Assessing the poplar photochemical response to high zinc concentrations by image processing and statistical approach

Exposure of plants to high-heavy metals concentration inhibits multiple metabolic processes in plants and leads to an oxidative stress commonly referred as heavy metal ion toxicity. Chlorophyll a fluorescence has enhanced understanding of heavy metal ion action on the photosynthetic system. A rapid and non-invasive technique involving imaging of chlorophyll fluorescence is a useful tool for early detection of plant responses to heavy metal ion toxicity. In this work chlorophyll fluorescence emission and photochemical parameters in plants of Populus x euramericana clone I-214 were investigated by the portable Imaging PAM fluorometer at different days after soil treatment with zinc. Custom software for analysis of the photochemical parameters images has been developed in order to gain a better assessing of the plant performance in response of metal stress. The imaging analysis allowed visualizing heterogeneity in plant response to high zinc concentrations. The heterogeneity of images suggests spatial differences in photochemical activity and changes in the antenna down-regulation.     

Imaging of the Blue,Green, and Red Fluorescence Emission of Plants: An Overview

An overview is given on the fluorescence imaging of plants. Emphasis is laid upon multispectral fluorescence imaging in the maxima of the fluorescence emission bands of leaves, i.e., in the blue (440 nm), green (520 nm), red (690 nm), and far-red (740 nm) spectral regions. Details on the origin of these four fluorescence bands are presented including emitting substances and emitting sites within a leaf tissue. Blue-green fluorescence derives from ferulic acids covalently bound to cell walls, and the red and far-red fluorescence comes from chlorophyll (Chl) a in the chloroplasts of green mesophyll cells. The fluorescence intensities are influenced (1) by changes in the concentration of the emitting substances, (2) by the internal optics of leaves determining the penetration of excitation radiation and partial re-absorption of the emitted fluorescence, and (3) by the energy distribution between photosynthesis, heat production, and emission of Chl fluorescence. The set-up of the Karlsruhe multispectral fluorescence imaging system (FIS) is described from excitation with UV-pulses to the detection with an intensified CCD-camera. The possibilities of image processing (e.g., formation of fluorescence ratio images) are presented, and the ways of extraction of physiological and stress information from the ratio images are outlined. Examples for the interpretation of fluorescence images are given by demonstrating the information available for the detection of different developmental stages of plant material, of strain and stress of plants, and of herbicide treatment. This novel technique can be applied for near-distance screening or remote sensing.     

Prolonged salt stress alters the ratios of protochlorophyllide spectral forms in dark-grown wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) and influences chlorophyll <Emphasis Type="Italic">a</Emphasis> accumulation following irradiation

To examine the effects of salt stress on dark-grown wheat (Triticum aestivum), seedlings of the salt-tolerant cultivar Sids 1 and the susceptible cultivar Giza 168 were grown in darkness for 14 days in nutrient solution with and without 200 mM of supplementary salt (100 mM of NaCl and 100 mM of KCl). During this time, we monitored their protochlorophyllide (Pchlide) contents, ratios of photoactive to non-photoactive forms of Pchlide (from 655/633-nm emission ratios in their 77 K fluorescence emission spectra) and (following flash irradiation) ratios of newly formed chlorophyllide (Chlide) to non-photoactive Pchlide. In addition, the accumulation of chlorophyll a in leaf sections was monitored during prolonged (24 h) irradiation. The results depended on the developmental state of the seedlings. However, the salt stress treatment caused marked increases in both Pchlide contents in dark-grown leaves and in Chlide contents following irradiation of leaf sections of both cultivars. The ratio of phototransformable to non-phototransformable Pchlide and the abundance of newly formed Chlide were also increased by the salt stress. Further, leaves of salt-stressed seedlings consistently accumulated more chlorophyll a than leaves of unstressed seedlings when floating on the nutrient solution (with or without supplementary salt) in continuous white light. The findings are consistent with the hypothesis that increased levels of the long-wavelength form of Pchlide contribute to protective mechanisms against salt stress.     

盐旱复合胁迫对小麦幼苗生长和水分吸收的影响

为明确盐害、干旱及盐旱复合胁迫对小麦幼苗生长和水分吸收的影响,从而为盐害和干旱胁迫下栽培调控提供理论依据。以2个抗旱性不同的小麦品种(扬麦16和耐旱型洛旱7号)为材料,采用水培试验,以NaCl和PEG模拟盐旱复合胁迫,研究了盐旱复合胁迫下小麦幼苗生长、根系形态、光合特性及水分吸收特性的变化。结果表明,盐、旱及复合胁迫下小麦幼苗的生物量、叶面积、总根长与根系表面积、叶绿素荧光和净光合速率均显著下降,但是复合胁迫处理的降幅却显著低于单一胁迫。盐旱复合胁迫下根系水导速率和根系伤流液强度显著大于单一胁迫,从而提高了小麦幼苗叶片水势和相对含水量。盐胁迫下小麦幼苗Na~+/K~+显著大于复合胁迫,但复合胁迫下ABA含量却显著小于单一的盐害和干旱胁迫。因此,盐旱复合胁迫可以通过增强根系水分吸收及降低根叶中ABA含量以维持较高光合能力,这是盐旱复合胁迫提高小麦适应性的重要原因。洛旱7号和扬麦16对盐及盐旱复合胁迫的响应基本一致,但在干旱胁迫下洛旱7号表现出明显的耐性。     

Photosynthetic activity of poikilochlorophyllous desiccation tolerant plant <Emphasis Type="Italic">Reaumuria soongorica</Emphasis> during dehydration and re-hydration

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as Chl content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub during dehydration and rehydration. The net photosynthetic rate (P _N), maximum photochemical efficiency of PS2 (variable to maximum fluorescence ratio, F_v/F_m), quantum efficiency of non-cyclic electron transport of PS2, and Chl content decreased, but non-photochemical quenching of fluorescence and carotenoid content increased in stems with the increasing of drought stress. 6 d after re-hydration, new leaves budded from stems. In the re-watered plants, the chloroplast function was restored and Chl a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in plant triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. Thus R. soongorica plants are able to sustain drought stress through leaf abscission and keep part of Chl content in stems.     

Salt tolerance in crop plants monitored by chlorophyll fluorescence in vivo

The potential of measurements of chlorophyll fluorescence in vivo to detect cellular responses to salinity and degrees of salt stress in leaves was investigated for three crop plants. Sugar beet (Beta vulgaris L.) (salt tolerant), sunflower (Helianthus annuus L.) (moderately salt tolerant), and bean (Phaseolus Vulgaris L. cv Canadian Wonder) (salt intolerant) were grown in pots and watered with mineral nutrient solution containing 100 millimolar NaCl. The fast rise in variable chlorophyll fluorescence yield that is correlated with photoreduction of photosystem II acceptors increased in leaves of sugar beet plants treated with salt suggesting stimulation of photosystem II activity relative to photosystem I. In sunflower, this fast rise was depressed by approximately 25% and the subsequent slow rate of quenching of the chlorophyll fluorescence was stimulated. These differences were more marked in the older mature leaves indicating an increasing gradient of salt response down the plant. The salt effect in vivo was reversible since chloroplasts isolated from mature leaves of salt-treated and control sunflower plants gave similar photosystem II activities. Unlike in sugar beet and sunflower, leaves of salt-treated bean progressively lost chlorophyll. The rate of slow quenching of chlorophyll fluorescence decreased indicating development of a partial block after photosystem II and possible initial stimulation of photosystem II activity. With further loss of chlorophyll photosystem II activity declined. It was concluded that measurements of chlorophyll fluorescence in vivo can provide a rapid means of detecting salt stress in leaves, including instances where photosynthesis is reduced in the absence of visible symptoms. The possible application to screening for salt tolerance is discussed.     

Leaf movement and photosynthetic plasticity of black locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis>) alleviate stress under different light and water conditions

Leaf morphological, physiological and biochemical characteristics of Robinia pseudoacacia L. seedlings were studied under different stress conditions. The plants were subjected to drought and shade stress for one month. Leaf inclination, chlorophyll fluorescence and chlorophyll content were measured at the first day (short-term stress) and at the end of the stress period (long-term stress) and in the recovery period. Leaf inclination was affected mainly by light; a low level of irradiance caused leaves to be arranged horizontally. Diurnal rhythmicity was lost after the long-term stress, but resumed, in part, in the recovery period. Drought stress caused leaves to tilt more obviously and decreased damage to the photosystem. Sun avoiding movement in a single leaf and sun tracking movement in the whole plant coexisted. Significant physiological changes occurred under different conditions of light. Increased energy dissipation and light capture were the main responses to high and low level of irradiance, respectively, and these were reflected by changes of chlorophyll fluorescence and chlorophyll content. Phenotypic plasticity in the leaflet enhanced the protective response to stress. These adaptive mechanisms may explain better survival of R. pseudoacacia seedlings in the understory, especially during the drought periods, and made it to be the preponderant reforestation species in Shandong Province of China.     

Photosynthesis and ion content of leaves and isolated chloroplasts of salt-stressed spinach

Spinach (Spinacia oleracea) plants were subjected to salt stress by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar. Plants were harvested 3 weeks after starting NaCl treatment. Fresh and dry weight of both shoots and roots was decreased more than 50% compared to control plants but the salt-stressed plants appeared healthy and were still actively growing. The salt-stressed plants had much thicker leaves. The salt-treated plants osmotically adjusted to maintain leaf turgor. Leaf K⁺ was decreased but Na⁺ and Cl⁻ were greatly increased.

The potential photosynthetic capacity of the leaves was measured at saturating CO₂ to overcome any stomatal limitation. Photosynthesis of salt-stressed plants varied only by about 10% from the controls when expressed on a leaf area or chlorophyll basis. The yield of variable chlorophyll a fluorescence from leaves was not affected by salt stress. Stomatal conductance decreased 70% in response to salt treatment.

Uncoupled rates of electron transport by isolated intact chloroplasts and by thylakoids were only 10 to 20% below those for control plants. CO₂-dependent O₂ evolution was decreased by 20% in chloroplasts isolated from salt-stressed plants. The concentration of K⁺ in the chloroplast decreased by 50% in the salt-stressed plants, Na⁺ increased by 70%, and Cl⁻ increased by less than 20% despite large increases in leaf Na⁺ and Cl⁻.

It is concluded that, for spinach, salt stress does not result in any major decrease in the photosynthetic potential of the leaf. Actual photosynthesis by the plant may be reduced by other factors such as decreased stomatal conductance and decreased leaf area. Effective compartmentation of ions within the cell may prevent the accumulation of inhibitory levels of Na⁺ and Cl⁻ in the chloroplast.

     

Chlorophyll fluorescence imaging for disease-resistance screening of sugar beet

Both biotic and abiotic stresses cause considerable crop yield losses worldwide (Chrispeels, Sadava Plants, genes, and crop biotechnology 2003; Oerke, Dehne Crop Prot 23:275–285 2004). To speed up screening assays in stress resistance breeding, non-contact techniques such as chlorophyll fluorescence imaging can be advantageously used in the quantification of stress-inflicted damage. In comparison with visual spectrum images, chlorophyll fluorescence imaging reveals cell death with higher contrast and at earlier time-points. This technique has the potential to automatically quantify stress-inflicted damage during screening applications. From a physiological viewpoint, screening stress-responses using attached plant leaves is the ideal approach. However, leaf growth and circadian movements interfere with time-lapse monitoring of leaves, making it necessary to fix the leaves to be studied. From this viewpoint, a method to visualise the evolution of chlorophyll fluorescence from excised leaf pieces kept in closed petri dishes offers clear advantages. In this study, the plant–fungus interaction sugar beet–Cercospora beticola was assessed both in attached leaf and excised leaf strip assays. The attached leaf assay proved to be superior in revealing early, pre-visual symptoms and to better discriminate between the lines with different susceptibility to Cercospora.     

Photosynthetic Responses for Vitis vinifera Plants Grown at Different Photon Flux Densities Under Field Conditions

In grapevine (Vitis vinifera L.) leaf chlorophyll (Chl) a and Chl b and carotenoid contents were higher in plants grown at low photon flux densities (PFD) than in those grown at medium and high PFD. The highest Chl a variable to maximum fluorescence ratio F_v/F_m was observed in plants grown at medium PFD while the minimum fluorescence F₀ was highest in those at high PFD. In isolated thylakoids, both high and low PFD caused marked inhibition of whole chain and photosystem 2 (PS2) activities. The artificial exogenous electron donor diphenyl carbazide significantly restored the loss of PS2 activity in low PFD leaves.     

Structural and Functional Changes in the Leaves of Plants from Steppe Communities as Affected by Aridization of the Eurasian Climate

Morphological and physiological characteristics of leaves from plant species collected in steppe communities in the various climatic zones in Eurasia were compared. The changes in leaf structure correlated with the major climatic factors. The mean thickness of leaves increased with increasing mean temperature of July and decreasing mean precipitation, which corresponded to aridity increase. The increased leaf thickness correlated with an increase in the specific leaf weight. The content of chlorophylls (a + b) in leaves greatly varied with plant habitats, whereas the chlorophyll a/b ratio remained unchanged. The chlorophyll content in leaf tissues had a general tendency to decrease with increasing leaf thickness. The leaf chlorophyll content positively correlated (R ² = 0.77) with the proportion of chlorenchyma in leaf tissues. It is concluded that steppe plants adapt to climate aridization at the structural level by increasing the proportion of protective heterotrophic components of the leaf without changing the functional activity of photosynthetic tissues.     

Effect of Benzylaminopurine on Rehydration of Bean Plants after Water Stress

The possibility of improving the recovery of plant photosynthesis after water stress by cytokinin-induced stimulation of stomatal opening or delay of leaf senescence was tested. The 6-benzylaminopurine (BAP) in concentrations 1 and 10 M was applied to the substrate (sand + nutrient solution) or sprayed on primary leaves of 14-d-old Phaseolus vulgaris L. plants sufficiently supplied with water or water-stressed for 4 d. The later ones having relative water content decreased to 69 % were fully rehydrated during the following three days. Parameters of photosynthesis and water relations were measured in primary leaves of 7-, 10-, 14-, and 17-d-old plants. Application of 1 M BAP slightly delayed leaf senescence: in 17-d-old control plants, net photosynthetic rate (P_N) and chlorophyll (Chl) content, and when sprayed on leaves also some of Chl a fluorescence kinetic parameters of BAP-treated leaves were slightly higher than those of untreated leaves. Both types of application of 1 M BAP slightly improved recovery of plants during rehydration after water stress in terms of increased g_ad, g_ab and P_N, i.e., parameters which were markedly decreased by mild water stress. However, contents of Chl a, Chl b and carotenoids and parameters of Chl a fluorescence kinetic were not markedly affected by mild water stress and after rehydration were not stimulated by 1 M BAP. 10 M BAP had mostly negative effects on the parameters measured.     

Changes in Chlorophyll Fluorescence in Maize Plants with Imposed Rapid Dehydration at Different Leaf Ages

A comparison of the effects of a rapidly imposed water deficit with different leaf ages on chlorophyll a fluorescence and gas exchange was performed in maize (Zea mays L.) plants. The relationships between photosynthesis and leaf relative turgidity (RT) and ion leakage were further investigated. Leaf dehydration substantially decreased net photosynthetic rate (A) and stomatal conductance (G _s), particularly for older leaves. With dehydration time, F _v /F _m maintained a relatively stable level for youngest leaves but significantly decreased for the older leaves. The electron transport rate (ETR) sharply decreased with intensifying dehydration and remained at lower levels during continuous dehydration. The photochemical quenching of variable chlorophyll fluorescence (q _P) gradually decreased with dehydration intensity for the older leaves but increased for the youngest leaves, whereas dehydration did not affect the nonphotochemical chlorophyll fluorescence quenching (NPQ) for the youngest leaves but remarkably decreased it for the older leaves. The leaf RT was significantly and positively correlated with its F _v /F _m, ETR, and q _P, and the leaf ion leakage was significantly and negatively correlated with F _v /F _m and NPQ. Our results suggest that the photosynthetic systems of young and old leaves decline at different rates when exposed to rapid dehydration.     

Improving low-temperature tolerance in sugarcane by expressing the ipt gene under a cold inducible promoter

Sugarcane is cultivated in tropical and subtropical regions where cold stress is not very common, but lower yields and reduced industrial quality of the plants are observed when it occurs. In our efforts to enhance cold tolerance in sugarcane, the gene encoding the enzyme isopentenyltransferase (ipt) under control of the cold inducible gene promoter AtCOR15a was transferred via biolistic transformation into sugarcane (Saccharum spp.) cv. RB855536. Semiquantitative RT-PCR using GAPDH encoding glyceraldehyde-3-phosphate dehydrogenase as the normalizer gene showed the increased expression of the ipt gene under cold stress. The detached leaves of genetically modified plants subjected to low temperatures showed visible reduction of leaf senescence in comparison to non-transgenic control plants. Induced overexpression of ipt gene also enhanced cold tolerance of non-acclimated whole plants. After being subjected to freezing temperature, leaf total chlorophyll contents of transgenic plants were up to 31 % higher than in wild type plants. Also, lower malondialdehyde content and electrolyte leakage indicated less damage induced by cold in transgenic plants. Thus, the expression of ipt driven by the stress inducible COR15a promoter did not affect plant growth while providing a greater tolerance to cold stress.     

Morphophysiological differences in leaves of <Emphasis Type="Italic">Lavoisiera campos</Emphasis>-<Emphasis Type="Italic">portoana</Emphasis> (Melastomataceae) enhance higher drought tolerance in water shortage events

Lavoisiera campos-portoana Barreto (Melastomataceae) has two kinds of leaves, pubescent and glabrous, and branches may have one or both types of leaves at the same moment. The plant is endemic to high altitude rocky fields in Brazil where rainfall is very seasonal. We predicted that these two leaf types are adaptations to different regimes of water availability. In experimental conditions of drought stress, we measured relative water content (RWC), photosynthetic pigments, chlorophyll a fluorescence and osmotic potential, and we counted stomates and measured stomatal conductance on both sides of leaves and compared these between the two leaf types. Stomatal conductance and electron transport rate at a given photosynthetic photon flux were greater in pubescent leaves than in glabrous leaves, and both declined during drought stress. Excessive photon flux density in glabrous leaves was greater during stress and after rehydration. Photosynthetic pigment content and RWC did not change between leaves, and values reduced during the stress period. Both types of leaves showed osmotic adjustment capacity, which occurred earlier in glabrous ones. These morphophysiological differences highlight the adaptation strategies of this plant to withstand drought, since the glabrous portion of the plant presents a preventive behavior, while the pubescent portion only shows the same responses in more advanced stages of drought stress.     

Spectral reflectance indices and pigment functions during leaf ontogenesis in six subtropical landscape plants

Pigment combinations are regulated during leaf ontogenesis. To better understand pigment function, alterations in chlorophyll, carotenoid and anthocyanin concentrations were investigated during different leaf development stages in six subtropical landscape plants, namely Ixora chinensis Lam, Camellia japonica Linn, Eugenia oleina Wight, Mangifera indica L., Osmanthus fragrans Lowr and Saraca dives Pierre. High concentrations of anthocyanin were associated with reduced chlorophyll in juvenile leaves. As leaves developed, the photosynthetic pigments (chlorophyll and carotenoid) of all six species increased while anthocyanin concentration declined. Chlorophyll fluorescence imaging of Φ_PSII (effective quantum yield of PSII) and of NPQ (non-photochemical fluorescence quenching) and determination of electron transport rate-rapid light curve (RLC) showed that maximum ETR (leaf electron transport rate), Φ_PSII and the saturation point in RLC increased during leaf development but declined as they aged. Juvenile leaves displayed higher values of NPQ and Car/Chl ratios than leaves at other developmental stages. Leaf reflectance spectra (400–800 nm) were measured to provide an in vivo non-destructive assessment of pigments in leaves during ontogenesis. Four reflectance indices, related to pigment characters, were compared with data obtained quantitatively from biochemical analysis. The results showed that the ARI (anthocyanin reflectance index) was linearly correlated to anthocyanin concentration in juvenile leaves, while a positive correlation of Chl NDI (chlorophyll normalized difference vegetation index) to chlorophyll a concentration was species dependent. Photosynthetic reflectance index was not closely related to Car/Chl ratio, while a structural-independent pigment index was not greatly altered by leaf development or species. Accordingly, it is suggested that the high concentration of anthocyanin, higher NPQ and Car/Chl ratio in juvenile leaves are important functional responses to cope with high radiation when the photosynthetic apparatus is not fully developed. Another two leaf reflectance indices, ARI and Chl NDI, are valuable for in vivo pigment evaluation during leaf development.     

Blue,Green and Red Fluorescence Signatures and Images of Tobacco Leaves*

Laser-induced fluorescence images of the leaf of an aurea mutant of Nicotiana tabacum were recorded for the blue and green fluorescence at 440 and 520 nm and the red chlorophyll fluorescence at 690 and 735 nm. The results obtained were compared with direct measurements of the fluorescence emission spectra of leaves using a conventional spectrofluorometer. The highest emission of blue (F440) and green fluorescence (F520) within the leaf was found in the leaf veins, particularly the main leaf vein. In contrast, the intercostal fields of leaves, which exhibited the highest chlorophyll content, showed only a very low blue and green fluorescence emission, which was much lower than the red and far-red chlorophyll fluorescence emission bands (F690 and F735). Correspondingly, the ratio of blue to red leaf fluorescence F440/F690 of upper and lower leaf side was much higher in the leaf veins (values 1.2 to 1.5) than in intercostal fields (values of 0.6 to 0.7). The results also demonstrated that in the intercostal fields the major part of the blue-green fluorescence was reabsorbed by chlorophylls and carotenoids. A partial reabsorption of the red fluorescence band near 690 nm by leaf chlorophyll took place, but did not affect the far-red fluorescence band near F735. As a consequence the chlorophyll fluorescence ratio F690/F735 exhibited significantly higher values in the chlorophyll-poor leaf vein regions (1.7 to 1.8) than in the chlorophyll-rich intercostal fields (0.8 to 1.3). Imaging spectroscopy of leaves was shown to be much more precise than the screening of fluorescence signatures by conventional fluorometers. It clearly demonstrated that the blue-green fluorescence and the red chlorophyll fluorescence of leaves exhibit an inverse contrast to each other. The advantage of the fluorescence imaging spectroscopy, which allows the simultaneous screening of the whole leaf surface and distinct parts of it, and its possible application in the detection of stress effects or local damage by insects and pathogens, is discussed.     

Extended depth-of-focus imaging of chlorophyll fluorescence from intact leaves

Imaging dynamic changes in chlorophyll a fluorescence provides a valuable means with which to examine localised changes in photosynthetic function. Microscope-based systems provide excellent spatial resolution which allows the response of individual cells to be measured. However, such systems have a restricted depth of focus and, as leaves are inherently uneven, only a small proportion of each image at any given focal plane is in focus. In this report we describe the development of algorithms, specifically adapted for imaging chlorophyll fluorescence and photosynthetic function in living plant cells, which allow extended-focus images to be reconstructed from images taken in different focal planes. We describe how these procedures can be used to reconstruct images of chlorophyll fluorescence and calculated photosynthetic parameters, as well as producing a map of leaf topology. The robustness of this procedure is demonstrated using leaves from a number of different plant species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chlorophyll fluorescence imaging of photosynthetic activity with the flash-lamp fluorescence imaging system

A flash-lamp chlorophyll (Chl) fluorescence imaging system (FL-FIS) is described that allows to screen and image the photosynthetic activity of several thousand leaf points (pixels) of intact leaves in a non-destructive way within a few seconds. This includes also the registration of several thousand leaf point images of the four natural fluorescence bands of plants in the blue (440 nm) and green (520 nm) regions as well as the red (near 690 nm) and far-red (near 740 nm) Chl fluorescence. The latest components of this Karlsruhe FL-FIS are presented as well as its advantage as compared to the classical single leaf point measurements where only the fluorescence information of one leaf point is sensed per each measurement. Moreover, using the conventional He-Ne-laser induced two-wavelengths Chl fluorometer LITWaF, we demonstrated that the photosynthetic activity of leaves can be determined measuring the Chl fluorescence decrease ratio, R_Fd (defined as Chl fluorescence decrease F_d from maximum to steady state fluorescence F_s:F_d/F_s), that is determined by the Chl fluorescence induction kinetics (Kautsky effect). The height of the values of the Chl fluorescence decrease ratio R_Fd is linearly correlated to the net photosynthetic CO₂ fixation rate P _N as is indicated here for sun and shade leaves of various trees that considerably differ in their P _N. Imaging the R_Fd-ratio of intact leaves permitted the detection of considerable gradients in photosynthetic capacity across the leaf area as well as the spatial heterogeneity and patchiness of photosynthetic quantum conversion within the control leaf and the stressed plants. The higher photosynthetic capacity of sun versus shade leaves was screened by Chl fluorescence imaging. Profile analysis of fluoresence signals (along a line across the leaf area) and histograms (the signal frequency distribution of the fluorescence information of all measured leaf pixels) of Chl fluorescence yield and Chl fluorescence ratios allow, with a high statistical significance, the quantification of the differences in photosynthetic activity between various areas of the leaf as well as between control leaves and water stressed leaves. The progressive uptake and transfer of the herbicide diuron via the petiole into the leaf of an intact plant and the concomitant loss of photosynthetic quantum conversion was followed with high precision by imaging the increase of the red Chl fluorescence F₆₉₀. Differences in the availability and absorption of soil nitrogen of crop plants can be documented via this flash-lamp fluorescence imaging technique by imaging the blue/red ratio image F₄₄₀/F₆₉₀, whereas differences in Chl content are detected by collecting images of the fluorescence ratio red/far-red, F₆₉₀/F₇₄₀.     

SALT REGULATION AND LEAF SENESCENCE IN AGING LEAVES OF JAUMEA CARNOSA (LESS.) GRAY (ASTERACEAE), A SALT MARSH SPECIES EXPOSED TO NaCl STRESS

The effects of increasing salt stress on leaf senescence and salt regulation were investigated in the halophyte Jaumea carnosa in hydroponic culture experiments. The plants were grown in Hoagland's nutrient solution plus additional NaCl salt (0, 300, 400, 500 mm NaCl). Decreases in nucleic acids, protein, and chlorophyll were used as indicators of leaf senescence. The results indicated no definitive pattern of acceleration in leaf senescence with increasing salt stress. Salt regulation was also unaffected as leaves aged under increasing NaCl concentrations. The results are consistent with those of previous studies of the halophyte which indicated that the species was very tolerant of high NaCl concentrations.