Systemic signalling of environmental cues in Arabidopsis leaves.

Light intensity and atmospheric CO2 partial pressure are two environmental signals known to regulate stomatal numbers. It has previously been shown that if a mature Arabidopsis leaf is supplied with either elevated CO2 (750 ppm instead of ambient at 370 ppm) or reduced light levels (50 micromol m-2 s-1 instead of 250 micromol m-2 s-1), the young, developing leaves that are not receiving the treatment grow with a stomatal density as if they were exposed to the treatment. But the signal(s) that it is believed is generated in the mature leaves and transmitted to developing leaves are largely unknown. Photosynthetic rates of treated, mature Arabidopsis leaves increased in elevated CO2 and decreased when shaded, as would be expected. Similarly, the levels of sugars (glucose, fructose, and sucrose) in the treated mature leaves increased in elevated CO2 and decreased with shade treatment. The levels of sugar in developing leaves were also measured and it was found that they mirrored this result even though they were not receiving the shade or elevated CO2 treatment. To investigate the effect of these treatments on global gene expression patterns, transcriptomics analysis was carried out using Affymetrix, 22K, and ATH1 arrays. Total RNA was extracted from the developing leaves after the mature leaves had received either the ambient control treatment, the elevated CO2 treatment, or the shade treatment, or both elevated CO2 and shade treatments for 2, 4, 12, 24, 48, or 96 h. The experiment was replicated four times. Two other experiments were also conducted, one to compare and contrast gene expression in response to plants grown at elevated CO2 and the other to look at the effect of these treatments on the mature leaf. The data were analysed and 915 genes from the untreated, signalled leaves were identified as having expression levels affected by the shade treatment. These genes were then compared with those whose transcript abundance was affected by the shade treatment in the mature treated leaves (1181 genes) and with 220 putative 'stomatal signalling' genes previously identified from studies of the yoda mutant. The results of these experiments and how they relate to environmental signalling are discussed, as well as possible mechanisms for systemic signalling.

The role of herbicides in the erosion of salt marshes in eastern England.

Laboratory studies and field trials were conducted to investigate the role of herbicides on saltmarsh vegetation, and their possible significance to saltmarsh erosion. Herbicide concentrations within the ranges present in the aquatic environment were found to reduce the photosynthetic efficiency and growth of both epipelic diatoms and higher saltmarsh plants in the laboratory and in situ. The addition of sublethal concentrations of herbicides resulted in decreased growth rates and photosynthetic efficiency of diatoms and photosynthetic efficiency of higher plants. Sediment stability also decreased due to a reduction in diatom EPS production. There was qualitative evidence that diatoms migrated deeper into the sediment when the surface was exposed to simazine, reducing surface sediment stability by the absence of a cohesive biofilm. Sediment loads on leaves severely reduced photosynthesis in Limonium vulgare. This, coupled with reduced carbon assimilation from the effects of herbicides, could have large negative consequences for plant productivity and over winter survival of saltmarsh plants. The data support the hypothesis that sublethal herbicide concentrations could be playing a role in the increased erosion of salt marshes that has occurred over the past 40 years.

Impact of folivory on photosynthesis is greater than the sum of its holes.

The effects of herbivores on plant production and fitness may not relate directly to the quantity of biomass removed because folivory may alter photosynthetic rates at a considerable distance from the damaged tissue [Welter, S. C. (1989) in Insect-Plant Interactions, ed. Bernays, E. A. (CRC, Boca Raton), pp. 135-151.]. An impediment to understanding the effects of leaf damage on photosynthesis has been an inability to map photosynthetic function within a single leaf. We developed an instrument for imaging chlorophyll fluorescence and used it to map the effects of caterpillar feeding on whole-leaf photosynthesis in wild parsnip. The adverse effects of caterpillar feeding on photosynthesis were found to extend well beyond the areas of the leaflet in which caterpillars removed tissue. These "indirectly" affected areas remained impaired for at least 3 days after the caterpillars were removed and were six times as large as the area directly damaged by the caterpillars. Although photosynthesis in indirectly affected areas was reduced and not eliminated, these areas accounted for three times as much of the overall reduction in photosynthesis as the area removed by the caterpillars. The size of the indirect effects was positively correlated with defense-related synthesis of furanocoumarins, suggesting that costs of chemical defense may be one factor that accounts for the indirect effects of herbivory on plants.

Primary sites of ozone-induced perturbations of photosynthesis in leaves: identification and characterization in Phaseolus vulgaris using high resolution chlorophyll fluorescence imaging.

High resolution imaging of chlorophyll a fluorescence was used to identify the sites at which ozone initially induces perturbations of photosynthesis in leaves of Phaseolus vulgaris. Leaves were exposed to 250 and 500 nmol mol(-1) ozone at a photosynthetically active photon flux density of 300 micromol m(-2) s(-1) for 3 h. Images of fluorescence parameters indicated that large decreases in both the maximum and operating quantum efficiencies of photosystem II had occurred in cells adjacent to stomata in the upper, but not lower, leaf surfaces. However, this treatment did not produce any significant changes in the maximum or operating quantum efficiencies of photosystem II in the leaves when estimated from fluorescence parameters measured with a conventional, integrating fluorometer. The localized decreases in photosystem II photochemical efficiencies were accompanied by an increase in the minimal fluorescence level, which is indicative of photoinactivation of photosystem II complexes and a decrease in stomatal conductance. Perturbations of photochemical efficiencies were not observed in cells associated with all of the stomata on the upper leaf surface or within cells distant from the upper leaf surface. It is concluded that ozone penetrates the leaf through stomata and initially damages only cells close to stomatal pores.

High resolution imaging of photosynthetic activities of tissues, cells and chloroplasts in leaves.

Through imaging of chlorophyll fluorescence, it is possible to produce parameterized fluorescence images that estimate the operating quantum efficiency of photosystem II (PSII) photochemistry and which can be used to reveal heterogeneous patterns of photosynthetic performance within leaves. The operating quantum efficiency of PSII photochemistry is dependent upon the effective absorption cross-section of the light-harvesting system of PSII and the photochemical capacity of PSII. The effective absorption cross-section is decreased by the process of down-regulation, which is widely thought to operate within the pigment matrices of PSII and which results in non-photochemical quenching of chlorophyll fluorescence. The photochemical capacity is non-linearly related to the proportion of PSII centres in the 'open' state and results in photochemical quenching of chlorophyll fluorescence. Examples of heterogeneity of the operating quantum efficiency of PSII photochemistry during the induction of photosynthesis in maize leaves and in the chloroplast populations of stomatal guard cells of a leaf of Tradescantia albifora are presented, together with analyses of the factors determining this heterogeneity. A comparison of the operating quantum efficiency of PSII photochemistry within guard cells and adjacent mesophyll cells of Commelina communis is also made, before and after stomatal closure through a change in ambient humidity.

An evaluation of the potential triggers of photoinactivation of photosystem II in the context of a Stern-Volmer model for downregulation and the reversible radical pair equilibrium model.

Photoinactivation of photosystem II (PS II) is a light-dependent process that frequently leads to break-down and replacement of the D1 polypeptide. Photoinhibition occurs when the rate of photoinactivation is greater than the rate at which D1 is replaced and results in a decrease in the maximum efficiency of PS II photochemistry. Downregulation, which increases non-radiative decay within PS II, also decreases the maximum efficiency of PS II photochemistry and plays an important role in protecting against photoinhibition by reducing the yield of photoinactivation. The yield of photoinactivation has been shown to be relatively insensitive to photosynthetically active photon flux density (PPFD). Formation of the P680 radical (P680+), through charge separation at PS II, generation of triplet-state P680 (3P680*), through intersystem crossing and charge recombination, and double reduction of the primary stable electron acceptor of PS II (the plastoquinone, Q(A)) are all potentially critical steps in the triggering of photoinactivation. In this paper, these processes are assessed using fluorescence data from attached leaves of higher plant species, in the context of a Stern-Volmer model for downregulation and the reversible radical pair equilibrium model. It is shown that the yield of P680+ is very sensitive to PPFD and that downregulation has very little effect on its production. Consequently, it is unlikely to be the trigger for photoinactivation. The yields of 3P680* generated through charge recombination or intersystem crossing are both less sensitive to PPFD than the yield of P680+ and are both decreased by down regulation. The yield of doubly reduced Q(A) increases with incident photon flux density at low levels, but is relatively insensitive at moderate to high levels, and is greatly decreased by downregulation. Consequently, 3P680* and doubly reduced Q(A) are both viable as triggers of photoinactivation.

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 (phiPSII) and CO2 assimilation (phiCO2) 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 phiPSII/phiCO2 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 alpha-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.

Light-dependent modification of Photosystem II in spinach leaves.

In dark-adapted spinach leaves approximately one third of the Photosystem II (PS II) reaction centers are impaired in their ability to transfer electrons to Photosystem I. Although these 'inactive' PS II centers are capable of reducing the primary quinone acceptor, QA, oxidation of QA (-) occurs approximately 1000 times more slowly than at 'active' centers. Previous studies based on dark-adapted leaves show that minimal energy transfer occurs from inactive centers to active centers, indicating that the quantum yield of photosynthesis could be significantly impaired by the presence of inactive centers. The objective of the work described here was to determine the performance of inactive PS II centers in light-adapted leaves. Measurements of PS II activity within leaves did not indicate any increase in the concentration of active PS II centers during light treatments between 10 s and 5 min, showing that inactive centers are not converted to active centers during light treatment. Light-induced modification of inactive PS II centers did occur, however, such that 75% of these centers were unable to sustain stable charge separation. In addition, the maximum yield of chlorophyll fluorescence associated with inactive PS II centers decreased substantially, despite the lack of any overall quenching of the maximum fluorescence yield. The effect of light treatment on inactive centers was reversed in the dark within 10-20 mins. These results indicate that illumination changes inactive PS II centers into a form that quenches fluorescence, but does not allow stable charge separation across the photosynthetic membrane. One possibility is that inactive centers are converted into centers that quench fluorescence by formation of a radical, such as reduced pheophytin or oxidized P680. Alternatively, it is possible that inactive PS II centers are modified such that absorbed excitation energy is dissipated thermally, through electron cycling at the reaction center.

Factors Associated with Depression of Photosynthetic Quantum Efficiency in Maize at Low Growth Temperature.

The photosynthetic productivity of maize (Zea mays) in temperate regions is often limited by low temperatures. The factors responsible for the sensitivity of photosynthesis in maize to growth at suboptimal temperature were investigated by measuring (a) the quantum yields of CO2 fixation and photosystem II (PSII) photochemistry, (b) the pigments of the xanthophyll cycle, (c) the concentrations of active and inactive PSII reaction centers, and (d) the synthesis of core components of PSII reaction centers. Measurements were made on fully expanded leaves grown at 14[deg]C, both before and during the first 48 h after transfer of these plants to 25[deg]C. Our findings indicate that zeaxanthin-related quenching of absorbed excitation energy at PSII is, quantitatively, the most important factor determining the depressed photosynthetic efficiency in 14[deg]C-grown plants. Despite the photoprotection afforded by zeaxanthin-related quenching of absorbed excitation energy, a significant and more persistent depression of photosynthetic efficiency appears to result from low temperature-induced inhibition of the rate at which damaged PSII centers can be replaced.

In situ evidence that chilling in the light does not cause uncoupling of photophosphorylation or detachment of coupling factor in chilling-sensitive plants.

The potential involvement of impaired photophosphorylation in the chilling sensitivity of photosynthesis in warm climate plant species has been a topic of investigation for more than two decades. With recent advances in the analysis of photosynthetic energy transduction in intact leaves, experiments are now possible that either address or avoid important uncertainties in the significance and interpretation of earlier in vitro work. Nevertheless, different laboratories using different techniques to analyze the effects of chilling in the light on photophosphorylation in intact cucumber (Cucumis sativus) leaves have come to very different conclusions regarding the role of impaired ATP formation capacity in the inhibition of net photosynthesis. In order to evaluate these discrepancies and bring this issue to a final resolution, in this investigation, we have made a detailed analysis of the decay of the flash-induced electrochromic shift and changes in chlorophyll fluorescence yield in cucumber leaves before, during and after a 5 h light-chill at chill temperatures of between 4 and 10°C. We feel that our findings address the major discrepancies in both data and interpretation as well as provide convincing evidence that photophosphorylation is not disrupted in cucumber leaves during or after light and chilling exposure. It follows that impaired photophosphorylation is not a contributing element to the inhibition of net photosynthesis that is widely observed in warm climate plants as a result of chilling in the light.

Characterisation of the effects of Antimycin A upon high energy state quenching of chlorophyll fluorescence (qE) in spinach and pea chloroplasts.

High energy state quenching of chlorophyll fluorescence (qE) is inhibited by low concentrations of the inhibitor antimycin A in intact and osmotically shocked chloroplasts isolated from spinach and pea plants. This inhibition is independent of any effect upon ΔpH (as measured by 9-aminoacridine fluorescence quenching). A dual control of qE formation, by ΔpH and the redox state of an unidentified chloroplast component, is implied. Results are discussed in terms of a role for qE in the dissipation of excess excitation energy within photosystem II.