Characteristics of a photorespiratory mutant of barley (Hordeum vulgare L.) deficient in phosphogly collate phosphatase

A barley mutant RPr84/90 has been isolated by selecting for plants which grow poorly in natural air, but normally in air enriched to 0.8% CO₂. After 5 minutes of photosynthesis in air containing¹⁴CO₂ this mutant incorporated 26% of the¹⁴C carbon into phosphoglycollate, a compound not normally labelled in wild type (cv. Maris Mink) leaves.The activity of phosphoglycollate phosphatase (EC 3.1.1.18) was 1.2 nkat mg^–1 protein at 30°C in RPr 84/90 compared to 19.2 nkat mg^–1 protein in the wild-type leaves. Phosphoglycollate phosphatase activity was not detected after protein separation by electrophoresis of leaf extracts from the mutant on polyacrylamide gels; on linear 5% acrylamide gels three bands with enzyme activity were separated from extracts of wild type plants. Gradient gel electrophoresis followed by activity staining showed two bands in Maris Mink tracks of MW 86,000 and 96,000, but no bands in 84/90. This is the first report of isozymes of phosphoglycollate phosphatase in barley which were absent in the mutant extracts. Our results confirm an earlier report of isozymes of this phosphatase in Phaseolus vulgaris [18].The photosynthetic rate of RPr 84/90 in 1% O₂, 350 l CO₂ l^–1 was 9–12 mg CO₂ dm^–2 h^–1 at 20°C, whereas the wild-type rate was 27–29 mg CO₂ dm^–2 h^–1 at 20°C. In 21% O₂, 350 l CO₂ l^–1 the rate was 2–3 mg CO₂ dm^–2 h^–1 in the mutant and 20 mg CO₂ dm^–2 h^–1 in the wild type.Genetic analysis has shown that the mutation segregates as a single recessive nuclear gene.     

C4 photosynthetic carbon metabolism in the leaves of aromatic tropical grasses — I. Leaf anatomy,CO2 compensation point and CO2 assimilation

A few species of Cymbopogon and Vetiveria are potentially important tropical grasses producing essential oils. In the present study, we report on the leaf anatomy and photosynthetic carbon assimilation in five species of Cymbopogon and Vetiveria zizanioides. Kranz-type leaf anatomy with a centrifugal distribution of chloroplasts and exclusive localization of starch in the bundle sheath cells were common among the test plants. Besides the Kranz leaf anatomy, these grasses displayed other typical C₄ characteristics including a low (0–5 µl/l) CO₂ compensation point, lack of light saturation of CO₂ uptake at high photon flux densities, high temperature (35°C) optimum of net photosynthesis, high rates of net photosynthesis (55–67 mg CO₂ dm^-2 leaf area h^-1), little or no response of net photosynthesis to atmospheric levels of O₂ and high leaf ¹³C/¹²C ratios. The biochemical studies with ¹⁴CO₂ indicated that the leaves of the above plant species synthesize predominantly malate during short term (5 s) photosynthesis. In pulse-chase experiments it was shown that the synthesis of 3-phosphoglycerate proceeds at the expense of malate, the major first formed product of photosynthesis in these plant species.     

Light quality affects photosynthesis and leaf anatomy of birch plantlets in vitro

Cultures in vitro of Betula pendula Roth were subjected to light of different spectral qualities. Photosynthetic capacity was highest when the plantlets were exposed to blue light (max recorded photosynthesis, 82 mol CO₂ dm^–2 h^–1) and lowest when irradiated with light high in red and/or far-red wave lengths (max recorded photosynthesis, 40 mol CO₂ dm^–2 h^–1). Highest chlorophyll content (2.2 mg dm^–2 leaf area) was found in cultures irradiated with blue light, which also enhanced the leaf area. Morphometric analysis of light micrographs showed that the epidermal cell areas were largest in plantlets subjected to blue light and smallest in those subjected to red light. Morphometric analysis of electron micrographs of palisade cells, showed that the functional chloroplast area was largest in chloroplasts of leaves subjected to blue light and smallest in those exposed to red light. We suggest that light quality affects photosynthesis both through effects on the composition of the photosynthetic apparatus and on translocation of carbohydrates from chloroplasts.     

Regulation of photosynthetic rates of submerged rooted macrophytes

Summary Fourteen temperate, submerged macrophytes were cultivated in the laboratory at high DIC levels (3.3–3.8 mM), 10.4–14.4 mol photons (PAR) m^-2 d^-1 and 15°C. Photosynthesis at photosaturation ranged between 0.59 and 17.98 mg O₂ g^-1 DW h^-1 at ambient pH (8.3) and were markedly higher between 1.76 and 47.11 mg O₂ g^-1 DW h^-1 at pH 6.5 under elevated CO₂ concentrations. Photosynthetic rates were significantly related to both the relative surface area and the chlorophyll content of the leaves. Consequently, the photosynthetic rate was much less variable among the species when expressed per surface area and chlorophyll content instead of dry mass. The chlorophyll content was probably a main predictor of photosynthesis of submerged leaves because of the direct relationship of chlorophyll to the light harvesting capacity and/or a coupling to the capacity for photosynthetic electron transport and carboxylation. A comparison with terrestrial leaves characterized the submerged leaves by their low chlorophyll concentrations and low photosynthetic rates per surface area due to the thin leaves. Photosynthetic rates per chlorophyll content in submerged leaves at CO₂ saturation, however, were at the same level as photosynthesis in terrestrial leaves measured at ambient CO₂ when appropriate corrections were made for differences in incubation temperature.     

Diurnal and Seasonal Patterns of Photosynthesis and Respiration by Stems of Populus tremuloides Michx

The photosynthetic and respiratory rates of 5- to 7-year-old aspen stems (Populus tremuloides Michx.) were monitored in the field for 1 year to determine the seasonal patterns. The stem was not capable of net photosynthesis, but the respiratory CO₂ loss from the stem was reduced by 0 to 100% depending on the time of year and the level of illumination as a result of bark photosynthesis. The monthly dark respiratory rate ranged from 0.24 mg CO₂/dm²· hr in January to a maximum 7.4 mg CO₂/dm²· hr in June. Individual measurements ranged from 0.02 mg CO₂/dm²· hr in February to 12.3 mg CO₂/dm²· hr in June. Gross photosynthesis followed a pattern similar to the dark respiratory rate. The mean monthly rate was highest in June (1.65 mg CO₂/dm²· hr) and lowest in December (0.02 mg CO₂/dm²· hr). Individual measurements ranged from 0.0 mg CO₂/dm²· hr in winter to 5.5 mg CO₂/dm²· hr in July.     

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)     

Effects of Irradiance during Growth on Adaptive Photosynthetic Characteristics of Velvetleaf and Cotton

We grew velvetleaf (Abutilon theophrasti Medic.) and cotton (Gossypium hirsutum L. var. Stoneville 213) at three irradiances and determined the photosynthetic responses of single leaves to a range of six irradiances from 90 to 2000 μeinsteins m⁻²sec⁻¹. In air containing 21% O₂, velvetleaf and cotton grown at 750 μeinsteins m⁻²sec⁻¹ had maximum photosynthetic rates of 18.4 and 21.9 mg of CO₂ dm⁻²hr⁻¹, respectively. Maximum rates for leaves grown at 320 and 90 μeinsteins m⁻²sec⁻¹ were 15.3 and 10.3 mg of CO₂ dm⁻²hr⁻¹ in velvetleaf and 12 and 6.7 mg of CO₂ dm⁻²hr⁻¹ in cotton, respectively. In 1 O₂, maximum photosynthetic rates were 1.5 to 2.3 times the rates in air containing 21% O₂, and plants grown at medium and high irradiance did not differ in rate. In both species, stomatal conductance was not significantly affected by growth irradiance. The differences in maximum photosynthetic rates were associated with differences in mesophyll conductance. Mesophyll conductance increased with growth irradiance and correlated positively with mesophyll thickness or volume per unit leaf area, chlorophyll content per unit area, and photosynthetic unit density per unit area. Thus, quantitative changes in the photosynthetic apparatus help account for photosynthetic adaptation to irradiance in both species. Net assimilation rates calculated for whole plants by mathematical growth analysis were closely correlated with single-leaf photosynthetic rates.     

Contrasting Effects of Carbon Dioxide and Irradiance on the Acclimation of Photosynthesis in Developing Soybean Leaves

Leaves developed at high irradiance (I) often have higher photosynthetic capacity than those developed at low I, while leaves developed at elevated CO₂ concentration [CO₂] often have reduced photosynthetic capacity compared with leaves developed at lower [CO₂]. Because both high I and elevated [CO₂] stimulate photosynthesis of developing leaves, their contrasting effects on photosynthetic capacity at maturity suggest that the extra photosynthate may be utilized differently depending on whether I or [CO₂] stimulates photosynthesis. These experiments were designed to test whether relationships between photosynthetic income and the net accumulation of soluble protein in developing leaves, or relationships between soluble protein and photosynthetic capacity at full expansion differed depending on whether I or [CO₂] was varied during leaf development. Soybean plants were grown initially with a photosynthetic photon flux density (PPFD) of 950 µmol m^–2 s^–1 and 350 µmol [CO₂] mol^–1, then exposed to [CO₂] ranging from 135 to 1400 µmol mol^–1 for the last 3 d of expansion of third trifoliolate leaves. These results were compared with experiments in which I was varied at a constant [CO₂] of 350 µmol mol^–1 over the same developmental period. Increases in area and dry mass over the 3 d were determined along with daily photosynthesis and respiration. Photosynthetic CO₂ exchange characteristics and soluble protein content of leaves were determined at the end of the treatment periods. The increase in leaflet mass was about 28 % of the dry mass income from photosynthesis minus respiration, regardless of whether [CO₂] or I was varied, except that very low I or [CO₂] increased this percentage. Leaflet soluble protein per unit of area at full expansion had the same positive linear relationship to photosynthetic income whether [CO₂] or I was varied. For variation in I, photosynthetic capacity varied directly with soluble protein per unit area. This was not the case for variation in [CO₂]. Increasing [CO₂] reduced photosynthetic capacity per unit of soluble protein by up to a factor of 2.5, and photosynthetic capacity exhibited an optimum with respect to growth [CO₂]. Thus CO₂ did not alter the relationship between photosynthetic income and the utilization of photosynthate in the net accumulation of soluble protein, but did alter the relationship between soluble protein content and photosynthetic characteristics in this species.     

Photosynthetic Metabolism and Activity of Carboxylating Enzymes in Emergent,Floating, and Submersed Leaves of Hydrophytes

Radioisotope techniques were used to compare photosynthetic CO₂ fixation, activities of carboxylating enzymes, and the composition of photosynthates in 42 species of aquatic plants (emergent, floating, and submersed hydrophytes) collected from rivers Sysert' and Iset' in Sverdlovsk oblast (Russia). The submersed leaves, in comparison with the emergent and floating leaves, featured lower rates of potential photosynthesis (by 2.2 mg CO²/(dm² h) on average), low content of the fraction I protein, and low activity of Rubisco and phosphoenolpyruvate carboxylase (PEPC). The averaged activities of Rubisco and PEPC were diminished in submersed leaves by 10 and 1 mg/(dm² h), respectively. Different hydrophyte groups showed similar composition of assimilates accumulated after 5-min photosynthesis and did not differ in this respect from terrestrial plants. However, the incorporation of ¹⁴C into sucrose and starch in submersed leaves (30 and 9% of total labeling, respectively) was lower than in emergent and floating leaves (45 and 15%, respectively). At the same time, the incorporation of ¹⁴C into C₄ acids (malate and aspartate) was 1.5 times higher in submersed leaves than in other leaf types. Analysis of leaf differentiation, the Rubisco/PEPC activity ratio, the PEPC activity, and the composition of primary photosynthates in the pulse–chase experiments revealed no evidence of the C₄ effect in the submersed hydrophytes examined. The adaptation of hydatophytes to specific conditions of an aquatic environment was structurally manifested in the reduction (by a factor of 3–5) in the number of chloroplasts per 1 cm² leaf area. This small number of chloroplasts was responsible for low photosynthetic rates in submersed leaves, although metabolic activities of individual chloroplasts were similar for all three hydrophyte groups.     

The Effect of High Temperatures on the Photosynthetic Activity of Intact Barley Leaves at Low and High Irradiance

Light curves of CO₂ fixation by barley seedling leaves preliminarily heated at 30–43°C for 5 min were measured. The slope of the linear part of the light curve decreased after leaf heating at temperatures above 35°C; whereas, at a high light level, the photosynthesis rate decreased only at temperatures of 40°C and higher. The linear relationships between the photosynthetic CO₂-fixation rate and a photon flux density up to 1400 mol/(m² s) were found in leaves preheated at 42°C; this indicates the strong nonphotochemical dissipation of absorbed light quanta. The lowering of the oxygen concentration from 21 to 1% led to a CO₂ fixation maximum quantum yield and a photosynthesis-rate increase at the highest light intensity in leaves preheated at temperatures above 40°C as compared to the control leaves. Nevertheless, the linear relationship between the photosynthetic CO₂ fixation and the light intensity was found in leaves heated at 42°C at O₂ concentrations of both 21 and 1%. The latter fact suggests that the proton gradient of the thylakoid membrane, which causes an increase in the nonphotochemical dissipation of the quanta absorbed, could also be formed due to the cyclic electron transport over photosystem I.     

Effect of temperature on net CO2 assimilation and photosystem II quantum yield of electron transfer of French bean (Phaseolus vulgaris L.) leaves during drought stress

The effect of leaf temperature on stomatal conductance and net CO₂ uptake was studied on French bean (Phaseolus vulgaris L.) using either dehydrated attached leaves (25–40% water deficit) or cut leaves supplied with 10^–4 M abscisic acid (ABA) solution to the transpiration stream. Decreasing leaf temperature caused stomatal opening and increased net CO₂ uptake (which was close to zero at around 25° C) to a level identical to that of control leaves (without water deficit) at around 15° C. (i) The ABA effect on stomatal closure was modulated by temperature and, presumably, ABA is at least partly responsible for stomatal closure of french bean submitted to a drought stress. (ii) For leaf temperatures lower than 15° C, net CO₂ uptake was no longer limited by water deficit even on very dehydrated leaves. This shows that dehydrated leaves retain a substantial part of their photosynthetic capacity which can be revealed at normal CO₂ concentrations when stomata open at low temperature. In contrast to leaves fed with ABA, decreasing the O₂ concentration from 21% to 1% O₂ did not increase either the rate of net CO₂ uptake or the thermal optimum for photosynthesis of dehydrated leaves. The quantum yield of PSII electron flow (measured by F/F_m) was lower in 1% O₂ than in 21% O₂ for each leaf pretreatment given (non-dehydrated leaves, dehydrated leaves, and leaves fed with ABA) even within a temperature range in which leaf photosynthesis at normal CO₂ concentration was the same in these two O₂ concentrations. It is concluded that this probably indicates an heterogeneity of photosynthesis, since this difference in quantum yield disappears when using high CO₂ concentrations during measurements.Abbreviations and Symbols ABA abscisic acid - F_m maximum chlorophyll fluorescence - F difference between steady-state chlorophyll fluorescence and F_m - PPFD photosynthetic photon flux density We would like to thank Dr. J.-M. Briantais (Laboratoire d'écologie végétale, Orsay, France) for help during fluorescence measurements and Ms. J. Liebert for technical assistance.     

Leaf cavity CO2 concentrations and CO2 exchange in onion,Allium cepa L.

Onion (Allium cepa L.) plants were examined to determine the photosynthetic role of CO₂ that accumulates within their leaf cavities. Leaf cavity CO₂ concentrations ranged from 2250 L L^–1 near the leaf base to below atmospheric (<350 L L^–1) near the leaf tip at midday. There was a daily fluctuation in the leaf cavity CO₂ concentrations with minimum values near midday and maximum values at night. Conductance to CO₂ from the leaf cavity ranged from 24 to 202 mol m^–2 s^–1 and was even lower for membranes of bulb scales. The capacity for onion leaves to recycle leaf cavity CO₂ was poor, only 0.2 to 2.2% of leaf photosynthesis based either on measured CO₂ concentrations and conductance values or as measured directly by ¹⁴CO₂ labeling experiments. The photosynthetic responses to CO₂ and O₂ were measured to determine whether onion leaves exhibited a typical C₃-type response. A linear increase in CO₂ uptake was observed in intact leaves up to 315 L L^–1 of external CO₂ and, at this external CO₂ concentration, uptake was inhibited 35.4±0.9% by 210 mL L^–1 O₂ compared to 20 mL L^–1 O₂. Scanning electron micrographs of the leaf cavity wall revealed degenerated tissue covered by a membrane. Onion leaf cavity membranes apparently are highly impermeable to CO₂ and greatly restrict the refixation of leaf cavity CO₂ by photosynthetic tissue.Abbreviations C_a external CO₂ concentration - C_i intercellular CO₂ concentration - CO₂ compensation concentration - PPFR photosynthetic photon fluence rate     

Phytoplankton primary production in a eutrophic cooling water pond

The seasonal variation of phytoplankton photosynthesis was measured with ¹⁴C-method in a warmed ice-free pond in central Finland. Simultaneously with in situ measurements the photosynthesis was also measured in an incubator with different water temperatures and constant light (ca. 16 W m^–2). The total annual photosynthesis was 57.2 C m^–2 a^–1. The portion of the winter and spring production of the annual photosynthesis was 18.4%, that of the autumn production ws 17.4%. Thus 64.3% of the total annual phytoplankton photosynthesis occurred in the three summer months. The range of the daily integrated photosynthesis per unit area was 1.9—563 mg C m^–2d^–1. The photosynthetic rate per unit chlorophyll a varied in situ from 0.94 to 33.1 mg C (mg chl. a)^–1 d^–1. The highest value was measured in the beginning of July and the lowest in mid-January. The photosynthetic rate increased in situ exponentially with increasing water temperature. In the incubator the highest photosynthetic rate values were also found in July and August (at+20 °C) when the phytoplankton population was increasing and the minimum values occurred after every diatom maximum both in spring and autumn. Light was a limiting factor for photosynthesis from September to Mid-January, low water temperature was a limiting factor from late January through May. The efficiency of the photosynthesis varied between 0.1 and 0.7% of P.A.R. According to the incubator experiments the Q₁₀ values for the photosynthesis were 2.45 and 2.44 for the winter population between 1 and 10° C and for the summer population between 5 and 15° C, respectively, but the Q₁₀ values decrease at the higher temperatures. The main effect of the warm effluents on the yearly photosynthesis was the increase of production in spring months due to the lack of ice cover. However, the increase of total annual phytoplankton photosynthesis was only ca. 10–15%, because the water temperature was during the spring months below 10° C.     

High photosynthetic rate of a chlorophyll mutant of cotton

In a chlorophyll mutant (virescent) and wild-type cotton (Gossypium hirsutum L.), a number of photosynthetic parameters have been measured and compared with those published for other chlorophyll mutants. (a) The photosynthetic rates at 230 w/m² (400-700 nm) from a tungsten lamp were 36.8 mg CO₂ fixed/dm²·hr (virescent) and 39.5 mg CO₂ fixed/dm²·hr (wild-type). On a chlorphyll basis, the photosynthetic rates were 36.8 and 12.1 mg CO₂ fixed/mg chl·hr, respectively. (b) The photosynthetic rates at 13 w/m² (400-700 nm) from a tungsten source were 7.1 mg CO₂ fixed/dm²·hr (virescent) and 7.4 mg CO₂ fixed/dm²·hr (wild-type). On a chlorophyll basis, the photosynthetic rates were 6.0 and 1.4 mg CO₂ fixed/mg chl·hr, respectively. (c) The chlorophyll a/b ratios of the virescent and wild-type leaves were 3.3 and 4.1 (d) The chlorophyll/carotenoid ratios for the virescent and wild-type leaves were 3.2 and 7.3, respectively. (e) The photosynthetic carbon metabolism of the chlorophyll mutant was through the reductive pentose phosphate cycle. (f) The CO₂ compensation points for the virescent and wild-type plants were similar. (g) The mutant and wild-type leaves have the same quantum yield in the red part of the visible spectrum, but the virescent leaves have a lower quantum yield in the blue part of the spectrum. (h) Virescent and wild-type leaves contain similar levels on a protein basis of several reductive pentose phosphate cycle enzymes.     

In situ CO2 gas-exchange in fruits of a tropical tree,Durio zibethinus Murray

We examined the in situ CO₂ gas-exchange of fruits of a tropical tree, Durio zibethinus Murray, growing in an experimental field station of the Universiti Pertanian Malaysia. Day and night dark respiration rates were exponentially related to air temperature. The temperature dependent dark respiration rate showed a clockwise loop as time progressed from morning to night, and the rate was higher in the daytime than at night. The gross photosynthetic rate was estimated by summing the rates of daytime dark respiration and net photosynthesis. Photosynthetic CO₂ refixation, which is defined as the ratio of gross photosynthetic rate to dark respiration rate in the daytime, ranged between 15 and 45%. The photosynthetic CO₂ refixation increased rapidly as the temperature increased in the lower range of air temperature T _c (T _c <28.5 °C), while it decreased gradually as the temperature increased in the higher range (T _c 28.5 °C). Light dependence of photosynthetic CO₂ refixation was approximated by a hyperbolic formula, where light saturation was achieved at 100 mol m^–2 s^–1 and the asymptotic CO₂ refixation was determined to be 37.4%. The estimated gross photosynthesis and dark respiration per day were 1.15 and 4.90 g CO₂ fruit^–1, respectively. Thus the CO₂ refixation reduced the respiration loss per day by 23%. The effect of fruit size on night respiration rate satisfied a power function, where the exponent was larger than unity.     

Ecological studies of Chloroflexis,a gliding photosynthetic bacterium

Summary Chloroflexis, a gliding, filamentous, photosynthetic bacterium, is present in the stratified algal-bacterial mats which occur in the 50°–70°C temperature range of alkaline hot spring effluents. The organism is in association with the alga in the upper, algal layer, and also forms thick, orange mats beneath the algal layer. Natural populations of Chloroflexis from these mats demonstrated light-stimulated uptake of some ¹⁴C-labelled organic compounds. Photosynthetic ¹⁴CO₂ fixation by natural samples of Chloroflexis was investigated with respect to temperature, light intensity and mat depth. Bacterial photosynthesis was determined in samples in which algae were present by use of the inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Bacterial photosynthesis was maximal at depths down to about 3 mm and then decreased rapidly to very low levels at greater depths. The greatest amounts of bacteriochlorophyll pigments were also concentrated in the top 3–4 mm of the mat. The optimum light intensity for bacterial photosynthesis (about 400 ft-c) was considerably lower than the normal summer light intensity at the surface of the mat (5000-8000 ft-c).The temperature optima for photosynthesis by the bacterial component of natural mat samples from several sites of different temperatures in a hot spring thermal gradient were determined. Temperature optima approximated the environmental temperatures, indicative of the occurrence of strains of Chloroflexis adapted to different temperatures. Although bacterial standing crop was greatest in the temperature range 50°–55°C, maximum photosynthetic efficiency was observed at about 45°C. Sulfide was stimulatory to photosynthetic ¹⁴CO₂ fixation by naturally occurring populations of Chloroflexis under field conditions. These data are consistent with the hypothesis that Chloroflexis may utilize sulfide as an electron donor for photosynthetic CO₂ reduction. However, it is also likely that Chloroflexis grows photoheterotrophically in these mats, obtaining organic compounds from algal excretory products.     

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.     

Cold-hardening results in increased activity of enzymes involved in carbon metabolism in leaves of winter rye (Secale cereale L.)

Light- and CO₂-saturated photosynthesis of nonhardened rye (Secale cereale L. cv. Musketeer) was reduced from 18.10 to 7.17 mol O₂·m^–2·s^–1 when leaves were transferred from 20 to 5°C for 30 min. Following cold-hardening at 5°C for ten weeks, photosynthesis recovered to 15.05 mol O₂·m^–2·s^–1,comparable to the nonhardened rate at 20°C. Recovery of photosynthesis was associated with increases in the total activity and activation of enzymes of the photosynthetic carbon-reduction cycle and of sucrose synthesis. The total hexose-phosphate pool increase by 30% and 120% for nonhardened and cold-hardened leaves respectively when measured at 5°C. The large increase in esterified phosphate in coldhardened leaves occurred without a limitation in inorganic phosphate supply. In contrast, the much smaller increase in esterified phosphate in nonhardened leaves was associated with an inhibition of ribulose-1,5-bisphosphate carboxylase/oxygenase and sucrose-phosphate synthase activation. It is suggested that the large increases in hexose phosphates in cold-hardened leaves compensates for the higher substrate threshold concentrations needed for enzyme activation at low temperatures. High substrate concentrations could also compensate for the kinetic limitations imposed by product inhibition from the accumulation of sucrose at 5°C. Nonhardened leaves appear to be unable to compensate in this fashion due to an inadequate supply of inorganic phosphate.Abbreviations DHAP dihydroxyacetone phosphate - Fru6P fructose-6-phosphate - Fru 1,6BP fructose-1,6-bisphosphate - Fru1,6BPase fructose-1,6-bisphosphatase - Glc6P glucose-6-phosphate - PGA 3-phosphoglycerate - PPFD photosynthetic photon flux density - CH cold-hardened rye grown at 5°C - NH nonhardened rye grown at 24°C - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - SPS sucrose-phosphate synthase - UDPGlc uridine 5-diphosphoglucose This work was supported by operating grants from the Swedish Natural Sciences Research Council to G.Ö. and P.G.     

The Regulatory Role of Inflorescence Leaves in Fruit-setting by Sweet Orange (Citrus sinensis)

Inflorescence leaves improve fruit set on sweet orange trees. We sought an explanation for this effect in terms of carbon demand by developing fruit versus potential supply from adjacent leaves. Our assessment was based upon measurements of fruit growth, leaf photosynthesis and ¹⁴C distribution patterns in plants grown under controlled conditions. Leafy inflorescences had sufficient foliar surface (1.24 dm²) and photosynthetic capacity (CO₂ 10.1 mg · dm^-2· h^-1) to support early development of fruits on the same shoot, and to make a substantial contribution towards subsequent growth. ¹⁴C-assimilates derived from new leaves were distributed towards adjacent fruit which showed strong competition for labelled substrate. By contrast, fruit borne on leafless inflorescences had to obtain all their assimilates from older leaves whose photosynthetic capacity (CO₂ 3.5–4.6 mg · dm^-2· h^-1) and individual area (0.2 dm²) were generally insufficient to wholly sustain fruit growth, so that a large number of old-leaves were needed; these fruit would be more susceptible to competition from other sinks.     

Photosynthetic performance of vegetative and reproductive structures of green hellebore (<Emphasis Type="Italic">Helleborus viridis</Emphasis> L. agg.)

Photosynthetic irradiance response of vegetative and reproductive structures of the green-flowered deciduous perennial green hellebore was studied by the comparative use of chlorophyll (Chl) fluorescence techniques and gas exchange measurements. All the Chl-containing organs (leaves, sepals, stalks, and fruits) examined were photosynthetically active showing high intrinsic efficiencies of photosystem 2 (F_v/F_m: 0.75–0.79) after dark adaptation. Even in the smaller fertile and sterile parts of the flower (nectaries and anthers) a remarkable photosynthetic competence was detected. With increasing photon flux densities (PFD) electron transport rates, actual quantum yields, and photochemical quenching coefficients of the main photosynthetic organs decreased in the order: leaf>sepal>fruit>stalk. At moderate to high PFDs the sepals achieved maximum electron transport rates corresponding to about 80 % of concomitant mature leaves. In contrast, maximum net photosynthetic rate of the sepals [2.3 mol(CO₂) m^–2 s^–1] were less than one fourth of the leaves [10.6 mol(CO₂) m^–2 s^–1]. This difference is explained by a 70–80 % lower stomatal density of sepals in comparison to leaves. As the basal leaves emerge late during fruit development, the photosynthetically active sepals are a major source of assimilates, contributing more than 60 % of whole-plant CO₂ gain in early spring. The ripening dehiscent fruits are characterized by an effective internal re-fixation of the respirational carbon loss and thus additionally improve the overall carbon budget.