Interspecific difference in the photosynthesis–nitrogen relationship: patterns,physiological causes,and ecological importance

The photosynthesis–nitrogen relationship is significantly different among species. Photosynthetic capacity per unit leaf nitrogen, termed as photosynthetic nitrogen-use efficiency (PNUE), has been considered an important leaf trait to characterise species in relation to their leaf economics, physiology, and strategy. In this review, I discuss (1) relations between PNUE and species ecology, (2) physiological causes and (3) ecological implications of the interspecific difference in PNUE. Species with a high PNUE tend to have high growth rates and occur in disturbed or high productivity habitats, while those with a low PNUE occur in stressful or low productivity habitats. PNUE is an important leaf trait that correlates with other leaf traits, such as leaf mass per area (LMA) and leaf life span, irrespective of life form, phylogeny, and biomes. Various factors are involved in the interspecific difference. In particular, nitrogen allocation within leaves and the mesophyll conductance for CO₂ diffusion are important. To produce tough leaves, plants need to allocate more biomass and nitrogen to make thick cell walls, leading to a reduction in the mesophyll conductance and in nitrogen allocation to the photosynthetic apparatus. Allocation of biomass and nitrogen to cell walls may cause the negative relationship between PNUE and LMA. Since plants cannot maximise both PNUE and leaf toughness, there is a trade-off between photosynthesis and persistence, which enables the existence of species with various leaf characteristics on the earth.     

The carboxylase activity of Rubisco and the photosynthetic performance in aquatic plants

Summary Activated carboxylase activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), as well as photosynthetic rates were measured for 42 species of freshwater and marine macrophytes. While the carboxylase activity varied greatly among the species investigated (0.2–12.5 mol CO₂ mg^–1 chlorophyll min^–1), the submersed freshwater plants showed significantly lower activities than emergent, floating leaved or secondary submersed forms. The variability in photosynthetic rates correlated with the carboxylase activity only for the marine macroalgae, and their photosynthesis to carboxylase activity ratios were close to 1. These plants also had a consistently high inorganic carbon transport capability, and it is suggested that ribulose-1,5-bisphosphate carboxylase/oxygenase activity is an important internal factor regulating the photosynthetic capacity within this plant group where, apparently, the internal CO₂ concentration is high and photorespiration is suppressed. Among the freshwater forms, it appears that their much lower inorganic carbon transport ability, rather than their carboxylase activity, limits the photosynthetic process.     

Morpho-anatomical and physiological leaf traits of two alpine herbs, Podophyllum hexandrum and Rheum emodi in the Western Himalaya under different irradiances

Morpho-anatomical leaf traits and photosynthetic activity of two alpine herbs, Podophyllum hexandrum (shade-tolerant) and Rheum emodi (light-requiring), were studied under field (PAR>2 000 μmol m⁻² s⁻¹) and greenhouse (PAR 500 μmol m⁻² s⁻¹) conditions. Mesophyll thickness, surface area of mesophyll cells facing intercellular spaces (S_mes), surface area of chloroplasts facing intercellular spaces (S_c), intercellular spaces of mesophyll cells (porosity), photon-saturated rate of photosynthesis per unit leaf area (P _Nmax), and ribulose-1,5-bisphosphate carboxylase/oxygenase activity decreased in the greenhouse with respect to the field and the decreases were significantly higher in R. emodi than in P. hexandrum. P. hexandrum had lower intercellular CO₂ concentration than R. emodi under both irradiances. The differences in acclimation of the two alpine herbs to low irradiance were due to their highly unlikely changes in leaf morphology, anatomy, and P _Nmax which indicated that the difference in radiant energy requirement related to leaf acclimation had greater impact under low than high irradiance.     

Cloning and expression of the d-ribulose-1,5-bisphosphate carboxylase/oxygenase form II gene from Thiobacillus intermedius in Escherichia coli

Abstract Both form I and II ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) genes were detected in Thiobacillus intermedius by heterologous hybridization using specific probes from Anacystis nidulans and Rhodobacter sphaeroides , respectively. However, only the previously reported from I enzyme could be demonstrated in cells grown under a number of different conditions. The reason(s) why the form II gene is not expressed in T. intermedius is/are not clear at this time. The form II gene was isolated from a lambda library by screening with the Rb. sphaeroides probe. A Sal I fragment from this clone was ligated into pUC8 and transformed into Escherichia coli DH5α. Subclones pTi20IIA and pTi20IIB representing both orientations relative to the lac promoter were isolated. Low levels of RuBisCO activity were detected in both induced and non-induced pTi20IIA indicating the probable expression from a T. intermedius promoter. Induced pTi20IIB produced much higher levels of enzyme activity. Analysis of cell-free extracts using sucrose density gradients confirmed the expression of a form II RuBisCO similar in size to that found in Rhodobacter capsulatus . Other Calvin cycle genes were not clustered with either the form I or form II genes.     

Variation in cellular ribulose-1,5-bisphosphate-carboxylase content in leaves of Triticum genotypes at three levels of ploidy

Ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 1.1.39) (RuBPCase) was quantified using polyacrylamide-gel electrophoresis in whole 9-d-old first leaves of 14 genotypes of Triticum, and cellular RuBPCase levels calculated. Diploids, tetraploids and hexaploids were analysed and it was confirmed that the RuBPCase level per cell is closely related to ploidy in wheat. Inter-genotypic variation in RuBPCase levels per cell and per leaf were surveyed. It was found that the interactions between leaf size, cell size and RuBPCase levels result in small variations in RuBPCase levels per unit leaf area between genotypes.Abbreviation RuBPCase ribulose-1,5-bisphosphate carboxylase/oxygenase     

Elevated CO<Subscript>2</Subscript> and temperature differentially affect photosynthesis and resource allocation in flag and penultimate leaves of wheat

Differences in acclimation to elevated growth CO₂ (700 μmol mol⁻¹, EC) and elevated temperature (ambient +4 °C, ET) in successive leaves of wheat were investigated in field chambers. At a common measurement CO₂, EC increased photosynthesis and the quantum yield of electron transport (Φ) early on in the growth of penultimate leaves, and later decreased them. In contrast, EC did not change photosynthesis, and increased Φ at later growth stages in the flag leaf. Contents of chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), and total soluble protein were initially higher and subsequently lower in penultimate than flag leaves. EC decreased RuBPCO protein content relative to soluble protein and Chl contents throughout the development of penultimate leaves. On the other hand, EC initially increased the RuBPCO:Chl and Chl a/b ratios, but later decreased them in flag leaves. In the flag leaves but not in the penultimate leaves, ET initially decreased initial and specific RuBPCO activities at ambient CO₂ (AC) and increased them at EC. Late in leaf growth, ET decreased Chl contents under AC in both kinds of leaves, and had no effect or a positive one under EC. Thus the differences between the two kinds of leaves were due to resource availability, and to EC-increased allocation of resources to photon harvesting in the penultimate leaves, but to increased allocation to carboxylation early on in growth, and to light harvesting subsequently, in the flag leaves.     

Photosynthetic nitrogen-use efficiency in leaves of woody and herbaceous species

1. Photosynthetic characteristics of an annual herb, Chenopodium album , and an evergreen tree, Quercus myrsinaefolia , were compared to clarify causes of the difference in photosynthetic nitrogen-use efficiency (photosynthetic capacity per unit nitrogen) between leaves of herbaceous and evergreen species.
2. When leaves with the same nitrogen content on an area basis were compared, photosynthetic capacity of C. album was twice as high as that of Q. myrsinaefolia . Gas-exchange measurements showed higher intercellular CO₂ concentration in C. album . Biochemical analyses indicated larger allocation of nitrogen into ribulose-1,5-bisphosphate carboxylase, a key enzyme of photosynthesis, and higher specific activity of ribulose-1,5-bisphosphate carboxylase in C. album . However, these differences were not large.
3. Compositional deviation of ¹³C in leaves of the two species suggested that the drop of CO₂ level between the intercellular space and the chloroplast was slightly larger in Q. myrsinaefolia when compared between the leaves with the same photosynthetic capacity.
4. It is concluded that the difference in photosynthetic nitrogen-use efficiency between C. album and Q. myrsinaefolia is not caused by a sole factor that is markedly different between the two species but by several factors each of which is slightly disadvantageous to Q. myrsinaefolia compared with C. album .     

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.     

Experimental and theoretical study on high temperature induced changes in chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence oscillations in barley leaves upon 2 % CO<Subscript>2</Subscript>

Oscillations in many of photosynthetic quantities with a period of about 1 min can be routinely measured with higher plant leaves after perturbation of the steady state by sudden change in gas phase. Among all hypotheses suggested so far to explain the oscillations, an effect of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activation status to control the oscillations is highly probable, at least upon high temperature (HT) treatment when in vivo RuBPCO activity controlled by RuBPCO activase (RuBPCO-A) decreases. Therefore, we measured the oscillations in fluorescence signal coming from barley leaves (Hordeum vulgare L. cv. Akcent) after their exposure for various time intervals to different HTs in darkness. We also evaluated steady state fluorescence and CO₂ exchange parameters to have an insight to functions of electron transport chain within thylakoid membrane and Calvin cycle before initiation of the oscillations. The changes in period of the oscillations induced by moderate HT (up to 43 °C) best correlated with changes in non-photochemical fluorescence quenching (q_N) that in turn correlated with changes in gross photosynthetic rate (P _G) and rate of RuBPCO activation (k_act). Therefore, we suggest that changes in period of the oscillations caused by moderate HT are mainly controlled by RuBPCO activation status. For more severe HT (45 °C), the oscillations disappeared which was probably caused by an insufficient formation of NADPH by electron transport chain within thylakoid membrane as judged from a decrease in photochemical fluorescence quenching (q_P). Suggestions made on the basis of experimental data were verified by theoretical simulations of the oscillations based on a model of Calvin cycle and by means of a control analysis of the model.     

Light and metabolite regulation of the synthesis of ribulose-1,5-bisphosphate carboxylase/oxygenase and the corresponding mRNAs in the unicellular alga Chlorogonium

In the unicellular green alga Chlorogonium elongatum, the synthesis of the plastid enzyme ribulose bisphosphate carboxylase/oxygenase (RuBPCase) and its mRNAs is under the control of light and acetate. Acetate is the sole metabolizable organic carbon source for this organism. Light greatly promotes the synthesis of RuBPCase and the increase in the concentration of the mRNAs of both subunits of the enzyme while acetate has a strong inhibitory effect on this process. There is a good agreement between RuBPCase synthesis and the amount of translateable RuBPCase mRNA present in cells which are cultured under different conditions (autotrophic, heterotrophic, mixotrophic). During the transition period after transfer of the cells from heterotrophic to autotrophic growth conditions the amounts of the large and small subunits of the enzyme increase well coordinated. In contrast to the protein subunits the two subunit-mRNAs accumulate with different kinetics.Abbreviations LSU large subunit of RuBPCase - poly(A)^- RNA - poly(A)⁺RNA non-, poly-adenylated RNA - RuBPCase ribulose-1,5-bisphosphate carboxylase/oxygenase EC 4.1.1.39 - SSU small subunit of RuBPCase     

Salicylic acid induced physiological and biochemical changes in wheat seedlings under water stress

Salicylic acid (SA) is an important signal molecule modulating plantresponses to stress. It is recently reported to induce multiple stresstolerancein plants including drought. An experiment was, therefore, conducted toascertain the effect of salicylic acid on the growth and metabolic profile ofwheat seedlings under water stress. Irrespective of the SA concentration(1–3 mM) and water stress, SA treated plants showed, ingeneral, a higher moisture content, dry mass, carboxylase activity of Rubisco,superoxide dismutase (SOD) activity and total chlorophyll compared to those ofuntreated seedlings. SA treatment, under water stress, protected nitratereductase (NR) activity and maintained, especially at 3 mM SAconcentration, the protein and nitrogen content of leaves compared to watersufficient seedlings. Results signify the role of SA in regulating the droughtresponse of plants and suggest that SA could be used as a potential growthregulator, for improving plant growth under water stress.     

Differential expression of the genes for ribulose-1,5-bisphosphate carboxylase and light-harvesting chlorophyll a/b protein in the developing barley leaf

The expression of genes in particular for light-harvesting chlorophyll a/b protein (LHCP) and ribulose-1,5-bisphosphate carboxylase (RuBPCase) has been studied in the developing barley leaf. This has been done by analysis of the occurrence of both proteins within the different regions (1 to 6, beginning from the base) of the primary 7-day-old leaf. It has been found that LHCP already appears in the base of the leaf, whereas RuBPCase is primarily expressed in the apical expanding part of the leaf. The distribution of the mRNAs for both proteins within this gradient is in accordance with that of the proteins themselves, indicating that gene expression is not regulated at the level of translation in both cases. The poly(A) mRNA for LHCP occurs mainly in the basic sections 2 and 3, whereas that for RuBPCase is found throughout the leaf but primarily in the apical sections of the leaf.Abbreviations LHCP light-harvesting chlorophyll a/b protein - RuBPCase ribulose-1,5-bisphosphate carboxylase - TCA trichloroacetic acid     

Analysis of differences in photosynthetic nitrogen use efficiency of alpine and lowland Poa species

This study investigates factors determining variation in photosynthetic nitrogen use efficiency (φ_N) in seven slow- and fast-growing Poa species from altitudinally contrasting sites. The species and their environmental origin were (in order of increasing relative growth rate): two alpine (Poa fawcettiae and P. costiniana), one sub-alpine (P. alpina) and three temperate lowland perennials (P. pratensis, P. compressa and P. trivialis), as well as one temperate lowland annual (P. annua). Plants were grown hydroponically under identical conditions with free access to nutrients in a growth room. Photosynthesis per unit leaf area measured at growth irradiance (500 μmol m⁻² s⁻¹) was slightly higher in the slow-growing alpine species. At saturating light intensities, photosynthesis was considerably higher in the alpine species than in the lowland species. Carboxylation capacity and Rubisco content per unit leaf area were also greater in the alpine species. Despite variation between the species, the in vivo specific activity of Rubisco showed little relationship to relative growth rate or photosynthetic rate. Both at light saturation and at the growth irradiance, φ_N was lowest in the slow-growing alpine species P. fawcettiae, P. costiniana and P. alpina, and highest in the fast-growing P. compressa and P. annua. The proportion of leaf nitrogen that was allocated to photosynthetic capacity and the in vivo catalytic constant of Rubisco accounted for most of the variation in φ_N at light saturation. Minor variations in intercellular CO₂ partial pressure also contributed to some extent to the variations in φ_N at light saturation. The low φ_N values at growth irradiance exhibited by the alpine species were additionally due to a lower percentage utilisation of their high photosynthetic capacity compared to the lowland species. Received: 28 May 1998 / Accepted: 28 March 1999     

Leaf phenology and seasonal variation of photosynthesis of invasive <Emphasis Type="Italic">Berberis thunbergii</Emphasis> (Japanese barberry) and two co-occurring native understory shrubs in a northeastern United States deciduous forest

Early leafing and extended leaf longevity can be important mechanisms for the invasion of the forest understory. We compared the leaf phenology and photosynthetic characteristics of Berberis thunbergii, an early leafing invasive shrub, and two co-occurring native species, evergreen Kalmia latifolia and late leafing Vaccinium corymbosum, throughout the 2004 growing season. Berberis thunbergii leafed out 1 month earlier than V. corymbosum and approximately 2 weeks prior to the overstory trees. The photosynthetic capacity [characterized by the maximum carboxylation rate of Rubisco (V _cmax) and the RuBP regeneration capacity mediated by the maximum electron transport rate (J _max)] of B. thunbergii was highest in the spring open canopy, and declined with canopy closure. The 2003 overwintering leaves of K. latifolia displayed high V _cmax and J _max in spring 2004. In new leaves of K. latifolia produced in 2004, the photosynthetic capacity gradually increased to a peak in mid-September, and reduced in late November. V. corymbosum, by contrast, maintained low V _cmax and J _max throughout the growing season. In B. thunbergii, light acclimation was mediated by adjustment in both leaf mass per unit area and leaf N on a mass basis, but this adjustment was weaker or absent in K. latifolia and V. corymbosum. These results indicated that B. thunbergii utilized high irradiance in the spring while K. latifolia took advantage of high irradiance in the fall and the following spring. By contrast, V. corymbosum generally did not experience a high irradiance environment and was adapted to the low irradiance understory. The apparent success of B. thunbergii therefore, appeared related to a high spring C subsidy and subsequent acclimation to varying irradiance through active N reallocation and leaf morphological modifications.     

Morphological and ecophysiological traits shaping altitudinal distribution of three Polylepis treeline species in the dry tropical Andes

Numerous species of the genus Polylepis form the highest treeline in the world, with striking dissimilarities in their upper altitudinal limits. The commonly accepted hypothesis is that growth at a treeline is limited by temperature. Here, using in situ records of various morphological and ecophysiological traits, we aimed to identify other factors influencing altitudinal distribution of three congeneric species from the dry tropical Andes: Polylepis rugulosa, Polylepis tarapacana and Polylepis tomentella. While P. tarapacana and P. tomentella reach their altitudinal limit at around 5000 m asl, P. rugulosa does not thrive above 4300 m, but precipitation is markedly lower in its distribution area. The three species responded to altitude by a change of morphological (e.g. decreased tree height and leaf size) and ecophysiological (e.g. decrease of transpiration rate, nutrient concentration or enrichment in the ¹³C isotope) traits, and this response was generally more pronounced in P. rugulosa. In comparison with P. tarapacana and P. tomentella, P. rugulosa displayed higher transpiration rates. Waxes from the abaxial (stomatous) leaf side of P. rugulosa were most strongly enriched in ¹³C. Furthermore, leaves of all species studied here had exceptionally low N and P concentrations. Trade-offs linked to changes in leaf area (e.g. bigger leaves, higher photosynthetic capacity but elevated transpiration) seem to drive differentiation and adaptations to altitude among these three congeneric species. We hypothesize that, while the upper distribution limit of P. tarapacana and P. tomentella is largely driven by low temperature, water is an important additional factor controlling the altitudinal distribution of P. rugulosa. Our results suggest that water stress needs to be taken into account among the factors shaping the altitudinal distribution of tropical treeline species.     

Effects of nitrogen supply on photosynthetic and anatomical changes in current-year needles of <Emphasis Type="Italic">Pinus koraiensis</Emphasis> seedlings grown under two irradiances

We investigated the responses of photon-saturated photosynthesis rate (P _sat) and its simultaneous acclimation of anatomy and nitrogen use patterns of current needles of Korean pine (Pinus koraiensis) seedlings grown under factorial combinations of two nitrogen levels and irradiances. Although N supply resulted in a significant increase of N content in needles under both irradiances, the increase of P _sat tended to be suppressed only in shade (S). The significant increase of P _sat in full sunlight (O) was associated with the increase of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and chlorophyll (Chl) contents. In contrast, small increase of Chl content and no increase of RuBPCO content were found in S (90 % cut of full irradiance), which would result in a small increase of P _sat. This result suggests that extra N is stocked in needles under shade for the growth in next season. With N supply, a significant decrease of specific leaf area (SLA) was detected only in O. This decrease of SLA was due to the increase of density of needle. Furthermore, the increase of needle density was not due to the increased number and size of mesophyll cells, but the increased density of each mesophyll cell. Therefore, although SLA changed in O, the change did not involve anatomical adaptation to use increased N effectively, at least observable by light microscopy. Hence, even though the SLA would change, N deposition will improve the photosynthetic capacity of Korean pine seedlings, not through the development of needle anatomy but through improvement of the allocation of N in both irradiances.     

Developmental regulation of photosynthate distribution in leaves of rice

mRNA expression patterns of genes for metabolic key enzymes sucrose phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPC), pyruvate kinase, ribulose 1,5-bisphosphate carboxylase/oxygenase, glutamine synthetase 1, and glutamine synthetase 2 were investigated in leaves of rice plants grown at two nitrogen (N) supplies (N_0.5, N_3.0). The relative gene expression patterns were similar in all leaves except for 9^th leaf, in which mRNA levels were generally depressed. Though increased N supply prolonged the expression period of each mRNA, it did not affect the relative expression intensity of any mRNA in a given leaf. SPS V_max, SPS limiting and PEPC activities, and carbon flow were examined. The ratio between PEPC activity and SPS V_max was higher in leaves developed at the vegetative growth stage (vegetative leaves: 5^th and 7^th leaves) than in leaves developed after the ear primordia formation stage (reproductive leaves: 9^th and flag leaves). PEPC activity and SPS V_max decreased with declining leaf N content. After using ¹⁴CO₂ the ¹⁴C photosynthate distribution in the amino acid fraction was higher in vegetative than in reproductive leaves when compared for the same leaf N status. Thus, at high PEPC/SPS activities ratio, more ¹⁴C photosynthate was distributed to the amino acid pool, whereas at higher SPS activity more ¹⁴C was channelled into the saccharide fraction. Thus, leaf ontogeny was an important factor controlling photosynthate distribution to the N- or C-pool, respectively, regardless of the leaf N status.     

The role of enzyme activation state in limiting carbon assimilation under variable light conditions

The mechanisms regulating transient photosynthesis by soybean (Glycine max) leaves were examined by comparing photosynthetic rates and carbon reduction cycle enzyme activities under flashing (saturating 1 s lightflecks separated by low photon flux density (PFD) periods of different durations) and continuous PFD. At the same mean PFD, the mean photosynthetic rates were reduced under flashing as compared to continuous light. However, as the duration of the low PFD period lengthened, the CO₂ assimilation attributable to a lightfleck increased. This enhanced lightfleck CO₂ assimilation was accounted for by a greater postillumination CO₂ fixation occurring after the lightfleck. The induction state of photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), fructose 1,6-bisphosphatase (FBPase) and ribulose 5-phosphate kinase (Ru5P kinase) activities all responded similarly and were all lower under flashing as compared to constant PFD of the same integrated mean value. However, the fast phase of induction and FBPase and Ru5P kinase activities were reduced more than were the slow phase of induction and rubisco activity. This was consistent with the role of the former enzymes in the fast induction component that limited RuBP regeneration. Competition for reducing power between carbon metabolism and thioredoxin-mediated enzyme activation may have resulted in lower enzyme activation states and hence lower induction states under flashing than continuous PFD, especially at low lightfleck frequencies (low mean PFD).Abbreviations FBPase fructose 1,6-bisphosphatase (EC 3.1.3.11) - LUE lightfleck use efficiency - P-glycerate 3-phosphoglycerate - PICF post-illumination CO₂ fixation - Ru5P kinase ribulose 5-phosphate kinase (EC 2.7.1.19) - RuBP ribulose 1,5-bisphosphate - rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) - SBpase sedoheptulose 1,7-bisphosphatase (EC 3.1.3.37)     

Effects of leaf longevity and retranslocation efficiency on the retention time of nutrients in the leaf biomass of different woody species

Summary A study was made of the retention times of N and P in the leaf biomass and their relationship with the retranslocation percentages and the leaf longevities in some woody species in Central Spain. The retention times of both nutrients were strongly related to the nutrient status of each species. These results suggest that a prolonged retention time is a way of increasing nutrient use efficiency in conditions of low nutrient availability. Plants can increase the retention time of nutrients in their leaf biomass by means of an increase in leaf longevity and/or by means of an increase in retranslocation efficiency. However, the effect of the retranslocation efficiency on retention times was almost negligible compared with the effect of leaf longevity. This suggests that an increase in leaf longevity is probably the best adaptation for increasing efficiency in the use of nutrients.     

The kinetics of ribulose-1,5-bisphosphate carboxylase/oxygenase in vivo inferred from measurements of photosynthesis in leaves of transgenic tobacco

Transgenic tobacco (Nicotiana tabacum L. cv. W38) with an antisense gene directed against the mRNA of the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit was used to determine the kinetic properties of Rubisco in vivo. The leaves of these plants contained only 34% as much Rubisco as those of the wild type, but other photosynthetic components were not significantly affected. Consequently, the rate of CO₂ assimilation by the antisense plants was limited by Rubisco activity over a wide range of CO₂ partial pressures. Unlike in the wild-type leaves, where the rate of regeneration of ribulose bisphosphate limited CO₂ assimilation at intercellular partial pressures above 400 ubar, photosynthesis in the leaves of the antisense plants responded hyperbolically to CO₂, allowing the kinetic parameters of Rubisco in vivo to be inferred. We calculated a maximal catalytic turnover rate, k_cat, of 3.5+0.2 mol CO₂·(mol sites)^–1·s^–1 at 25° C in vivo. By comparison, we measured a value of 2.9 mol CO₂·(mol sites)^–1·^–1 in vitro with leaf extracts. To estimate the Michaelis-Menten constants for CO₂ and O₂, the rate of CO₂ assimilation was measured at 25° C at different intercellular partial pressures of CO₂ and O₂. These measurements were combined with carbon-isotope analysis (¹³C/¹²C) of CO₂ in the air passing over the leaf to estimate the conductance for transfer of CO₂ from the substomatal cavities to the sites of carboxylation (0.3 mol·m^–2·s^–1·bar^–1) and thus the partial pressure of CO₂ at the sites of carboxylation. The calculated Michaelis-Menten constants for CO₂ and O₂ were 259 ±57 bar (8.6±1.9M) and 179 mbar (226 M), respectively, and the effective Michaelis-Menten constant for CO₂ in 200 mbar O₂ was 549 bar (18.3 M). From measurements of the photocompensation point (_* = 38.6 ubar) we estimated Rubisco's relative specificity for CO₂, as opposed to O₂ to be 97.5 in vivo. These values were dependent on the size of the estimated CO₂-transfer conductance.Abbreviations and Symbols A CO₂-assimilation rate - g_w conductance for CO₂ transfer from the substomatal cavities to the sites of carboxylation - K_c, K_o Michaelis-Menten constants for carboxylation, oxygenation of Rubisco - k_cat V_cmax/[active site] - O partial pressure of O₂ at the site of carboxylation - p_c partial pressure of CO₂ at the site of carboxylation - p_i intercellular CO₂ partial pressure - R_d day respiration (non-photorespiratory CO₂ evolution) - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - S_c/o relative specificity factor for Rubisco - SSu small subunit of Rubisco - V_cmax, V_omax maximum rates of Rubisco carboxylation, oxygenation - _* partial pressure of CO₂ in the chloroplast at which photorespiratory CO₂ evolution equals the rate of carboxylation