Why does photosynthesis decrease with needle age in Pinus pinaster?

Rates of photosynthesis vary with foliage age and typically decline from full-leaf expansion until senescence occurs. This age-related decline in photosynthesis is especially important in species that retain foliage for several years, yet it is not known whether the internal conductance to CO₂ movement (g _i) plays any role. More generally, g _i has been measured in only a few conifers and has never been measured in leaves or needles older than 1 year. The effect of ageing on g _i was investigated in Pinus pinaster, a species that retains needle for 4 or more years. Measurements were made in autumn when trees were not water limited and after leaf expansion was complete. Rates of net photosynthesis decreased with needle age, from 8 μmol m⁻² s⁻¹ in fully expanded current-year needles to 4.4 μmol m⁻² s⁻¹ in 3-year-old needles. The relative limitation due to internal conductance (0.24–0.35 out of 1) was in all cases larger than that due to stomatal conductance (0.13–0.19 out of 1). Internal conductance and stomatal conductance approximately scaled with rates of photosynthesis. Hence, there was no difference among year-classes in the relative limitations posed by internal and stomatal conductance or evidence that they cause the age-related decline in photosynthesis. There was little evidence that the age-related decline in photosynthesis was due to decreases in contents of N or Rubisco. The decrease in rates of photosynthesis from current-year to older needles was instead related to a twofold decrease in rates of photosynthesis per unit nitrogen and V _cmax/Rubisco (i.e., in vivo specific activity).     

Control of photosynthesis by the carbohydrate level in leaves of the C4 plant Amaranthus edulis L.

Photosynthesis was studied in relation to the carbohydrate status in intact leaves of the C₄ plant Amaranthus edulis. The rate of leaf net CO₂ assimilation, stomatal conductance and intercellular partial pressure of CO₂ remained constant or showed little decline towards the end of an 8-h period of illumination in ambient air (340 bar CO₂, 21% O₂). When sucrose export from the leaf was inhibited by applying a 4-h cold-block treatment (1°C) to the petiole, the rate of photosynthesis rapidly decreased with time. After the removal of the cold block from the petiole, further reduction in photosynthetic rate occurred, and there was no recovery in the subsequent light period. Although stomatal conductance declined with time, intercellular CO₂ partial pressure remained relatively constant, indicating that the inhibition of photosynthesis was not primarily caused by changes in stomatal aperture. Analysis of the leaf carbohydrate status showed a five- to sixfold increase in the soluble sugar fraction (mainly sucrose) in comparison with the untreated controls, whereas the starch content was the same. Leaf osmotic potential increased significantly with the accumulation of soluble sugars upon petiole chilling, and leaf water potential became slightly more negative. After 14 h recovery in the dark, photosynthesis returned to its initial maximum value within 1 h of illumination, and this was associated with a decline in leaf carbohydrate levels overnight. These data show that, in Amaranthus edulis, depression in photosynthesis when translocation is impaired is closely related to the accumulation of soluble sugars (sucrose) in source leaves, indicating feedback control of C₄ photosynthesis. Possible mechanisms by which sucrose accumulation in the leaf may affect the rate of photosynthesis are discussed with regard to the leaf anatomy of C₄ plants.Abbreviations and symbols A net CO₂ assimilation rate - Ci intercellular CO₂ partial pressure - PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate - water potential - osmotic pressure     

Photosynthetic and respiratory characterization of field grown tomato

The photosynthetic responses of tomato (Lycopersicum esculentum Mill.) leaves to environmental and ontogenetic factors were determined on plants grown in the field under high radiation and high nitrogen fertilization. Response curves showed net photosynthesis to only approach light saturation at a photosynthetic photon flux density (PPFD) of 2200 mol m^-2 s^-1, with rates of approx. 40 mol CO₂ m^-2 s^-1. A broad temperature optimum was observed between 25° and 35°C, with 50% of the photosynthetic rates remaining even at 47°C. The high rate, the lack of saturation at the equivalent of full sunlight, and the tolerance to high temperature of tomato were unusual in light of the literature on this C₃ species. Apparently, acclimation to the field environment of high radiation and hot daytime temperature, coupled with the high nitrogen nutrition, made possible the high photosynthetic performance normally associated with C₄ species.Photosynthetic ability of the leaf reached a maximum near the time of its full expansion and declined steadily thereafter, regardless of the time of leaf initiation. Leaf nitrogen content showed a similar decline with leaf ontogeny. Photosynthesis was linearly correlated with nitrogen content, whether the nitrogen variation was due to leaf age or rates of nitrogen fertilization. Internal CO₂ concentrations (C_i) of the leaf indicated that stomatal function was well coordinated with photosynthetic capacity as leaf age and fluence rate varied down to a PPFD of 500 mol m^-2 s^-1. As PPFD decreased further, there was less stomatal control and C_i increased to as high as 320 bar bar^-1.Dark respiration was highest for expanding leaves and increased nearly exponentially with temperature. Respiration was also highest for young and expanding fruits, and next highest for fruits just turning pink. Fruit respiration increased approximately linearly with temperature, and was estimated to be an important component of the CO₂ flux of the plant near maturity because of the heavy fruit load and low leaf photosynthesis at that time. The results are significant for model simulation of tomato productivity in the field.     

Leaf and canopy responses to elevated CO2 in a pine forest under free-air CO2 enrichment

Physiological responses to elevated CO₂ at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO₂ using a free-air CO₂ enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO₂ over an 8-day exposure period was similar to that of trees exposed to current ambient CO₂ under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO₂-exposed trees showed minor (7%) but significant reductions in relative sap flux density compared to trees under ambient CO₂ conditions. Short-term (minutes) direct stomatal responses to elevated CO₂ were also relatively weak (5% reduction in stomatal aperture in response to high CO₂ concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO₂ for nearly 80 days compared to foliage grown under current ambient CO₂, so intrinsic leaf water-use efficiency at elevated CO₂ was enhanced primarily by direct responses of photosynthesis to CO₂. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO₂ (A _net-C _i curves) for Pinus taeda foliage grown under elevated CO₂ (550 mol mol^–1) for 50–80 days compared to those for foliage grown under current ambient CO₂ from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 mol mol^–1 CO₂ was enhanced by approximately 65% compared to the rate at ambient CO₂ (350 mol mol^–1). A similar level of enhancement under elevated CO₂ was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO₂ during the study period appears to be primarily attributable to direct photosynthetic responses to CO₂ in the pine forest, longer-term CO₂ responses and feedbacks remain to be evaluated.     

Leaf and canopy photosynthetic CO2 uptake of a stand of Echinochloa polystachya on the Central Amazon floodplain

The C₄ grass Echinochloa polystachya, which forms dense and extensive monotypic stands on the Varzea floodplains of the Amazon region, provides the most productive natural higher plant communities known. The seasonal cycle of growth of this plant is closely linked to the annual rise and fall of water level over the floodplain surface. Diurnal cycles of leaf photosynthesis and transpiration were measured at monthly intervals, in parallel with measurements of leaf area index, canopy light interception and biomass. By artificial manipulation of the light flux incident on leaves in the field light-response curves of photosynthesis at the top and near to the base of the canopy were generated. Fitted light-response curves of CO₂ uptake were combined with information of leaf area index, incident light and light penetration of the canopy to estimate canopy rates of photosynthesis. Throughout the period in which the floodplains were submerged photosynthetic rates of CO₂ uptake (A) for the emergent leaves were high with a mean of c. 30 mol m^-2 s^-1 at mid-day and occasional values of 40 mol m^-2 s^-1. During the brief dry phase, when the floodplain surface is uncovered, there was a significant depression of A, with mid-day mean values of c. 17 mol m^-2 s^-1. This corresponded with a c. 50% decrease in stomatal conductance, and a c. 35% depression in the ratio of the leaf inter-cellular to external CO₂ concentration (c _i/c _a). During the dry phase, a midday depression of rates of CO₂ assimilation was observed. The lowest leaf area index (F) was c. 2 in November–December, when the flood plain was dry, and again in May, when the rising floodwaters were submerging leaves faster than they were replaced. The maximum F of c. 5 was in August when the floodwaters were receding rapidly. Canopy light interception efficiency varied from 0.90 to 0.98. Calculated rates of canopy photosynthesis exceeded 18 mol C m^-2 mo^-1 throughout the period of flooding, with a peak of 37 mol C m^-2 mo^-1 in August, but declined to 13 mol C m^-2 mo^-1 in November during the dry phase. Estimated uptake of carbon by the canopy from the atmosphere, over 12 months, was 3.57 kg C m^-2. This was insufficient to account for the 3.99 kg C m^-2 of net primary production, measured simultaneously by destructive harvesting. It is postulated that this discrepancy might be accounted for by internal diffusion of CO₂ from the CO₂-rich waters and sediments via the roots and stems to the sites of assimilation in the leaves.     

The efficiency of water use in water stressed plants is increased due to ABA induced stomatal closure

Gas exchange and abscisic acid content of Digitalis lanata EHRH. have been examined at different levels of plant water stress. Net photosynthesis, transpiration and conductance of attached leaves declined rapidly at first, then more slowly following the withholding of irrigation. The intercellular partial pressure of CO₂ decreased slightly. The concentration of 2-cis(S)ABA increased about eight-fold in the leaves of non-irrigated plants as compared with well-watered controls. A close linear correlation was found between the ABA content of the leaves and their conductance on a leaf area basis. In contrast, the plot of net assimilation versus ABA concentration was curvilinear, leading to an increased efficiency of water use during stress. After rewatering, photosynthesis reached control values earlier than transpiration, leaf conductance and ABA content. From these data it is concluded that transpiration through the stomata is directly controlled by the ABA content, whereas net photosynthesis is influenced additionally by other factors.Possible reasons for the responses of photosynthesis and water use efficiency to different stress and ABA levels are discussed.Abbreviations A net CO₂ assimilation - ABA abscisic acid - C_i intercellular CO₂ concentration - g stomatal conductance - T transpiration - WUE water use efficiency     

Analysis of inhibition of photosynthesis due to water stress in the C3 species Hordeum vulgare and Vicia faba: Electron transport, CO2 fixation and carboxylation capacity

A C₃ monocot, Hordeum vulgare and C₃ dicot, Vicia faba, were studied to evaluate the mechanism of inhibition of photosynthesis due to water stress. The net rate of CO₂ fixation (A) and transpiration (E) were measured by gas exchange, while the true rate of O₂ evolution (J _O2) was calculated from chlorophyll fluorescence analysis through the stress cycle (10 to 11 days). With the development of water stress, the decrease in A was more pronounced than the decrease in J _O2 resulting in an increased ratio of Photosystem II activity per CO₂ fixed which is indicative of an increase in photorespiration due to a decrease in supply of CO₂ to Rubisco. Analyses of changes in the J _O2 A ratios versus that of CO₂ limited photosynthesis in well watered plants, and RuBP pool/RuBP binding sites on Rubisco and RuBP activity, indicate a decreased supply of CO₂ to Rubisco under both mild and severe stress is primarily responsible for the decrease in CO₂ fixation. In the early stages of stress, the decrease in C _i (intercellular CO₂) due to stomatal closure can account for the decrease in photosynthesis. Under more severe stress, CO₂ supply to Rubisco, calculated from analysis of electron flow and CO₂ exchange, continued to decrease. However, C _i, calculated from analysis of transpiration and CO₂ exchange, either remained constant or increased which may be due to either a decrease in mesophyll conductance or an overestimation of C _i by this method due to patchiness in conductance of CO₂ to the intercellular space. When plants were rewatered after photosynthesis had dropped to 10–30% of the original rate, both species showed near full recovery within two to four days.Abbreviations A- net CO₂ assimilation rate - A ^*- net CO₂ assimilation rate plus dark respiration - ATP- adenosine triphosphate - CABP- carboxyarabinitol 1,5-bisphosphate - C _a- ambient CO₂ concentration - C _c- CO₂ concentration in the chloroplast - C _i- intercellular CO₂ concentration - E- transpiration rate - g _m- mesophyll conductance - g _s- stomatal conductance - J _O2 true rate of O₂ evolution - LSD- least significant difference - PPFD- photosynthetic photon flux density - PS II- Photosystem II - R _n- dark respiration rate - Rubisco- ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP- ribulose 1,5-bisphosphate - RWC- relative water content - _c- rate of carboxylation - _o- rate of oxygenation - _PSII- quantum yield of Photosystem II - - CO₂ compensation point in the absence of R _n - - water potential     

Acclimation of photosynthesis to increasing atmospheric CO2: The gas exchange perspective

The nature of photosynthetic acclimation to elevated CO₂ is evaluated from the results of over 40 studies focusing on the effect of long-term CO₂ enrichment on the short-term response of photosynthesis to intercellular CO₂ (the A/C_i response). The effect of CO₂ enrichment on the A/C_i response was dependent on growth conditions, with plants grown in small pots (< 5 L) or low nutrients usually exhibiting a reduction of A at a given C_i, while plants grown without nutrient deficiency in large pots or in the field tended to exhibit either little reduction or an enhancement of A at a given C_i following a doubling or tripling of atmospheric CO₂ during growth. Using theoretical interpretations of A/C_i curves to assess acclimation, it was found that when pot size or nutrient deficiency was not a factor, changes in the shape of A/C_i curves which are indicative of a reallocation of resources within the photosynthetic apparatus typically were not observed. Long-term CO₂ enrichment usually had little effect or increased the value of A at all C_i. However, a minority of species grown at elevated CO₂ exhibited gas exchange responses indicative of a reduced amount of Rubisco and an enhanced capacity to metabolize photosynthetic products. This type of response was considered beneficial because it enhanced both photosynthetic capacity at high CO₂ and reduced resource investment in excessive Rubisco capacity. The ratio of intercellular to ambient CO₂ (the C_i/C_a ratio) was used to evaluate stomatal acclimation. Except under water and humidity stress, C_i/C_a exhibited no consistent change in a variety of C₃ species, indicating no stomatal acclimation. Under drought or humidity stress, C_i/C_a declined in high-CO₂ grown plants, indicating stomata will become more conservative during stress episodes in future high CO₂ environments.Abbreviations A net CO₂ assimilation rate - C_i (C_a) intercellular (ambient) partial pressure of CO₂ - operational C_i intercellular partial pressure of CO₂ at a given ambient partial pressure of CO₂ - g_s stomatal conductance - normal CO₂ current atmospheric mole fraction of CO₂ (330 to 355 mol mol^–1) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase     

Impacts of the C4 sedge Cyperus papyrus L. on carbon and water fluxes in an African wetland

Fluxes of CO₂ and H₂O vapour were measured by eddy covariance from a stand of the C₄ emergent sedge Cyperus papyrus (papyrus), which formed a fringing swamp on the north-west shore of Lake Naivasha, Kenya. The fluxes of CO₂ and H₂O vapour between the papyrus swamp and the atmosphere were large but variable, depending on the hydrology of the wetland system and the condition of the vegetation. These measurements, combined with simulation modelling of annual fluxes of CO₂, show that papyrus swamps have the potential to sequester large amounts of the carbon (1.6 kg C m^–2 y^–1) when detritus accumulates under water in anaerobic conditions, but they are a net source of carbon release to the atmosphere (1.0 kg C m^–2 y^–1) when water levels fall to expose detritus and rhizomes to aerobic conditions. Evapotranspiration from papyrus swamps (E) was frequently lower than evaporation from open water surfaces (E _o) and plant factors have a strong influence on the flux of water to the atmosphere. For the period of measurement E/E_o was 0.36.     

Photosynthesis,water use and growth of a C4 grass stand at high CO2 concentration

Leaf photosynthesis rate of the C₄ species Paspalum plicatulum Michx was virtually CO₂-saturated at normal atmospheric CO₂ concentration but transpiration decreased as CO₂ was increased above normal concentrations thereby increasing transpiration efficiency. To test whether this leaf response led growth to be CO₂-sensitive when water supply was restricted, plants were grown in sealed pots of soil as miniature swards. Water was supplied either daily to maintain a constant water table, or at three growth restricting levels on a 5-day drying cycle. Plants were either in a cabinet with normal air (340 mol (CO₂) mol^-1 (air)) or with 250 mol mol^-1 enrichment. Harvesting was by several cycles of defoliation.With abundant water supply high CO₂ concentration did not cause increased growth, but it did not cause an increase in growth over a wide range of growth-limiting water supplies either. Only when water supply was less than 30–50% of the amount used by the stand with a water-table was there evidence that dry weight growth was enhanced by high CO₂. In addition, with successive regrowth, the enhancing effect under a regime of minimal water allocations, became attenuated. Examination of leaf gas exchange, growth and water use data showed that in the long term stomatal conductance responses were of little significance in matching plant water use to low water allocation; regulation of leaf area was the mechanism through which consumption matched supply. Since high CO₂ effects operate principally via stomatal conductance in C₄ species, we postulate that for this species higher CO₂ concentrations expected globally in future will not have much effect on long term growth.     

Net CO2 assimilation of cacao seedlings during periods of plant water deficit

The effect of leaf water potential () on net CO₂ assimilation rate (A), stomatal conductance (g), transpiration (E) and water-use efficiency (WUE) was measured for three cultivars of cacao (Theobroma cacao L.) seedlings during three recurrent drought cycles. Net assimilation varied greatly at high water potentials, but as dropped below approximately -0.8 and -1.0 MPa, A was reduced to less than 1.5 mol CO₂ m^-2 s^-1. The relation between g and A was highly significant and conformed to an asymptotic exponential model, with A approaching maximal values at stomatal conductances of 55–65 mmol H₂O m^-2 s^-1. Net assimilation varied linearly (r=0.95) with transpiration, and the slope of the A-E relation (WUE) was approximately 3.0 mol CO₂ mmol^-1 H₂O throughout the range of stomatal conductances observed. C _i was insensitive to water stress, even though both g and A were strongly affected. Under the experimental conditions used here, mesophyll photosynthesis did not appear to control g through changes in C _i. As stress intensified within each drying cycle, WUE of nonirrigated seedlings did not decline relative to that of controls even though CO₂ and water vapor exchange rates underwent large displacements. The effect of seed source was highly significant for WUE, and the basis for observed differences among genotypes is discussed.Abbreviations ABA Abscisic Acid     

Effect of increased salinity on CO2 assimilation,O2 evolution and the δ13C values of leaves of Plantago maritima L. developed at low and high NaCl levels

The effect of increased salinity on photosynthesis was studied in leaves of Plantago maritima L. that developed while plants were at low and high NaCl levels. In leaves that developed while plants were grown at 50 mol·m^-3, exposure to 200 and 350 mol·m^-3 NaCl resulted in reductions in net CO₂ assimilation and stomatal conductance. The decline in CO₂ assimilation in plants at 200 and 350 mol·m^-3 NaCl occurred almost exclusively at high intercellular CO₂ concentrations. The initial slope of the CO₂ assimilation-intercellular CO₂ (A-C _i) curve, determined after salinity was increased, was identical or very similar to that measured initially. In contrast to the reductions observed in CO₂ assimilation, there were no significant differences in O₂ evolution rates measured at 5% CO₂ among leaves from plants exposed to higher salinity and plants remaining at low salinity.Leaves that developed while plants were at increased salinity levels also had significantly lower net CO₂ assimilation rates than plants remaining at 50 mol·m^-3 NaCl. The lower CO₂ assimilation rates in plants grown at 200 and 350 mol·m^-3 NaCl were a result of reduced stomatal conductance and low intercellular CO₂ concentration. There were no significant differences among treatments for O₂ evolution rates measured at high CO₂ levels. The increased stomatal limitation of photosynthesis was confirmed by measurements of the ¹³C/¹²C composition of leaf tissue. Water-use efficiency was increased in the plants grown at high salinity.Abbreviations and symbols A net CO₂ assimilation rate - C _a ambient CO₂ concentration - C _i intercellular CO₂ concentration - ¹³C isotopic ratio (¹³C/¹²C) expressed relative to a standard - RuBP ribulose-1,5-bisphosphate     

The relative contributions of reduced photorespiration,and improved water-and nitrogen-use efficiencies,to the advantages of C3−C4 intermediate photosynthesis in Flaveria

Summary Analyses of carbon-assimilation patterns in response to intercellular CO₂ concentrations, and the photosynthetic water-and nitrogen-use efficiencies, were conducted for a C₃, a C₄, and three C₃–C₄ species in the genus Flaveria in order to determine some of the advantages and disadvantages of C₃–C₄ intermediate photosynthesis. Operational intercellular CO₂ partial pressures (pi), determined when the atmospheric CO₂ partial pressure (pa) was approximately 330 bar, in the C₃–C₄ species were generally equal to, or greater than, those observed in the C₃ species under well-watered or water-stressed conditions. This reflects equal, or lower, water-use efficiencies (WUEs) in the C₃–C₄ species. The only case in which higher WUEs were observed in the C₃–C₄ species, compared to the C₃ species, was when photosynthesis rates were limited by available nitrogen and were less than 12.5 mol CO₂ m^-2s^-1. At higher photosynthesis rates, the C₃–C₄ species exhibited lower values of photosynthesis rate for equal values of stomatal conductance (lower WUE), compared to the C₃ species. Comparing slopes for the linear regions of the relationship between leaf nitrogen content and net photosynthesis rate (taken as an index of photosynthetic nitrogen-use efficiency, NUE), the C₄ species exhibited the highest NUE, followed by the C₃–C₄ species, F. ramosissima, with the other two C₃–C₄ species and the C₃ species being equal and exhibiting the lowest NUEs. The lack of consistent advantages in NUE and WUE in the C₃–C₄ species F. pubescens and F. floridana suggest that in some C₃–C₄ Flaveria species C₄-like anatomy and biochemistry do not provide the same gas exchange advantages that we typically attribute to the CO₂-concentrating mechanism of fully-expressed C₄ plants.     

Gas exchange of Ginkgo biloba leaves at different CO2 concentration levels

One and a half year-old Ginkgo saplings were grown for 2 years in 7 litre pots with medium fertile soil at ambient air CO₂ concentration and at 700 μmol mol⁻¹ CO₂ in temperature and humidity-controlled cabinets standing in the field. In the middle of the 2nd season of CO₂ enrichment, CO₂ exchange and transpiration in response to CO₂ concentration was measured with a mini-cuvette system. In addition, the same measurements were conducted in the crown of one 60-year-old tree in the field. Number of leaves/tree was enhanced by elevated CO₂ and specific leaf area decreased significantly.CO₂ compensation points were reached at 75–84 μmol mol⁻¹ CO₂. Gas exchange of Ginkgo saplings reacted more intensively upon CO₂ than those of the adult Ginkgo. On an average, stomatal conductance decreased by 30% as CO₂ concentration increased from 30 to 1000 μmol mol⁻¹ CO₂. Water use efficiency of net photosynthesis was positively correlated with CO₂ concentration levels. Saturation of net photosynthesis and lowest level of stomatal conductance was reached by the leaves of Ginkgo saplings at >1000 μmol mol⁻¹ CO₂. Acclimation of leaf net CO₂ assimilation to the elevated CO₂ concentration at growth occurred after 2 years of exposure. Maximum of net CO₂ assimilation was 56% higher at ambient air CO₂ concentration than at 700 μmol mol⁻¹ CO₂.     

Simultaneous and independent effects of abscisic acid on stomata and the photosynthetic apparatus in whole leaves

(±)-Abscisic acid (ABA) at 10^-5 M was added to the transpiration stream of leaves of 16 species (C₃ and C₄, monocotyledons and dicotyledons). Stomatal responses followed one of three patterns: i) stomata that were wide and insensitive to CO₂ initially, closed partially and became sensitive to CO₂; ii) for stomata that were sensitive to CO₂ before the application of ABA, the range of highest sensitivity to CO₂ shifted from high to low intercellular partial pressures of CO₂, for instance in leaves of Zea mays from 170–350 to 70–140 bar; iii) when stomata responded strongly to ABA, their conductance was reduced to a small fraction of the initial conductance, and sensitivity to CO₂ was lost. The photosynthetic apparatus was affected by applications of ABA to various degrees, from no response at all (in agreement with several previous reports on the absence of effects of ABA on photosynthesis) through a temporary decrease of its activity to a lasting reduction. Saturation curves of photosynthesis with respect to the partial pressure of CO₂ in the intercellular spaces indicated that application of ABA could produce three phenomena: i) a reduction of the initial slope of the saturation curve (which indicates a diminished carboxylation efficiency); ii) a reduction of the level of the CO₂-saturated rate of assimilation (which indicates a reduction of the ribulose-1,5-bisphosphate regeneration capacity); and iii) an increase of the CO₂ compensation point. Photosynthesis of isolated mesophyll cells was not affected by ABA treatments. Responses of the stomatal and photosynthetic apparatus were usually synchronous and often proportional to each other, with the result that the partial pressure of CO₂ in the intercellular spaces frequently remained constant in spite of large changes in conductance and assimilation rate. Guard cells and the photosynthetic apparatus were able to recover from effects of ABA applications while the ABA supply continued. Recovery was usually partial, in the case of the photosynthetic apparatus occasionally complete. Abscisic acid did not cause stomatal closure or decreases in the rate of photosynthesis when it was applied during a phase of stomatal opening and induction of photosynthesis that followed a transition from darkness to light.Abbreviations and symbols A rate of CO₂ assimilation - ABA (±)-abscisic acid - c _a partial pressure of CO₂ in the ambient air or in the gas supplied to the leaf chambers - c _i partial pressure of CO₂ in the intercellular spaces of a leaf - e _a partial pressure of H₂O in the air - g conductance for water vapor - J quantum flux - T ₁ leaf temperature     

Sensitivity to far-red radiation in stomata of Phaseolus vulgaris L.: Rhythmic effects on conductance and photosynthesis

The influence of far-red (FR; 700–800 nm) radiation on steady-state stomatal conductance and net photosynthesis in P. vulgaris has been studied. Whereas FR radiation alone was relatively ineffective, addition of FR to a background of white light (WL; predominantly 400–700 nm) resulted in increased stomatal conductance. Stomata exhibited a marked diurnal sensitivity to FR. The action maximum for enhancing stomatal conductance was near 714 nm. A combination of FR and infra-red (IR; >800 nm) enhanced net photosynthesis when added to a background of WL. When IR alone was added to WL, there was a net decrease in photosynthesis, indicating that it is the FR waveband which is responsible for the observed photosynthetic effects. Naturally occurring levels of FR radiation (235 mol·m^-2·s^-1) in vegetation-canopy shade enhanced net photosynthetic CO₂ gain by 28% when added to a background of 55 mol·m^-2·s^-1 WL.Abbreviations BL blue - FR far-red - IR infra-red - PAR photosynthetically active radiation - R red - WL white light     

Seasonal changes in net photosynthesis rates and photosynthetic capacity in leaves of Cistus salvifolius,a European mediterranean semi-deciduous shrub

Summary During five different periods between Nov. 1982 and Aug. 1983, the diurnal patterns exhibited in photosynthetic CO₂ uptake and stomatal conductance were observed under natural conditions on twigs of Cistus salvifolius, a Mediterranean semi-deciduous shrub which retains a significant proportion of its leaves through the summer drought. During the same periods, net photosynthesis at saturating CO₂ partial pressure was measured on the same twigs as a function of irradiance at different temperatures. From these data, photosynthetic capacity, defined here as the CO₂- and light-saturated net photosynthesis rate, was obtained as a function of leaf temperature. C. salvifolius is a winter growing species, shoot growth being initiated in Nov. and continuing through May. Photosynthetic capacity was quite high in Nov., March and June, exceeding 40 mol m^-2 s^-1 at optimum temperature. In Dec., photosynthetic capacity was somewhat reduced, perhaps due to low night-time temperatures (<5°C) during the measurement period. In Aug., capacity in oversummering shoots at optimum temperature fell to less than 8 mol m^-2 s^-1, due to water trees and perhaps leaf aging. Seasonal changes in maximal photosynthetic rates under ambient conditions were similar, and like those found in co-occurring evergreen sclerophylls. Like the evergreens, Cistus demonstrated considerable stomatal control of transpirational water loss, particularly in oversummering leaves. During each measurement period except Aug. when capacity was quite low, the maximum rates of net photosynthesis measured under ambient conditions were less than half the measured photosynthetic capacities at comparable temperatures, suggesting an apparent excess nitrogen investment in the photosynthetic apparatus.     

Photosynthetic characteristics of Cymbidium plantlet in vitro

The photosynthetic characteristics of the Cymbidium plantlet in vitro cultured on Hyponex-agar medium with 2% sucrose were determined based on the measurements of CO₂ concentration inside and outside of the culture vessels. The CO₂ measurements were made with a gas chromatograph at a PPF (photosynthetic photon flux) of 35, 102 and 226 mol m^-2 s^-1, a chamber air temperature of 15, 25 and 35°C and a CO₂ concentration outside the vessel of approximately 350, 1100 and 3000 ppm. The net photosynthetic rates were determined on individual plantlets and were expressed on a dry weight basis. The steady-state CO₂ concentration during the photoperiod was lower inside the vessel than outside the vessel at any PPF greater than 35 mol m^-2s^-1 and at any chamber air temperature. The photosynthetic response curves relating the net photosynthetic rate, PPF, and CO₂ concentration in the vessel and chamber air temperature were similar to those for Cymbidium plants grown outside and other C₃ plants grown outside under shade. The results indicate that CO₂ enrichment for the plantlets in vitro at a relatively high PPF would promote photosynthesis and hence the growth of chlorophyllous shoots/plantlets in vitro and that the plantlets in vitro would make photoautotrophic growth under environmental conditions favorable for photosynthesis.Abbreviations C_in CO₂ concentration in the culture vessel - C_out CO₂ concentration outside the vessel (in the culture room) - PPF photosynthetic photon flux     

Effects of low and elevated CO2 on C3 and C4 annuals

Abutilon theophrasti (C₃) and Amaranthus retroflexus (C₄), were grown from seed at four partial pressures of CO₂: 15 Pa (below Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current), and 70 Pa (future) in the Duke Phytotron under high light, high nutrient, and wellwatered conditions to evaluate their photosynthetic response to historic and future levels of CO₂. Net photosynthesis at growth CO₂ partial pressures increased with increasing CO₂ for C₃ plants, but not C₄ plants. Net photosynthesis of Abutilon at 15 Pa CO₂ was 70% less than that of plants grown at 35 Pa CO₂, due to greater stomatal and biochemical limitations at 15 Pa CO₂. Relative stomatal limitation (RSL) of Abutilon at 15 Pa CO₂ was nearly 3 times greater than at 35 Pa CO₂. A photosynthesis model was used to estimate ribulose-1,5-bisphosphate carboxylase (rubisco) activity (Vc_max), electron transport mediated RuBP regeneration capacity (J _max), and phosphate regeneration capacity (PiRC) in Abutilon from net photosynthesis versus intercellular CO₂ (A–C _i) curves. All three component processes decreased by approximately 25% in Abutilon grown at 15 Pa compared with 35 Pa CO₂. Abutilon grown at 15 Pa CO₂ had significant reductions in total rubisco activity (25%), rubisco content (30%), activation state (29%), chlorophyll content (39%), N content (32%), and starch content (68%) compared with plants grown at 35 Pa CO₂. Greater allocation to rubisco relative to light reaction components and concomitant decreases in J _max and PiRC suggest co-regulation of biochemical processes occurred in Abutilon grown at 15 Pa CO₂. There were no significant differences in photosynthesis or leaf properties in Abutilon grown at 27 Pa CO₂ compared with 35 Pa CO₂, suggesting that the rise in CO₂ since the beginning of the industrial age has had little effect on the photosynthetic performance of Abutilon. For Amaranthus, limitations of photosynthesis were balanced between stomatal and biochemical factors such that net photosynthesis was similar in all CO₂ treatments. Differences in photosynthetic response to growth over a wide range of CO₂ partial pressures suggest changes in the relative performance of C₃ and C₄ annuals as atmospheric CO₂ has fluctuated over geologic time.     

Light-emitting diodes as a light source for photosynthesis research

Light-emitting diodes (LED) can provide large fluxes of red photons and so could be used to make lightweight, efficient lighting systems for photosynthetic research. We compared photosynthesis, stomatal conductance and isoprene emission (a sensitive indicator of ATP status) from leaves of kudzu (Pueraria lobata (Willd) Ohwi.) enclosed in a leaf chamber illuminated by LEDs versus by a xenon arc lamp. Stomatal conductance was measured to determine if red LED light could sufficiently open stomata. The LEDs produced an even field of red light (peak emission 656±5 nm) over the range of 0–1500 mol m^-2 s^-1. Under ambient CO₂ the photosynthetic response to red light deviated slightly from the response measured in white light and stomatal conductance followed a similar pattern. Isoprene emission also increased with light similar to photosynthesis in white light and red light. The response of photosynthesis to CO₂ was similar under the LED and xenon arc lamps at equal photosynthetic irradiance of 1000 mol m^-2 s^-1. There was no statistical difference between the white light and red light measurements in high CO₂. Some leaves exhibited feedback inhibition of photosynthesis which was equally evident under irradiation of either lamp type. Photosynthesis research including electron transport, carbon metabolism and trace gas emission studies should benefit greatly from the increased reliability, repeatability and portability of a photosynthesis lamp based on light-emitting diodes.