Effects of Nitrate Application on Amaranthus powellii Wats. : III. Optimal Allocation of Leaf Nitrogen for Photosynthesis and Stomatal Conductance

Optimal allocation of leaf nitrogen maximizes daily CO₂ assimilation for a given leaf nitrogen concentration. According to the hypothesis of optimization, this condition occurs when the partial derivative of assimilation rate with respect to leaf nitrogen concentration is constant. This hypothesis predicts a linear increase of assimilation rate with leaf nitrogen concentration under constant conditions. Plants of Amaranthus powellii Wats. were grown at 1, 5, 10, or 45 millimolar nitrate to obtain leaves with different nitrogen concentrations. Assimilation rate at 340 microbar CO₂/bar, stomatal conductance, CO₂- and light-saturated net photosynthetic rate, the initial slope of the CO₂ response of photosynthesis, ribulose-1,5′-bisphosphate carboxylase activity, and phosphoenolpyruvate carboxylase activity were linearly related to estimated or actual leaf nitrogen concentration. The data are consistent with the optimal use of leaf nitrogen. This hypothesis and the hypothesis of optimal stomatal conductance were combined to determine the relationship between conductance and leaf nitrogen concentration. The slope of conductance versus leaf nitrogen concentration was not significantly different than the slope predicted by the combination of the two hypotheses. Stomatal conductance was linearly related to leaf nitrogen in the field and the slope decreased with lower xylem pressure potentials in a manner consistent with the hypotheses. Finally, apparent maximum stomatal aperture of isolated abaxial epidermal strips was linearly related to leaf nitrogen suggesting stomatal conductance and assimilation rate are controlled in parallel by leaf nitrogen concentration or some factor correlated with leaf nitrogen.     

Regulation of Soybean Net Photosynthetic CO(2) Fixation by the Interaction of CO(2), O(2), and Ribulose 1,5-Diphosphate Carboxylase

Kinetic properties of soybean net photosynthetic CO₂ fixation and of the carboxylase and oxygenase activities of purified soybean (Glycine max [L.] Merr.) ribulose 1, 5-diphosphate carboxylase (EC 4.1.1.39) were examined as functions of temperature, CO₂ concentration, and O₂ concentration. With leaves, O₂ inhibition of net photosynthetic CO₂ fixation increased when the ambient leaf temperature was increased. The increased inhibition of CO₂ fixation at higher temperatures was caused by a reduced affinity of the leaf for CO₂ and an increased affinity of the leaf for O₂. With purified ribulose 1,5-diphosphate carboxylase, O₂ inhibition of CO₂ incorporation and the ratio of oxygenase activity to carboxylase activity increased with increased temperature. The increased O₂ sensitivity of the enzyme at higher temperature was caused by a reduced affinity of the enzyme for CO₂ and a slightly increased affinity of the enzyme for O₂. The similarity of the effect of temperature on the affinity of intact leaves and of ribulose 1,5-diphosphate carboxylase for CO₂ and O₂ provides further evidence that the carboxylase regulates the O₂ response of photosynthetic CO₂ fixation in soybean leaves. Based on results reported here and in the literature, a scheme outlining the stoichiometry between CO₂ and O₂ fixation in vivo is proposed.     

Control analysis of photosynthetic CO2 fixation

The potential of control analysis to aid our understanding of regulation and control of photosynthetic carbon metabolism is investigated. Methods of metabolic control analysis are used to determine flux control coefficients of photosynthetic reactions from enzyme elasticities. Equations expressing control coefficients symbolically by enzyme elasticities are derived, and general properties of these expressions are analysed. Suggestions for experimental determination of flux control coefficients from enzyme elasticities are given. A simplified model of the Calvin-Benson cycle is used to illustrate interrelations between patterns of photosynthetic metabolites and that of control coefficients.Abbreviations GAPDH glyceraldehyde phosphate dehydrogenase - PGA 3-phosphoglycerate - PGK 3-phosphoglycerate kinase - Pi inorganic phosphate - PRK phosphoribulokinase - RuBP ribulose-1,5-bisphosphate(_{total, free}) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - Ru5P ribulose-5-phosphate     

The effect of CaCl<Subscript>2</Subscript> on calcium content,photosynthesis, and chlorophyll fluorescence of tung tree seedlings under drought conditions

The study investigated the effects of different CaCl₂ concentrations (2, 5, and 10 mM) on photosynthetic enzymatic activities, photosynthesis, and chlorophyll fluorescence of tung tree seedlings under drought conditions. Plants were sprayed with either CaCl₂ or distilled water until run-off. Irrigation was then withheld to induce drought stress. The strength of drought stress was evaluated by relative leaf water content and soil water content, which was 27.3 and 9.5% on day 0 and day 12, respectively. Drought stress decreased activities of ribulose-1,5-bisphosphate carboxylase/oxygenase and phosphoenolpyruvate carboxylase, chlorophyll (a+b) content, net photosynthetic rate, stomatal conductance, transpiration rate, electron transport rate, the maximal quantum yield of PSII photochemistry, and effective quantum yield of PSII in tung tree seedlings. The CaCl₂ pretreatments alleviated the negative effect of drought stress to some degree on all the parameters mentioned above.     

Improving cucumber photosynthetic capacity under NaCl stress by grafting onto two salt-tolerant pumpkin rootstocks

Cucumber plants were either self-grafted or grafted onto two salt-tolerant pumpkin rootstocks Chaojiquanwang (Cucurbita moschata Duch), and Figleaf Gourd (Cucurbita ficifolia Bouche). Plants were grown hydroponically in 0, 30, 60, or 90 mM NaCl for 16 d in greenhouse. Salinity induced a smaller decrease in plant shoot dry mass, leaf area, net photosynthetic rate, and stomatal conductance in the two rootstock-grafted plants compared to the self-grafted plants. In addition, a significant increase in intercellular CO₂ concentration, as well as a significant decrease in the initial and total ribulose-1,5-bisphosphate carboxylase/oxygenase activities were observed only in the self-grafted plants under 90 mM NaCl treatment. These results suggest that the use of salt tolerant rootstock can improve cucumber photosynthetic capacity under salt stress through both stomatal and non-stomatal pathways.     

Phosphoglycerate dehydrogenase from soybean nodules : partial purification and some kinetic properties

The relationship between single leaf photosynthesis and conductance was examined in cotton (Gossypium hirsutum L.) across a range of environmental conditions. The purpose of this research was to separate and define the degree of stomatal and nonstomatal limitations in the photosynthetic process of field-grown cotton.

Photosynthetic rates were related to leaf conductance of upper canopy leaves in a curvilinear manner. Increases in leaf conductance of CO₂ in excess of 0.3 to 0.4 mole per square meter per second did not result in significant increases in gross or net photosynthetic rates. No tight coupling between environmental influences on photosynthetic rates and those affecting conductance levels was evident, since photosynthesis per unit leaf conductance did not remain constant. Slowly developing water stress caused greater reductions in photosynthesis than in leaf conductance, indicating nonstomatal limitations of photosynthesis.

Increases in external CO₂ concentration to levels above ambient did not produce proportional increases in photosynthesis even though substomatal or intercellular CO₂ concentration increased. The lack of a linear increase in photosynthetic rate in response to increases in leaf conductance and in response to increases in external CO₂ concentration demonstrated that nonstomatal factors are major photosynthetic rate determinants of cotton under field conditions.

     

The F<Subscript>O</Subscript>F<Subscript>1</Subscript> ATP synthase: from atomistic three-dimensional structure to the rotary-chemical function

There are numerous studies describing how growth conditions influence the efficiency of C₄ photosynthesis. However, it remains unclear how changes in the biochemical capacity versus leaf anatomy drives this acclimation. Therefore, the aim of this study was to determine how growth light and nitrogen availability influence leaf anatomy, biochemistry and the efficiency of the CO₂ concentrating mechanism in Miscanthus × giganteus. There was an increase in the mesophyll cell wall surface area but not cell well thickness in the high-light (HL) compared to the low-light (LL) grown plants suggesting a higher mesophyll conductance in the HL plants, which also had greater photosynthetic capacity. Additionally, the HL plants had greater surface area and thickness of bundle-sheath cell walls compared to LL plants, suggesting limited differences in bundle-sheath CO₂ conductance because the increased area was offset by thicker cell walls. The gas exchange estimates of phosphoenolpyruvate carboxylase (PEPc) activity were significantly less than the in vitro PEPc activity, suggesting limited substrate availability in the leaf due to low mesophyll CO₂ conductance. Finally, leakiness was similar across all growth conditions and generally did not change under the different measurement light conditions. However, differences in the stable isotope composition of leaf material did not correlate with leakiness indicating that dry matter isotope measurements are not a good proxy for leakiness. Taken together, these data suggest that the CO₂ concentrating mechanism in Miscanthus is robust under low-light and limited nitrogen growth conditions, and that the observed changes in leaf anatomy and biochemistry likely help to maintain this efficiency.     

C4 Photosynthesis (The Effects of Leaf Development on the CO2-Concentrating Mechanism and Photorespiration in Maize)

The effect of O2 on photosynthesis was determined in maize (Zea mays) leaves at different developmental stages. The optimum level of O2 for maximum photosynthetic rates was lower in young and senescing tissues (2-5 kPa) than in mature tissue (9 kPa). Inhibition of photosynthesis by suboptimal levels of O2 may be due to a requirement for functional mitochondria or to cyclic/pseudocyclic photophosphorylation in chloroplasts; inhibition by supraoptimal levels of O2 is considered to be due to photorespiration. Analysis of a range of developmental stages (along the leaf blade and at different leaf ages and positions) showed that the degree of inhibition of photosynthesis by supraoptimal levels of O2 increased rapidly once the ribulose-1,5-bisphosphate carboxylase/oxygenase and chlorophyll contents were below a critical level and was similar to that of C3 plants. Tissue having a high sensitivity of photosynthesis to O2 may be less effective in concentrating CO2 in the bundle sheath cells due either to limited function of the C4 cycle or to higher bundle sheath conductance to CO2. An analysis based on the kinetic properties of ribulose-1,5-bisphosphate carboxylase/oxygenase was used to predict the maximum CO2 level concentrated in bundle sheath cells at a given degree of inhibition of photosynthesis by supraoptimal levels of O2.     

Partitioning of photosynthetic electron flow between CO2 and O2 reduction in a C3 leaf (Phaseolus vulgaris L.) at different CO2 concentrations and during drought stress

Photosystem II chlorophyll fluorescence and leaf net gas exchanges (CO₂ and H₂O) were measured simultaneously on bean leaves (Phaseolus vulgaris L.) submitted either to different ambient CO₂ concentrations or to a drought stress. When leaves are under photorespiratory conditions, a simple fluorescence parameter F/ F_m (B. Genty et al. 1989, Biochem. Biophys. Acta 990, 87–92; F = difference between maximum, F_m, and steady-state fluorescence emissions) allows the calculation of the total rate of photosynthetic electron-transport and the rate of electron transport to O₂. These rates are in agreement with the measurements of leaf O₂ absorption using ¹⁸O₂ and the kinetic properties of ribulose-1,5bisphosphate carboxylase/oxygenase. The fluorescence parameter, F/F_m, showed that the allocation of photosynthetic electrons to O₂ was increased during the desiccation of a leaf. Decreasing leaf net CO₂ uptake, either by decreasing the ambient CO₂ concentration or by dehydrating a leaf, had the same effect on the partitioning of photosynthetic electrons between CO₂ and O₂ reduction. It is concluded that the decline of net CO₂ uptake of a leaf under drought stress is only due, at least for a mild reversible stress (causing at most a leaf water deficit of 35%), to stomatal closure which leads to a decrease in leaf internal CO₂ concentration. Since, during the dehydration of a leaf, the calculated internal CO₂ concentration remained constant or even increased we conclude that this calculation is misleading under such conditions.Abbreviations Ca, Ci ambient, leaf internal CO₂ concentrations - F_m, F_o, F_s maximum, minimal, steady-state fluorescence emission - F_v variable fluorescence emission - PPFD photosynthetic photon flux density - q_p, q_N photochemical, non-photochemical fluorescence quenching - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase     

S‐nitrosoglutathione spraying improves stomatal conductance,Rubisco activity and antioxidant defense in both leaves and roots of sugarcane plants under water deficit

Water deficit is a major environmental constraint on crop productivity and performance and nitric oxide (NO) is an important signaling molecule associated with many biochemical and physiological processes in plants under stressful conditions. This study aims to test the hypothesis that leaf spraying of S‐nitrosoglutathione (GSNO), an NO donor, improves the antioxidant defense in both roots and leaves of sugarcane plants under water deficit, with positive consequences for photosynthesis. In addition, the roles of key photosynthetic enzymes ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) in maintaining CO₂ assimilation of GSNO‐sprayed plants under water deficit were evaluated. Sugarcane plants were sprayed with water or GSNO 100 μM and subjected to water deficit, by adding polyethylene glycol (PEG‐8000) to the nutrient solution. Sugarcane plants supplied with GSNO presented increases in the activity of antioxidant enzymes such as superoxide dismutase in leaves and catalase in roots, indicating higher antioxidant capacity under water deficit. Such adjustments induced by GSNO were sufficient to prevent oxidative damage in both organs and were associated with better leaf water status. As a consequence, GSNO spraying alleviated the negative impact of water deficit on stomatal conductance and photosynthetic rates, with plants also showing increases in Rubisco activity under water deficit.     

The Regulation of Photosynthesis in Leaves of Field-Grown Spring Wheat (Triticum aestivum L., cv Albis) at Different Levels of Ozone in Ambient Air

Wheat (Triticum aestivum L. cv Albis) was grown in open-top chambers in the field and fumigated daily with charcoal-filtered air (0.015 microliters per liter O₃), nonfiltered air (0.03 microliters per liter O₃), and air enriched with either 0.07 or 0.10 microliters per liter ozone (seasonal 8 hour/day [9 am-5 pm] mean ozone concentration from June 1 until July 10, 1987). Photosynthetic ¹⁴CO₂ uptake was measured in situ. Net photosynthesis, dark respiration, and CO₂ compensation concentration at 2 and 21% O₂ were measured in the laboratory. Leaf segments were freeze-clamped in situ for the determination of the steady state levels of ribulose 1,5-bisphosphate, 3-phosphoglycerate, triose-phosphate, ATP, ADP, AMP, and activity of ribulose, 1,5-bisphosphate carboxylase/oxygenase. Photosynthesis of flag leaves was highest in filtered air and decreased in response to increasing mean ozone concentration. CO₂ compensation concentration and the ratio of dark respiration to net photosynthesis increased with ozone concentration. The decrease in photosynthesis was associated with a decrease in chlorophyll, soluble protein, ribulose bisphosphate carboxylase/oxygenase activity, ribulose bisphosphate, and adenylates. No decrease was found for triose-phosphate and 3-phosphoglycerate. The ratio of ATP to ADP and of triosephosphate to 3-phosphoglycerate were increased suggesting that photosynthesis was limited by pentose phosphate reductive cycle activity. No limitation occurred due to decreased access of CO₂ to photosynthetic cells since the decrease in stomatal conductance with increasing ozone concentration did not account for the decrease in photosynthesis. Ozonestressed leaves showed an increased degree of activation of ribulose bisphosphate carboxylase/oxygenase and a decreased ratio of ribulose bisphosphate to initial activity of ribulose bisphosphate carboxylase/oxygenase. Nevertheless, it is suggested that photosynthesis in ozone stressed leaves is limited by ribulose bisphosphate carboxylation possibly due to an effect of ozone on the catalysis by ribulose bisphosphate carboxylase/oxygenase.     

A thirty percent increase in UV-B has no impact on photosynthesis in well-watered and droughted pea plants in the field

It has been suggested that field experiments which increase UV-B irradiation by a fixed amount irrespective of ambient light conditions (‘square-wave’), may overestimate the response of photosynthesis to UV-B irradiation. In this study, pea (Pisum sativum L.) plants were grown in the field and subjected to a modulated 30% increase in ambient UK summer UV-B radiation (weighted with an erythemal action spectrum) and a mild drought treatment. UV-A and ambient UV control treatments were also studied. There were no significant effects of the UV-B treatment on the in situ CO₂ assimilation rate throughout the day or on the light-saturated steady-state photosynthesis. This was confirmed by an absence of UV-B effects on the major components contributing to CO₂ assimilation; photosystem II electron transport, ribulose 1,5-bisphosphate regeneration, ribulose 1,5-bisphosphate carboxylase/oxygenase carboxylation, and stomatal conductance. In addition to the absence of an effect on photosynthetic activities, UV-B had no significant impact on plant biomass, leaf area or partitioning. UV-B exposure increased leaf flavonoid content. The UV-A treatment had no observable effect on photosynthesis or productivity. Mild drought resulted in reduced biomass, a change in partitioning away from shoots to roots whilst maintaining leaf area, but had no observable effect on photosynthetic competence. No UV-B and drought treatment interactions were observed on photosynthesis or plant biomass. In conclusion, a 30% increase in UV-B had no effects on photosynthetic performance or productivity in well-watered or droughted pea plants in the field.     

Nitrogen enhanced photosynthesis of Miscanthus by increasing stomatal conductance and phosphoenolpyruvate carboxylase concentration

Miscanthus is one of the most promising bioenergy crops with high photosynthetic nitrogen-use efficiency (PNUE). It is unclear how nitrogen (N) influences the photosynthesis in Miscanthus. Among three Miscanthus genotypes, the net photosynthetic rate (P _N) under the different light intensity and CO₂ concentration was measured at three levels of N: 0, 100, and 200 kg ha^?1. The concentrations of chlorophyll, soluble protein, phosphoenolpyruvate carboxylase (PEPC), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) large subunit, leaf anatomy and carbon isotope discrimination (Δ) in the leaf were analyzed to probe the response of photosynthesis in Miscanthus genotypes to N levels. P _N in all genotypes rose significantly as N application increased. The initial slope of response curves of P _N to C _i was promoted by N application in all genotypes. Both stomatal conductance and C _i were increased with increased N supply, indicating that stomatal factors played an important role in increasing P _N. At a given C _i, P _N in all genotypes was enhanced by N, implying that nonstomatal factors might also play an important role in increasing P _N. Miscanthus markedly regulated N investment into PEPC rather than the Rubisco large subunit under higher N conditions. Bundle sheath leakiness of CO₂ was constant at about 0.35 for all N levels. Therefore, N enhanced the photosynthesis of Miscanthus mainly by increasing stomatal conductance and PEPC concentration.     

Specific density of leaf as a characteristic of the photosynthetic apparatus

At early stages of ontogeny (up to 50-60% of the maximum leaf area) of wheat (Triticum aestivum L.), meadow fescue (Festuca pratensis Huds.), reed fescue (F. arindinacea Schreb.), and sugar beet (Beta vulgaris L. var. saccharifera (Alef) Krass), there is correlation between changes in the specific leaf density (SLD), rate of photosynthetic CO2 assimilation; activity of the key photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39); and concentration of chlorophyll (Chl) a, Chl b, carotenoids, and soluble leaf proteins. However, there is no correlation of SLD with the activity of phospho(enol)pyruvate carboxylase (EC 4.1.1.31). Senescence of leaves was accompanied by a decrease in the SLD value. Treatment with cytokininomimetics (6-benzylaminopurine and Metribuzin) caused an increase in the SLD value. The specific leaf density is suggested to be a structural and functional characteristic of the photosynthetic apparatus of agricultural plants.     

Does a Low Nitrogen Supply Necessarily Lead to Acclimation of Photosynthesis to Elevated CO2?

Long-term exposure of plants to elevated partial pressures of CO₂ (pCO₂) often depresses photosynthetic capacity. The mechanistic basis for this photosynthetic acclimation may involve accumulation of carbohydrate and may be promoted by nutrient limitation. However, our current knowledge is inadequate for making reliable predictions concerning the onset and extent of acclimation. Many studies have sought to investigate the effects of N supply but the methodologies used generally do not allow separation of the direct effects of limited N availability from those caused by a N dilution effect due to accelerated growth at elevated pCO₂. To dissociate these interactions, wheat (Triticum aestivum L.) was grown hydroponically and N was added in direct proportion to plant growth. Photosynthesis did not acclimate to elevated pCO₂ even when growth was restricted by a low-N relative addition rate. Ribulose-1, 5-bisphosphate carboxylase/oxygenase activity and quantity were maintained, there was no evidence for triose phosphate limitation of photosynthesis, and tissue N content remained within the range recorded for healthy wheat plants. In contrast, wheat grown in sand culture with N supplied at a fixed concentration suffered photosynthetic acclimation at elevated pCO₂ in a low-N treatment. This was accompanied by a significant reduction in the quantity of active ribulose-1, 5-bisphosphate carboxylase/oxygenase and leaf N content.     

Effect of inorganic carbon supply on the photosynthetic physiology of Gracilaria tenuistipitata

Gracilaria tenuistipitata Zhang et Xia was cultured for 15 d at low, normal and high inorganic carbon concentrations under constant light, temperature and nutrient conditons. Carbonic anhydrase (CA; EC 4.2.1.1.) activity, ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco; EC 4.1.1.39) content, pigment content and C/N ratio were measured, and also the photosynthesis and growth rates. Both Rubisco content and CA activity increased under conditions of low inorganic carbon (C_i) but decreased at high C_i with respect to the control. The amount of pigments declined considerably at high C_i and was slightly higher at low C_i. The maximum rate of photosynthesis and the photosynthetic efficiency increased in low C_i and the opposite was found at high C_i concentration. The effects of C_i concentration on maximum rate of photosynthesis and photosynthetic efficiency are discussed in relation to the variation in pigment and Rubisco contents and CA activity. The data indicate that C_i may be an important factor controlling the photosynthetic physiology of G. tenuistipitata with regard, not only to the enzymes of C_i metabolism, but also to the pigment content.Abbreviations APS_max maximum apparent photosynthetic rate - CA carbonic anhydrase - Chl chlorophyll - C_i inorganic carbon - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase This work has been supported by grants No. PB91-0962 and No. MAR90-0365 from Spanish Direction for Science and Technology (DIGICYT). M.J. G-S holds a fellowship from the DIGICYT.     

Control of steady-state photosynthesis in sunflowers growing in enhanced CO2

Control coefficients were used to describe the degree to which ribulose bisphosphate carboxylase/oxygenase (Rubisco) limits the steady-state rate of CO₂ assimilation in sunflower leaves from plants grown at high (800 μmol mol⁻¹) and low (350 μmol mol⁻¹) CO₂. The magnitude of a control coefficient is approximately the percentage change in the flux that would result from a 1% rise in enzyme active site concentration. In plants grown at low CO₂, leaves of different ages varied considerably in their photosynthetic capacities. In a saturating light flux and an ambient CO₂ concentration of 350 μmol mol⁻¹, the Rubisco control coefficient was about 0.7 in all leaves, indicating that Rubisco activity largely limited the assimilation flux. The Rubisco control coefficient for leaves grown at 350 μmol mol⁻¹ CO₂ dropped to about zero when the ambient CO₂ concentration was raised to 800 μmol mol⁻¹. In relatively young, fully expanded leaves of plants grown at high CO₂, the Rubisco control coefficient was also about 0.7 at a saturating light flux and at the CO₂ concentration at which the plants were grown (800 μmol mol⁻¹). This apparently resulted from a decrease in the concentration of Rubisco active sites. In older leaves, however, the control coefficient was about 0.2. Because, on the whole, Rubisco activity still largely limits the assimilation flux in plants grown at high CO₂, the kinetics of this enzyme can still be used to model photosynthesis under these conditions. The relatively high Rubisco control coefficient under enhanced CO₂ indicates that the young sunflower leaves have the capacity to acclimate their photosynthetic biochemistry in a way consistent with an optimal use of protein resources.     

The Specific Density of a Leaf as a Characteristic of the Photosynthetic Apparatus

At early stages of ontogeny (up to 50–60% of the maximum leaf area) of wheat (Triticum aestivumL.), meadow fescue (Festuca pratensisHuds.), reed fescue (F. arindinaceaSchreb.), and sugar beet (Beta vulgarisL. var. saccharifera(Alef) Krass), there is a correlation between changes in the specific leaf density (SLD); photosynthetic CO₂assimilation rate; activity of the key photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39); and the concentrations of chlorophyll (Chl) a, Chl b, carotenoids, and soluble leaf proteins. However, the SLD does not correlate with the activity of phospho(enol)pyruvate carboxylase (EC 4.1.1.31). Senescence was accompanied by a decrease in the leaf SLD. Treatment with cytokininomimetics (6-benzylaminopurine and Metribuzin) caused an increase in the SLD. The specific leaf density is suggested to be a structural and functional characteristic of the photosynthetic apparatus of agricultural plants.     

Rubisco activity in Mediterranean species is regulated by the chloroplastic CO2 concentration under water stress

Water stress decreases the availability of the gaseous substrate for ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) by decreasing leaf conductance to CO(2). In spite of limiting photosynthetic carbon assimilation, especially in those environments where drought is the predominant factor affecting plant growth and yield, the effects of water deprivation on the mechanisms that control Rubisco activity are unclear. In the present study, 11 Mediterranean species, representing different growth forms, were subject to increasing levels of drought stress, the most severe one followed by rewatering. The results confirmed species-specific patterns in the decrease in the initial activity and activation state of Rubisco as drought stress and leaf dehydration intensified. Nevertheless, all species followed roughly the same trend when Rubisco activity was related to stomatal conductance (g(s)) and chloroplastic CO(2) concentration (C(c)), suggesting that deactivation of Rubisco sites could be induced by low C(c), as a result of water stress. The threshold level of C(c) that triggered Rubisco deactivation was dependent on leaf characteristics and was related to the maximum attained for each species under non-stressing conditions. Those species adapted to low C(c) were more capable of maintaining active Rubisco as drought stress intensified.     

Effects of root flooding and stage of development on the growth and photosynthesis of field bean (Vicia faba L. minor)

Field bean plants were subjected to flooding stress for 7 days, during two stages of development: at the vegetative phase (4-week-old seedlings) and at the generative phase (8-week-old plants). The height of plants, total area of leaves, the number of undamaged leaves, dry plant matter, chlorophyll content, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activity, the maximum quantum yield of PS2 photochemistry (Fv/Fm ratio), the photosynthesis rate (P _N) and stomatal conductance (g _s) were determined. A strong reduction in stem elongation and leaf area as well as in dry matter production was observed as a result of flooding. The responses from vegetative plants were greater than in generative plants. Waterlogging decreased chlorophyll a and b in leaves, notably at the vegetative stage, and persisted after cessation of flooding. After flooding, photosynthesis was strongly reduced and positively correlated with decreased stomatal conductance. Damage to the photosynthetic apparatus resulted in a lower Fv/Fm especially in young seedlings. In vegetative plants Fv/Fm quickly returned to the control levels after the soil was drained. The results show that an excess of water in the soil limits growth and injures the photosynthetic apparatus in field beans, but that the extent of the injury is strongly age dependent.