation in the chloroplast.

Cadmium transport, resistance, and toxicity in bacteria, algae, and fungi

Abstract: Cadmium is an important environmental pollutant and a potent toxicant to bacteria, algae, and fungi. Mechanisms of Cd toxicity and resistance are variable, depending on the organism. It is very clear that the form of the metal and the environment it is studied in, play an important role in how Cd exerts its effect and how the organism(s) responds. A wide range of Cd concentrations have been used to designate resistance in organisms. To date, no concentration has been specified that is applicable to all species studied under standardized conditions. Cadmium exerts its toxic effect(s) over a wide range of concentrations. In most cases, algae and cyanobacteria are the most sensitive organisms, whereas bacteria and fungi appear to be more resistant. In some bacteria, plasmid-encoded resistance can lead to reduced Cd2+ uptake. However, some Gram-negative bacteria without plasmids are just as resistant to Cd as are bacteria containing plasmids encoding for Cd resistance. According to Silver and Misra (1984), there is no evidence for enzymatic or chemical transformations associated with Cd resistance. Insufficient information is available on the genetics of Cd uptake and resistance in cyanobacteria and algae. Mechanisms remain largely unknown at this point in time. Cadmium is toxic to these organisms, causing severe inhibition of such physiological processes as growth, photosynthesis, and nitrogen fixation at concentrations less than 2 ppm, and often in the ppb range (Tables 2 and 3). Cadmium also causes pronounced morphological aberrations in these organisms, which are probably related to deleterious effects on cell division. This may be direct or indirect, as a result of Cd effects on protein synthesis and cellular organelles such as mitochondria and chloroplasts. Cadmium is accumulated internally in algae (Table 4) as a result of a two-phase uptake process. The first phase involves a rapid physicochemical adsorption of Cd onto cell wall binding sites, which are probably proteins and (or) polysaccharides. This is followed by a lag period and then a slow, steady intracellular uptake. This latter phase is energy dependent and may involve transport systems used to accumulate other divalent cations, such as Mn2+ and Ca2+. Some data indicate that Cd resistance, and possibly uptake, in algae and cyanobacteria is controlled by a plasmid-encoded gene(s). Although considerable information is available on Cd toxicity to, and uptake in fungi, further work is clearly needed in several areas. There is little information about Cd uptake by filamentous fungi, and even in yeasts, information on the specificity, kinetics, and mechanisms of Cd uptake is limited.(ABSTRACT TRUNCATED AT 400 WORDS)

The relationship between phosphate status and photosynthesis in leaves : Effects on intracellular orthophosphate distribution, photosynthesis and assimilate partitioning.

Abstract: Photosynthesis, assimilate partitioning and intracellular distribution of orthophosphate (Pi) in barly (Hordeum vulgare L.) leaves were measured in plants grown with either 25, 1 or 0 mmol· 1−1 nutrient phosphate supply. Phosphate deficiency resulted in a significant decrease in the leaf Pi, diminished rates of photosynthesis and a decrease in the sucrose/starch ratio in the leaves. Changes in the cytoplasmic Pi content were relatively small in comparison with the large variations in vacuolar Pi. The cytoplasmic Pi concentration was slightly higher in the leaves of plants grown at 25 mmol·l−1 Pi than in those grown at 1 mmol·l−1 Pi and was decreased in the phosphate-deficient plants in which photosynthesis was inhibited. With barley plants grown in phosphate-deficient media, very little, if any, Pi was present in the vacuole. All of the cellular Pi was in the cytoplasm. Barley, spinach (Spinacia oleracea L.) and soya (Glycine max L.) plants were grown to a comparative stage of phosphate deficiency as measured by leaf Pi levels. These species showed a uniform response to phosphate deficiency by increasing starch synthesis relative to sucrose but the accompanying limitation on photosynthetic capacity varied considerably among the species. Interspecific differences in assimilate partitioning between starch and sucrose were maintained over a wide range of Pi supply.

Adaptation toHydrogen Sulfide ofOxygenic andAnoxygenic Photosynthesis among Cyanobacteria

Abstract: Four different types of adaptation to sulfide among cyanobacteria are described based on the differential toxicity to sulfide of photosystems I and II and the capacity for the induction of anoxygenic photosynthesis. Most cyanobacteria are highly sensitive to sulfide toxicity, and brief exposures to low concentrations cause complete and irreversible cessation of CO2 photoassimilation. Resistance of photosystem II to sulfide toxicity, allowing for oxygenic photosynthesis under sulfide, is found in cyanobacteria exposed to low H2S concentrations in various hot springs. When H2S levels exceed 200 μM another type of adaptation involving partial induction of anoxygenic photosynthesis, operating in concert with partially inhibited oxygenic photosynthesis, is found in cyanobacterial strains isolated from both hot springs and hypersaline cyanobacterial mats. The fourth type of adaptation to sulfide is found at H2S concentrations higher than 1 mM and involves a complete replacement of oxygenic photosynthesis by an effective sulfide-dependent, photosystem II-independent anoxygenic photosynthesis. The ecophysiology of the various sulfide-adapted cyanobacteria may point to their uniqueness within the division of cyanobacteria.

Adaptation to Hydrogen Sulfide of Oxygenic and Anoxygenic Photosynthesis among Cyanobacteria.

Abstract: Four different types of adaptation to sulfide among cyanobacteria are described based on the differential toxicity to sulfide of photosystems I and II and the capacity for the induction of anoxygenic photosynthesis. Most cyanobacteria are highly sensitive to sulfide toxicity, and brief exposures to low concentrations cause complete and irreversible cessation of CO2 photoassimilation. Resistance of photosystem II to sulfide toxicity, allowing for oxygenic photosynthesis under sulfide, is found in cyanobacteria exposed to low H2S concentrations in various hot springs. When H2S levels exceed 200 μM another type of adaptation involving partial induction of anoxygenic photosynthesis, operating in concert with partially inhibited oxygenic photosynthesis, is found in cyanobacterial strains isolated from both hot springs and hypersaline cyanobacterial mats. The fourth type of adaptation to sulfide is found at H2S concentrations higher than 1 mM and involves a complete replacement of oxygenic photosynthesis by an effective sulfide-dependent, photosystem II-independent anoxygenic photosynthesis. The ecophysiology of the various sulfide-adapted cyanobacteria may point to their uniqueness within the division of cyanobacteria.

Limitation of Photosynthesis by Carbon Metabolism: II. O2-Insensitive CO2 Uptake Results from Limitation Of Triose Phosphate Utilization

Abstract: The occurrence of O2-insensitive photosynthesis at high quantum flux and moderate temperature in Spinacia oleracea was characterized by analytical gas exchange measurements on intact leaves In addition photosynthetic metabolite pools were measured in leaves which had been rapidly frozen under defined gas conditions Upon switching to low O2 in O2-insensitive conditions the ATP/ADP ratio fell dramatically within one minute The P-glycerate pool increased over the same time Ribulose bisphosphate initially declined, then increased and exceeded the pool size measured in air The pools of hexose monophosphates and UDPglucose were higher at a partial pressure of O2 of 21 millibars than at 210 millibars These results are consistent with the hypothesis that the rate of sucrose synthesis limited the overall rate of assimilation under O2-insensitive conditions

Effects of Phosphorus Nutrition on Ribulose-1,5-Bisphosphate Carboxylase Activation, Photosynthetic Quantum Yield and Amounts of Some Calvin-Cycle Metabolites in Spinach Leaves

Abstract: When spinach plants were transferred to nutrient solutions without phosphorus, the photosynthetic rate per unit leaf area gradually declined. Stomatal conductance also decreased but was not the sole cause of the decreased photosynthetic rate because the partial pressure of CO2 in the intercellular spaces (CI) was unaltered. Measurements of the photosynthetic rate as a function of CI indicated reductions in both ribulosebisphosphate (RuP2) carboxylase activity and RuP2 regeneration capacity. From assays of RuP2 carboxylase activity in vitro and 'percentage activation', it was concluded that low-P leaves had less enzyme per unit area than controls and that the enzyme was also less activated. The photosynthetic quantum yield was reduced by phosphorus deficiency with no effect on leaf absorptance or chlorophyll content. The reduced quantum yield was accompanied by changes in chlorophyll fluorescence of photosystems I and II measured at 77K. However, since phosphorus deficiency did not affect the uncoupled rate of whole-chain electron transport in vitro, some factor(s) other than photoinhibition probably contributed to the reduced quantum yield. The lack of effect on this electron-transport rate also indicates that the maximal RuP2 regeneration rate in low-P leaves was not limited by the amount of electon-transport components. At ambient [CO2], low-P leaves had significantly less RuP2 and 3-phosphoglycerate (PGA) than controls and the response of photosynthesis to low [O2] was similar to control leaves. Therefore photosynthesis did not appear to be limited by triose-phosphate utilization. The low concentrations of RuP2 and PGA (and presumably other Calvin-cycle intermediates) might have reduced the rate of the Calvin cycle. After returning low-P plants to nutrient solutions with PO4, the percentage activation of RuP2 carboxylase, amounts of RuP2 and PGA, quantum yield and maximal RuP2 regeneration rate increased within 24 h. The quantum yield and photosynthetic rate at higher irradiance also increased when leaf discs from low-P plants were floated on 10 mM PO4 solutions for 2 h.

Salinity and Nitrogen Effects on Photosynthesis, Ribulose-1,5-Bisphosphate Carboxylase and Metabolite Pool Sizes in Phaseolus vulgaris L.

Abstract: Salinity (100 millimolar NaCl) was found to reduce photosynthetic capacity independent of stomatal closure in Phaseolus vulgaris. This reduction was shown to be a consequence of a reduction in the efficiency of ribulose-1,5-bisphosphate (RuBP) carboxylase (RuBPCase) rather than a reduction in the leaf content of photosynthetic machinery. In control plants, photosynthesis became RuBP-limited at approximately 1.75 moles RuBP per mole 2-carboxyarabinitol bisphosphate binding sites. Salinization caused the RuBP pool size to reach this limiting value for CO(2) fixation at much lower values of intercellular CO(2). Plants grown at low nitrogen and +/- NaCl became RuBP limited at similar RuBP pool sizes as the high nitrogen-grown plants. At limiting RuBP pool sizes and equal values of intercellular CO(2) photosynthetic capacity of salt-stressed plants was less than control plants. This effect of salinity on RuBPCase activity could not be explained by deactivation of the enzyme or inhibitor synthesis. Thus, salinity reduced photosynthetic capacity by reducing both the RuBP pool size by an effect on RuBP regeneration capacity and RuBPCase activity by an unknown mechanism when RuBP was limiting.

Mechanism of photoinhibition: photochemical reaction center inactivation in system II of chloroplasts

Abstract: Photoinhibition of photosynthesis is manifested at the level of the leaf as a loss of CO2 fixation and at the level of the chloroplast thylakoid membrane as a loss of photosystem II electron-transport capacity. At the photosystem II level, photoinhibition is manifested by a lowered chlorophyll a variable fluorescence yield, by a lowered amplitude of the light-induced absorbance change at 320 nm (ΔA320) and 540-minus-550 nm (ΔA540–550), attributed to inhibition of the photoreduction of the primary plastoquinone QA molecule. A correlation of the kinetics of variable fluorescence yield loss with the inhibition of QA photoreduction suggested that photoinhibited reaction centers are incapable of generating a stable charge separation but are highly efficient in the trapping and non-photochemical dissipation of absorbed light. The direct effect of photoinhibition on primary photochemical parameters of photosystem II suggested a permanent reaction center modification the nature of which remains to be determined.

Photosynthesis in vivo can be limited by phosphate supply

Abstract: SUMMARY Measurements of photosynthetic CO2 and 02 exchange, and associated chlorophyll fluorescence were made on leaves (from plants grown in complete nutrient medium) while the internal inorganic phosphate concentration was increased or decreased by feeding solutions through the vascular tissue. The data indicate that orthophosphate supply to the chloroplast can, in some circumstances, become the process that limits photosynthesis in vivo.

Chlorophyll a Fluorescence and Photosynthetic and Growth Responses of Pinus radiata to Phosphorus Deficiency, Drought Stress, and High CO2

Abstract: Needles from phosphorus deficient seedlings of Pinus radiata D. Don grown for 8 weeks at either 330 or 660 microliters CO(2) per liter displayed chlorophyll a fluorescence induction kinetics characteristic of structural changes within the thylakoid chloroplast membrane, i.e. constant yield fluorescence (F(O)) was increased and induced fluorescence ([F(P)-F(I)]/F(O)) was reduced. The effect was greatest in the undroughted plants grown at 660 mul CO(2) L(-1). By week 22 at 330 mul CO(2) L(-1) acclimation to P deficiency had occurred as shown by the similarity in the fluorescence characteristics and maximum rates of photosynthesis of the needles from the two P treatments. However, acclimation did not occur in the plants grown at 660 mul CO(2) L(-1). The light saturated rate of photosynthesis of needles with adequate P was higher at 660 mul CO(2) L(-1) than at 330 mul CO(2) L(-1), whereas photosynthesis of P deficient plants showed no increase when grown at the higher CO(2) concentration. The average growth increase due to CO(2) enrichment was 14% in P deficient plants and 32% when P was adequate. In drought stressed plants grown at 330 mul CO(2) L(-1), there was a reduction in the maximal rate of quenching of fluorescence (R(Q)) after the major peak. Constant yield fluorescence was unaffected but induced fluorescence was lower. These results indicate that electron flow subsequent to photosystem II was affected by drought stress. At 660 mul CO(2) L(-1) this response was eliminated showing that CO(2) enrichment improved the ability of the seedlings to acclimate to drought stress. The average growth increase with CO(2) enrichment was 37% in drought stressed plants and 19% in unstressed plants.

Light-temperature interactions in the control of photosynthesis in Antarctic phytoplankton

Abstract: During October/November 1983 photosynthetic responses of natural phytoplankton from the Scotia Sea and Bransfield strait to light and temperature were examined in incubators. Both assimilation numbers at saturating light levels and the slopes of the light-limited portions of the photosynthesis versus irradiance curves were smaller than in algae from lower latitudes. However, both parameters increased significantly with rising temperatures. Light-saturated photosynthesis on the average exhibited a Q10-value of ca. 4.2 between-1.5°C and +2°C. Light-limited photosynthesis between-1.5°C and +5°C rose at a rate corresponding to a Q10-value of roughly 2.6. Above +5°C, temperature enhancement of both light-saturated and light-limited photosynthetic rates was minimal or absent. Our results suggest that under extremely low temperatures light-limited photosynthetic rates become temperature-dependent due to changes in maximum quantum yields.

Light absorption by plants and its implications for photosynthesis

Abstract: Summary The preceding account has attempted to examine the interactions between light absorption and photosynthesis, with reference to both unicellular and multicellular terrestrial and aquatic plants. There are, however, some notable plant groups to which no direct reference has been made, e.g. mosses, liverworts and lichens. Although many have similar optical properties to terrestrial vascular plants (Gates, 1980) and apparently similar photosynthetic responses (see Green & Snelgar, 1982; Kershaw, 1984) they may possess subtle, as yet unknown differences. For instance, the lichen thallus has a high surface reflectance although the transmittance is virtually zero (Gates, 1980; Osborne, unpublished results). It is envisaged, however, that differences in optical properties between species will reflect differences in degree not kind. Although not all variation in photosynthesis is due to differences in light absorption a number of accounts suggest that this is a contributing factor. Variations in leaf absorptance have been found to account for most of the variation in leaf photosynthesis at low Jis (see Ehleringer & Bjorkman, 1978a; Osborne & Garrett, 1983). There is, however, little direct experimental evidence on light absorption and photosynthesis in either microalgal species or aquatic macrophytes. We also do not know over what range of incident photon flux densities photosynthesis is determined largely by changes in light absorption. Plants growing under natural conditions also experience large diurnal and seasonal fluctuations in Ji, unlike species grown under laboratory conditions. The occurrence of transitory peaks in Ji tends to overshadow the fact that the average Ji is often lower than the J1 required to saturate photosynthesis, i.e. 1500–2000 μmol m-2 s-1, depending on the growth treatment. Using the data of Monteith (1977) and I W m2= 5 μmol m-2 s-1, and with photosynthetically active radiation 50% of total solar radiation, the daily mean value for Britain is approximately 450 μmol m-2 s-1, with a maximum in June of 1000μmol m-2 s-1 and a minimum during the winter of 75 μmol m-2 s-1. Such values could be even lower on shaded understory leaves and considerably lower for aquatic species. Based on average values of net photosynthesis for a terrestrial plant leaf, light saturation would only be expected in June while for most of the year the average values would lie largely on the light-limited portion of the photosynthesis light response curve. Although the daily average values in tropical climates may be higher during the winter months, they are remarkably similar throughout the world for the respective summers in the northern and southern hemispheres, because the increased daylength at high latitudes compensates for the lower Jis. The expected lower dark respiration rates during the winter may also partially offset the effects of a lower light level. There is therefore a trade-off between high Jis for a short period of time against a lower Ji for a longer period of time. We might expect different photosynthetic responses to these two very different conditions. Importantly, a low Ji with a long daylength may enable a plant to photosynthesize at or near its maximum photon efficiency for most of the day. Although the response of the plant to fluctuations in Ji is complicated because it is affected by the previous environmental conditions, this may indicate that light absorption has a much greater significance under natural conditions, particularly for perennial species. The bias in many laboratories towards research on terrestrial vascular plants also tends to ignore the fact that a number of multicellular and unicellular aquatic species survive in very low light environments. Furthermore, the direct extrapolation of photosynthetic responses from measurements on single leaves to those of whole plants is clearly erroneous. Although this is obvious, many physiological ecologists have attributed all manner of things to the photosynthetic responses of ‘primary’ leaves. Most researchers have ignored problems associated with composite plant tissues and internal light gradients. Clearly caution is required in interpreting the photosynthesis light-response curve of multicellular tissues based on biochemical features alone. Also, the importance of cell structure on light absorption and photosynthesis has generally been ignored and attributed solely to the effects of structural features on CO2 diffusion. In doing so the work of two or three generations of plant physiologists has been ignored. Haberlandt (1914) at the turn of the century probably first implicated the role of cell structure in leaf optics, and Heath (1970) stressed that in order to completely understand the role of light in photosynthesis we need to know the flux incident on the chloroplast itself. Even this suggestion may need modification because of the capacity of the internal chloroplast membranes for scattering light. It is worth emphasizing the importance of light gradients within tissues and their role in regulating photosynthesis, particularly at light saturation. Measurements of light gradients are fraught with problems because of experimental difficulties and the majority (few) are based on reflectance and transmittance measurements. Seyfried & Fukshansky (1983) have shown that light incident on the lower surface of a Cucurbita cotyledon produced a larger light gradient than light incident from above, indicating the importance of the spatial arrangement of the tissues with respect to the light source. Also, light incident on the lower surface of leaves of Picea sitchensis was less ‘effective’ in photosynthesis than light from above (Leverenz & Jarvis, 1979). Clearly, two tissues could have the same gross absorptance but different photosynthetic rates because of differences in the internal light environment. Fisher & Fisher (1983) have recently found asymmetries in the light distribution within leaves, which they related to asymmetries in photosynthetic products due to differences in solar elevation. Such modifications in light distribution could be important for a number of solar-tracking species. Changes in light absorption are brought about by a whole gamut of physiological, morphological and behavioural responses which serve to optimize the amount of light absorbed. Perhaps the simplest way of regulating the amount of light absorbed is by restricting growth either to particular times of the year or to conditions when the light climate is favourable. We are still largely ignorant of many details of these modifications. In particular, differences in tissue structure such as the size and number of vacuoles or the effects of organelles on the scattering component of the internal light environment of photosynthetic tissues are not understood. A better understanding of the interaction of light with plants in aquatic systems is also required. It is unfortunate that light-absorptance measurements are not routinely made in photosynthetic studies, and this is quite clearly a neglected area of study. That these measurements are not made is even more surprising, since techniques have been available for over sixty years (Ulbricht, 1920). Absorptance measurements are of particular importance in the photosynthetic adaptation of microalgae, where only a small proportion of the incident photon flux density is absorbed. For multicellular species more detailed information is required on internal light gradients and their variability. Light-absorptance measurements are also important in any study relating kinetic data on CO2 fixation to in vivo photosynthesis, especially when there are large variations in the morphology and structure of the photosynthetic organ.

Effect of low root temperature on net photosynthesis, stomatal conductance and carbohydrate concentration in Engelmann spruce (Picea engelmannii Parry ex Engelm.) seedlings.

Abstract: Summary The effect of low root temperature on net photosynthesis, stomata1 conductance and carbohydrate concentration in potted Engelmann spruce (Picea engelmanii Parry ex Engelm.) seedlings was examined under controlled growth-chamber conditions. Root temperature had no effect on net photosynthesis between 10 and 2o”C, however, conductance and photosynthesis declined sharply below 8°C. Net photosynthesis and stomata1 conductance decreased to 50 and 34% of the initial values after 7 days at a root temperature of 0.7”C. Low root temperature also caused a decrease in photosynthetic utilization of internal CO,, carboxylation efficiency and apparent quantum yield, and it was concluded that the decrease in photosynthetic rate was caused primarily by nonstomatal limitations. Root chilling caused a small increase in starch content in needles and stems and induced the hydrolysis of starch to glucose in roots. The increase in glucose concentration may enable continued root growth at low temperatures.

Inhibition of photosynthesis in Phaseolus vulgaris by treatment with toxic concentrations of zinc: effects on electron transport and photophosphorylation

Abstract: Dwarf beans (Phaseolus vulgaris L. cv. Limburgse Vroege) were grown on a nutrient medium containing a toxic non-lethal ZnSO4 concentration. The electron transport and photophosphorylation activities of chloroplasts, isolated from these beans, and from control plants, grown under standard nutrient conditions, were compared. Electron transport was significantly inhibited by Zn2+ treatment. Photosystem 2 activity proved to be more sensitive than photosystem 1 activity. Inhibition was dependent on electron flow rate. Activity was fully restored with semicarbazide. EDTA-washed thylakoid membranes were strongly manganese-deficient. The results suggest that photolysis of water was primarily inhibited, due to a zinc-induced deficiency in loosely bound manganese at the water-splitting site. Manganese is probably substituted by zinc, since the zinc content of thylakoids increased five-fold. Non-cyclic photophosphorylation capacity was also limited as a result of inhibition of electron transport. Phosphorylation efficiency (ATP/2e ratio) involving both energy conserving sites was hardly affected.

Biosynthetic Cause of in Vivo Acquired Thermotolerance of Photosynthetic Light Reactions and Metabolic Responses of Chloroplasts to Heat Stress

Abstract: Thermotolerance of photosynthetic light reactions in vivo is correlated with a decrease in the ratio of monogalactosyl diacylglycerol to digalactosyl diacylglycerol and an increased incorporation into thylakoid membranes of saturated digalactosyl diacylglycerol species. Although electron transport remains virtually intact in thermotolerant chloroplasts, thylakoid protein phosphorylation is strongly inhibited. The opposite is shown for thermosensitive chloroplasts in vivo. Heat stress causes reversible and irreversible inactivation of chloroplast protein synthesis in heat-adapted and nonadapted plants, respectively, but doe not greatly affect formation of rapidly turned-over 32 kilodalton proteins of photosystem II. The formation on cytoplasmic ribosomes and import by chloroplasts of thylakoid and stroma proteins remain preserved, although decreased in rate, at supraoptimal temperatures. Thermotolerant chloroplasts accumulate heat shock proteins in the stroma among which 22 kilodalton polypeptides predominate. We suggest that interactions of heat shock proteins with the outer chloroplast envelope membrane might enhance formation of digalactosyl diacylglycerol species. Furthermore, a heat-induced recompartmentalization of the chloroplast matrix that ensures effective transport of ATP from thylakoid membranes towards those sites inside the chloroplast and the cytoplasm where photosynthetically indispensable components and heat shock proteins are being formed is proposed as a metabolic strategy of plant cells to survive and recover from heat stress.