Measurement of gradients of absorbed light in spinach leaves from chlorophyll fluorescence profiles

Profiles of chlorophyll fluorescence were measured in spinach leaves irradiated with monochromatic light. The characteristics of the profiles within the mesophyll were determined by the optical properties of the leaf tissue and the spectral quality of the actinic light. When leaves were infiltrated with 10^?4M DCMU [3‐(3,4‐dichlorophenyl)‐1, 1‐dimethyl‐urea] or water, treatments that minimized light scattering, irradiation with 2000 μmol m^?2 s^?1 green light produced broad Gaussian‐shaped fluorescence profiles that spanned most of the mesophyll. Profiles for chlorophyll fluorescence in the red (680 ± 16 nm) and far red (λ > 710 nm) were similar except that there was elevated red fluorescence near the adaxial leaf surface relative to far red fluorescence. Fluorescence profiles were narrower in non‐infiltrated leaf samples where light scattering increased the light gradient. The fluorescence profile was broader when the leaf was irradiated on its adaxial versus abaxial surface due to the contrasting optical properties of the palisade and spongy mesophyll. Irradiation with blue, red and green monochromatic light produced profiles that peaked 50, 100 and 150 μm, respectively, beneath the irradiated surface. These results are consistent with previous measurements of the light gradient in spinach and they agree qualitatively with measurements of carbon fixation under monochromatic blue, red and green light. These results suggest that chlorophyll fluorescence profiles may be used to estimate the distribution of quanta that are absorbed within the leaf for photosynthesis.     

Photosynthesis within isobilateral Eucalyptus pauciflora leaves

Adult Eucalyptus pauciflora leaves are vertically displayed. They have multiple palisade cell layers beneath both surfaces, interrupted by numerous oil glands. Here, we characterized light absorption, chlorophyll, photosynthetic capacity and CO2 fixation profiles through these leaves. Multiple chlorophyll fluorescence images of leaves viewed in cross-section were made by applying light from different directions. 14CO2 labelling, followed by paradermal cryosectioning, was used to measure profiles of photosynthesis. Photosynthetic capacity peaked 75 microm into the mesophyll beneath each surface and was lowest in the centre of the 600-microm-thick leaf. Predictions by a multilayer model using Beer's law matched the observed profiles of 14C fixation. When constrained to the horizontal, a vertically acclimated leaf gains only 79% of the daily photosynthesis achieved by a horizontally acclimated leaf. However, it outperforms the horizontally acclimated leaf when both are oriented vertically. Each half of the observed profile of photosynthetic capacity closely matches the profile of light absorption through the leaf with unilateral illumination to that surface. Derivation of biochemical parameters from gas exchange measured under unilateral illumination would underestimate the real photosynthetic capacity of these leaves by 21%.     

竹柏2种颜色叶片的光合特性研究

为了解竹柏(Podocarpus nagi)的光合特性,以3 a生全绿叶和花叶竹柏为材料,测定其光合色素含量和气体交换参数。结果表明,全绿叶竹柏叶片的叶绿素a、叶绿素b、类胡萝卜素、叶绿素a+b、叶绿素a/b和叶绿素a+b/类胡萝卜素均显著高于花叶竹柏;全绿叶竹柏叶片的初始量子效率、最大光合速率和暗呼吸速率均显著高于花叶,而光饱和点和光补偿点均显著低于花叶;全绿叶竹柏叶片的初始羧化效率、光合速率、CO2饱和点和光呼吸速率均高于花叶,而CO2补偿点低于花叶。2种颜色叶片的气孔导度、蒸腾速率和水分利用效率均随着光合有效辐射的增大而增大,且均表现为全绿叶花叶,而胞间CO2浓度则相反,表现为花叶全绿叶。因此,全绿叶竹柏利用弱光的能力强于花叶竹柏,而花叶竹柏利用强光的能力更强,在园林绿化配置中,可根据2种颜色叶片的光合特性合理配置。     

Characteristics of gas exchange and chlorophyll fluorescence in red and green leaves of Begonia semperflorens

To determine the effects of leaf colour on gas exchange and chlorophyll fluorescence, two genotypes of Begonia semperflorens with green leaves or red leaves were compared. The red leaves showed a high accumulation of anthocyanins and high absorbance at 282 and 537 nm while the green leaves exhibited a higher net photosynthetic rate and lower thermal dissipation of light energy. It seems likely that anthocyanins in the vacuoles restricted the absorption of green light to the chloroplasts, leading to a decrease in the efficiency of excitation capture by open PS 2 centres, photochemical quenching and CO₂ assimilation.     

Simulation of photon transport in a three-dimensional leaf: implications for photosynthesis

A model to evaluate photon transport within leaves and the implications for photosynthesis are investigated. A ray tracing model, Raytran, was used to produce absorption profiles within a virtual dorsiventral plant leaf oriented in two positions (horizontal/vertical) and illuminated on one of its two faces (adaxial/abaxial). Together with chlorophyll profiles, these absorption profiles feed a simple photosynthesis model that calculates the gross photosynthetic rate as a function of the incident irradiance. The differences observed between the four conditions are consistent with the literature: horizontal‐adaxial leaves, which are commonly found in natural conditions, have the greatest light use efficiency. The absorption profile obtained with horizontal‐abaxial leaves lies below this, but above those obtained for vertical leaves. The latter present similar gross photosynthetic rates when irradiated on either the adaxial or abaxial surfaces. Vertical profiles of photosynthetic rates across the leaf confirm that carbon fixation occurs mainly in the palisade parenchyma, that the leaf anatomy is integral to its function and that leaves cannot be considered as a single homogeneous unit. Finally, the relationships between leaf structure, orientation and photosynthesis are discussed.     

Photosynthetic and structural acclimation to light direction in vertical leaves of Silphium terebinthinaceum

The azimuth of vertical leaves of Silphium terebinthinaceum profoundly influenced total daily irradiance as well as the proportion of direct versus diffuse light incident on the adaxial and abaxial leaf surface. These differences caused structural and physiological adjustments in leaves that affected photosynthetic performance. Leaves with the adaxial surface facing East received equal daily integrated irradiance on each surface, and these leaves had similar photosynthetic rates when irradiated on either the adaxial or abaxial surface. The adaxial surface of East-facing leaves was also the only surface to receive more direct than diffuse irradiance and this was the only leaf side which had a clearly defined columnar palisade layer. A potential cost of constructing East-facing leaves with symmetrical photosynthetic capcity was a 25% higher specific leaf mass and increased leaf thickness in comparison to asymmetrical South-facing leaves. The adaxial surface of South-facing leaves received approximately three times more daily integrated irradiance than the abaxial surface. When measured at saturating CO₂ and irradiance, these leaves had 42% higher photosynthetic rates when irradiated on the adaxial surface than when irradiated on the abaxial surface. However, there was no difference in photosynthesis for these leaves when irradiated on either surface when measurements were made at ambient CO₂. Stomatal distribution (mean adaxial/abaxial stomatal density = 0.61) was unaffected by leaf orientation. Thus, the potential for high photosynthetic rates of adaxial palisade cells in South-facing leaves at ambient CO₂ concentrations may have been constrained by stomatal limitations to gas exchange. The distribution of soluble protein and chlorophyll within leaves suggests that palisade and spongy mesophyll cells acclimated to their local light environment. The protein/chlorophyll ratio was high in the palisade layers and decreased in the spongy mesophyll cells, presumably corresponding to the attentuation of light as it penetrates leaves. Unlike some species, the chlorophyll a/b ratio and the degree of thylakoid stacking was uniform throughout the thickness of the leaf. It appears that sun-shade acclimation among cell layers of Silphium terebinthinaceum leaves is accomplished without adjustment to the chlorophyll a/b ratio or to thylakoid membrane structure.     

Profiles of light absorption and chlorophyll within spinach leaves from chlorophyll fluorescence

Chlorophyll fluorescence was used to estimate profiles of absorbed light within chlorophyll solutions and leaves. For chlorophyll solutions, the intensity of the emitted fluorescence declined in a log–linear manner with the distance from the irradiated surface as predicted by Beer's law. The amount of fluorescence was proportional to chlorophyll concentration for chlorophyll solutions given epi‐illumination on a microscope slide. These relationships appeared to hold for more optically complex spinach leaves. The profile of chlorophyll fluorescence emitted by leaf cross sections given epi‐illumination corresponded to chlorophyll content measured in extracts of leaf paradermal sections. Thus epifluorescence was used to estimate relative chlorophyll content through leaf tissues. Fluorescence profiles across leaves depended on wavelength and orientation, reaching a peak at 50–70 µm depth. By infiltrating leaves with water, the pathlengthening due to scattering at the airspace : cell wall interfaces was calculated. Surprisingly, the palisade and spongy mesophyll had similar values for pathlengthening with the value being greatest for green light (550 > 650 > 450 nm). By combining fluorescence profiles with chlorophyll distribution across the leaf, the profile of the apparent extinction coefficient was calculated. The light profiles within spinach leaves could be well approximated by an apparent extinction coefficient and the Beer–Lambert/Bouguer laws. Light was absorbed at greater depths than predicted from fibre optic measurements, with 50% of blue and green light reaching 125 and 240 µm deep, respectively.     

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.     

Effects of light quality on the chloroplastic ultrastructure and photosynthetic characteristics of cucumber seedlings

To understand how light quality influences plant photosynthesis, we investigated chloroplastic ultrastructure, chlorophyll fluorescence and photosynthetic parameters, Rubisco and chlorophyll content and photosynthesis-related genes expression in cucumber seedlings exposed to different light qualities: white, red, blue, yellow and green lights with the same photosynthetic photon flux density of 100 μmol m^?2 s^?1. The results revealed that plant growth, CO₂ assimilation rate and chlorophyll content were significantly reduced in the seedlings grown under red, blue, yellow and green lights as compared with those grown under white light, but each monochromatic light played its special role in regulating plant morphogenesis and photosynthesis. Seedling leaves were thickened and slightly curled; Rubisco biosynthesis, expression of the rca, rbcS and rbcL, the maximal photochemical efficiency of PSII (Fv/Fm) and quantum yield of PSII electron transport (Ф_PSII) were all increased in seedlings grown under blue light as compared with those grown under white light. Furthermore, the photosynthetic rate of seedlings grown under blue light was significantly increased, and leaf number and chlorophyll content of seedlings grown under red light were increased as compared with those exposed to other monochromatic lights. On the contrary, the seedlings grown under yellow and green lights were dwarf with the new leaves etiolated. Moreover, photosynthesis, Rubisco biosynthesis and relative gene expression were greatly decreased in seedlings grown under yellow and green light, but chloroplast structural features were less influenced. Interestingly, the Fv/Fm, Ф_PSII value and chlorophyll content of the seedlings grown under green light were much higher than those grown under yellow light.     

Photosynthetic response and adaptation to high temperature in desert plants : a comparison of gas exchange and fluorescence methods for studies of thermal tolerance

The temperature threshold for the onset of irreversible loss of photosynthetic capacity of leaves was examined in studies of net CO₂ exchange and by chlorophyll fluorescence techniques. Close agreement was found between the temperature threshold for a dramatic increase in the fluorescence of chlorophyll from intact leaves and the leaf temperature at which the capacity for photosynthetic CO₂ fixation (measured at rate saturating light intensity by infrared gas analysis) began to be temperature unstable (i.e. decline with time of exposure to a constant temperature). This decline in CO₂ uptake was not a result of a stomatal response yielding a reduced intercellular CO₂ concentration at high temperature, and it is interpreted as an indication of progressive damage to some essential component(s) of the leaf. The temperature-dependent change in chlorophyll fluorescence apparently also indicated the onset of this damage. The fluorescence assay could be conducted with discs of leaves collected from remote locations and kept moist while they were transported to a central location, allowing assessment of the high temperature tolerance of leaves which developed under natural field conditions. These assays were verified using a mobile laboratory to study gas exchange of attached leaves in situ. The high temperature sensitivity of leaves of plants growing under natural conditions were similar to those of the same species grown in controlled environments of similar thermal regimes. High temperature in controlled environment studies brought about acclimation responses which increased the threshold for high temperature damage as measured by gas exchange. Studies of fluorescence versus temperature confirmed that this method could be used to quantify these responses, and permitted the kinetics of the acclimation response to be examined. Gas exchange studies, while providing similar estimates of thermal stability, required more time, more elaborate instrumentation, and are particularly difficult to conduct with field plants growing in situ.     

The geometry of light interception by shoots of Heteromeles arbutifolia: morphological and physiological consequences for individual leaves

The influence of leaf orientation and position within shoots on individual leaf light environments, carbon gain, and susceptibility to photoinhibition was studied in the California chaparral shrub Heteromeles arbutifolia with measurements of gas exchange and chlorophyll fluorescence, and by application of a three-dimensional canopy architecture model. Simulations of light absorption and photosynthesis revealed a complex pattern of leaf light environments and resulting leaf carbon gain within the shoots. Upper, south-facing leaves were potentially the most productive because they intercepted greater daily photon flux density (PFD) than leaves of any other orientation. North-facing leaves intercepted less PFD but of this, more was received on the abaxial surface because of the steep leaf angles. Leaves differed in their response to abaxial versus adaxial illumination depending on their orientation. While most had lower photosynthetic rates when illuminated on their abaxial as compared to adaxial surface, the photosynthetic rates of north-facing leaves were independent of the surface of illumination. Because of the increasing self-shading, there were strong decreases in absorbed PFD and daily carbon gain in the basipetal direction. Leaf nitrogen per unit mass also decreased in the basipetal direction but on a per unit area basis was nearly constant along the shoot. The decrease in leaf N per unit mass was accounted for by an increase in leaf mass per unit area (LMA) rather than by movement of N from older to younger leaves during shoot growth. The increased LMA of older lower leaves may have contributed directly to their lower photosynthetic capacities by increasing the limitations to diffusion of CO₂ within the leaf to the sites of carboxylation. There was no evidence for sun/shade acclimation along the shoot. Upper leaves and especially south-facing upper leaves had a potential risk for photoinhibition as demonstrated by the high PFDs received and the diurnal decreases in the fluorescence ratio F _v/F _m. Predawn F _v/F _m ratios remained high (>0.8) indicating that when in their normal orientations leaves sustained no photoinhibition. Reorientation of the leaves to horizontal induced a strong sustained decrease in F _v/F _m and CO₂ exchange that slowly recovered over the next 10–15?days. If leaves were also inverted so that the abaxial surface received the increased PFDs, then the reduction in F _v/F _m and CO₂ assimilation was much greater with no evidence for recovery. The heterogeneity of responses was due to a combination of differences between leaves of different orientation, differences between responses on their abaxial versus adaxial surfaces, and differences along the shoot due to leaf age and self-shading effects.     

Definition and Evaluation of the Spatio‐Temporal Variations in Chlorophyll Fluorescence during the Phases of CAM and during Endogenous Rhythms in Continuous Light,in Thick Leaves of Kalanchoë daigremontiana5

Abstract: We used chlorophyll fluorescence imaging to examine the homogeneity of photosynthetic metabolism during CAM in the thick leaves of Kalanchoë daigremontiana Hamet et Perrier de la Bǎthie. Intense, persistent fluorescence from a DCMU treated thin leaf of Clematis sp placed beneath a K. daigremontiana leaf was readily detected through the thick leaf. Evidently reabsorption of fluorescence was qualitatively unimportant in the system used. Chlorophyll fluorescence images from 7 mm tissue discs excised from Kalanchoë leaves were collected at 60 s intervals during 20 min transients elicited by red excitation light. Information about patchiness and subsurface processes was gained by statistical factor analysis and Fourier transform. Although small, highly resolved rings of bright chlorophyll fluorescence surrounding discs of low fluorescence were observed from cells near the surface, no independent regional temporal variation in fluorescence was evident in the surface‐biased images. Temporally independent chlorophyll fluorescence was present in images biased towards sub‐epidermal sources, in most phases of CAM, and during endogenous rhythm. These asynchronous changes were several millimetres apart. This patchy fluorescence was confirmed when attached leaves were excited with blue light in a leaf chamber while CO₂ and H₂O exchange was monitored. Large spatio‐temporal variations in the efficiency of photosystem II were always observed during phases II and IV of CAM, when both CO₂ fixation cycles are active, and during the maximum rate of CO₂ fixation during the endogenous rhythm in continuous light. These data are discussed in terms of metabolic isolation in the thick but uniform tissues in which gas diffusion may be largely confined to wet cell walls, thereby rendering the tissue functionally heterobaric. Prolonged, but in some instances, reversible alterations in PSII efficiency could be produced by injection of metabolic inhibitors, confirming that patchy fluorescence may reflect the differing energy costs of photosynthesis in different CAM phases.     

Photosynthesis and Chlorophyll Fluorescence Characteristics in Relationship to Changes in Pigment and Element Composition of Leaves of Platanus occidentalis L. during Autumnal Leaf Senescence

The loss of chlorophyll and total leaf nitrogen during autumnal senescence of leaves from the deciduous tree Platanus occidentalis L. was accompanied by a marked decline in the photosynthetic capacity of O₂ evolution on a leaf area basis. When expressed on a chlorophyll basis, however, the capacity for light-and CO₂-saturated O₂ evolution did not decline, but rather increased as leaf chlorophyll content decreased. The photon yield of O₂ evolution in white light (400-700 nanometers) declined markedly with decreases in leaf chlorophyll content below 150 milligrams of chlorophyll per square meter on both an incident and an absorbed basis, due largely to the absorption of light by nonphotosynthetic pigments which were not degraded as rapidly as the chlorophylls. Photon yields measured in, and corrected for the absorptance of, red light (630-700 nanometers) exhibited little change with the loss of chlorophyll. Furthermore, PSII photochemical efficiency, as determined from chlorophyll fluorescence, remained high, and the chlorophyll a/b ratio exhibited no decline except in leaves with extremely low chlorophyll contents. These data indicate that the efficiency for photochemical energy conversion of the remaining functional components was maintained at a high level during the natural course of autumnal senescence, and are consistent with previous studies which have characterized leaf senescence as being a controlled process. The loss of chlorophyll during senescence was also accompanied by a decline in fluorescence emanating from PSI, whereas there was little change in PSII fluorescence (measured at 77 Kelvin), presumably due to decreased reabsorption of PSII fluorescence by chlorophyll. Nitrogen was the only element examined to exhibit a decline with senescence on a dry weight basis. However, on a leaf area basis, all elements (C, Ca, K, Mg, N, P, S) declined in senescent leaves, although the contents of sulfur and calcium, which are not easily retranslocated, decreased to the smallest extent.     

Characteristics of a virescent cotton mutant

The virescent cotton (Gossypium hirsutum) mutant described here differs from normal cultivated cotton by a single mutation in the nucleus. The mutant exhibits nuclear control of chlorophyll and carotenoid development. Young leaves are distinctly yellow and become green with age. There is no unusual photometabolism of ¹⁴CO₂ or ¹⁴C-acetate in this mutant. It is probable that the nuclear virescent mutation is in a locus concerned with making structural units. The yellow leaves do show a high photosynthetic capacity on a chlorophyll basis. At saturating light intensity the rate of CO₂ fixation is 8 fold higher than the green control leaves. Thus, impaired pigment synthesis which could be lethal is offset by a high photosynthetic capacity in the virescent leaves.     

Profiles of photosynthesis within red and green leaves of Quintinia serrata

We have measured photosynthesis at the cellular, tissue, and whole leaf levels to understand the role of anthocyanin pigments on patterns of light utilization. Profiles of chlorophyll fluorescence through sections of red and green leaves of Quintinia serrata showed that anthocyanins in the mesophyll restricted absorption of green light to the uppermost palisade mesophyll. The distribution was further restricted when anthocyanins were also present in the upper epidermis. Mesophyll cells located beneath a cyanic light-filter assumed the characteristic photosynthetic features of shade-adapted cells. As a result, red leaves showed a 23% reduction in CO₂ assimilation under light-saturating conditions, and a lower threshold irradiance for light-saturation, relative to those of green leaves. The photosynthetic characteristics of red leaves are comparable to those of shade-acclimated plants.     

Enhancement of photosynthesis in Sorghum bicolor by ultraviolet radiation

We assessed the influence of ultraviolet radiation (UV) on net photosynthetic CO₂ assimilation rate (P_n) in Sorghum bicolor, with particular attention to examining whether UV can enhance P_n via direct absorption of UV and absorption of UV‐induced blue fluorescence by photosynthetic pigments. A polychromatic UV response spectrum of leaves was constructed by measuring P_n under different UV supplements using filters that had sharp transmission cut‐offs from 280 to 382 nm, against a background of non‐saturating visible light. When the abaxial surface was irradiated, P_n averaged 4.6% higher with the UV supplement that cut‐off UV at 311 nm, compared to lower and higher UV wavelength supplements. This former supplement differed from higher wavelength supplements by primarily providing more UV between 320 and 350 nm. To assess the possibility of direct absorption of UV by photosynthetic pigments, we measured the absorbance of extracted chlorophylls. Chlorophyll a had absorbance peaks at 340 and 389 nm that were 49 and 72% of that at the sorét peak. Chlorophyll b had absorbance peaks at 315 and 346 nm that were both 35% of that at the sorét peak. Since the epidermis transmits some UV, the strong UV absorbance of chlorophyll implies a potential role for irradiance beyond the bounds of the conventionally defined photosynthetically active radiation waveband (400–700 nm). To assess the role of absorption of UV‐induced blue fluorescence, we measured the UV‐induced fluorescence excitation and emission spectra of leaves. Abaxial excitation peaked at 328 nm, while emission peaked at 446 nm. In this analysis, we used our abaxial fluorescence excitation spectrum and the UV photosynthetic inhibition spectrum of Caldwell et al. (1986) to weight the UV irradiance with each cut‐off filter, thereby estimating the potential contribution of UV‐induced blue fluorescence to photosynthesis and the inhibitory effects of UV irradiance on photosynthesis, respectively. With a non‐saturating visible background, we estimate that the absorption of UV‐induced blue fluorescence and the direct absorption of UV by photosynthetic pigments maximally enhanced photosynthesis by about 1% each with the UV supplement that cut‐off UV at 311 nm. We suggest that a portion of the incident UV on the S. bicolor leaves was used to drive photosynthesis.     

Measurement of Differences in Red Chlorophyll Fluorescence and Photosynthetic Activity between Sun and Shade Leaves by Fluorescence Imaging

With a flash-lamp chlorophyll (Chl) fluorescence imaging system (FL-FIS) the photosynthetic activity of several thousand image points of intact shade and sun leaves of beech were screened in a non-destructive way within a few seconds. The photosynthetic activity was determined via imaging the Chl fluorescence at maximum F_p and steady state fluorescence F_s of the induction kinetics (Kautsky effect) and by a subsequent determination of the images of the fluorescence decrease ratio R_Fd and the ratio F_p/F_s. Both fluorescence ratios are linearly correlated to the photosynthetic CO₂ fixation rates. This imaging method permitted to detect the gradients in photosynthetic capacity and the patchiness of photosynthetic quantum conversion across the leaf. Sun leaves of beech showed a higher photosynthetic capacity and differential pigment ratios (Chl a/b and Chls/carotenoids) than shade leaves. Profile analysis and histogram of the Chl fluorescence yield and the Chl fluorescence ratios allow to quantify the differences in photosynthetic activity between different leaf parts and between sun and shade leaves with a high statistical significance.     

Photosynthetic light-response curves

The shapes of photosynthetic light-response curves for leaves of Eucalyptus maculata (Hook) and E. pauciflora (Sieber ex Sprengel) were examined. Three different methods were used to measure photosynthesis: CO₂ and H₂O-vapour exchange, O₂ evolution at a 5-kPa CO₂ partial pressure, and chlorophyll fluorescence. The three methods were compared and gave good agreement when measured under equivalent conditions. However, O₂ evolution was inhibited by high CO₂ partial pressures. A non-rectangular hyperbolic curve has been used widely to describe photosynthetic light-response curves. It has three variables which define the maximum quantum yield (photosynthetic rate divided by absorbed irradiance at very low irradiances), the maximum capacity and the curvature (Θ). We found that Θ was affected by the CO₂ partial pressure, declining to a minimum of about 0.6 as CO₂ partial pressure increased to 100 Pa. Further increases in the CO₂ partial pressure began to inhibit the rate of O₂ evolution at 2000 μmol quanta · m^?2·^?1 and Θ increased back to 0.95 by 5 kPa CO₂ partial pressure. At low irradiances, photosynthesis is limited by the rate of electron transport while at high irradiances, photosynthesis is frequently limited by the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco). The dependence of Θ on CO₂ partial pressure arises because the transition between limitations changes as a function of the CO₂ partial pressure. The light-response curve is truncated by the transition to a Rubisco limitation and the lower the irradiance at the transition, the higher the value of Θ. There is a gradient in light absorption through the leaf which influences the photosynthetic capacity of different layers within the leaf. The gradient in photosynthetic capacity can be demonstrated by the fact that the shape of the light-response curve changes when the leaf is illuminated unilaterally onto either the adaxial or abaxial surface. We compared two Eucalyptus species which had either isolateral or dorsiventral leaf anatomy. Leaves were able to reverse completely the gradients in photosynthetic capacity following inversion of the leaves for a week, irrespective of their anatomy.     

Light scattering,chlorophyll fluorescence and state of the adenylate system in illuminated spinach leaves

Scattering of green light and chlorophyll fluorescence by spinach leaves kept in a stream of air or nitrogen were compared with leaf adenylate levels during illumination with blue, red or far-red light. Energy charge and ATP-ADP ratios exhibited considerable variability in different leaves both in the dark and in the light. Variability is explained by different possible states of the reaction oxidizing triose phosphate or reducing 3-phosphoglycerate. Except when oxygen levels were low, there was an inverse relationship between light scattering and chlorophyll fluorescence during illumination with blue or red light. When CO₂ was added to a stream of CO₂-free air, chlorophyll fluorescence increased, sometimes after a transient decrease, and both light scattering and leaf $\text{ATP}\text{ADP}$ ratios decreased. Similar observations were made when air was replaced by nitrogen under blue or high-intensity red light. Under these conditions, over-reduction caused inhibition of electron transport and phosphorylation in chloroplasts. However, when air was replaced by nitrogen during illumination with low-intensity red light or far-red light, light scattering increased instead of decreasing. Under these light conditions, $\text{ATP}\text{ADP}$ ratios were maintained in the light. They decreased drastically only after darkening. Although $\text{ATP}\text{ADP}$ ratios responded faster than light scattering or the slow secondary decline of chlorophyll fluorescence due to illumination, it appeared that in the steady state, light scattering and chlorophyll fluorescence are useful indicators of the phosphorylation state of the leaf adenylate system at least under aerobic conditions, when chloroplast and extrachloroplast adenylate systems can effectively communicate.     

Photoinhibition of isolated mesophyll cells from cold-hardened and non-hardened winter rye

Cold-hardened rye leaves have been shown to be more resistant to low temperature photoinhibition than non-hardened rye leaves. Isolated mesophyll cells from winter rye (Secale cereale L. cv. Musketeer) were exposed to photoinhibitory light conditions to estimate the importance of leaf morphology and leaf optical properties in the resistance of cold-hardened rye leaves to photoinhibition. Cold-hardened rye cells showed more resistance to photoinhibition than non-hardened rye cells when monitored with chlorophyll a variable to maximal fluorescence ratio (F_v/F_m). Thus, leaf morphology does not contribute to the resistance of cold-hardened rye leaves to low temperature photoinhibition. However, cold-hardened and non-hardened rye cells showed a similar extent of photoinhibition when photsynthetic CO₂ fixation rates were measured. They also showed the same capacity to recover from photoinhibition. During both photoinhibition and recovery, F_v/F_m and light limited CO₂ fixation rates showed different kinetics. We propose that inactivation and subsequent reactivation during recovery of some light activated Calvin cycle enzymes explain the greater extent of photoinhibition of light limited CO₂ fixation and its faster recovery compared to F_v/F_m kinetics during photoinhibition.