Apparent Electron Transport Research Articles

Effects of drought stress on photosynthetic gas exchange, chlorophyll fluorescence and stem diameter of soybean plants

Changes in plant growth, photosynthetic gas exchange, chlorophyll fluorescence and stem diameter of soybean [Glycine max (L.) Merr.] plants under drought stress were studied. Total plant dry mass was reduced by 30 % compared to well-watered control plants. Leaf water potential was slightly decreased by water stress. Water stress induced daytime shrinkage and reduced night-time expansion of stem. Photosynthetic rate, stomatal conductance and transpiration rate were significantly declined by water stress, while the intercellular CO2 concentration was changed only slightly at the initiation of stress treatment. The maximum photochemical efficiency of photosystem 2 and apparent photosynthetic electron transport rate were not changed by water stress.

The effects of sooty mold on photosynthesis and mesophyll structure of mahogany (Swietenia macrophylla King., Meliaceae)

The aim of present study was to evaluate the effects of the sooty mold on anatomy and photochemical activity of mahogany (Swietenia macrophylla) leaves. The photochemical features of shade-developed leaves with or without sooty mold were compared to those of sun leaves using chlorophyll a fluorescence measurements. Leaf anatomy was also evaluated using conventional techniques. The degree of blockage of the photosynthetic active photon flux density (PPFD) by sooty mold and its effect on photochemistry were evaluated. Sun leaves showed thick mesophyll with palisade parenchyma disposed in a uniseriate layer, whereas shade leaves showed narrow mesophyll, independently of sooty mold presence. The effective quantum yield (deltaF/Fm') and the apparent electron transport rate (ETR) of sun leaves were higher than those of shade leaves. The values of ETR suggested that photochemistry saturation occurred at lower PPFD in shade-grown plants. Lower values of the deltaF/Fm' and, consequently, lower values of ETR were observed in leaves with sooty mold. A reduction of 40% of the incident light was seen due to physical blockage by sooty mold which is presumably responsible for an additional decrease of ETR values. Our data indicated that sooty mold did not directly damage the leaf, but reduce leaf photochemistry capacity, by decreasing light availability.

Physiological changes and UV protection in the aquatic liverwort Jungermannia exsertifolia subsp. cordifolia along an altitudinal gradient of UV-B radiation.

Here we report the effects of a natural altitudinal gradient of UV-B radiation, from 1140 to 1816 m altitude, on the physiology of the aquatic liverwort Jungermannia exsertifolia Steph. subsp. cordifolia (Dumort.) Váña collected in mountain streams. Photosynthetic pigments, net photosynthesis and dark respiration rates, chlorophyll fluorescence, protein concentration, sclerophylly, and UV-absorbing compounds [both global UV absorbance of methanol-extractable UV-absorbing compounds (MEUVAC) and concentrations of five individual compounds] were measured. Two new caffeic acid derivatives were discovered: 5″-(7″,8″-dihydroxycoumaroyl)-2-caffeoylmalic acid and 5″-(7″,8″-dihydroxy-7-O-β-glucosyl-coumaroyl)-2-caffeoylmalic acid, whereas three additional compounds were already known in other species: p-coumaroylmalic acid, phaselic acid (both compounds in their cis- and trans- forms) and feruloylmalic acid. Most physiological variables changed considerably along the altitudinal gradient, but only six showed significant linear relationships with altitude: MEUVAC levels, the concentrations of the two new secondary compounds, the maximal apparent electron transport rate through PSII (ETRmax) and the maximal non-photochemical quenching (NPQmax) increased with altitude, whereas photoinhibition percentage decreased. A principal components analysis (PCA) was conducted to rank the values of the physiological and ecological variables obtained along the altitudinal transect, showing that those variables correlated with altitude were responsible for the ordination of the sampling points. The liverwort was not adversely affected by the changing conditions along the altitudinal gradient and, in particular, by the increasing UV-B irradiance, probably because the characteristics shown by high-altitude populations may confer tolerance to high UV-B levels. The response to UV-B of the two new compounds suggests that they could be used as indicators of the spatial changes in UV-B radiation.

Susceptibility to low-temperature photoinhibition in three conifers differing in successional status

Susceptibility to photoinhibition of the evergreen conifers Abies alba Mill., Picea abies (L.) Karst. and Pinus mugo Turra was investigated in an unheated greenhouse during winter and spring 2003. Photosynthetic performance of the seedlings was assessed by chlorophyll a fluorescence and analyses of chlorophyll and total carotenoid concentrations in needles. During winter months, maximum quantum yield of PSII photochemistry (ratio of variable to maximum fluorescence, Fv/Fm) was significantly greater in A. alba than in P. abies and P. mugo. Abies alba also sustained higher maximum apparent electron transport rate (ETRmax) than P. abies and P. mugo. Total concentrations of chlorophyll and carotenoids in needles decreased during the winter in P. mugo and P. abies, but remained stable in A. alba. For all species, Fv/Fm decreased from December until February and then increased to a maximum in April. Photoinhibition was greatest (Fv/Fm < 0.80) in all seedlings in February, the month with the lowest mean temperature. Saturating photosynthetic photon flux (PPFsat) and ETRmax were positively related to air temperature. All species had lower values of ETRmax and PPFsat in winter than in spring. Non-photochemical quenching of chlorophyll fluorescence (NPQ) was highest at low air temperatures. Differences among species in susceptibility to winter photoinhibition resulted from their specific light preferences and led to different mechanisms to cope with photoinhibitory stress. The more shade-tolerant A. alba sustained a higher photosynthetic capacity in winter than P. abies and P. mugo. Winter photoinhibition in P. abies, P. mugo and, to a lesser extent, in A. alba may reflect adaptive photoprotection of the photosynthetic apparatus in winter.

Seasonal photosynthetic performance and nutrients relations of Butea monosperma Taub. in comparison to two other woody species of a seasonal deciduous forest in Se-Rajasthan and to planted trees in the area

Butea monosperma Taub. (Fabaceae) is a widespread dominant tree of seasonally dry open forest on rocky hill sites in SE-Rajasthan. It is of ecological importance for habitat-stabilization and of commercial value for its timber and pharmacological applications. Physiological ecology of photosynthesis and water use was studied in B. monosperma in comparison to two less abundant woody species, the tall shrub Capparis sepiaria L. and the straggling, scandent shrub Cocculus hirsutus (L.) Diels. In a natural forest reserve. Levels of essential nutrients in the soil and leaves of these species are presented. Comparisons are also made with a variety of planted trees. B. monosperma tended to have the highest apparent photosynthetic electron transport rates (ETRmax). It had a high capacity for non-photochemical quenching (NPQ). Stable isotope signatures (13C, 18O) indicated flexibility for operating at a wide range of internal/atmospheric CO2 concentrations (ci/ca) and a high water use efficiency (WUE). Thus, ecophysiologically B. monosperma shows traits well suited for the arid rupestrian fields to which it is adapted. This explains its dominance in the area and makes B. monosperma a very suitable tree for afforestation and wasteland reclamation as well as a useful crop for a large variety of pharmacological and other applications.

Ecophysiology of selected tree species in different plant communities at the periphery of the Atlantic Forest of SE—Brazil III. Three legume trees in a semi-deciduous dry forest

Three legume tree species (Fabaceae) occurring abundantly in a semi-deciduous tropical dry forest of the Atlantic forest complex in southeastern Brazil were subjected to a comparative ecophysiological study at the end of the dry season/beginning of the wet season. The trees chosen were morphologically very similar: Caesalpinia echinata Lam. and Caesalpinia ferrea Mart. ex. Tul., both 10–20 m of height, of the sub-family Caesalpinioideae, and the somewhat smaller, 2–4 m tall, Machaerium obovatum Kuhlm. & Hoehne of the sub-family Faboideae. Despite their similarities with respect to their geographic distribution restricted to Brazilian dry forests, their comparable abundance in the study site and their phylogenetic proximity, the three species display distinctly different ecophysiological behaviour. Compared to the other two species, C. ferrea had the highest photosynthetic capacity (maximum apparent photosynthetic electron transport rate, ETRmax) and higher saturation light-intensity, was less subject to photoinhibition as indicated by potential quantum yield of photosystem II (F v/F m) and had the lowest bulk N content of which soluble non-protein N compounds were only 1.5%. It showed stronger sun plant characteristics. C. echinata had lower photosynthetic capacity, was under chronic photoinhibition and had high bulk N content of which 6.1% were soluble N compounds with high concentrations of proline. In addition to proline, high concentrations of sugars may serve as osmoprotectants. M. obovatum also showed lower photosynthetic capacity and was under chronic photoinhibition. Here, arginine may have a function as osmoprotectant. The ecophysiological differences between the three species are not related to local abundance. However, the observations presented highlight a contrasting behaviour of the otherwise very similar compatriot species.

Photosynthetic responses of tropical tree species from different successional groups under contrasting irradiance conditions

This study evaluated the photosynthetic responses of seven tropical trees of different successional groups under contrasting irradiance conditions, taking into account changes in gas exchange and chlorophyll a fluorescence. Although early successional species have shown higher values of CO2 assimilation (A) and transpiration (E), there was not a defined pattern of the daily gas exchange responses to high irradiance (FSL) among evaluated species. Cariniana legalis (Mart.) Kuntze (late secondary) and Astronium graveolens Jacq. (early secondary) exhibited larger reductions in daily-integrated CO2 assimilation (DIA) when transferred from medium light (ML) to FSL. On the other hand, the pioneer species Guazuma ulmifolia Lam. had significant DIA increase when exposed to FSL. The pioneers Croton spp. trended to show a DIA decrease around 19%, while Cytharexyllum myrianthum Cham. (pioneer) and Rhamnidium elaeocarpum Reiss. (early secondary) trended to increase DIA when transferred to FSL. Under this condition, all species showed dynamic photoinhibition, except for C. legalis that presented chronic photoinhibition of photosynthesis. Considering daily photosynthetic processes, our results supported the hypothesis of more flexible responses of early successional species (pioneer and early secondary species). The principal component analysis indicated that the photochemical parameters effective quantum efficiency of photosystem II and apparent electron transport rate were more suitable to separate the successional groups under ML condition, whereas A and E play a major role to this task under FSL condition.

Growth, photosynthesis, and needle structure of silver fir ( Abies alba Mill.) seedlings under different canopies

As part of fir reintroduction program in the Karkonosze Mountains (S. Poland) a study was undertaken to investigate growth and ecophysiological performance of young firs ( Abies alba) under varying degrees of natural shade. Three-year-old fir seedlings were planted in five forest stands dominated by Picea abies, Fagus sylvatica, Betula pendula, Pinus sylvestris or Larix decidua. Over four growing seasons, annual apical increments became progressively greater under the light permeable canopy of Larix, but remained stunned under the deep shade of Picea, while intermediate irradiance at other sites resulted in intermediate growth responses. Similarly, apical dominance ratio was positively influenced by irradiance. These two contrasting sites were subsequently used for ecophysiological comparisons. Fir seedlings under the Larix canopy had consistently higher photosynthetic rates throughout the day compared to those under Picea, but in both stands light adapted quantum yield of PSII remained mostly above 0.8 indicating lack of photoinhibition. Quantum yield declined transiently only during sunflecks, which were more frequent under Larix. Light dependence of apparent electron transport rates in both sites was linear showing lack of light saturation, while slope differences between sites were only slight. In contrast, depression of PSII quantum yield by light was less pronounced under Larix, showing an acclimation to higher irradiance. Fir needles produced under Larix were thicker and contained more chlorophyll, carotenoid and nitrogen on area basis (but, except for nitrogen, not on dry mass basis) suggesting that structural modification is a major component of acclimation to light. On anatomical levels, needles produced under Larix had the traits typical for sun-acclimated ones, while those under Picea for shade-acclimated. The magnitude of modifications was, however, less than that reported in literature for sun and shade needles. Young seedlings were able to acclimate to higher irradiance under Larix without showing light stress symptoms. This suggests that success of stand regeneration may be enhanced by adjusting light penetration through the overstorey. Fir seedlings found best growth conditions under the relatively light permeable crowns of light-demanding tree species ( Larix decidua, Pinus sylvestris) suggesting that stands dominated by these trees are more suitable for transformation using silver fir seedlings than those with shade-tolerant tree overstoreys.

Comparative stomatal conductance and chlorophyll a fluorescence in leaves vs. fruits of the cerrado legume tree, Dalbergia miscolobium

The aim of this study was to compare water vapor conductance and chlorophyll a fluorescence between leaflets and fruits of Dalbergia miscolobium, the Jacaranda tree. The frequency of stomata on the leaflets was 20 times higher than that observed on the fruits, and this was related with the lower conductance of the fruits in comparison with the leaflets. The potential quantum yield of PSII (Fv /Fmax) was significantly lower in fruits than in leaflets. The Fv /Fmax values for leaflets increased to over 0.8 during the afternoon, indicating the occurrence of dynamic photoinhibition. In contrast, Fv /Fmax values for fruits remained low even at early morning, indicating the occurrence of chronic photoinhibition. The maximum values of effective quantum yield (deltaF/F'm), and of the apparent electron transport rate (ETRmax) were higher in leaflets than in fruits. It was concluded that, like other green tissues, the pericarp of D. miscolobium was photosynthetically active, and therefore can contribute to the maintenance of the fruits and/or to the development of the seeds.

Chlorophyll fluorescence of tropical tree species in a semi-deciduous forest gap

The characterization of different ecological groups in a forest formation/succession is unclear. To better define the different successional classes, we have to consider ecophysiological aspects, such as the capacity to use or dissipate the light energy available. The main objective of this work was to assess the chlorophyll fluorescence emission of tropical tree species growing in a gap of a semi-deciduous forest. Three species of different ecological groups were selected: Croton floribundus Spreng. (pioneer, P), Astronium graveolens Jacq. (early secondary, Si), and Esenbeckia febrifuga A. Juss. (late secondary, St). The potential (Fv/Fm) and effective (deltaF/Fm') quantum efficiency of photosystem II, apparent electron transport rate (ETR), non-photochemical (qN) and photochemical (qP) quenching of fluorescence were evaluated, using a modulated fluorometer, between 7:30 and 11:00 h. Values of Fv/Fm remained constant in St, decreasing in P and Si after 9:30 h, indicating the occurrence of photoinhibition. Concerning the measurements taken under light conditions (deltaF/Fm', ETR, qP and qN), P and Si showed better photochemical performance, i.e., values of deltaF/Fm', ETR and qP were higher than St when light intensity was increased. Values of qN indicated that P and Si had an increasing tendency of dissipating the excess of energy absorbed by the leaf, whereas the opposite was found for St. The principal component analysis (PCA), considering all evaluated parameters, showed a clear distinction between St, P and Si, with P and Si being closer. The PCA results suggest that chlorophyll fluorescence may be a potential tool to differentiate tree species from distinct successional groups.

Photosynthesis, Chlorophyll Fluorescence, and Carbohydrate Content of Illicium Taxa Grown under Varied Irradiance

Tolerance to high solar irradiation is an important aspect of stress tolerance for landscape plants, particularly for species native to understory conditions. The objective of this study was to evaluate differential tolerance to high solar irradiation and underlying photosynthetic characteristics of diverse taxa of Illicium L. grown under full sun or 50% shade. Eleven commercially available taxa of Illicium were evaluated for light tolerance by measuring light-saturated photosynthetic capacity (Amax), dark-adapted quantum efficiency of photosystem II (Fv/Fm), and relative chlorophyll content using a SPAD chlorophyll meter. Comparisons of Amax indicated that three of the 11 taxa (I. anisatum L., I. parviflorum Michx. ex Vent., and I. parviflorum `Forest Green') maintained similar rates of light-saturated carbon assimilation when grown in either shade or full sun. All other taxa experienced a significant reduction in Amax when grown in full sun. Chlorophyll fluorescence analysis demonstrated that Fv/Fm was similar between sun and shade plants for the same three taxa that were able to maintain Amax. These taxa appeared to experience less photoinhibition than the others and maintained greater maximum photochemical efficiency of absorbed light. SPAD readings were not significantly reduced in these three taxa either, whereas most other taxa experienced a significant reduction. In fact, SPAD readings were significantly higher in I. parviflorum `Forest Green' when grown under full sun, which also maintained the highest Amax of all the taxa. These results suggest that there is considerable variation in light tolerance among these taxa, with I. parviflorum `Forest Green' demonstrating superior tolerance to high light among the plants compared. A more rigorous examination of I. parviflorum `Forest Green' (high light tolerance) and I. floridanum Ellis (low-light tolerance) demonstrated that I. parviflorum `Forest Green' had a considerably higher Amax, a higher light saturation point, greater potential photosynthetic capacity, reduced susceptibility to photoinhibition as indicated by superior PSII efficiency following light exposure, greater capacity for thermal de-excitation as indicated by a higher rate of nonphotochemical quenching (NPQ) under full sun, greater apparent electron transport rate (ETR) at mid-day, and higher concentrations of the free-radical scavenger myo-inositol. All of these factors contribute potentially to a greater capacity to use light energy for carbon fixation while minimizing photodamage.

High temperature effects on the response of photosynthesis to light in sweet orange plants infected with Xylella fastidiosa

The objective of this study was to evaluate the high temperature effects on the response of photosynthesis to light in sweet orange plants infected with Xylella fastidiosa. This vascular bacterium is the causal agent of the citrus variegated chlorosis that causes severe economical losses to the Brazilian citrus industry. The responses of the photosynthetic oxygen evolution and the parameters related to chlorophyll a fluorescence to the increase in light intensity were evaluated at 35ºC and 45ºC in both healthy and infected leaf discs. The increase in temperature affected the photosynthetic apparatus of both healthy and infected plants, although infected plants showed higher photochemical sensitivity at the higher temperature (e.g. in the potential quantum efficiency of photosystem II, maximum and basal fluorescence yield, and in the relation between variable and basal fluorescence yield). This higher sensitivity of infected plants was not reflected in the overall photosynthetic reaction, since photosynthetic oxygen evolution values did not vary at 45ºC. Healthy and infected plants showed differences in photosynthetic oxygen evolution but displayed similar effective quantum efficiency of photosystem II as well as apparent electron transport rates at 35ºC. These results suggest that the limitations in photosynthesis observed on the infected plants might arise through impaired biochemical reactions.

Comparative field study of Quercus ilex and Phillyrea latifolia: photosynthetic response to experimental drought conditions

Quercus ilex and Phillyrea latifolia growing in a holm oak forest in Prades mountains (NE Spain) were subjected to experimental drought conditions. Soil water availability was reduced about 15% by plastic strips and funnels that partially excluded rain throughfall and by ditch exclusion of water runoff. Diurnal courses of maximum photochemical efficiency of PSII (Fv/Fm), apparent photosynthetic electron transport rate (ETR), net photosynthetic rate (A), transpiration rate (E) and water use efficiency (WUE) were measured in sunlit and shade leaves of both species during 2 years. Moreover, the responses of photosynthetic rates to PPFD and CO 2 concentrations were also measured. Q. ilex experienced lower E rates and higher A rates and WUE than P. latifolia throughout the experimental period, but during summer drought these differences disappeared. Q. ilex exhibited a less cold sensitive behavior whereas P. latifolia showed a more heat–drought resistant behavior. Under severe summer drought conditions none of the two species was able to reach a positive carbon gain. Drought treatment produced a slight decrease in Fv/Fm values of Q. ilex plants and a strong decrease in Fv/Fm values of P. latifolia only in winter 2000, when drought stress coincided with cold stress. Drought treatment produced also a slight decrease in ETR values of both species. During midday, A and E rates decreased in drought plots in both species associated to lower photochemical efficiencies. In those drought plots, only P. latifolia was able to increase WUE by reducing transpiration losses during midday. Both species tended to present higher A rates for a given soil humidity in drought than in control plots. However, whereas Q. ilex A rates increased with soil humidity, P. latifolia A rates did not increase above 17% soil humidity, showing no water availability response above such threshold. It is very likely that mesic species such as Q. ilex lose competitive advantage in the drier environment forecasted for next decades than the more xeric P. latifolia.

Early photosynthetic responses of sweet orange plants infected with Xylella fastidiosa

Photosynthetic processes of sweet orange plants [ Citrus sinensis (L.) Osbeck cv. Pera] infected with Xylella fastidiosa were investigated. Simultaneous measurements of leaf gas exchanges and chlorophyll a fluorescence were taken in healthy and infected leaves before development of symptoms. The photosynthetic oxygen evolution response to different light intensities and curves of photosynthesis induction were carried out in leaf discs extracted from the same leaves used for gas exchange measurements. Infected plants showed decreased CO 2 assimilation, transpiration and stomatal conductance. No differences between healthy and infected intact leaves regarding chlorophyll a fluorescence parameters were observed, except for the potential quantum yield of photosystem II which was higher in infected plants. Values of photosynthetic oxygen evolution, effective quantum yield of photosystem II, apparent electron transport rate, and photochemical fluorescence quenching were higher in healthy leaf discs than in infected ones. Our results suggest that low photosynthetic rates of non-symptomatically sweet orange leaves infected with X. fastidiosa were caused by low stomatal conductance, biochemical alterations of photosynthetic machinery and increase in alternative electron sinks.

Different physiological responses of two aquatic bryophytes to enhanced ultraviolet-B radiation

Two aquatic bryophytes, the moss Fontinalis antipyretica Hedw. and the foliose liverwort Jungermannia exsertifolia Steph. subsp. cordifolia (Dumort.) Váña were analysed with regard to their ultraviolet-B (UV-B) tolerance. The plants were cultivated in the laboratory for 36 days under three different radiation regimes which were set up using appropriate lamps and filters: P (PAR alone, control), PA (PAR + UV-A) and PAB (PAR + UV-A + UV-B). The two bryophytes responded differently to the enhancement in UV-B radiation, while UV-A radiation had only a slight biological effect. The samples of the moss which were irradiated with UV-B showed, with respect to the control, decreases in the chlorophyll and carotenoids concentration, the chlorophyll a/b quotient, the chlorophylls/phaeopigments ratios, the net photosynthesis rates, the light saturation point, the maximum quantum yield of photosystem II (Fv/Fm) and the apparent electron transport rate (ETR). They also showed increases in the sclerophylly index and the dark respiration rates. The majority of these changes are indicative of plant stress and some of them had been found previously in bryophytes exposed to enhanced UV-B radiation. However, the UV-B-irradiated samples of the liverwort only showed a decrease in Fv/Fm, which might be the most sensitive physiological variable to UV-B, together with an increase in the concentration of UV-absorbing compounds. This defence mechanism, rarely described in bryophytes, would enable the liverwort to have a higher tolerance than the moss against UV-B radiation, at least under the specific experimental conditions used in this study. Also, the increment of UV-absorbing compounds in the liverwort, which seems to be a specific response to the enhancement in UV-B radiation, might be a useful ecophysiological tool in the bioindication of this phenomenon; long-term field research is needed to confirm this usefulness. The different UV-B tolerances of the species studied may be related, to a certain extent, with their different distribution and ecology. Given that UV-B tolerance depends on the species, bryophytes should not be grouped as a single functional type in the prediction of the consequences of a hypothetic enhancement in UV-B radiation.

Changes in photosynthesis during leaf expansion in Corymbia gummifera

Growth, pigment levels and various photosynthesis parameters were measured in expanding leaves of Corymbia gummifera (Solander ex Gaertner) Hochreutiner. C. gummifera trees were studied growing in sandstone plateau woodland communities in Royal National Park, New South Wales, in a recently burned open habitat. Young leaves (horizontally oriented to maximise light exposure) were found to be conspicuously red until they reached approximately 75% of their full size. As the leaves expanded, anthocyanin content declined and chlorophyll levels proportionately increased. Young red leaves showed net negative carbon assimilation rates, although CO2 assimilation rate, transpiration rate, stomatal conductance, actual quantum yield of PSII (ΦPSII) and apparent electron transport rate (ETR) all increased in a similar pattern as the leaves expanded. Measurements of maximum quantum yield of dark-adapted leaves (Fv/Fm) were also correlated with leaf area. Younger leaves had lower Fv/Fm ratios than did mature leaves, whether measured at midday or 2 h after sunset, indicating that young leaves exhibited some degree of chronic photoinhibition. It is concluded that C. gummifera exhibits a transient red pattern of anthocyanin expression and that photosynthesis is limited in young leaves because of low stomatal conductance, low chlorophyll content, immature chloroplasts and an attenuation of light caused by anthocyanins.

Comparative seasonal gas exchange and chlorophyll fluorescence of two dominant woody species in a Holm Oak Forest

Summary Diurnal courses of net CO 2 uptake rate, stomatal conductance (g S ), maximum photochemical efficiency of PSII (Fv/Fm) and apparent photosynthetic electron transport rate (ETR) were measured in Quercus ilex and Phillyrea latifolia in a holm oak forest throughout the seasons of the year. These measurements were complemented with response curves of photosynthetic rates to PPFD and CO 2 concentrations. P. latifolia was better adapted to drought and warm conditions and showed higher net CO 2 uptake, g S and Fv/Fm values than Q. ilex in summer. But in autumn and specially in winter P. latifolia was more sensitive to low temperatures and experienced lower net CO 2 uptake, g S , ETR and Fv/Fm values than Q. ilex . The maximum net CO 2 uptake values for P. latifolia occurred under summer high temperatures whereas maximum net CO 2 uptake values for Q. ilex occurred under winter low temperatures. However, in summer during midday, both species presented null or slightly negative net CO 2 uptake rates. Since in the summer season both species experienced similar ETR values, the lower net CO 2 uptake values of Q. ilex suggest that Q. ilex presented greater photorespiration rates. During winter, very low Fv/Fm values were found especially for P. latifolia , indicating that maximal photochemical efficiency of PSII is very sensitive to low temperatures. However, they were not accompanied by low net CO 2 uptake rates showing that cold photoinhibition determined a potential but not an actual decrease in photosynthetic performance. Under well watered conditions and with high CO 2 concentration and saturated PPFD, Q. ilex was able to increase its photosynthetic rates whereas P. latifolia had lower plasticity to make a profit of optimal environmental conditions. These results show different strategies between these two dominant co-occurring species. They also indicate that the warmer and drier conditions expected for the Mediterranean region in the near decades as a result of climate change will favour drought resistant species with lower photosynthetic rates such as P. latifolia in detriment of more mesic species such as Q. ilex .

Assessment of photoprotection mechanisms of grapevines at low temperature.

The photosynthetic response of grapevine leaves (Vitis vinifera L. cv. Riesling) to low temperature was studied in the field and laboratory. Light-saturated rates of photosynthetic electron transport were lower and non-photochemical energy dissipation was higher when leaves were subject to low morning temperatures than to high afternoon temperatures under field conditions. These responses to low temperatures occurred without sustained reduction of quantum efficiency of PSII as measured by the variable to maximum chlorophyll fluorescence yield ratio, Fv/Fm, after dark adaptation. The temperature dependence of light-saturated apparent electron transport rate, gas exchange and non-photochemical quenching (NPQ) was also examined in laboratory experiments with glasshouse-grown material. NPQ reached saturation at lower light intensity with decreasing temperature. The relationship between the quantum efficiency of PSII and CO2 fixation at 25°C (2-21% O2) and 10°C (2-21% O2) indicated a decreased dependence of electron transport on both photorespiration and the Mehler reaction at the lower temperature. The calculated percentage of electron flow to the Mehler reaction declined faster than photorespiration at low temperature. Warm- and cold-treated leaf discs under saturating light showed very little photoinhibition as measured by sustained reduction in Fv/Fm, which was linearly related to the percentage of functional PSII reaction centres. However, the addition of dithiothreitol greatly enhanced the rate of photoinhibition, indicating a potentially strong dependence on xanthophyll de-epoxidation for photoprotection at low temperature.

Photosynthetic response of citrus grown under reflective aluminized polypropylene shading nets

High temperatures and high atmospheric vapor pressure deficits (VPDs) are usually encountered in greenhouses in hot climates. For citrus, these environmental conditions can lead to decreases in photosynthetic activity with detrimental effects on plant growth. The aim of this study was to evaluate the use of reflective aluminized polypropylene shading nets on photosynthetic performance of citrus plants, by measuring CO 2 assimilation, transpiration rate, stomatal conductance and chlorophyll a fluorescence. Incident photosynthetically active radiation (PAR) levels and leaf temperatures were reduced when the reflective nets were used. Higher stomatal conductance and higher CO 2 assimilation rates were observed in shaded plants, so that integrated daily net CO 2 uptake was approximately 20% higher than in exposed plants. The better performance of shaded plants, however, was observed only during the middle of the day, being PAR-limited in early morning and late afternoon. The reflective net was also effective in preventing photoinhibition of photosynthesis in shaded plants, which sustained higher maximum ( F v/ F m) and effective (Δ F/ F m′) quantum yield with higher apparent electron transport rates (ETRs) than exposed plants. Observed photoinhibition in exposed plants was transient, probably reflecting photosynthetic regulatory responses to excess absorbed light energy. Therefore, the results clearly showed that photosynthetic performance of citrus cultivated in greenhouses can be improved by the use of reflective nets. Favorable effects comprised not only the maintenance of proper stomatal aperture for gas exchange but also a better functioning of the photosystem II (PSII) under non-photoinhibitory conditions.

Atividade fotossintética e peroxidação de lipídios de membrana em plantas de aroeira-do-sertão sob estresse hídrico e após reidratação

Avaliaram-se os efeitos do estresse hídrico e da reidratação sobre parâmetros de fluorescência, condutância estomática e peroxidação de lipídios em plantas de Myracrodruon urundeuva Fr. All. (Anacardiaceae). As plantas foram cultivadas em casa de vegetação e a seca foi induzida por suspensão da irrigação durante 14 dias. Sob estresse hídrico, observou-se queda acentuada no conteúdo relativo de água foliar, o qual alcançou valores próximos a 60 %. Essa queda foi acompanhada por grande diminuição na condutância estomática e na taxa aparente de transporte de elétrons (ETR) e extensivos danos às membranas evidenciados por aumentos na peroxidação de lipídios. Apesar do decréscimo nos valores de ETR, não se observaram alterações nos valores do rendimento quântico potencial do fotossistema II (Fv/Fm) em conseqüência do estresse hídrico. Durante a fase de recuperação, as plantas apresentaram valores de condutância estomática, parâmetros de fluorescência e peroxidação lipídica similares àqueles verificados antes do período de seca. Todos os parâmetros fotossintéticos investigados mostraram valores normais 6h após o início de reidratação. O ajuste linear entre os valores de condutância e ETR mostrou alta correlação entre essas duas variáveis, sugerindo que os estômatos representam o principal fator limitante da fotossíntese em plantas de Myracrodruon urundeuva sob estresse hídrico.