Photosynthetic Gas Exchange and Chlorophyll a Fluorescence of Three Wild Soybean Species in Response to NaCl Treatments

Responses of photosynthetic gas exchange and chlorophyll (Chl) a fluorescence of three wild soybeans, Glycine soja, G. tomentella, and G. tabacina occurring in different habitats of Taiwan, to four NaCl treatments, 0S, LS, MS, and HS (i.e. 0, 17, 51, and 85 mM NaCl) were compared. In G. soja following exposure to NaCl treatment for one month, the photon saturated photosynthetic rate (P _N), the ratio of variable to maximum fluorescence (F_v/F_m), the quantum yield of photosystem 2 (_PS2), and the electron transport rate (ETR) decreased dramatically. These reductions increased with increasing concentration of NaCl treatment. Plants of MS and HS treatments did not survive after extending the treatment to two months. Reductions in P _N, _PS2, and ETR (but not in F_v/F_m) were found in G. tabacina after two months of exposure to MS and HS treatments, but the reduction was not as severe as that in G. soja. In G. tomentella, significant reductions in P _N and g _s were found only in HS plants after two months of treatment, but no significant differences in F_v/F_m, _PS2, and ETR were found among plants of the four treatments. Thus the three wild soybeans in Taiwan have differentiated in their photosynthetic susceptibility to salinity, G. tomentella being the least susceptible, G. soja the most sensitive, and G. tabacina the intermediate. Different mechanisms are attributed to the inhibition effect of salinity on photosynthesis of the three species.     

Gas Exchange and Chlorophyll Fluorescence of C3 and C4 Saltmarsh Species

Spartina maritima (Curtis) Fernald, Spartina densiflora Brong, Arthrocnemum perenne (Miller) Moss, and Arthrocnemum fruticosum (L.) Moq are very frequent halophytes on the coasts of SW Europe. The first two are perennial Gramineae with C₄ metabolism; the last two are perennial Chenopodiaceae with C₃ metabolism. Controlled garden experiments were carried out with the four species to compare their physiological response, i.e., water potential (Ψ), net photosynthetic rate (P_N), transpiration rate (E), stomatal conductance (g_s), intercellular CO₂ concentration (Ci), and chlorophyll fluorescence of photosystem (PS) 2 under saline and non-saline conditions. S. maritima behaves as an osmoconformer species, the other three as osmoregulators. In the four species, P_N, E, and g_s improved following freshwater irrigation. The variations in P_N might be related with biochemical changes (which appear not to affect PS2), but not with significant stomatal fluctuations, which are associated with a lower water use efficiency in the case of Arthrocnemum. The species were segregated into two groups (not depending on their C₃ or C₄ photosynthetic pathway), in relation with the topographic level of this species in natural conditions: the relative responses of P_N in S. maritima and A. perenne were lower than those of S. densiflora and A. fruticosum. The salt-tolerance index supports such segregation. S. densiflora demonstrated the best competitive possibilities against salt-tolerant glycophytes, with its more flexible response in saline or brackish environments, which explains its spreading along the rivers draining into the estuaries of the SW Iberian Peninsula. This revised version was published online in June 2006 with corrections to the Cover Date.     

Response of Spinach Leaves (Spinacia oleracea L.) to Ozone Measured by Gas Exchange,Chlorophyll a Fluorescence,Antioxidant Systems,and Lipid Peroxidation

Spinach (Spinacia oleracea L. cv. Clermont) leaves grown in open-top chambers and exposed to three different concentrations of ozone were measured for gas exchange, chlorophyll a fluorescence, antioxidant systems, and lipid peroxidation at the end of growing season. High O₃ concentration reduced F_v/F_m, indicating that the efficiency in the energy conversion of photosystem 2 (PS2) was altered. The rate of non-cyclic electron transport rate and the capacity to reduce the quinone pool were also affected. The development of non-photochemical quenching was not high enough to decrease the photon excess in the PS2. The limitation of photosynthetic activity was probably correlated with stomata closure and with an increase in intercellular CO₂ concentration. Under oxidative stress, superoxide dismutase (SOD) activity was stimulated in parallel with lipid peroxidation. We did not find any differences in the ascorbate (AsA) pool and ascorbate peroxidase (APX) or glutathione reductase (GR) activities between air qualities. Small, but similar responses were observed in spinach leaves exposed to ambient ozone concentration.     

转C4光合酶基因水稻的CO2交换和荧光特性

用转PEPC、PPDK、NADP-ME、PEPC+PPDK酶基因水稻(Oryza sativa L.)及原种为材料 ,研究了光合作用对光照、温度、CO2的响应和光抑制条件下的叶绿素荧光特性,结果如下: 1.转C4光合酶基因水稻的饱和光合速率比原种高,其中转PEPC、PEPC+PPDK双基因水稻的光饱和点比原种高200 μmol*m-2*s-1,饱和光合速率比原种分别高51.6%和 58.5%;转PEPC基因水稻的羧化效率比原种高49.3%,CO2补偿点降低26.2%;在高温(35 ℃)下,转PEPC基因水稻的光合速率比原种高17.5%.2.经光抑制处理8 d后,转PEPC、PEPC +PPDK酶基因水稻的PSⅡ光化学效率(Fv/Fm)和光化学猝灭(qP)下降20%- 30%,非光化学猝灭(qN)增加了约30%;但原种的Fv/Fm和qP下降了5 0%多,qN变化不明显,表明转C4光合基因水稻耐光抑制能力增强.这些结果为用生物技术提高水稻光合效率研究提供了新的依据和途径.     

Effects of Nitrogen Deficiency on Gas Exchange,Chlorophyll Fluorescence,and Antioxidant Enzymes in Leaves of Rice Plants

Gas exchange, chlorophyll (Chl) fluorescence, and contents of photosynthetic pigments, soluble proteins (ribulose-1,5-bisphosphate carboxylase/oxygenase, RuBPCO), and antioxidant enzymes were characterized in the fully expanded 6^th leaves in rice seedlings grown on either complete (CK) or on nitrogen-deficient nutrient (N-deficiency) solutions during a 20-chase period. Compared with the control plants, the lower photosynthetic capacity at saturation irradiance (P _max) was accompanied by an increase in intercellular CO₂ concentration (C_i), indicating that in N-deficient plants the decline in P _max was not due to stomatal limitation but due to the reduced carboxylation efficiency. The fluorescence parameters _PS2, F_v/F_m, electron transport rate (ETR), and q_P showed the same tendency as P _max in N-deficient plants. Correspondingly, a higher q_N paralleled the rise of the ratio of carotenoid (Car) to Chl contents. However, F_v/F_m was still diminished, suggesting that photoinhibition did occur in the photosystem 2 (PS2) reaction centres. In addition, the activities of antioxidant enzymes on a fresh mass basis were gradually lowered, leading to the aggravation of membrane lipid peroxidation with the proceeding N-deficiency. The accumulation of malonyldialdehyde resulted in the lessening of Chl and soluble protein content. Analyses of regression showed PS2 excitation pressure (1 - q_P) was linearly correlated with the content of Chl and inversely with soluble protein (particularly RuBPCO) content. There was a lag phase in the increase of PS2 excitation pressure compared to the decrease of RuBPCO content. Therefore, the increased excitation pressure under N-deficiency is probably the result of saturation of the electron transport chain due to the limitation of the use of reductants by the Calvin cycle. Rice plants responded to N-deficiency and high irradiance by decreasing light-harvesting capacity and by increasing thermal dissipation of absorbed energy.     

The quantum yield for CO2 uptake in C3 and C4 grasses

The quantum yield for CO₂ uptake was measured in C₃ and C₄ monocot species from several different grassland habitats. When the quantum yield was measured in the presence of 21% O₂ and 340 cm³ m^-3 CO₂, values were very similar in C₃ monocots, C₃ dicots, and C₄ monocots (0.045–0.056 mole CO₂ · mole^-1 quanta absorbed). In the presence of 2% O₂ and 800 cm³ m^-3 CO₂, enhancements of the quantum yield values occurred for the C₃ plants (both monocots and dicots), but not for C₄ monocots. A dependence of the quantum yield on leaf temperature was observed in the C₃ grass, Agropyron smithii, but not in the C₄ grass, Bouteloua gracilis, in 21% O₂ and 340 cm³ m^-3 CO₂. At leaf temperatures between 22–25°C the quantum yield values were approximately equal in the two species.     

Chlorophyll Fluorescence and Photosynthetic Gas Exchange Responses to Irradiance of Tree of Heaven (Ailanthus Altissima) in Contrasting Urban Environments

Sun-and shade-adapted plants of Ailanthus altissima utilized thermal-dissipative photoprotection (NPQ) across a range of photosynthetic photon flux densities (PPFD), with higher NPQ and lower maximum quantum yield of photosystem 2 photochemistry (F_v/F_m) in sun-adapted individuals, suggesting increased engagement of antennae-based quenching. Photosynthetic quantum requirements (Q_req; number of photons per CO₂) were similar in sun and shade plants, but were low and comparable to forest understory species. Diurnal measurements showed that PPFDs in both habitats were consistently above photosynthetic compensation irradiance, and frequently exceeded saturating values. In addition, sun- and shade-adapted individuals possessed stomata that tracked short-term fluctuations in PPFD. Thus A. altissima may be unique in that it couples high, shade-plant like photosynthetic efficiency with high photosynthetic capacity in high-irradiance, while stomatal attributes that optimize water use efficiency are maintained in the shade. These features may contribute to success of A. altissima in establishing in disturbance-prone urban systems, and facilitate its spread into more PPFD-limited and competitive natural ecosystems.     

Photorespiration in the context of Rubisco biochemistry,CO2 diffusion and metabolism

Photorespiratory metabolism is essential for plants to maintain functional photosynthesis in an oxygen‐containing environment. Because the oxygenation reaction of Rubisco is followed by the loss of previously fixed carbon, photorespiration is often considered a wasteful process and considerable efforts are aimed at minimizing the negative impact of photorespiration on the plant’s carbon uptake. However, the photorespiratory pathway has also many positive aspects, as it is well integrated within other metabolic processes, such as nitrogen assimilation and C₁ metabolism, and it is important for maintaining the redox balance of the plant. The overall effect of photorespiratory carbon loss on the net CO₂ fixation of the plant is also strongly influenced by the physiology of the leaf related to CO₂ diffusion. This review outlines the distinction between Rubisco oxygenation and photorespiratory CO₂ release as a basis to evaluate the costs and benefits of photorespiration.     

Effects of Salinity on Chlorophyll Fluorescence and Photosynthesis of Barley (Hordeum Vulgare L.) Grown Under a Triple-Line-Source Sprinkler System in the Field

In flag leaves of four cultivars of barley (Hordeum vulgare L.) grown in the field under a triple-line-source sprinkler system, that produces a linear soil salinity gradient, a decrease in net carbon dioxide assimilation rate (P_N) and stomatal conductance for water vapour (gs) was found. These changes were related to salinity tolerance at moderate salinity. With increasing salinity, P_N was saturated at low irradiances and stomatal frequencies increased. A decrease in photosystem 2 (PS2) efficiency was not found in the field after dark adaptation even at high salinity. Salinity induced only small decreases in the actual PS2 efficiency at midday steady-state photosynthesis, indicating that the photosynthetic electron transport was little affected by salinity. Therefore, using PS2 efficiency estimates in attached leaves is probably not a useful tool to screen barley genotypes grown under saline conditions in the field for salinity tolerance. In contrast, excised flag leaves from high salinity plots, once in the laboratory, exhibited a decrease in the variable to maximum chlorophyll fluorescence ratio as compared to excised leaves from control plants. On the other hand, the PN rate might allow for a good discrimination between tolerant and non-tolerant cultivars. This revised version was published online in June 2006 with corrections to the Cover Date.     

Simultaneous Measurements of Steady State Chlorophyll a Fluorescence and CO(2) Assimilation in Leaves: The Relationship between Fluorescence and Photosynthesis in C(3) and C(4) Plants

Rates of CO₂ assimilation and steady state chlorophyll a fluorescence were measured simultaneously at different intercellular partial pressures of CO₂ in attached cotton (Gossypium hirsutum L. cv Deltapine 16) leaves at 25°C. Electron transport activity for CO₂ assimilation plus photorespiration was calculated for these experiments. Under light saturating (1750 microeinsteins per square meter per second) and light limiting (700 microeinsteins per square meter per second) conditions there was a good correlation between fluorescence and the calculated electron transport activity at 19 and 200 millibars O₂, and between fluorescence and rates of CO₂ assimilation at 19 millibars but not 200 millibars O₂. The values of fluorescence measured at about 220 microbars intercellular CO₂ were not greatly affected by increasing O₂ from 19 to 800 millibars. Fluorescence increased with light intensity at any one intercellular CO₂ partial pressure. But the values obtained for fluorescence, expressed as a ratio of the maximum fluorescence obtained in DCMU-treated tissue, over the same range of CO₂ partial pressure at 500 microeinsteins per square meter per second were similar to those obtained at 1000 and 2000 microeinsteins per square meter per second. There were two phases in the observed correlation between fluorescence and calculated electron transport activity: an initial inverse relationship at low CO₂ partial pressures which reversed to a positive correlation at higher values of CO₂ partial pressures. Similar results were observed in the C₃ species Helianthus annuus L., Phaseolus vulgaris L., and Brassica chinensis. In all C₄ species (Zea mays L., Sorghum bicolor L., Panicum maximum Jacq., Amaranthus edulis Speg., and Echinochloa frumentacea [Roxb.] Link) examined changes in fluorescence were directly correlated with changes in CO₂ assimilation rates. The nature and the extent to which Q (primary quencher) and high-energy state (q_E) quenching function in determining the steady state fluorescence obtained during photosynthesis in leaves is discussed.     

Acclimation of photosynthesis, H2O2 content and antioxidants in maize (Zea mays) grown at sub-optimal temperatures

Maize plants were grown at 14, 18 and 20 °C until the fourth leaf had emerged. Leaves from plants grown at 14 and 18 °C had less chlorophyll than those grown at 20 °C. Maximal extractable ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was decreased at 14 °C compared with 20 °C, but the activation state was highest at 14 °C. Growth at 14 °C increased the abundance (but not the number) of Rubisco breakdown products. Phosphoenolpyruvate carboxylase (PEPC) activity was decreased at 14 °C compared with 20 °C but no chilling-dependent effects on the abundance of the PEPC protein were observed. Maximal extractable NADP-malate dehydrogenase activity increased at 14 °C compared with 20 °C whereas the glutathione pool was similar in leaves from plants grown at both temperatures. Foliar ascorbate and hydrogen peroxide were increased at 14 °C compared with 20 °C. The foliar hydrogen peroxide content was independent of irradiance at both growth temperatures. Plants grown at 14 °C had decreased rates of CO₂ fixation together with decreased quantum efficiencies of photosystem (PS) II in the light, although there was no photo-inhibition. Growth at 14 °C decreased the abundance of the D1 protein of PSII and the PSI psaB gene product but the psaA gene product was largely unaffected by growth at low temperatures. The relationships between the photosystems and the co-ordinate regulation of electron transport and CO₂ assimilation were maintained in plants grown at 14 °C.     

Photosynthetic Characteristics of the Palisade Mesophyll and Spongy Mesophyll in the CAM/C4 Intermediate Plant,Peperomia camptotricha*

Several photosynthetic parameters were examined in the different tissue layers of leaves from the recently characterized crassulacean acid metabolism/C₄ intermediate plant, Peperomia camptotricha (Nishio and Ting, 1987). Light appears to control the development of certain photosynthetic characteristics within the tissue layers, while factors other than light seem to dictate others. Analysis of the chlorophyll content (including P700) and chlorophyll-proteins indicated that more light harvesting chlorophylls were associated with reaction centers in the tissues that were shaded by overlying tissue. Electron transport activity and chlorophyll-protein analysis indicated that the ratio of photosystem I to photosystem II in the spongy mesophyll (the abaxial tissue type and hence most shaded) was elevated relative to the overlying chlorophyll-rich median palisade mesophyll. The elevated photosystem I relative to photosystem II in the spongy mesophyll of Peperomia camptotricha may be related to C₄ metabolism and an increased requirement for ATP.     

Yield responses of wild C3 and C4 crop progenitors to subambient CO2: a test for the role of CO2 limitation in the origin of agriculture

Limitation of plant productivity by the low partial pressure of atmospheric CO₂ (C_a) experienced during the last glacial period is hypothesized to have been an important constraint on the origins of agriculture. In support of this hypothesis, previous work has shown that glacial C_a limits vegetative growth in the wild progenitors of both C₃ and C₄ founder crops. Here, we present data showing that glacial C_a also reduces grain yield in both crop types. We grew four wild progenitors of C₃ (einkorn wheat and barley) and C₄ crops (foxtail and broomcorn millets) at glacial and postglacial C_a, measuring grain yield and the morphological and physiological components contributing to these yield changes. The C₃ species showed a significant increase in unthreshed grain yield of ~50% with the glacial to postglacial increase in C_a, which matched the stimulation of photosynthesis, suggesting that increases in photosynthesis are directly translated into yield at subambient levels of C_a. Increased yield was controlled by a higher rate of tillering, leading to a larger number of tillers bearing fertile spikes, and increases in seed number and size. The C₄ species showed smaller, but significant, increases in grain yield of 10–15%, arising from larger seed numbers and sizes. Photosynthesis was enhanced by C_a in only one C₄ species and the effect diminished during development, suggesting that an indirect mechanism mediated by plant water relations could also be playing a role in the yield increase. Interestingly, the C₄ species at glacial C_a showed some evidence that photosynthetic capacity was upregulated to enhance carbon capture. Development under glacial C_a also impacted negatively on the subsequent germination and viability of seeds. These results suggest that the grain production of both C₃ and C₄ crop progenitors was limited by the atmospheric conditions of the last glacial period, with important implications for the origins of agriculture.     

Photosynthetic Assimilation of Sun versus Shade Norway Spruce [Picea abies (L.) Karst] Needles Under the Long-Term Impact of Elevated CO2 Concentration

The long-term impact of elevated concentration of CO₂ on assimilation activity of sun-exposed (E) versus shaded (S) foliage was investigated in a Norway spruce stand [Picea abies (L.) Karst, age 14 years] after three years of cultivation in two domes with adjustable windows (DAW). One DAW was supplied with ambient air [AC, ca. 350 µmol(CO₂) mol^–1) and the second with elevated CO₂ concentration [EC = AC plus 350 µmol(CO₂) mol^–1]. The pronounced vertical profile of the photosynthetic photon flux density (PPFD) led to the typical differentiation of the photosynthetic apparatus between the shaded and sun needles. Namely, photon-saturated values of maximal net photosynthetic rate (P _Nmax) and apparent quantum yield () were significantly higher/lower for E-needles as compared with the S-ones. The prolonged exposure to EC was responsible for the apparent assimilatory activity stimulation observed mainly in deeply shaded needles. The degree of this stimulation decreases in the order: S-needles dense part > S-needles sparse part > E-needles dense part > E-needles sparse part. In exposed needles some signals on a manifestation of the acclimation depression of the photosynthetic activity were found. The long-term effect of EC was responsible for the decrease of nitrogen content of needles and for its smoother gradient between E- and S-needles. The obtained results indicate that the E- and S-foliage respond differently to the long-term impact of EC.     

Effects of Potato Virus YNTN Infection on Gas Exchange and Photosystem 2 Function in Leaves of Solanum tuberosum L.

Photosynthetic responses of potato (Solanum tuberosum L. cv. Chunzao) were examined during potato virus Y (PVY^NTN) infection. PVY^NTN infection significantly reduced net photosynthetic rate and stomatal conductance, but had little influence on intercellular CO₂ concentration. As the disease developed, the maximum carboxylation velocity of ribulose-1,5-bisphosphate carboxylase/oxygenase and the maximum electron transport rate contributing to ribulose-1,5-bisphosphate regeneration gradually decreased, followed by substantial reductions in the relative quantum efficiency of photosystem 2 (PS2) electron transport, the efficiency of excitation energy capture by open PS2 reaction centres, and photochemical quenching, but not in sustained photoinhibition. Thus PVY^NTN depressed photosynthesis mainly by interfering with the enzymatic processes in the Calvin cycle which resulted in a down-regulation of electron transport.     

Different Responses of Norway Spruce Needles from Shaded and Exposed Crown Layers to the Prolonged Exposure to Elevated CO2 Studied by Various Chlorophyll a Fluorescence Techniques

The acclimation depression of capacity of photon utilisation in photochemical reactions of photosystem 2 (PS2) can develop already after three months of cultivation of the Norway spruces (Picea abies [L.] Karst.) under elevated concentrations of CO₂ (i.e., ambient, AC, + 350 µmol(CO₂) mol^–1 = EC) in glass domes with adjustable windows. To examine the role that duration of EC plays in acclimation response, we determined pigment contents, rate of photosynthesis, and parameters of chlorophyll a fluorescence for sun and shade needles after three seasons of EC exposure. We found responses of shaded and exposed needles to EC. Whereas the shaded needles still profited from the EC and revealed stimulated electron transport, for the exposed needles the stimulation of both electron transport activity and irradiance saturated rate of CO₂ assimilation (P _Nmax) under EC already disappeared. No signs of the PS2 impairment were observed as judged from high values of potential quantum yield of PS2 photochemistry (F_V/F_M) and uniform kinetics of Q_A reoxidation for all variants. Therefore, the long-term acclimation of the sun-exposed needles to EC is not necessarily accompanied with the damage to the PS2 reaction centres. The eco-physiological significance of the reported differentiation between the responses of shaded and sun exposed needles to prolonged EC may be in changed contribution of the upper and lower crown layers to the production activity of the tree. Whereas for the AC spruces, P _Nmax of shaded needles was only less than 25 % compared to exposed ones, for the EC spruces the P _Nmax of shaded needles reached nearly 40 % of that estimated for the exposed ones. Thus, the lower shaded part of the crown may become an effective consumer of CO₂.     

The Nitrogen Use Efficiency of C(3) and C(4) Plants: II. Leaf Nitrogen Effects on the Gas Exchange Characteristics of Chenopodium album (L.) and Amaranthus retroflexus (L.)

The effect of leaf nitrogen (N) on the photosynthetic capacity and the light and temperature response of photosynthesis was studied in the ecologically similar annuals Chenopodium album (C₃) and Amaranthus retroflexus (C₄). Photosynthesis was linearly dependent on leaf N per unit area (N_a) in both species. A. retroflexus exhibited a greater dependence of photosynthesis on N_a than C. album and at any given N_a, it had a greater light saturated photosynthesis rate than C. album. The difference between the species became larger as N_a increased. These results demonstrate a greater photosynthetic N use efficiency in A. retroflexus than C. album. However, at a given applied N level, C. album allocated more N to a unit of leaf area so that photosynthetic rates were similar in the two species. Leaf conductance to water vapor increased linearly with N_a in both species, but at a given photosynthetic rate, leaf conductance was higher in C. album. Thus, A. retroflexus had a greater water use efficiency than C. album. Water use efficiency was independent of leaf N in C. album, but declined with decreasing N in A. retroflexus.     

Characteristics of Slow Induction Curve of Chlorophyll Fluorescence and CO2 Exchange for the Assessment of Plant Heat Tolerance at Various Levels of Light Intensity

The heat tolerance of wheat (Triticum aestivum L.) and radish (Raphanus sativus L. var. minor) cenoses exposed to elevated and damaging air temperatures (35°C for 20 h, 45°C for 7 h) under photoculture conditions at various levels of photosynthetically active radiation (PAR) was assessed by measuring characteristics of the slow induction curve of chlorophyll fluorescence at 682 and 734 nm and the CO₂ exchange rate. Irrespective of the illumination level, the exposure of the cenoses to 35°C did not induce irreversible changes in the plant photosynthetic apparatus. The lowest extent of damage to wheat and radish cenoses exposed to 45°C was observed at 150 W/m² of PAR, whereas the highest damage of the plants was observed at an illumination level that was close to the compensation point of the cenose photosynthesis (50–70 W/m² of PAR at air temperature of 24°C). Viability index proved to be the most sensitive characteristic, compared to other characteristics, which were determined by measuring the slow phase of fluorescence induction at 682 and 734 nm. In the cenoses studied, the pattern of changes in the viability index in response to a stress factor was close to the changes in the photosynthetic rate.     

High-Resolution Chlorophyll Fluorescence Imaging Serves as a Non-Invasive Indicator to Monitor the Spatio-Temporal Variations of Metabolism during the Day-Night Cycle and during the Endogenous Rhythm in Continuous Light in the CAM Plant Kalanchoë daigremontiana

Abstract: Chlorophyll fluorescence imaging is a powerful tool to monitor temporal and spatial dynamics of photosynthesis and photosynthesis-related metabolism. In this communication, we use high resolution chlorophyll fluorescence imaging techniques under strictly controlled conditions to quantify day courses of relative effective quantum yield (φ_PSII) of an entire leaf of the crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana at different light intensities. Careful interpretation of the combined gas exchange and fluorescence data, in combination with micro malate samples, allow the interpretation of underlying metabolic properties, such as leaf internal CO₂ concentration (ci_CO2) and energy demand of the cells. Spatial variations of φ_PSII, which occur as running wave fronts at the transition from phase III to phase IV of CAM, may reflect spatial differences of ci_CO2, which are preserved in the tightly packed mesophyll cells of K. daigremontiana. An endogenous rhythm is driven by a master switch which mediates between malate storage and malate release to and from the vacuole, however, using fluorescence techniques, four different metabolic states can be distinguished which also account for the activity of phosphoenolpyruvate carboxylase.     

Low growth temperatures modify the efficiency of light use by photosystem II for CO2 assimilation in leaves of two chilling-tolerant C4 species, Cyperus longus L. and Miscanthus x giganteus

Two C4 plants, Miscanthus x giganteus and Cyperus longus L., were grown at suboptimal growth temperatures and the relationships between the quantum efficiencies of photosynthetic electron transport through photosystem II (PSII) (PSII operating efficiency; Fq'/Fm') and CO2 assimilation (phiCO2) in leaves were examined. When M. x giganteus was grown at 10 degrees C, the ratio of the PSII operating efficiency to phiCO2 increased relative to that found in leaves grown at 14 and 25 degrees C. Similar increases in the Fq'/Fm': phiCO2 occurred in the leaves of two C. longus ecotypes when the plants were grown at 17 degrees C, compared to 25 degrees C. These elevations of Fq'/Fm': phiCO2 at low growth temperatures were not attributable to the development of anthocyanins, as has been suggested for maize, and were indicative of the operation of an alternative sink to CO2 assimilation for photosynthetic reducing equivalents, possibly oxygen reduction via a Mehler reaction, which would act as a mechanism for protection of PSII from photoinactivation and damage. Furthermore, in M. x giganteus grown at 10 degrees C, further protection of PSII was effected by a 20-fold increase in zeaxanthin content in dark-adapted leaves, which was associated with much higher levels of non-photochemical quenching of excitation energy, compared to that observed in leaves grown at 14 and 25 degrees C. These differences may explain the long growing season and remarkable productivity of this C4 plant in cool climates, even in comparison to other C4 species such as C. longus, which occur naturally in such climates.