On the enrichment of H2 18O in the leaves of transpiring plants

Summary The vapor pressure difference between H₂ ¹⁸O and H₂ ¹⁶O is the reason for the accumulation of the heavy molecule in transpiring leaves. Since photosynthesis on land is the main source of atmospheric oxygen, this mechanism is important for the remarkable enrichment of¹⁸O in atmospheric O₂ (Dole effect). Using a simple box model for transpiring leaves a quantitative understanding of the isotope fractionation is possible which is well confirmed by the results of model experiments as well as by measurements on trees. Maximum enrichment of H₂ ¹⁸O in the water of leaves (relative to soil water) is 25 (theoretically, for dry air) and was found under natural conditions to be 21 (for 28 % relative humidity); minimum theoretical enrichment is zero (observed 2.5 ).     

Influence of vegetation and soil CO2 exchange on the concentration and stable oxygen isotope ratio of atmospheric CO2 within a Pinus resinosa canopy

Measurements were made of the concentration and stable oxygen isotopic ratio of carbon dioxide in air samples collected on a diurnal basis at two heights within a Pinus resinosa canopy. Large changes in CO₂ concentration and isotopic composition were observed during diurnal time courses on all three symple dates. In addition, there was strong vertical stratification in the forest canopy, with higher CO₂ concentrations and more negative ¹⁸O values observed closer to the soil surface. The observed daily increases in ¹⁸O values of forest CO₂ were dependent on relative humidity consistent with the modelled predictions of isotopic fractionation during photosynthetic gas exchange. During photosynthetic gas exchange, a portion of the CO₂ that enters the leaf and equilibrates with leaf water is not fixed and diffuses back out of the leaf with an altered oxygen isotopic ratio. The oxygen isotope ratio of CO₂ diffusing out of a leaf depends primarily on the ¹⁸O content of leaf water which changes in response to relative humidity. In contrast, soil respiration caused a decline in the ¹⁸O values of forest CO₂ at night, because CO₂ released from the soil has equilibrated with soil water which has a lower ¹⁸O content than leaf water. The observed relationship between diurnal changes in CO₂ concentration and oxygen isotopic composition in the forest environment were consistent with a gas mixing model that considered the relative magnitudes of CO₂ fluxes associated with photosynthesis, respiration and turbulent exchange between the forest and the bulk atmosphere.     

<Superscript>18</Superscript> O-rich Oxygen from Land Photosynthesis

ATMOSPHERIC oxygen is not in equilibrium with sea water with respect to the isotope exchange illustration but has an ¹⁸O excess of about 22‰ compared to sea water¹. This could be due to isotope fractionation during respiration². Another large contribution to the effect has been overlooked up to now. Photosynthesis on land takes place in transpiring leaves, where the difference in the vapour pressure of ¹⁶OH₂ and ¹⁸OH₂ concentrates the heavy molecules in their stationary water content. Since the free oxygen stems from the water in which photosynthesis takes place^3–8 (with only a very small shift in isotopic composition⁹), photosynthesis on land is an ¹⁸O source for atmospheric O₂. We have begun to study this effect quantitatively.

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Hydrogen-isotope composition of leaf water in C3 and C4 plants: its relationship to the hydrogen-isotope composition of dry matter

The natural abundance hydrogen-isotope composition of leaf water ( ) and leaf organic matter ( _D ^org ) was measured in leaves of C₃ and C₄ dicotyledons and monocotyledons. The value of leaf water showed a marked diurnal variation, greatest enrichment being observed about midday. However, this variation was greater in the more slowly transpiring C₄ plants than in C₃ plants under comparable environmental conditions. A model based on analogies with a constant feed pan of evaporating water was developed and the difference between C₃ and C₄ plants expressed in terms of either differences in kinetic enrichment or different leaf morphology. Microclimatic and morphological features of the leaves which may be associated with this factor are discussed. There was no daily excursion in the _D ^org value in leaves of either C₃ or C₄ plants. When _D ^org values were referenced to the mean values during the period of active photosynthesis, the discrimination against deuterium during photosynthetic metabolism (D) was greater in C₃ plants (-117 to -121) than in C₄ plants (-86 to -109).These results show that the different water use strategies of C₃ and C₄ plants are responsible for the measured difference in deuterium-isotope composition of leaf water. However, it is unlikely that these physical processes account fully for the differences in hydrogen-isotope composition of the products of C₃ and C₄ photosynthetic metabolism.Symbols Hydrogen-isotope composition of leaf water - _D ^org hydrogen-isotope composition of leaf organic matter     

Stable oxygen isotopes in<Emphasis Type="Italic"> Porites</Emphasis> corals monitor weekly temperature variations in the northern Gulf of Aqaba,Red Sea

In order to assess the ability of Porites corals to accurately record environmental variations, high-resolution (weekly/biweekly) coral ¹⁸O records were obtained from four coral colonies from the northern Gulf of Aqaba, which grew at depths of 7, 19, 29, and 42 m along one transect. Adjacent to each colony, hourly temperatures, biweekly salinities, and monthly ¹⁸O of seawater were continuously recorded over a period of 14 months (April 1999 to June 2000). Contrary to water temperature, which shows a regular and strong seasonal variation and change with depth, seawater ¹⁸O exhibits a weak seasonality and little change with depth. Positive correlations between seawater ¹⁸O and salinity were observed. The two parameters were related to each other by the equation ¹⁸O _Seawater (, VSMOW) = 0.281 × Salinity – 9.14. The high-resolution coral ¹⁸O records from this study show a regular pattern of seasonality and are able to capture fine details of the weekly average temperature records. They resolve more than 95% of the weekly average temperature range. On the other hand, attenuation and amplification of coral seasonal amplitudes were recorded in deep, slow-growing corals, which were not related to environmental effects (temperature and/or seawater ¹⁸O) or sampling resolution. We propose that these result from a combined effect of subannual variations in extension rate and variable rates of spine thickening of skeletal structures within the tissue layer. However, no smoothing or distortion of the isotopic signals was observed due to calcification within the tissue layer in shallow-water, fast-growing corals. The calculations from coral ¹⁸O calibrations against the in situ measurements show that temperature (T) is related to coral ¹⁸O ( _c ) and seawater ¹⁸O ( _w ) by the equation T (°C) = –5.38 ( _c – _w ) –1.08. Our results demonstrate that coral ¹⁸O from the northern Gulf of Aqaba is a reliable recorder of temperature variations, and that there is a minor contribution of seawater ¹⁸O to this proxy, which could be ignored.     

Emission of gaseous nitrogen oxides from an extensively managed grassland in NE Bavaria, Germany.

We analysed the stable isotope composition of emitted N₂O in a one-year field experiment (June 1998 to April 1999) in unfertilized controls, and after adding nitrogen by applying slurry or mineral N (calcium ammonium nitrate). Emitted N₂O was analysed every 2–4 weeks, with additional daily sampling for 10 days after each fertilizer application. In supplementary soil incubations, the isotopic composition of N₂O was measured under defined conditions, favouring either denitrification or nitrification. Soil incubated for 48 h under conditions favouring nitrification emitted very little N₂O (0.024 mol g_dw ^–1) and still produced N₂O from denitrification. Under denitrifying incubation conditions, much more N₂O was formed (0.91 mol g_dw ^–1 after 48 h). The isotope ratios of N₂O emitted from denitrification stabilized at ¹⁵N = –40.8 ± 5.7 and ¹⁸O = 2.7 ± 6.3. In the field experiment, the N₂O isotope data showed no clear seasonal trends or treatment effects. Annual means weighted by time and emission rate were ¹⁵N = –8.6 and ¹⁸O = 34.7 after slurry application, ¹⁵N = –4.6 and ¹⁸O = 24.0 after mineral fertilizer application and ¹⁵N = –6.4 and ¹⁸O = 35.6 in the control plots, respectively. So, in all treatments the emitted N₂O was ¹⁵N-depleted compared to ambient air N₂O (¹⁵N = 11.4 ± 11.6, ¹⁸O = 36.9 ± 10.7). Isotope analyses of the emitted N₂O under field conditions per se allowed no unequivocal identification of the main N₂O producing process. However, additional data on soil conditions and from laboratory experiments point to denitrification as the predominant N₂O source. We concluded (1) that the isotope ratios of N₂O emitted from the field soil were not only influenced by the source processes, but also by microbial reduction of N₂O to N₂ and (2) that N₂O emission rates had to exceed 3.4 mol N₂O m^–2 h^–1 to obtain reliable N₂O isotope data.     

Factors influencing the stable carbon and oxygen isotopic composition ofPorites lutea coral skeletons from Phuket,South Thailand

We determined the ¹⁸O and ¹³C composition of the same fixed growth increment in severalPorites lutea coral skeletons from Phuket, South Thailand. Skeletal growth rate and ¹⁸O are inversely related. We explain this in terms of McConnaughey's kinetic isotopic disequilibria model. Annual trends in ¹⁸O cannot be solely explained by observed variations in seawater temperature or salinity and may also reflect seasonal variations in calcification rate. Coral tissue chlorophylla content and ¹³C of the underlying 1 mm of skeleton are positively related, suggesting that algal modification of the dissolved inorganic carbonate pool is the main control on skeletal ¹³C. However, in corals that bleached during a period of exceptionally high seawater temperatures in the summer of 1991, ¹³C of the outer 1 mm of skeleton and skeletal growth rate (over 9 months up to and including the bleaching event) are inversely related. Seasonal variations in °¹³C may reflect variations in calcification rate, zooxanthellae photosynthesis or in seawater ¹³C composition. Bleached corals had reduced calcification over the 9-month period up to and including the bleaching event and over the event they deposited carbonate enriched in¹³C and¹⁸O compared with unaffected corals. However, calcification during the event was limited and insufficient material was deposited to influence significantly the isotopic signature of the larger seasonal profile samples. In profile, overall decreases in ¹⁸O and ¹³C were observed, supporting evidence that positive temperature anomalies caused the bleaching event and reflecting the loss of zooxanthellae photosynthesis.     

Partitioning evapotranspiration fluxes from a Colorado grassland using stable isotopes: Seasonal variations and ecosystem implications of elevated atmospheric CO2

The stable isotopic composition of soil water is controlled by precipitation inputs, antecedent conditions, and evaporative losses. Because transpiration does not fractionate soil water isotopes, the relative proportions of evaporation and transpiration can be estimated using a simple isotopic mass balance approach. At our site in the shortgrass steppe in semi-arid northeastern Colorado, ¹⁸O values of soil water were almost always more enriched than those of precipitation inputs, owing to evaporative losses. The proportion of water lost by evaporation (E/ET) during the growing season ranged from nil to about 40% (to >90% in the dormant season), and was related to the timing of precipitation inputs. The sum of transpiration plus evaporation losses estimated by isotopic mass balance were similar to actual evapotranspiration measured from a nearby Bowen ratio system. We also investigated the evapotranspiration response of this mixed C₃/C₄ grassland to doubled atmospheric [CO₂] using Open-Top Chambers (OTC). Elevated atmospheric [CO₂] led to increased soil-water conservation via reduced stomatal conductance, despite greater biomass growth. We used a non-invasive method to measure the ¹⁸O of soil CO₂ as a proxy for soil water, after establishing a strong relationship between ¹⁸O of soil CO₂ from non-chambered control (NC) plots and ¹⁸O of soil–water from an adjacent area of native grassland. Soil–CO₂ ¹⁸O values showed significant treatment effects, particularly during a dry summer: values in ambient chambers (AC) were more enriched than in NC and elevated chamber (EC) plots. During the dry growing season of 2000, transpiration from the EC treatment was higher than from AC and lower than from NC treatments, but during 2001, transpiration was similar on all three treatments. Slightly higher evaporation rates from AC than either EC or NC treatments in 2000 may have resulted from increased convection across the soil surface from the OTC blowers, combined with lower biomass and litter cover on the AC treatment. Transpiration-use efficiency, or the amount of above-ground biomass produced per mm water transpired, was always greatest on EC and lowest on NC treatments.     

Enrichment of oxygen heavy isotopes during photosynthesis in phytoplankton

Some of the oxygen produced during oxygenic photosynthesis is consumed but little is known about the extent of the processes involved. We measured the ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios in O₂ produced by certain marine and freshwater phytoplankton representing important groups of primary producers. When the cells were performing photosynthesis under very low dissolved oxygen concentrations (<3 μM), we observed significant enrichment in both ¹⁸O and ¹⁷O with respect to the substrate water. The difference in δ¹⁸O between O₂ and water was about 4.5, 3, 5.5, and 7‰ in the diatom Phaeodactylum tricornutum, Nannochloropsis sp. (Eustigmatophyceae), the coccolithophore Emiliania huxleyi and the green alga Chlamydomonas reinhardtii, respectively. The difference in δ¹⁷O was about 0.52 that of δ¹⁸O. As explained, the observed enrichments most probably stem from considerable oxygen consumption during photosynthesis even when major O₂-consuming reactions such as photorespiration were minimized. These enrichments increased linearly with rising O₂ levels but with different δ¹⁷O/δ¹⁸O slopes for the various organisms, suggesting engagements of different O₂-consuming reactions with rising O₂ levels. Consumption of O₂ may be important for energy dissipation during photosynthesis. The isotope enrichment observed here, not accounted for in earlier assessments, closes an important gap in our understanding of the difference between the isotopic compositions of atmospheric oxygen and that of seawater, i.e., the Dole effect.     

On the controls of leaf-water oxygen isotope ratios in the atmospheric Crassulacean acid metabolism epiphyte Tillandsia usneoides

Previous theoretical work showed that leaf-water isotope ratio (δ¹⁸O_L) of Crassulacean acid metabolism epiphytes was controlled by the δ¹⁸O of atmospheric water vapor (δ¹⁸O_a), and observed δ¹⁸O_L could be explained by both a non-steady-state model and a “maximum enrichment” steady-state model (δ¹⁸O_L-M), the latter requiring only δ¹⁸O_a and relative humidity (h) as inputs. δ¹⁸O_L, therefore, should contain an extractable record of δ¹⁸O_a. Previous empirical work supported this hypothesis but raised many questions. How does changing δ¹⁸O_a and h affect δ¹⁸O_L? Do hygroscopic trichomes affect observed δ¹⁸O_L? Are observations of changes in water content required for the prediction of δ¹⁸O_L? Does the leaf need to be at full isotopic steady state for observed δ¹⁸O_L to equal δ¹⁸O_L-M? These questions were examined with a climate-controlled experimental system capable of holding δ¹⁸O_a constant for several weeks. Water adsorbed to trichomes required a correction ranging from 0.5‰ to 1‰. δ¹⁸O_L could be predicted using constant values of water content and even total conductance. Tissue rehydration caused a transitory change in δ¹⁸O_L, but the consequent increase in total conductance led to a tighter coupling with δ¹⁸O_a. The non-steady-state leaf water models explained observed δ¹⁸O_L (y = 0.93*x − 0.07; r² = 0.98) over a wide range of δ¹⁸O_a and h. Predictions of δ¹⁸O_L-M agreed with observations of δ¹⁸O_L (y = 0.87*x − 0.99; r² = 0.92), and when h > 0.9, the leaf did not need to be at isotopic steady state for the δ¹⁸O_L-M model to predict δ¹⁸O_L in the Crassulacean acid metabolism epiphyte Tillandsia usneoides.Tropical and subtropical epiphytic higher plants have long been a curiosity to plant physiologists because of the multiple and unique constraints on physiology that the epiphytic lifeform represents (Mez, 1904; Benzing, 1970; Medina and Troughton, 1974). While the competition for light has no doubt led the plants to the tree tops, the concomitant loss of roots effectively removed the plants from an environment of abundant rainfall to one of arid conditions (Griffiths et al., 1986; Smith et al., 1986a; Winter and Smith, 1996). In a sense, the plants shifted biomes by immigrating vertically instead of horizontally. This shift led to morphological adaptations like the development of modified leaf hairs to absorb water and the formation of tanks via the overlapping of leaf bases as well as physiological adaptations such as maintaining high leaf osmotic potential and, perhaps most notably, the evolution of the CO₂-concentrating mechanism known as Crassulacean acid metabolism (CAM) photosynthesis (Medina and Troughton, 1974; Benzing et al., 1976; Griffiths and Smith, 1983; Smith et al., 1986b; Martin and Schmitt, 1989; Martin et al., 2004; Ohrui et al., 2007). This unique physiology of tropical and subtropical CAM epiphytes also presents an intriguing contrast to the general way we view oxygen isotope ratios (δ¹⁸O) in plant water and organic material (Helliker and Griffiths, 2007; Helliker and Noone, 2009).The enrichment of ¹⁸O in leaf water during plant transpiration leads to a suite of physiology-based tracers that inform us of current and past plant-environment interactions. Leaf water δ¹⁸O (δ¹⁸O_L) labels CO₂ and allows for partitioning of the gross components of net CO₂ flux from ecosystem to regional scales (Yakir and Wang, 1996; Ciais et al., 1997; Styles et al., 2002; Cuntz et al., 2003; Ogee et al., 2004; Helliker et al., 2005). The production of isotopically distinct O₂ by photosynthesis allows for global-scale estimates of productivity over millennia (Guy et al., 1993; Luz et al., 1999; Hoffmann et al., 2004). The isotopic record of δ¹⁸O_L in leaf and tree ring cellulose allows for the reconstruction of growth environment and/or physiological responses to that growth environment (Epstein et al., 1977; Anderson et al., 1998; Switsur and Waterhouse, 1998; Barbour and Farquhar, 2000; Barbour et al., 2000; Roden and Ehleringer, 2000; Ferrio and Voltas, 2005; Poussart and Schrag, 2005; Helliker and Richter, 2008). Much of the error associated with the above δ¹⁸O applications could be decreased with better estimates of the isotope ratio of atmospheric water vapor (δ¹⁸O_a), a primary control on leaf-water ¹⁸O enrichment (Farquhar and Cernusak, 2005; Helliker and Griffiths, 2007), and the study of δ¹⁸O_L in CAM epiphytes may offer better estimates of δ¹⁸O_a through time and space (Helliker and Griffiths, 2007).The epiphyte, and specifically the CAM vascular epiphyte, offers an extreme in the way δ¹⁸O in plant water and organic material is viewed. For most vascular plants, δ¹⁸O_L is controlled by a balance of soil water δ¹⁸O and δ¹⁸O_a; soil water comes into the leaves via the root system, and atmospheric vapor diffuses into the leaf through open stomata. The balance of the two water sources is determined by relative humidity (h): if h is high, then δ¹⁸O_L is controlled proportionally more by δ¹⁸O_a; if h is unity, then δ¹⁸O_L is at equilibrium with δ¹⁸O_a. If these plants happen to lose water during a humid night, then leaf water exchange can lead to control by, and possibly equilibrium with, δ¹⁸O_a (Lai et al., 2008). Typically, the dramatic increase of transpiration during the daytime moves the δ¹⁸O_L away from equilibrium with δ¹⁸O_a. Hence, the nocturnal movement of δ¹⁸O_L toward equilibrium with δ¹⁸O_a in most vascular plants is not likely recorded in plant organic material and is only important to the isotopic composition of CO₂ during nocturnal respiration (Cernusak et al., 2004). In the CAM epiphyte, rainwater and dewfall rehydrate epiphyte tissue, and due to stomata being open at night, when h is near unity, rehydration water is quickly exchanged with atmospheric water vapor. This leads to the situation, hypothesized by Helliker and Griffiths (2007), where rainfall and dewfall (which are in equilibrium with δ¹⁸O_a; Gat, 1996) control tissue water status, and the δ¹⁸O of this tissue water is continuously controlled by δ¹⁸O_a through subsequent nocturnal vapor exchange. Because stomata are typically closed during the day, organic material synthesized by the photosynthetic carbon reduction cycle should obtain the δ¹⁸O signature of δ¹⁸O_a-controlled leaf water (Helliker and Griffiths, 2007). It is this situation that yields new applications for δ¹⁸O that are unique to epiphytes: the reconstruction of physiological responses to the epiphytic growth habit (Reyes-Garcia et al., 2008) and the reconstruction of δ¹⁸O_a (Helliker and Griffiths, 2007). It was recently shown through empirical and theoretical work that lichen thalli obtain equilibrium with δ¹⁸O_a even under nonsaturating conditions (Hartard et al., 2009). The lichen system of equilibrium with δ¹⁸O_a represents an important contrast to that of CAM epiphytes, and this is discussed later. The theoretical work presented by Hartard et al. (2009), however, is extremely helpful for the interpretation of isotopic signals in CAM epiphytes.Helliker and Griffiths (2007) developed the theoretical underpinnings for δ¹⁸O_a as a control on CAM epiphyte δ¹⁸O_L. They extended the thoughts of Farquhar and Cernusak (2005) to show that at the high nocturnal h experienced by CAM epiphytes, gross exchange fluxes of water vapor into the leaf from the atmosphere can be several times the transpirational flux out of the leaf. Model simulations showed that constant environmental conditions and continuous water loss led to an isotopic steady state where both δ¹⁸O_L and transpired water (δ¹⁸O_E) converged to a single isotopic value controlled by the exchange of δ¹⁸O_a and described by the following “maximum enrichment” equation (Farquhar and Gan, 2003; Helliker and Griffiths, 2007; Hartard et al., 2009):where the steady-state δ¹⁸O_L (R_L) is determined solely by δ¹⁸O_a (R_a), h, the temperature-dependent equilibrium fractionation factor α*, and the balance of the ratio of diffusivities of light to heavy water molecules through the stomata and through the leaf boundary layer, α^K (Flanagan et al., 1991; Farquhar and Lloyd, 1993; Farquhar and Cernusak, 2005). The non-steady-state analytical solution for δ¹⁸O_L developed by Hartard et al. (2009; Eq. 6 in “Materials and Methods,” hereafter referred to as the Hartard-Cuntz solution) clearly demonstrates that, through time in the nonsteady state, δ¹⁸O_L is continuously moving toward the value of R_l-M (δ¹⁸O_l-M). In a similar manner, the simulations of Helliker and Griffiths (2007) suggested that even at values of nocturnal h of 0.8, which is low for a CAM epiphyte (Garth, 1964; Smith et al., 1986b), δ¹⁸O_L was controlled almost entirely by the isotope ratio of atmospheric water vapor and the maximum enrichment equation above would ultimately predict δ¹⁸O_L. The simulations also showed that the approach to steady state was faster and occurred through less water loss as h increased. In general, their simulations showed that δ¹⁸O_L approached the steady-state value, or δ¹⁸O_l-M, much faster than δ¹⁸O_E, and this yields an important prediction that is tested by our study: the leaf does not need to be at full isotopic steady state for δ¹⁸O_L to be at or near the steady-state, maximum enrichment value (δ¹⁸O_l-M).There are many unresolved questions as to the agreement between models and observations that underpin the efficacy of using δ¹⁸O_L to reconstruct δ¹⁸O_a. The empirical work of Helliker and Griffiths (2007) did show support for the modeling exercises, but only over a narrow range of conditions. Their experimental setup was limited, as it was developed to demonstrate a preliminary proof of concept. Also, the invariably high h and noisy δ¹⁸O_a (±1.2‰) of Helliker and Griffiths (2007) could lead one to erroneously conclude that δ¹⁸O_L was in direct equilibrium with δ¹⁸O_a even when h was less than unity, which should not be the case. The primary controls of CAM epiphyte δ¹⁸O_L are h and δ¹⁸O_a, and the manner in which changes in δ¹⁸O_a and h affect δ¹⁸O_L through time must be assessed. Like many CAM epiphytes, the study species Tillandsia usneoides has a heavy covering of hygroscopic trichomes. While the water adsorbed to these trichomes does not lead to water uptake by living cells at subsaturating conditions (Martin and Schmitt, 1989), the water is inevitably sampled and extracted for δ¹⁸O_L determination. Hence, the isotopic offset caused by these trichomes must be determined. Total plant conductance to water loss (g_tot; stomatal and boundary layer conductance) controls the rate of exchange of tissue water with water vapor and the relative water content (RWC) of the plant. Therefore, the effect of changes in RWC on δ¹⁸O_L, through both water loss and rehydration, must be determined. Previous work has shown that changes in water volume are not important to predictions of δ¹⁸O_L (Cernusak et al., 2002; Cuntz et al., 2007), and a similar finding in CAM epiphytes could greatly simplify our interpretation of observed δ¹⁸O_L. In summary, the goal of this study was to construct controlled-environment experiments to assess how well observed δ¹⁸O_L could be predicted over a range of conditions through both steady-state and non-steady-state approaches of Helliker and Griffiths (2007) and Hartard et al. (2009).     

Shade adaptation of photosynthesis in Coffea arabica

The effect of irradiance on the rate of net photosynthesis was measured for mature leaves of coffee grown under five levels of radiation from 100% to 5% daylight. The rate of light-saturated photosynthesis per unit leaf area (P_Nmax) increased from 2 mol CO₂ m^-2 s^-1 under 5% daylight to 4.4 mol CO₂ m^-2 s^-1 under 100% daylight. The photon flux density (PAR, photosynthetically active radiation) needed for 50% saturation of photosynthesis, as well as the light compensation point, also increased with increasing levels of irradiation during growth. The quantum efficiency of photosynthesis (), measured by the initial slope of the photosynthetic response to increasing irradiance, was greater under shaded growth conditions. The rate of dark respiration was greatest for plants grown in full daylight. On the basis of the increase in the quantal efficiency of photosynthesis and the low light compensation point when grown under shaded conditions, coffee shows high shade adaptation. Plants adjusted to shade by an increased ability to utilize short-term increases in irradiance above the level of the growth irradiance (measured by the difference between photosynthesis at the growth irradiance, P_Ng, and P_Nmax).     

Effect of carbon dioxide and temperature on photosynthetic CO2 uptake and photorespiratory CO2 evolution in sunflower leaves

Using an open gas-exchange system, apparent photosynthesis, true photosynthesis (TPS), photorespiration (PR) and dark respiration of sunflower (Helianthus annuus L.) leaves were determined at three temperatures and between 50 and 400 l/l external CO₂. The ratio of PR/TPS and the solubility ratio of O₂/CO₂ in the intercellular spaces both decreased with increasing CO₂. The rate of PR was not affected by the CO₂ concentration in the leaves and was independent of the solubility ratio of oxygen and CO₂ in the leaf cell. At photosynthesis-limiting concentrations of CO₂, the ratio of PR/TPS significantly increased from 18 to 30°C and the rate of PR increased from 4.3 mg CO₂ dm^-2 h^-1 at 18°C to 8.6 mg CO₂ dm^-2 h^-1 at 30°C. The specific activity of photorespired CO₂ was CO₂-dependent but temperature-independent, and the carbon traversing the glycolate pathway appeared to be derived both from recently fixed assimilate and from older reserve materials. It is concluded that PR as a percentage of TPS is affected by the concentrations of O₂ and CO₂ around the photosynthesizing cells, but the rate of PR may also be controlled by other factors.Abbreviations APS apparent photosynthesis (net CO₂ uptake) - PR photorespiration (CO₂ evolution in light) - RuBP ribulose-1,5-bisphosphate - TPS true photosynthesis (true CO₂ uptake)     

Recovery from photoinhibition: effect of light and inhibition of protein synthesis of 32-kD chloroplast protein

Recovery from photoinhibition of photosynthesis in intact Lemna gibba was studied in presence of the protein synthesis inhibitors chloramphenicol and cycloheximide. Exposure to an irradiance of 1000 mol m^-2s^-1 in N₂ for 90 min induced 80% photoinhibition. The plants recovered photosynthesis when transfered to normal irradiances (210 mol m^-2s^-1) and air. Chloramphenicol added to the medium was taken up by the plant and reduced photosynthesis slightly. Recovery from photoinhibition was more inhibited than photosynthesis. Cycloheximide was also taken up by the plants and reduced synthesis of light harvesting chlorophyll protein: however, neither photosynthesis nor recovery were much affected. Synthesis of 32-kD chloroplast protein during recovery was inhibited by chloramphenicol, but not by cycloheximide. Synthesis of 32-kD protein was enhanced by 20–210 mol m^-2s^-1 light. The results support the hypothesis that synthesis of 32-kD protein is important for recovery of photosynthesis after photoinhibition.     

C4 plants of high biomass in arid regions of asia-occurrence of C4 photosynthesis in Chenopodiaceae and Polygonaceae from the Middle East and USSR

Summary Measurements of ¹³C values were used as a diagnostic test for the possible occurrence of C₄ photosynthesis in 175 species of the Chenopodiaceae and 18 species of the genus Calligonum (Polygonaceae) from deserts of the Middle East and USSR. Eighty percent of the Chenopodiaceae (predominantly members of the genera Aellenia, Anabasis, Haloxylon, Salsola and Suaeda) and all species of the genus Calligonum showed C₄-like ¹³C values. Several features of these plants disclose some new facets of C₄ photosynthesis. Some of the Haloxylon and Calligonum species are trees or tall shrubs and in Middle Asia are dominant members of plant communities characterized by high biomass. Many C₄ species of the Chenopodiaceae and Polygonaceae in their natural, Middle and Central Asian desert habitats, experience temperatures far below the freezing point for a long period of the year. Several of these C₄ species are of considerable economic value.     

Time-resolved FTIR difference spectroscopy in combination with specific isotope labeling for the study of A1, the secondary electron acceptor in photosystem 1

A phylloquinone molecule (2-methyl, 3-phytyl, 1, 4-naphthoquinone) occupies the A₁ binding site in photosystem 1 particles from Synechocystis sp. 6803. In menB mutant photosystem 1 particles from the same species, plastoquinone-9 occupies the A₁ binding site. By incubation of menB mutant photosystem 1 particles in the presence of phylloquinone, it was shown in another study that phylloquinone will displace plastoquinone-9 in the A₁ binding site. We describe the reconstitution of unlabeled (¹⁶O) and ¹⁸O-labeled phylloquinone back into the A₁ binding site in menB photosystem 1 particles. We then produce time-resolved Fourier transform infrared (FTIR) difference spectra for these menB photosystem 1 particles that contain unlabeled and ¹⁸O-labeled phylloquinone. By specifically labeling only the phylloquinone oxygen atoms we are able to identify bands in FTIR difference spectra that are due to the carbonyl (CO) modes of neutral and reduced phylloquinone. A positive band at 1494 cm⁻¹ in the FTIR difference spectrum is found to downshift 14 cm⁻¹ and decreases in intensity on ¹⁸O labeling. Vibrational mode frequency calculations predict that an antisymmetric vibration of both CO groups of the phylloquinone anion should display exactly this behavior. In addition, phylloquinone that has asymmetrically hydrogen bonded carbonyl groups is also predicted to display this behavior. The positive band at 1494 cm⁻¹ in the FTIR difference spectrum is therefore due to the antisymmetric vibration of both CO groups of one electron reduced phylloquinone. Part of a negative band at 1654 cm⁻¹ in the FTIR difference spectrum downshifts 28 cm⁻¹ on ¹⁸O labeling. Again, vibrational mode frequency calculations predict this behavior for a CO mode of neutral phylloquinone. The negative band at 1654 cm⁻¹ in the FTIR difference spectrum is therefore due to a CO mode of neutral phylloquinone. More specifically, calculations on a phylloquinone model molecule with the C₄O group hydrogen bonded predict that the 1654 cm⁻¹ band is due to the non hydrogen bonded C₁O mode of neutral phylloquinone.     

Exogenous inorganic carbon sources for photosynthesis in seawater by members of the Fucales and the Laminariales (Phaeophyta): ecological and taxonomic implications

Summary Characteristics of inorganic carbon assimilation by photosynthesis in seawater were investigated in six species of the Fucales (five Fucaceae, one Cystoseiraceae) and four species of the Laminariales (three Laminariaceae, one Alariaceae) from Arbroath, Scotland. All of the algae tested could photosynthesise faster at high external pH values than the uncatalysed conversion of HCO ₃ ^- to CO₂ can occur, i.e. can use external HCO ₃ ^- . They all had detectable extracellular carbonic anhydrase activity, suggesting that HCO ₃ ^- use could involve catalysis of external CO₂ production, a view supported to some extent by experiments with an inhibitor of carbonic anhydrase. All of the algae tested had CO₂ compensation concentrations at pH 8 which were lower than would be expected from diffusive entry of CO₂ supplying RUBISCO as the initial carboxylase, consistent with the operation of energized entry of HCO ₃ ^- and / or CO₂ acting as a CO₂ concentrating mechanism. Quantitative differences among the algae examined were noted with respect to characteristics of inorganic C assimilation. The most obvious distinction was between the eulittoral Fucaceae, which are emersed for part of, or most of, the tidal cycle, and the other three families (Cystoseiraceae, Laminariaceae, Alariaceae) whose representatives are essentially continually submersed. The Fucaceae examined are able to photosynthesise at high pH values, and have lower CO₂ compensation concentrations, and lower K_1/2 values for inorganic C use in photosynthesis, at pH 8, than the other algae tested. Furthermore, the Fucaceae are essentially saturated with inorganic C for photosynthesis at the normal seawater concentration at pH 8 and 10°C. These characteristics are consistent with the dominant role of a CO₂ concentrating mechanism in CO₂ acquisition by these plants. Other species tested have characteristcs which suggest a less effective HCO ₃ ^- use and CO₂ concentrating mechanism, with the Laminariaceae being the least effective; unlike the Fucaceae, photosynthesis by these algae is not saturated with inorganic C in normal seawater. Taxonomic and ecological implications of these results are considered in relation to related data in the literature.     

The specific radioactivity of glycolic acid in relation to the specific activity of carbon dioxide evolved in light in photosynthesizing sunflower leaves

Attached leaves of sunflower (Helianthus annuus L.) were exposed to ¹⁴CO₂ during steady-state photosynthesis for 2 to 30 min in 345 l/l CO₂ and 21% O₂ at 29° C and a light intensity of 1300 E m^-2s^-1. Glycolic acid was extracted with water and diethyl ether, and was determined in the aqueous residue by high-pressure liquid column chromatography. The relative specific radioactivity of the glycolic acid synthesized during photosynthesis reached about 100% after 30 min of photosynthesis and was almost equal to that of the CO₂ evolved during photorespiration, their ratio at all times being nearly one. These results provide strong in-vivo evidence that the glycolic acid is the substrate for CO₂ evolved by sunflower leaves in light.     

Effects of environmental conditions on isoprene emission from live oak

Live-oak plants (Quercus virginiana Mill.) were subjected to various levels of CO₂, water stress or photosynthetic photon flux density to test the hypothesis that isoprene biosynthesis occurred only under conditions of restricted CO₂ availability. Isoprene emission increases as the ambient CO₂ concentration decreased, independent of the amount of time that plants had photosynthesized at ambient CO₂ levels. When plants were water-stressed over a 4-d period photosynthesis and leaf conductance decreased 98 and 94%, respectively, while isoprene emissions remained constant. Significant isoprene emissions occurred when plants were saturated with CO₂, i.e., below the light compensation level for net photosynthesis (100 mol m^-2 s^-1). Isoprene emission rates increased with photosynthetic photon flux density and at 25 and 50 mol m^-2 s^-1 were 7 and 18 times greater than emissions in the dark. These data indicate that isoprene is a normal plant metabolite and not — as has been suggested — formed exclusively in response to restricted CO₂ or various stresses.Abbreviation PPFD photosynthetic photon flux density