Influence of variation potential on resistance of the photosynthetic machinery to heating in pea

Electrical signals [action potentials (APs) and variation potentials (VPs)] induced by local stimuli are a mechanism that underlies rapid plant response to environmental factors. Such signals induce a number of functional responses, including changes in photosynthesis. Ultimately, these responses are considered to increase plant resistance to stress factors, but this question has been poorly investigated. We studied the influence of VP on photosynthesis and resistance of the photosynthetic machinery to heating in leaves of pea (Pisum sativum). Localized burning induced a VP that decreased photosynthesis parameters [CO₂ assimilation rate and quantum yields of photosystem I (PSI) and photosystem II (PSII)]. The photosynthetic response was initiated by a decrease in photosynthesis dark‐stage activity, which in turn increased resistance of PSI to heating. Three results supported this hypothesized mechanism: (1) the magnitude of VP‐induced decrease in CO₂ assimilation and enhanced PSI resistance to heating were highly correlated; (2) the VP influence on PSI resistance to heating was suppressed under a low external CO₂ concentration and (3) decreasing external CO₂ concentration imitated the VP‐induced photosynthetic response and increased PSI resistance to heating.     

The role of the intra- and extracellular protons in the photosynthetic response induced by the variation potential in pea seedlings

Local burning induces generation and propagation of variation potential (VP) in higher plants. VP induces transient inactivation of photosynthesis, which is possibly connected with proton signal in plant cell. Analysis of the role of changes in intracellular and extracellular pH in the VP-induced photosynthetic response in pea seedlings was the aim of this work. It was shown that local burning induced VP propagation, which was accompanied with a decrease of intracellular pH and increase of extracellular pH. VP induced photosynthesis inactivation that included an increase in the nonphotochemical fluorescence quenching and a decrease in the CO₂ assimilation rate. Analysis of photosynthetic responses under control and low external CO₂ concentration and changes in pH showed that there were two components in the responses. The first component appeared as a fast decrease of the CO₂ assimilation and increase of nonphotochemical quenching. It depended on the activity of the dark stage of photosynthesis and was connected with apoplast alkalization. The second component was presented as a slow increase of nonphotochemical quenching. It weakly depended on a dark stage and was connected with a decrease of intracellular pH.     

Effect of leaf surface wetness and wettability on photosynthesis in bean and pea

Leaf surface wetness that occurs frequently in natural environments has a significant impact on leaf photosynthesis. However, the physiological mechanisms for the photosynthetic responses to wetness are not well understood. The responses of leaf CO₂ assimilation rate (A) to 72 h of artificial mist of a wettable (bean; Phaseolus vulgaris) and a non‐wettable species (pea; Pisum sativum) were compared. Stomatal and non‐stomatal limitations to A were investigated. A 28% inhibition of A was observed in the bean leaves as a result of a 16% decrease in stomatal conductance and a 55% reduction in the amount of Rubisco. The decrease of Rubisco was mainly due to its partial degradation. In contrast to the bean leaves, a 22% stimulation of A was obtained in the 72 h mist‐treated pea leaves. Mist treatment increased stomatal conductance by 12.5% and had no effect on the amount of Rubisco. These results indicated that a positive photosynthetic response to wetness occurred only in non‐wettable species and is due to the change in stomatal regulation.     

Decrease of mesophyll conductance to CO<Subscript>2</Subscript> is a possible mechanism of abscisic acid influence on photosynthesis in seedlings of pea and wheat

Treatment of plants by phytohormones is a perspective method of regulation of the plant stress resistance and productivity. However, the mechanisms of phytohormone effects on physiological processes require investigations. The aim of this work was the analysis of the influence of exogenous abscisic acid (ABA) on photosynthesis in seedlings of pea and wheat, and in particular, an estimation of the involvement of the mesophyll conductance to CO₂ in the realization of the ABA effects. A standard system for recording of photosynthetic parameters and a system for intracellular measurements of electrical activity were used in the experiments. It was shown that the effect of exogenous ABA on photosynthesis was most prominent 1 day after spraying of a plant. A detailed analysis of photosynthetic processes showed that ABA decreased photosynthetic assimilation of CO₂ and increased cyclic electron flow in plants under study; these processes were interconnected. A decrease of the mesophyll conductance to CO₂ was probably a mechanism of the decrease in the photosynthetic assimilation of CO₂, because ABA did not significantly influence water conductance of a leaf and parameters of the CO₂ fixation in Calvin–Benson cycle. It is likely that the decrease of the mesophyll conductance to CO₂ was related with a decrease of the plasma membrane conductance to carbon dioxide. Inactivation of H⁺-ATP-ase and changes in extracellular pH can be a mechanism of this decrease. A decrease of the metabolic component of the electrical resting potential after the ABA treatment testifies in favor of this possibility.     

A branch bag technique for simultaneous CO2 enrichment and assimilation measurements on beech (Fagus sylvatica L.)

A cheap CO₂ enrichment system was designed to perform continuous gas exchange measurements of branches of mature European beech trees (Fagus sylvatica L.). Branches were grown at ambient (350 cm³ m^-3) and elevated CO₂ (700cm³ m^-3) during the whole 1992 leafy period. Leaks resulting from airtightness defaults in the system appeared to be low enough to measure accurately net CO₂ assimilation and transpiration rates during the day. However, the CO₂ exchange rates during the night (respiration) were too low to allow accurate measurements. Elevated CO₂ had a great effect on the net assimilation rate of branches via its influence on both the C₃ photosynthetic pathway and the shade-tolerance of beech trees (85% increase). The A/Ca curves showed no acclimation effect to high CO₂, both control and enriched branches increasing their net assimilation in the same way. The decrease of net assimilation rates in mature leaves was similar for both control and enriched branches. The pattern of daily transpiration rates remained the same for both control and enriched branches, hence we can assume that there was no visible CO₂ effect on stomata.     

Transmission characteristics and some other properties of bean yellow vein-banding virus, and its association with pea enation

Bean yellow vein-banding virus (BYVBV) has been found occasionally in mixed infection with pea enation mosaic virus (PEMV) in spring-sown field beans (Vicia faba minor) in southern England. Glasshouse tests confirmed that, like PEMV, BYVBV is transmissible by manual inoculation and by aphids in the persistent manner. However, BYVBV can be transmitted by aphids only from plants that are also infected with a helper virus, usually PEMV. Thus after separation from PEMV by passage through Phaseolus vulgaris it was no longer aphid-transmissible. It became aphid-transmissible again only after re-mixing in plants with PEMV or with a substitute helper, bean leaf roll virus (BLRV). It was not transmitted by aphids that fed sequentially on plants singly infected with PEMV and BYVBV. Thus the interaction between BYVBV and PEMV (or BLRV) that enables BYVBV to be transmitted by aphids seems to occur only in doubly infected plants. However, it was not transmitted by aphids from plants doubly infected with BYVBV and broad bean wilt virus (BBWV). BYVBV and PEMV were transmitted more readily by Acyrthosiphon pisum than by Myzus persicae; neither virus was transmitted by Aphis fabae. Phenol extracts of BYVBV-infected leaves were more infective than phosphate buffer or bentonite-clarified extracts and were sometimes infective when diluted to 1/1000. The infectivity of BYVBV in phosphate buffer extracts of leaves singly infected with BYVBV, unlike that in extracts of leaves doubly infected with BYVBV and PEMV (or BLRV), was destroyed by treatment with organic solvents. BYVBV infected 11 of 28 plant species that were inoculated with phenol extracts; seven of the infected species were legumes. No transmission of BYVBV was detected through seed harvested from infected field bean plants. Isometric particles c. 30 nm in diameter were seen in extracts of plants doubly infected with BYVBV and PEMV but not in extracts of plants infected with BYVBV alone. Leaves of plants infected with BYVBV, alone or with PEMV, contained membrane-bound structures c. 50–90 nm in diameter associated with the tonoplast in cell vacuoles. These structures were not found in healthy leaves. BYVBV has several properties in common with other known aphid-borne viruses that are helper-dependent and transmitted in a persistent manner. Possibly, as suggested for some of them, aphid transmission of BYVBV depends on the coating of its nucleic acid with helper virus coat protein.     

Modeling leaf CO2 assimilation and Photosystem II photochemistry from chlorophyll fluorescence and the photochemical reflectance index

We present a simple model to assess the quantum yield of photochemistry (Φ_P) and CO₂ assimilation rate from two parameters that are detectable by remote sensing: chlorophyll (chl) fluorescence and the photochemical reflectance index (PRI). Φ_P is expressed as a simple function of the chl fluorescence yield (Φ_F) and nonphotochemical quenching (NPQ): Φ_P = 1–bΦ_F(1 + NPQ). Because NPQ is known to be related with PRI, Φ_P can be remotely assessed from solar‐induced fluorescence and the PRI. The CO₂ assimilation rate can be assessed from the estimated Φ_P value with either the maximum carboxylation rate (V_cmax), the intercellular CO₂ concentration (C_i), or parameters of the stomatal conductance model. The model was applied to experimental data obtained for Chenopodium album leaves under various environmental conditions and was able to successfully predict Φ_F values and the CO₂ assimilation rate. The present model will improve the accuracy of assessments of gas exchange rates and primary productivity by remote sensing.     

Evaluation of the role of damage to photosystem II in the inhibition of CO2 assimilation in pea leaves on exposure to UV-B radiation

Mature pea (Pisum sativum L., cv. Meteor) leaves were exposed to two levels of UV-B radiation, with and without supplementary UV-C radiation, during 15 h photoperiods. Simultaneous measurements of CO₂ assimilation and modulated chlorophyll fluorescence parameters demonstrated that irradiation with UV-B resulted in decreases in CO₂ assimilation that are not accompanied by decreases in the maximum quantum efficiency of photosystem II (PSII) primary photochemistry. Increased exposure to UV-B resulted in a further loss of CO₂ assimilation and decreases in the maximum quantum efficiency of PSII primary photochemistry, which were accompanied by a loss of the capacity of thylakoids isolated from the leaves to bind atrazine, thus demonstrating that photodamage to PSII reaction centres had occurred. Addition of UV-C to the UV-B treatments increased markedly the rate of inhibition of photosynthesis, but the relationships between CO₂ assimilation and PSII characteristics remained the same, indicating that UV-B and UV-C inhibit leaf photosynthesis by a similar mechanism. It is concluded that PSII is not the primary target site involved in the onset of the inhibition of photosynthesis in pea leaves induced by irradiation with UV-B.     

Effects of moderate high-temperature stress on photosynthesis in three saplings of the constructive tree species of subtropical forest

During the exposition to moderate high-temperature stress, photosynthetic rates and fluorescence of chlorophyll a were measured with a photosynthetic measurement system (Li-Cor 6400) and leaf chamber fluorometer (Li-Cor6400 LCF), respectively, in leaves of saplings, sun-adapted species (Schima superba), shade-adapted species (Cryptocarya concinna), and in mesophytic plant (Castanopsis hystrix) (42°C). The results showed that moderate high-temperature stress led to a decrease in F_v/F_>m, namely the primary photochemical quantum efficiency, indicating that moderate high-temperature stress causes a partial inhibition of PSII. It also showed that such an effect was more severe in the shade-adapted plant C. concinna than in the sun-adapted species S. superba. However, except for the sun-grown leaves of C. concinna, the moderate high-temperature stress increased the photosynthetic rate of leaves at high light intensity. Simultaneously, less photoinhibition was found to occur under high-light intensity, suggesting that the capacity of resistant-photoinhibition was stimulated by moderate high-temperature stress. The quantum yield of PSII (?_PSII) decreased in the sun-grown leaves of S. superba and C. hystrix but did not show any significant change in leaves of the shade plant C. concinna and shade-grown leaves of sun plant S. superba or the mesophytic plant C. hystrix because they already had a very low ?_PSII under this condition. Moderate high-temperature stress led to a decrease in ?_PSII/?_CO2 ratios, an estimate of the quantum requirement for CO2 assimilation, in the sun plant S. superba and the mesophytic plant C. hystrix because they were associated with the dissipation of a lower fraction of excitation energy. However, no significant changes were found in shade plant C. concinna and in shade-grown leaves of the other examined plants. The effect of moderate high-temperature (42°C) on photosynthesis depends on species and leaf type (sun and shade leaves) in the saplings of subtropical broad-leaved trees.     

High-light-like photosynthetic responses of Cucumis sativus leaves acclimated to fluorescent illumination with a high red:far-red ratio: interaction between light quality and quantity

This study evaluated the photosynthetic responses of Cucumis sativus leaves acclimated to illumination from three-band white fluorescent lamps with a high red:far-red (R:FR) ratio (R:FR = 10.5) and the photosynthetic responses of leaves acclimated to metal-halide lamps that provided a spectrum similar to that of natural light (R:FR = 1.2) at acclimation photosynthetic photon flux density (PPFD) of 100 to 700 μmol m^?2 s^?1. The maximum gross photosynthetic rate (P _G) of the fluorescent-acclimated leaves was approximately 1.4 times that of the metal-halide-acclimated leaves at all acclimation PPFDs. The ratio of quantum efficiency of photosystem II (Φ_PSII) of the fluorescent-acclimated leaves to that of the metal-halide-acclimated leaves tended to increase with increasing acclimation PPFD, whereas the corresponding ratios for the leaf mass per unit area tended to decrease with increasing acclimation PPFD. These results suggest that the greater maximum P _G of the fluorescent-acclimated leaves resulted from an interaction between the acclimation light quality and quantity, which was mainly caused by the greater leaf biomass for photosynthesis per area at low acclimation PPFDs and by the higher Φ_PSII as a result of changes in characteristics and distribution of chloroplasts, or a combination of these factors at high acclimation PPFDs.     

Effects of moderate high-temperature stress on photosynthesis in three saplings of the constructive tree species of subtropical forest

During the exposition to moderate high-temperature stress, photosynthetic rates and fluorescence of chlorophyll a were measured with a photosynthetic measurement system (Li-Cor 6400) and leaf chamber fluorometer (Li-Cor6400 LCF), respectively, in leaves of saplings, sun-adapted species (Schima superba), shade-adapted species (Cryptocarya concinna), and in mesophytic plant (Castanopsis hystrix) (42°C). The results showed that moderate high-temperature stress led to a decrease in F_v/F_m, namely the primary photochemical quantum efficiency, indicating that moderate high-temperature stress causes a partial inhibition of PSII. It also showed that such an effect was more severe in the shade-adapted plant C. concinna than in the sun-adapted species S. superba. However, except for the sun-grown leaves of C. concinna, the moderate high-temperature stress increased the photosynthetic rate of leaves at high light intensity. Simultaneously, less photoinhibition was found to occur under high-light intensity, suggesting that the capacity of resistant-photoinhibition was stimulated by moderate high-temperature stress. The quantum yield of PSII (ϕ_PSII) decreased in the sun-grown leaves of S. superba and C. hystrix but did not show any significant change in leaves of the shade plant C. concinna and shade-grown leaves of sun plant S. superba or the mesophytic plant C. hystrix because they already had a very low ϕ_PSII under this condition. Moderate high-temperature stress led to a decrease in ϕ_PSII/ϕ_CO2 ratios, an estimate of the quantum requirement for CO₂ assimilation, in the sun plant S. superba and the mesophytic plant C. hystrix because they were associated with the dissipation of a lower fraction of excitation energy. However, no significant changes were found in shade plant C. concinna and in shade-grown leaves of the other examined plants. The effect of moderate high-temperature (42°C) on photosynthesis depends on species and leaf type (sun and shade leaves) in the saplings of subtropical broad-leaved trees.     

Necrosis in field beans (Vicia faba) induced by interactions between bean leaf roll, pea early-browning and pea enation mosaic viruses

Mixed infections with two or three viruses - bean leaf roll (BLRV), pea early-browning (PEBV) and pea enation mosaic (PEMV) - were detected in plants showing leaf curling, stunting and necrosis in a crop of field beans grown for seed in 1980. In glasshouse tests, field bean plants infected with any one of these viruses showed no necrosis, and plants infected with PEBV and PEMV together showed symptoms of PEMV only. However, mixed infection with BLRV and PEMV almost invariably induced severe stunting and leaf necrosis, and infection with BLRV and PEBV often induced both leaf and stem necrosis and sometimes caused early death. Thus it seems that the necrotic symptoms seen in the field were induced by interactions between BLRV and the other viruses. No transmission of PEBV was detected through seed harvested from the crop, but up to 5% transmission was detected through seed from experimentally-infected plants. The infected seedlings were symptomless.     

CO2 assimilation, ribulose-1,5-bisphosphate carboxylase/oxygenase, carbohydrates and photosynthetic electron transport probed by the JIP-test, of tea leaves in response to phosphorus supply

Background  

Although the effects of P deficiency on tea (Camellia sinensis (L.) O. Kuntze) growth, P uptake and utilization as well as leaf gas exchange and Chl a fluorescence have been investigated, very little is known about the effects of P deficiency on photosynthetic electron transport, photosynthetic enzymes and carbohydrates of tea leaves. In this study, own-rooted 10-month-old tea trees were supplied three times weekly for 17 weeks with 500 mL of nutrient solution at a P concentration of 0, 40, 80, 160, 400 or 1000 μM. This objective of this study was to determine how P deficiency affects CO₂ assimilation, Rubisco, carbohydrates and photosynthetic electron transport in tea leaves to understand the mechanism by which P deficiency leads to a decrease in CO₂ assimilation.     

In vivo photosynthetic electron transport does not limit photosynthetic capacity in phosphate-deficient sunflower and maize leaves

The effects of extreme phosphate (Pi) deficiency during growth on the contents of adenylates and pyridine nucleotides and the in vivo photochemical activity of photosystem II (PSII) were determined in leaves of Helianthus annuus and Zea mays grown under controlled environmental conditions. Phosphate deficiency decreased the amounts of ATP and ADP per unit leaf area and the adenylate energy charge of leaves. The amounts of oxidized pyridine nucleotides per unit leaf area decreased with Pi deficiency, but not those of reduced pyridine nucleotides. This resulted in an increase in the ratio of reduced to oxidized pyridine nucleotides in Pi-deficient leaves. Analysis of chlorophyll a fluorescence at room temperature showed that Pi deficiency decreased the efficiency of excitation capture by open PSII reaction centres (φ_e), the in vivo quantum yield of PSII photochemistry (φ_PSII) and the photochemical quenching co-efficient (q_P), and increased the non-photochemical quenching co-efficient (q_N) indicating possible photoinhibitory damage to PSII. Supplying Pi to Pi-deficient sunflower leaves reversed the long-term effects of Pi-deficiency on PSII photochemistry. Feeding Pi-sufficient sunflower leaves with mannose or FCCP rapidly produced effects on chlorophyll a fluorescence similar to long-term Pi-deficiency. Our results suggest a direct role of Pi and photophosphorylation on PSII photochemistry in both long-and short-term responses of photosynthetic machinery to Pi deficiency. The relationship between φ_PSII and the apparent quantum yield of CO₂ assimilation determined at varying light intensity and 21 kPa O₂ and 35 Pa CO₂ partial pressures in the ambient air was linear in Pi-sufficient and Pi-deficient leaves of sunflower and maize. Calculations show that there was relatively more PSII activity per mole of CO₂ assimilated by the Pi-deficient leaves. This indicates that in these leaves a greater proportion of photosynthetic electrons transported across PSII was used for processes other than CO₂ reduction. Therefore, we conclude that in vivo photosynthetic electron transport through PSII did not limit photosynthesis in Pi-deficient leaves of sunflower and maize and that the decreased CO₂ assimilation was a consequence of a smaller ATP content and lower energy charge which restricted production of ribulose, 1-5, bisphosphate, the acceptor for CO₂.     

Far‐red light enhances photochemical efficiency in a wavelength‐dependent manner

Linear electron transport depends on balanced excitation of photosystem I and II. Far‐red light preferentially excites photosystem I (PSI) and can enhance the photosynthetic efficiency when combined with light that over‐excites photosystem II (PSII). The efficiency of different wavelengths of far‐red light exciting PSI was quantified by measuring the change in quantum yield of PSII (Φ_PSII) of lettuce (Lactuca sativa) under red/blue light with narrowband far‐red light added (from 678 to 752 nm, obtained using laser diodes). The Φ_PSII of lettuce increased with increasing wavelengths of added light from 678 to 703 nm, indicating longer wavelengths within this region are increasingly used more efficiently by PSI than by PSII. Adding 721 nm light resulted in similar Φ_PSII as adding 703 nm light, but Φ_PSII tended to decrease as wavelength increased from 721 to 731 nm, likely due to decreasing absorptance and low photon energy. Adding 752 nm light did not affect Φ_PSII. Leaf chlorophyll fluorescence light response measurements showed lettuce had higher Φ_PSII under halogen light (rich in far‐red) than under red/blue light (which over‐excites PSII). Far‐red light is more photosynthetically active than commonly believed, because of its synergistic interaction with light of shorter wavelengths.     

Relationship between CO2 Assimilation, Photosynthetic Electron Transport, and Active O2 Metabolism in Leaves of Maize in the Field during Periods of Low Temperature

Measurements of the quantum efficiencies of photosynthetic electron transport through photosystem II (_PSII) and CO₂ assimilation (_CO2) were made simultaneously on leaves of maize (Zea mays) crops in the United Kingdom during the early growing season, when chilling conditions were experienced. The activities of a range of enzymes involved with scavenging active O₂ species and the levels of key antioxidants were also measured. When leaves were exposed to low temperatures during development, the ratio of _PSII/_CO2 was elevated, indicating the operation of an alternative sink to CO₂ for photosynthetic reducing equivalents. The activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and superoxide dismutase and the levels of ascorbate and α-tocopherol were also elevated during chilling periods. This supports the hypothesis that the relative flux of photosynthetic reducing equivalents to O₂ via the Mehler reaction is higher when leaves develop under chilling conditions. Lipoxygenase activity and lipid peroxidation were also increased during low temperatures, suggesting that lipoxygenase-mediated peroxidation of membrane lipids contributes to the oxidative damage occurring in chill-stressed leaves.     

Higher electron transport rate observed at low intercellular CO2 concentration in long-term drought-acclimated leaves of Japanese mountain birch (Betula ermanii)

The influence of long‐term drought stress on photosynthesis of Japanese mountain birch (Betula ermanii Cham.) was examined using chlorophyll fluorescence and gas exchange measurements. Drought stress was imposed in potted plants by reducing irrigation frequency from daily (control) to twice‐weekly and once‐weekly. Thirty‐day‐old leaves, which had developed under fully stressed conditions, were used for the measurements. The decline in net CO₂ assimilation rate (A) observed in situ in drought‐stressed plants resulted from a lower intercellular CO₂ concentration (C_i) due to stomatal closure but the carboxylation efficiency was not affected as there was no difference in the initial slope of the A/C_i response after watering. Although there were no treatment differences in A at C_i below 270 μmol mol^?1 (with ambient air at 360 μmol mol^?1 CO₂), higher electron transport rate (ETR), photochemical quenching (q_P) and the efficiency of energy conversion of open PSII (F_v′/F_m′), and similar or even lower non‐photochemical quenching (NPQ) were observed at a given C_i in drought‐stressed plants (of both twice‐ and once‐weekly irrigation), suggesting a higher fraction of open PSII resulting from energy dissipation achieved through higher electron flow rather than through thermal dissipation in PSII antennae. The once‐weekly watered plants showed a lower ratio of gross carbon assimilation rate to ETR (A*/ETR), suggesting an enhanced alternative pathway of electron flow probably involving the Mehler‐peroxidase (MP) reaction as indicated by a higher Φ_PSII at a given Φ_CO2 under non‐photorespiratory conditions. On the other hand, plants of twice‐weekly watering exhibited almost the same A*/ETR and Φ_PSII–Φ_CO2 relationship as control plants, indicating no enhanced alternative pathways under mild drought stress.     

Improvements in the detection of pea seed-borne mosaic virus by ELISA

The detection by ELISA of pea seed-borne mosaic virus (PSbMV) in pea leaves and seeds was improved by the addition of cellulase or Triton X-100 to the extraction fluid, probably because the additives aided the release of virus particles from host materials. With leaf extracts the additives were most effective at 0.1%. In initial tests cellulase was used with macerozyme, but the latter enzyme was then shown to decrease the effectiveness of cellulase. Triton X-100 was as effective as cellulase and the absorbance values obtained in ELISA of infected leaf extracts, diluted to 1/10 in extraction fluid containing the additive, were about six times greater than those of infected extracts diluted in normal extraction fluid. Five named isolates of PSbMV, in addition to the homologous isolate, were readily detected in infected leaves extracted in fluid containing Triton X-100. In tests on seeds and seedlings of seven infected seed lots of pea cv. Waverex, using Triton X-100 in the extraction fluid, PSbMV was detected in five times as many seeds as seedlings, probably mainly because in many infected seeds the virus was in the testa and not in the embryo. About 9% of infected seedlings were without recognisable symptoms 4 wk after emergence.     

Separation and measurement of direct and indirect effects of light on stomata

Conductance for water vapor, assimilation of CO₂, and intercellular CO₂ concentration of leaves of five species were determined at various irradiances and ambient CO₂ concentrations. Conductance and assimilation were then plotted as functions of irradiance and intercellular CO₂ concentration. The slopes of these curves allowed us to estimate infinitesimal changes in conductance (and assimilation) that occurred when irradiance changed and intercellular CO₂ concentration was constant, and when CO₂ concentration changed and irradiance was constant. On leaves of Xanthium strumarium L., Gossypium hirsutum L., Phaseolus vulgaris L., and Perilla frutescens (L.), Britt., the stomatal response to light was determined to be mainly a direct response to light and to a small extent only a response to changes in intercellular CO₂ concentration. This was also true for stomata of Zea mays L., except at irradiances < 150 watts per square meter, when stomata responded primarily to the depletion of the intercellular spaces of CO₂ which in turn was caused by changes in the assimilation of CO₂.     

Reduction of ribulose-1,5-bisphosphate carboxylase/oxygenase content by antisense RNA reduces photosynthesis in transgenic tobacco plants

A complementary DNA for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was cloned from tobacco (Nicotiana tabacum) and fused in the antisense orientation to the cauliflower mosaic virus 35S promoter. This antisense gene was introduced into the tobacco genome, and the resulting transgenic plants were analyzed to assess the effect of the antisense RNA on Rubisco activity and photosynthesis. The mean content of extractable Rubisco activity from the leaves of 10 antisense plants was 18% of the mean level of activity of control plants. The soluble protein content of the leaves of anti-small subunit plants was reduced by the amount equivalent to the reduction in Rubisco. There was little change in phosphoribulokinase activity, electron transport, and chlorophyll content, indicating that the loss of Rubisco did not affect these other components of photosynthesis. However, there was a significant reduction in carbonic anhydrase activity. The rate of CO₂ assimilation measured at 1000 micromoles quanta per square meter per second, 350 microbars CO₂, and 25°C was reduced by 63% (mean value) in the antisense plants and was limited by Rubisco activity over a wide range of intercellular CO₂ partial pressures (p_i). In control leaves, Rubisco activity only limited the rate of CO₂ assimilation below a p_i of 400 microbars. Despite the decrease in photosynthesis, there was no reduction in stomatal conductance in the antisense plants, and the stomata still responded to changes in p_i. The unchanged conductance and lower CO₂ assimilation resulted in a higher p_i, which was reflected in greater carbon isotope discrimination in the leaves of the antisense plants. These results suggest that stomatal function is independent of total leaf Rubisco activity.