Acclimation to changing light conditions of long-term shade-grown beech (Fagus sylvatica L.) seedlings of different geographic origins

 Effects of changing light conditions on the ecophysiological condition behind survival were examined on beech from two different populations. Plants were grown in a greenhouse under simulated understorey and canopy gap light conditions. Upon exposure to high light maximum photosynthesis of shade-acclimated leaves increased followed by a reduction over several days to between high- and low-light control rates. In the reciprocal transfer, the decrease in maximum photosynthesis was rapid during the first 2–3 days and then levelled off to values comparable to low-light controls. Seedlings from Sicily (Madonie) showed generally higher maximum photosynthetic rates than those from Abetone. Leaf conductance varied in the same direction as photosynthesis in high- to low-light seedlings but to a lesser degree. Leaves grown under low light and exposed to high light experienced photoinhibition. The Abetone population was more susceptible to photoinhibitory damage than the seedlings from Sicily. Exposure to high light of shade-acclimated seedlings resulted in intermediate chlorophyll concentrations between levels of the high-light and low-light seedlings. Carotenoid concentration was unaffected by treatments. Seedlings grew more in high light, but had a lower leaf area ratio. Light-limited seedlings showed a shift in carbon allocation to foliage. Leaves formed in the new light regime maintained the same anatomy that had been developed before transfer. Seedlings from Sicily had thicker leaves than those of seedlings from Abetone. Seedlings from Abetone were found to be more susceptible to changing light conditions than seedlings from Sicily. We conclude that small forest gaps may represent a favorable environment for photosynthesis and growth of beech regeneration as a result of the limited ability of seedlings to acclimate to sudden increases in high irradiance and because of the moderate levels of light stress in small gaps. Received: 11 April 1997 / Accepted: 11 December 1997     

Differences in light energy utilisation and dissipation between dipterocarp rain forest tree seedlings

 The light environment within tropical rain forests varies considerably both spatially and temporally, and photon flux density (PFD) is considered to be an important factor determining the growth and survival of rain forest tree seedlings. In this paper we examine the ability of four ecologically contrasting dipterocarps (Dryobalanops lanceolata, Shorea leprosula, Hopea nervosa and Vatica oblongifolia) to utilise and dissipate light energy when grown in different light environments in lowland dipterocarp rain forest in the Danum Valley Conservation Area, Sabah, East Malaysia. Specifically we report (i) photosynthetic light response curves and associated fluorescence characteristics, including quantum yield (ΦPSII) and non-photochemical quenching (qN) and (ii) the extent to which photoinhibition occurs when plants grown in either high or low light are exposed to short bursts of high PFD. When grown in low light (artificial or forest shade) all four species had low light saturated rates of photosynthesis which were achieved at low PFDs. In addition, values of ΦPSII and qN were similar over a range of measurement PFDs. D. lanceolata and S. leprosula were also grown at high PFD and showed marked differences in their responses. S. leprosula demonstrated an ability to increase its rate of photosynthesis and there was a small increase in capacity to dissipate excess light energy non-photochemically at high PFDs. Partitioning of this qN into its fast, photo-protective (qE) and slow, photoinhibitory (qI) components indicated that there was an increase in qE quenching. In contrast, although D. lanceolata survived in the high light environment, greater rates of photosynthesis were not observed and the plants showed a greater capacity to dissipate energy non-photochemically. Partitioning of qN revealed that the majority of this increase was attributable to the slower relaxing phases. Received: 10 February 1996 / Accepted: 14 June 1996     

Effects of blue light deficiency on acclimation of light energy partitioning in PSII and CO2 assimilation capacity to high irradiance in spinach leaves

Blue light effects on the acclimation of energy partitioningcharacteristics in PSII and CO₂ assimilation capacity in spinachto high growth irradiance were investigated. Plants were grownhydroponically in different light treatments that were a combinationof two light qualities and two irradiances, i.e. white lightand blue-deficient light at photosynthetic photon flux densities(PPFDs) of 100 and 500 µmol m^–2 s^–1. The CO₂assimilation rate, the quantum efficiency of PSII (PSII) andthermal dissipation activity / in young, fully expanded leaves were measured under 1,600 µmol m^–2 s^–1white light. The CO₂ assimilation rate and PSII were higher,while / was lower in plants grown under high irradiancethan in plants grown under low irradiance. These responses wereobserved irrespective of the presence or absence of blue lightduring growth. The extent of the increase in the CO₂ assimilationrate and PSII and the decrease in / by high growth irradiance was smaller under blue light-deficient conditions. These resultsindicate that blue light helps to boost the acclimation responsesof energy partitioning in PSII and CO₂ assimilation to highirradiance. Similarly, leaf N, Cyt f and Chl contents per unitleaf area increased by high growth irradiance, and the extentof the increment in leaf N, Cyt f and Chl was smaller underblue light-deficient conditions. Regression analysis showedthat the differences in energy partitioning in PSII and CO₂assimilation between plants grown under high white light andhigh blue-deficient light were closely related to the differencein leaf N.     

Nitrogen supply as a factor influencing photoinhibition and photosynthetic acclimation after transfer of shade-grown Solanum dulcamara to bright light

We have compared the ability of shadegrown clones of Solamum dulcamara L. from shade and sun habitats to acclimate to bright light, as a function of nitrogen nutrition before and after transfer to bright light. Leaves of S. dulcamara grown in the shade with 0.6 mM NO ₃ ^- have similar photosynthetic properties as leaves of plants grown with 12.0 mM NO ₃ ^- . When transferred to bright light for 1–2 d the leaves of these plants show substantial photoinhibition which is characterized by about 50% decrease in apparent quantum yield and a reduction in the rate of photosynthesis in air at light saturation. Photoinhibition of leaf photosynthesis is associated with reduction in the variable component of low-temperature fluorescence emission, and with loss of in-vitro electron transport, especially of photosystem II-dependent processes.We find no evidence for ecotypic differentiation in the potential for photosynthetic acclimation among shade and sun clones of S. dulcamara, or of differentiation with respect to nitrogen requirements for acclimation. Recovery from photoinhibition and subsequent acclimation of photosynthesis to bright light only occurs in leaves of plants provided with 12.0 mM NO ₃ ^- . In these, apparent quantum yield is fully restored after 14 d, and photosynthetic acclimation is shown by an increase in light-saturated photosynthesis in air, of light-and CO₂-saturated photosynthesis, and of the initial slope of the CO₂-response curve. The latter changes are highly correlated with changes in ribulose-bisphosphate-carboxylase activity in vitro. Plants supplied with 0.6 mM NO ₃ ^- show incomplete recovery of apparent quantum yield after 14 d, but CO₂-dependent leaf photosynthetic parameters return to control levels.Symbols and abbreviations F_o initial level of fluorescence at 77 K - F_m maximum level of fluorescence at 77 K - F_v variable components of fluorescence at 77 K (F_v=F_m-F_o) - PSI, PSII photosystem I and II, respectively - RuBP ribulose-1,5-bisphosphate - RuBPCase ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39)     

Ectomycorrhizal colonisation of Sitka spruce [Picea sitchensis (Bong.) Carr] seedlings in a Scottish plantation forest

 The ectomycorrhizal (ECM) colonisation of seedling Sitka spruce (Picea sitchensis) was examined in an uneven-aged plantation forest in southern Scotland. The extent of ECM colonisation of individual seedlings was 43.8–97.2%, with an overall mean of 80.3 ± 1.1%. A total of 13 ECM morphotypes were differentiated, with 1–4 ECM types colonising an individual seedling. ECM colonisation was dominated by a single species, Tylospora fibrillosa, which accounted for 72.4–97.7% of the ECM colonisation recorded, on a plot mean basis. Other ECM types appeared to be distributed very patchily, only two types (Lactarius sp. and Mycelium radicis atrovirens Melin) exceeding a mean of 10% colonisation in any one plot. No significant correlations were recorded between ECM colonisation and seedling growth, or between ECM colonisation and soil pH, loss-on-ignition, or water content. Accepted: 16 October 1997     

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.     

Rapid photosynthetic acclimation of Shorea johorensis seedlings after logging disturbance in Central Kalimantan

This study examined the photosynthetic acclimation of pre-existing Shorea johorensis (Dipterocarpaceae) seedlings to the change in conditions that occurs at the time of logging in Central Kalimantan, Indonesia. The hypothesis was that the seedlings would be unable to acclimate beyond partially open conditions after canopy disturbance caused by logging, therefore limiting the potential for regeneration in the most open areas. Bleaching and reductions in the predawn ratio of variable to maximum fluorescence (F _v /F _m) indicated chronic photoinhibition and damage to the previously shade-adapted leaves of seedlings in an area logged 2 weeks earlier. The majority of seedlings in partially open and open environments of an area logged 3 months earlier were already growing fast. Leaves that had developed in the new environment showed only small reductions in predawn F _v /F _m and large increases in the light saturated rate of photosynthesis (A _max) per unit area when compared to shaded seedlings. Leaves in the most open environments had higher but more variable nitrogen concentrations, A _max per unit area and A _max per unit mass when compared to seedlings in partially open environments. Increases in dark respiration were disproportionately large compared to increases in A _max, and may have been the result of increased investment in photoprotective mechanisms. The response of stomatal conductance to the vapour pressure deficit and leaf temperature was examined, but it suggested only a 10% reduction in daily leaf level carbon gain in open environments. The ratio of leaf area to fine root mass was highest in shade-suppressed and newly exposed seedlings, suggesting a potential hydraulic limitation to transpiration during acclimation. However, rainfall during this period was high and leaf water potentials did not differ between disturbed and undisturbed environments. S. johorensis seedlings were capable of significant acclimation to conditions more extreme than partial canopy opening. Low seedling density after logging during the wet season cannot be explained by a limited potential for photosynthetic acclimation. Received: 14 September 1998 / Accepted: 12 August 1999     

Genetics and cytology of a new genic male-sterile soybean [Glycine max (L.) Merr.]

 Genetic and cytological studies were conducted with a new male-sterile, female-fertile soybean [Glycine max (L.) Merr.] mutant. This mutant was completely male sterile and was inherited as a single-recessive gene. No differences in female or male gamete transmission of the recessive allele were observed between reciprocal cross-pollinations in the F₁ or F₂ generations. This mutant was not allelic to any previously identified soybean genic male-sterile mutants: ms1, ms2, ms3, ms4, ms5, or ms6. No linkage was detected between sterility and flower color (W1 locus), or between sterility and pubescence color (T1 locus). Light microscopic and cytological observations of microsporogenesis in fertile and sterile anthers were conducted. The structure of microspore mother cells (MMC) in male-sterile plants was identical to the MMCs in male-fertile plants. Enzyme extraction analyses showed that there was no callase activity in male-sterile anthers, and this suggests that sterility was caused by retention of the callose walls, which normally are degraded around tetrads at the late tetrad stage. The tapetum from male-sterile anthers also showed abnormalities at the tetrad stage and later stages, which were expressed by an unusual formation of vacuoles, and by accumulation of densely staining material. At maturity, anthers from sterile plants were devoid of pollen grains. Received: 13 May 1996 / Revision accepted: 19 August 1996     

Inhibition of photosynthesis in current-year needles of unfertilized and fertilized Norway spruce [Picea abies (L.) Karst.] during autumn and early winter

Inhibition of photosynthesis was followed during autumn and early winter in current-year sun and shade needles of unfertilized and fertilized Norway spruce [Picea abies (L.) Karst.] by simultaneous measurements of photosynthetic O₂ evolution and chlorophyll a fluorescence at 20 °C. The CO₂-saturated rate of O₂ evolution was generally higher in sun needles of fertilized trees than in those of unfertilized trees over a wide range of incident photon flux densities (PFDs). Furthermore, the maximum photo-chemical efficiency of photosystem (PS) II, as indicated by the ratio of variable to maximum fluorescence (F_V/F_M) was generally higher for sun needles of fertilized trees. The depression of f_v/f_m during frost periods was more pronounced in sun needles than in shade needles, indicating that winter inhibition in Norway spruce is strongly light-dependent. However, the inhibition of the rate of O₂ evolution at high PFDs in needles of fertilized trees during early winter was partly independent of the light regime experienced by those needles in the field, which appeared to result in a pronounced decrease in the proportion of oxidized PS II reaction centres in shade needles. A nearly identical linear relationship between the quantum yield of PS II electron transport determined by chlorophyll fluorescence and the quantum yield of O₂ evolution (gross rate of O₂ evolution/PFD) was obtained for the investigated types of needles during autumn and early winter. Except for shade needles of fertilized trees, this appeared to be largely achieved by adjustments in thermal energy dissipation within PS II.     

Photosynthetic acclimation of the liana Stigmaphyllon lindenianum to light changes in a tropical dry forest canopy

Tropical plant canopies show abrupt changes in light conditions across small differences in spatial and temporal scales. Given the canopy light heterogeneity, plants in this stratum should express a high degree of plasticity, both in space (allocation to plant modules as a function of opportunity for resource access) and time (photosynthetic adjustment to temporal changes in the local environment). Using a construction crane for canopy access, we studied light acclimation of the liana Stigmaphyllon lindenianum to sun and shade environments in a tropical dry forest in Panama during the wet season. Measured branches were randomly distributed in one of four light sequences: high- to low-light branches started the experiment under sun and were transferred to shade during the second part of the experiment; low- to high-light branches (LH) were exposed to the opposite sequence of light treatments; and high-light and low-light controls , which were exposed only to sun and shade environments, respectively, throughout the experiment. Shade branches were set inside enclosures wrapped in 63% greenhouse shade cloth. After 2 months, we transferred experimental branches to opposite light conditions by relocating the enclosures. Leaf mortality was considerably higher under shade, both before and after the transfer. LH branches reversed the pattern of mortality by increasing new leaf production after the transfer. Rates of photosynthesis at light saturation, light compensation points, and dark respiration rates of transferred branches matched those of controls for the new light treatment, indicating rapid photochemical acclimation. The post-expansion acclimation of sun and shade foliage occurred with little modification of leaf structure. High photosynthetic plasticity was reflected in an almost immediate ability to respond to significant changes in light. This response did not depend on the initial light environment, but was determined by exposure to new light conditions. Stigmaphyllon responded rapidly to light changes through the functional adjustment of already expanded foliage and an increase in leaf production in places with high opportunity for carbon gain. Received: 24 April 1998 / Accepted: 11 May 1999     

Specifying the introgressed regions from H. argophyllus in cultivated sunflower (Helianthus annuus L.) to mark Phomopsis resistance genes

A method based upon targetting of intro-gressed markers in a Phomopsis-resistant line (R) of cultivated sunflower, issuing from a H. argophyllus cross was used to mark the Phomopsis resistance regions. Our study was based upon 203 families derived from a cross between an inbred line susceptible to Phomopsis (S1) and the introgressed resistant line (R). Families were checked for Phomopsis resistance level in a design with replicated plots and natural infection was re-inforced by pieces of contaminated stems. Thirty four primers were employed for RAPD analysis. Out of 102 polymorphic fragments between (S1) and H. argophyllus, seven were still present in (R) suggesting that they marked introgressions of H. argophyllus into (R). The plants were scored for the presence or absence of 19 fragments obtained from five primers, and the relationships between the presence/absence of fragments in plants and Phomopsis resistance/susceptiblity in the progenies was determined by using an analysis of variance. We found that at least two introgressed regions, as well as favourable factors from sunflower, contributed to the level of Phomopsis resistance in cultivated sunflower. Received: 28 June 1996 / Accepted: 5 July 1996     

Genetic variation in grain-filling ability in dwarf pearl millet [Pennisetum glaucum (L.) R. Br.] restorer lines

The d2 dwarfing gene in pearl millet [Pennisetum glaucum (L.) R. Br.] carries a yield penalty due to an associated reduction in individual grain mass. This reduction, however, varies with genetic background, indicating that it may be possible to select against poor grain filling in d2 dwarfbackgrounds, given an effective measure of grain filling. This study was conducted to assess genetic variability forgrain-filling ability (in contrast to simply grain size),and its relationship to grain yield,indwarf pearl milletrestorer (R) lines. The grain-filling ability (GFA) of an individual R line was defined as the least squares estimate of its effect on individual grain mass in the analysis of variance, following a linear covariance adjustment for grain number. The study was based on 93dwarf hybrids involving31 d2 dwarfR-lines, evaluated over 3 years. Half of the variation in individual grain mass in the 93 hybrids was related to variation in grain number. Covariance adjustment in individual grain mass for grain number resulted in highly significant differences among hybrids and R lines in GFA. The R-line combining ability for GFA accounted for 26% of the variation in the R-line combining ability for yield, compared to 46% for the combining ability for grain number, and just 8% for the combining ability of individual grain mass. The combining ability for GFA was independent of the combining ability for various pre-flowering effects, including grain number, but was related to the combining ability for individual grain mass and harvest index. Improvement in individual grain mass achieved through selection for GFA should translate directly into yield improvement, whereasimprovement by direct selectionfor individual grain mass is less-likely to do so. Received: 9 April 2000 / Accepted: 16 May 2000     

Changes in chlorophyll fluorescence and chlorophyll content in suppressed Norway spruce [Picea abies (L.) Karst.] in response to release cutting

Summary In order to study physiological strain caused by release cutting, suppressed Norway spruce on mesic and moist sites was completely released from overstory birch, or 500 birches per hectare were left as a shelter. The treatments were conducted in late June in 1988 and in 1989. The spruce's reaction to the environmental change was monitored by measurements of fast chlorophyll fluorescence kinetics and analysis of chlorophyll content. This was done before treatment, 1 week after treatment, 2 months after treatment, and twice during the following growth period. Complete release resulted in a more pronounced decrease in the ratio of variable fluorescence to maximal fluorescence (F_v/F_m) than partial release. There was also a tendency for the build-up of chlorophyll content in needles to be more affected when the spruce was completely released. Released spruces on moist sites tended to be more affected by the release than released spruces on mesic sites. The results suggest that in this kind of stand the risk of damage to the spruces is greatest when the spruces are completely released on moist sites. Furthermore, it is shown that the weather conditions prevailing shortly before and after the release have a large influence on the spruce's reaction to the release. The results also indicate some adjustment to the new environment in mature needles.     

Photoprotection mechanisms in European beech (Fagus sylvatica L.) seedlings from diverse climatic origins

This study shows that beech leaves adapt to their light environment by inducing dramatic changes to antioxidant systems and pigment composition. Thus, ascorbate, tocopherol, glutathione, β-carotene and xanthophyll cycle pigments are much more concentrated in sun leaves, while α-carotene is much less concentrated than in shade leaves. These characteristics were used to identify the inherent potential of beech cotyledons from three contrasting climatic origins to tolerate light stress. The antioxidant content was initially different in the three provenances tested, but these initial differences tended to reduce with leaf ageing. The higher antioxidant and de-epoxidized xanthophyll content found in developing cotyledons indicated a superior potential for tolerance to photo-oxidative damage in those plants collected from the stressful climate of the Pyrenees. Nevertheless under an experimental high irradiation treatment no differences in light stress tolerance were observed between provenances. Received: 31 May 1999 / Accepted: 16 November 1999     

The concentrations of Fe, Zn and Co in successive needle age classes of Norway spruce [Picea abies (L.) Karst.]

The concentrations of Fe, Zn and Co were determined in up to five successive needle age classes in 54 individual Norway spruce trees from eight different sites (soil pH 3.1–7.7). Fe concentrations (12–25 μg in needles from the current year) were lower than most published values, due to the removal of surface contamination prior to analysis. Fe showed a significant positive correlation with Al. Successive needle age classes either had constant values or showed an increase for Fe concentrations; individual trees on a given site were rather uniform in their behaviour. Zn concentrations were 19–40 μg/g. On acid sites, they showed a positive correlation with total soil concentrations. The majority of trees showed decreasing Zn concentrations in successive needle age classes, but constant or increasing concentrations were also found; site homogeneity was less than with Fe. Co concentrations differed between trees on a neutral soil (12 ng/g) and on acid soils (41–174 ng/g). They showed a significant positive correlation with Mn needle concentrations. The changes of Co with needle age in most, but not all, trees were similar to those of Zn. The different changes of Fe, Zn and Co with needle age may be due to a different retranslocation. A modest retranslocation of Fe as opposed to a high retranslocation of Zn and Co (in most trees) is consistent with the observed behaviour. Received: 10 May 1999 / Accepted: 8 September 1999     

Seasonal acclimation in metabolism reduces light requirements of eelgrass (Zostera marina)

We investigated the ability of eelgrass (Zostera marina) to adjust light requirements to seasonal changes in temperature, light and nutrient conditions through changes in metabolism, pigment and nutrient content. In agreement with expectations we found that rates of respiration and light saturated photosynthesis of summer acclimated plants peaked at higher temperatures (5 °C and 2 °C higher, respectively), and were lower than of winter acclimated plants, both at sub- and supra-optimal temperatures. Moreover respiration rates were generally more sensitive to increasing temperatures than photosynthetic rates, especially so for cold acclimated plants in February (36% higher Q₁₀-values). These changes were accompanied by a reduction in chlorophyll a and nitrogen concentrations in leaves by 35% and 60% respectively from February to August. The critical light requirement (E_C) of Z. marina to maintain a positive carbon balance increased exponentially with increasing temperature but less so for summer-acclimated than for winter-acclimated plants. However, combining E_C vs temperature models for whole-plants with data on daily light availability showed that seasonal acclimation in metabolism increased the annual period, when light requirements were meet at the 2-5 m depth interval, by 32-66 days. Hence, acclimation is an important mechanism allowing eelgrass to grow faster and penetrate to deeper waters. Critical depth limits estimated for different combinations of summer temperatures and water clarity in a future climate scenario, suggested that expected increases in temperature and nutrient run-off have synergistic negative effects, especially in clear waters, stressing the importance of continued efforts to improve water clarity of coastal waters.     

The impact of ozone fumigation and fertilization on chlorophyll fluorescence of birch leaves (Betula pendula)

 The impact of ozone fumigation on chlorophyll a fluorescence parameters and chlorophyll content of birch trees grown at high and low fertilization were studied for 6-, 8-, and 12-week old leaves. Fluorescence parameters were measured with a portable fluorometer with its fibre optics tightly inserted in a gas exchange cuvette at light intensities from 0 to 220 μmol photons m⁻² s⁻¹. Ozone caused significant changes of primary photosynthetic reactions: a decrease of the quantum yield of photosystem II and an increase of non-photochemical quenching. In all leaves a biphasic light response of non-photochemical quenching was observed. Ozone fumigation shifted the onset of the second phase from a PFD of about 60 μmol m⁻² s⁻¹ to about 30 μmol m⁻² s⁻¹. While the fertilizer concentration had no influence on this character, high fertilization supply of plants partially reduced O₃-induced damage. The light responses of Ft, Fm′ and NPQ observed in birch leaves grown in O₃-free air indicate the existence of at least two different processes governing energy conversion of the photosynthetic apparatus at PS II in the range of PFD 0–200 μmol photons m⁻² s⁻¹. The first phase was attributed to a rather slowly relaxing type of non-photochemical quenching, which, at least at low PFD, is thought to be related to a state 1–2 transition. The further changes of the fluorescence parameters studied at higher PFD might be explained by an increase of energy-dependent quenching, connected with the energization of the thylakoid membrane and zeaxanthin synthesis. A major effect of ozone treatment was a lowering of PS II quantum yield. This reflects a reduction of PS II electron transport and corresponds to the reduction of CO₂-fixation observed in ozonated leaves. Received: 24 September 1996 / Accepted: 27 January 1999     

A new monitoring PAM fluorometer (<Emphasis Type="Italic">MONI-PAM</Emphasis>) to study the short- and long-term acclimation of photosystem II in field conditions

We present and evaluate the performance of a new field monitoring PAM fluorometer (MONI-PAM) which is intended for short- and long-term monitoring of the acclimation of photosystem II (PSII). The instrument measures chlorophyll fluorescence, photosynthetic photon flux density (PPFD), and temperature in the field, and monitors exactly the same leaf area over prolonged periods of time, facilitating the estimation of both rapidly reversible and sustained non-photochemical quenching (NPQ). The MONI-PAM performance is evaluated in the lab and under natural conditions in a Scots pine canopy during spring recovery of photosynthesis. The instrument provides a new tool to study in detail the acclimation of PSII to the environment under natural field conditions.     

Photosynthetic carbon metabolism in leaves of transgenic tobacco (Nicotiana tabacum L.) containing decreased amounts of fructose 2,6-bisphosphate

The aim of this work was to examine the role of fructose 2,6-bisphosphate (Fru-2,6-P₂) in photosynthetic carbon partitioning. The amount of Fru-2,6-P₂ in leaves of tobacco (Nicotiana tabacum L. cv. Samsun) was reduced by introduction of a modified mammalian gene encoding a functional fructose-2,6-bisphosphatase (EC 3.1.3.46). Expression of this gene in transgenic plants reduced the Fru-2,6-P₂ content of darkened leaves to between 54% and 80% of that in untransformed plants. During the first 30 min of photosynthesis sucrose accumulated more rapidly in the transgenic lines than in the untransformed plants, whereas starch production was slower in the transgenic plants. On illumination, the proportion of ¹⁴CO₂ converted to sucrose was greater in leaf disks of transgenic lines possessing reduced amounts of Fru-2,6-P₂ than in those of the control plants, and there was a corresponding decrease in the proportion of carbon assimilated to starch in the transgenic lines. Furthermore, plants with smaller amounts of Fru-2,6-P₂ had lower rates of net CO₂ assimilation. In illuminated leaves, decreasing the amount of Fru-2,6-P₂ resulted in greater amounts of hexose phosphates, but smaller amounts of 3-phosphoglycerate and dihydroxyacetone phosphate. These differences are interpreted in terms of decreased inhibition of cytosolic fructose-1,6-bisphosphatase resulting from the lowered Fru-2,6-P₂ content. The data provide direct evidence for the importance of Fru-2,6-P₂ in co-ordinating chloroplastic and cytosolic carbohydrate metabolism in leaves in the light. Received: 8 February 2000 / Accepted: 25 April 2000     

Leaf water relations of black alder [Alnus glutinosa (L.) Gaertn.] growing at neighbouring sites with different water regimes

 The tree species black alder [Alnus glutinosa (L.) Gaertn.] typically inhabits wet sites in central Europe but is also successful on well drained soils. To test the physiological adjustment of the species in situ, conductances, transpiration rates and water potentials (Scholander pressure chamber) of black alder leaves were investigated at two neighbouring sites with different water regimes: alder trees at an occasionally water logged alder forest and alder shrubs in a nearby, much drier hedgerow. Additional experiments with alder cuttings in nutrient culture showed that leaf conductances and gas exchange were both strongly influenced by the substrate water potential. In situ however, there was little spatial variability within the different parts of a crown and we found that physiological regulation at leaf level was hardly influenced by different site water regimes or different tree sizes. Diurnal courses of leaf water relations as well as their regulation at the leaf level (e.g. the hyperbolic relationship between conductances and ΔW) were strikingly similar at both sites. Leaf water potential in black alder was shown to be a consequence of immediate transpiration rates, which were high in comparison to other tree species (up to 4 mmol H₂O m^–2 s^–1), rather than the water potentials being a factor that influenced conductance and, therefore, transpiration. The always high leaf conductances and consequent high transpiration rates are interpreted as a strategy to maximise productivity through low stomatal limitation at sites where water supply is usually not limited. However, at the same time this behaviour restricts black alder to sites where at least the deep-going roots can exploit water. Received: 10 September 1998 / Accepted: 12 January 1999