Remembering Melvin Calvin (1911-1997), a highly versatile scientist of the 20th century.

Melvin Calvin (1911-1997) was the recipient of the 1961 Nobel Prize in Chemistry for the discovery of the canonical photosynthetic carbon reduction cycle. We present here a very brief glimpse of this extraordinary American scientist, who in his time was a preeminent force in physical and organic chemistry. Besides natural photosynthesis, Calvin's prolific career included artificial photosynthesis, colors of organic substances, the origin of life, cancer, moon rocks, molecular basis of learning, and plant lipids & algal hydrocarbons as potential renewable sources of transport fuels.

Tribute Roland Douce, 1939-2018.

On November 4, 2018, Roland Douce, Professor Emeritus at the University of Grenoble, France, died at the age of 79. In Grenoble, where he spent most of his scientific career, Roland Douce created a world-renowned school of plant science, studying the structure, functions, and interactions of plant organelles involved in photosynthesis, respiration, and photorespiration. His main achievements concern the chemical and functional characterization of chloroplast envelope membranes, the demonstration of the uniqueness of plant mitochondria, and the integration of metabolism within the plant cell, among manifold activities. Roland Douce devoted his whole life to science and research with passion and enthusiasm: he was a true charismatic leader.

Peter Heinrich Böger, 1935-2015.

On October 22, 2015, Professor Peter Heinrich Böger, an excellent and internationally highly regarded plant scientist, died in Constance, Germany, at the age of 80 years. He was a broadly oriented researcher of photosynthetic processes, with emphasis on the mode of action of herbicides in chloroplasts and on the biodiversity of nitrogen-fixing cyanobacteria. He was a very active, much committed person, who advanced not only plant science research, but also scientific communication, international cooperation and the promotion of young scientists. His scientific career, his manifold activities as editor and board member, and his merits and honors are described in this tribute.

Andrew A. Benson, 1917-2015.

UNLABELLED: On January 16, 2015, Professor Andrew Alm Benson, one of the leading plant biochemists of the twentieth century, died in La Jolla, California, at the age of 97; he was born on September 24, 1917. Benson was known especially for his pioneering studies on photosynthesis (CO2 assimilation, carbon reduction cycle) and plant lipids (phospholipid phosphatidyl glycerol; and the sulfolipid, sulfoquinovosyl diglyceride). A photograph of Benson is shown in Fig. 1. Fig. 1 Photograph of Andrew A. Benson. SOURCE: Annual Review of Plant Biology, Vol. 53, 2002, published with permission.

Uptake of diuron and concomitant loss of photosynthetic activity in leaves as visualized by imaging the red chlorophyll fluorescence.

The principles of the chlorophyll (Chl) fluorescence induction kinetics (known as Kautsky effect) and their change by the photosystem II herbicide diuron are presented together with the Chl fluorescence emission spectra of a normal and diuron-inhibited leaf. By imaging the Chl fluorescence emission of green leaves the successive uptake of diuron and the concomitant loss of photosynthetic quantum conversion from the leaf base to the leaf tip are documented.

Chlorophyll fluorescence kinetics, photosynthetic activity, and pigment composition of blue-shade and half-shade leaves as compared to sun and shade leaves of different trees.

The chlorophyll (Chl) fluorescence induction kinetics, net photosynthetic CO2 fixation rates P N, and composition of photosynthetic pigments of differently light exposed leaves of several trees were comparatively measured to determine the differences in photosynthetic activity and pigment adaptation of leaves. The functional measurements were carried out with sun, half-shade and shade leaves of seven different trees species. These were: Acer platanoides L., Ginkgo biloba L., Fagus sylvatica L., Platanus x acerifolia Willd., Populus nigra L., Quercus robur L., Tilia cordata Mill. In three cases (beech, ginkgo, and oak), we compared the Chl fluorescence kinetics and photosynthetic rates of blue-shade leaves of the north tree crown receiving only blue sky light but no direct sunlight with that of sun leaves. In these cases, we also determined in detail the pigment composition of all four leaf types. In addition, we determined the quantum irradiance and spectral irradiance of direct sunlight, blue skylight as well as the irradiance in half shade and full shade. The results indicate that sun leaves possess significantly higher mean values for the net CO2 fixation rates P N (7.8-10.7 µmol CO2 m(-2) s(-1) leaf area) and the Chl fluorescence ratio R Fd (3.85-4.46) as compared to shade leaves (mean P N of 2.6-3.8 µmol CO2 m(-2) s(-1) leaf area.; mean R Fd of 1.94-2.56). Sun leaves also exhibit higher mean values for the pigment ratio Chl a/b (3.14-3.31) and considerably lower values for the weight ratio total chlorophylls to total carotenoids, (a + b)/(x + c), (4.07-4.25) as compared to shade leaves (Chl a/b 2.62-2.72) and (a + b)/(x + c) of 5.18-5.54. Blue-shade and half-shade leaves have an intermediate position between sun and shade leaves in all investigated parameters including the ratio F v/F o (maximum quantum yield of PS2 photochemistry) and are significantly different from sun and shade leaves but could not be differentiated from each other. The mean values of the Chl fluorescence decrease ratio R Fd of blue-shade and half-shade leaves fit well into the strong linear correlation with the net photosynthetic rates P N of sun and shade leaves, thus unequivocally indicating that the determination of the Chl fluorescence decrease ratio R Fd is a fast and indirect measurement of the photosynthetic activity of leaves. The investigations clearly demonstrate that the photosynthetic capacity and pigment composition of leaves and chloroplasts strongly depend on the amounts and quality of light received by the leaves.

Comparison of photosynthetic induction and transient limitations during the induction phase in young and mature leaves from three poplar clones.

We tested the hypothesis that leaf age affects photosynthetic induction, because conductance to CO2 diffusion usually decreases with increasing leaf age. Photosynthetic inductions, primarily determined by the light modulation of Rubisco activity and stomatal opening, were investigated in both young and mature leaves, as defined by leaf plastochron index (LPI), from three poplar clones: Populus alba L., P. nigra L. and P. x euramericana (Dode) Guinier. In all clones, maximum assimilation rates (A max), maximum stomatal conductance (G Smax) and dark respiration rates (RD) were higher in young leaves (LPI = 3-5) than in mature leaves (LPI = 10-14), and A max decreased from P. alba via P. x euramericana to P. nigra. The clones with high photosynthetic capacity had low induction states 60 s after leaf illumination (IS60; indicating a slow initial induction phase), and required less time to reach 90% photosynthetic induction (T90). In contrast, the clone with the lowest photosynthetic capacity (P. nigra) exhibited high IS60 (high initial induction state) but a long induction time (high T90). A comparison of mature leaves with young leaves revealed significantly (P < 0.01) lower IS60 values in mature leaves of P. nigra only, and significantly higher T90 values in mature leaves of P. alba only. In all clones, young leaves exhibited a lower percentage of maximum transient stomatal limitation during photosynthetic induction (4-9%) compared with mature leaves (16-30%). Transient biochemical limitation, assessed on the basis of the time constants of Rubisco activation (tau), was significantly higher in mature leaves than in young leaves of P. alba; whereas there were no significant differences in tau between young and mature leaves of the other poplar clones. Thus, our hypothesis that leaf age affects photosynthetic induction was confirmed at the level of transient stomatal limitation, which was significantly higher in mature leaves than in young leaves in all clones. For the induction parameters IS60, T90 and tau, photosynthetic induction was more clone-specific and was dependent on leaf age only in some cases, an observation that may apply to other tree species.

Long-term acclimation of plants to elevated CO2 and its interaction with stresses.

Rising atmospheric carbon dioxide (CO(2)) concentration and air temperature are of major concern when considering the possible effects of global climate change on vegetations. Although production has been found to increase in many cases, other experiments have also indicated increased hazards for plant growth because of the increased frequency of weather extremes, such as droughts, floods, and extreme temperatures. Thus at the same time elevated CO(2) and the extreme climatic events, intra- and interannual climatic variability alone can be foreseen as an indirect constraint, which separately influences significantly the carbon cycling in ecosystems, too. In the shorter term the effect of CO(2) is direct and is mediated by photosynthesis. In the longer term the effects of elevated CO(2) became more and more indirect and its effects are mediated by source-sink interactions within plants, resources (nutrients, water), temperature, microbes, herbivores, and land-use management practice. In fact, the plants can make use of their general stress coping mechanisms to avoid or compensate possible negative effects of elevated CO(2). One has to consider that all the classical abiotic, biotic, and anthropogenic stressors are threatening plant growth and development also under elevated CO(2), although at possibly different doses compared to ambient CO(2) concentrations. Therefore, the knowledge of the general stress coping, stress avoiding, and tolerance mechanisms is needed to understand the regulation of the plants' metabolism under normal and elevated CO(2) levels.

Differences in pigment composition, photosynthetic rates and chlorophyll fluorescence images of sun and shade leaves of four tree species.

The differential pigment composition and photosynthetic activity of sun and shade leaves of deciduous (Acer pseudoplatanus, Fagus sylvatica, Tilia cordata) and coniferous (Abies alba) trees was comparatively determined by studying the photosynthetic rates via CO(2) measurements and also by imaging the Chl fluorescence decrease ratio (R(Fd)), which is an in vivo indicator of the net CO(2) assimilation rates. The thicker sun leaves and needles in all tree species were characterized by a lower specific leaf area, lower water content, higher total chlorophyll (Chl) a+b and total carotenoid (Cars) content per leaf area unit, as well as higher values for the ratio Chl a/b compared to the much thinner shade leaves and needles that possess a higher Chl a+b and Cars content on a dry matter basis and higher values for the weight ratio Chls/Cars. Sun leaves and needles exhibited higher rates of maximum net photosynthetic CO(2) assimilation (P(Nmax)) measured at saturating irradiance associated with higher maximum stomatal conductance for water vapor efflux. The differences in photosynthetic activity between sun and shade leaves and needles could also be sensed via imaging the Chl fluorescence decrease ratio R(Fd), since it linearly correlated to the P(Nmax) rates at saturating irradiance. Chl fluorescence imaging not only provided the possibility to screen the differences in P(N) rates between sun and shade leaves, but in addition permitted detection and quantification of the large gradients in photosynthetic rates across the leaf area existing in sun and shade leaves.

Induction of photosynthesis and importance of limitations during the induction phase in sun and shade leaves of five ecologically contrasting tree species from the temperate zone.

We examined the principal differences in photosynthetic characteristics between sun and shade foliage and determined the relative importance of biochemical and stomatal limitations during photosynthetic induction. Temperate-zone broadleaf and conifer tree species, ranging widely in shade tolerance, were investigated from one locality in the Czech Republic. The study species included strongly shade-tolerant Abies alba Mill. and Tilia cordata Mill., less shade-tolerant Fagus sylvatica L. and Acer pseudoplatanus L. and sun-demanding Picea abies (L.) Karst. In the fully activated photosynthetic state, sun foliage of all species had significantly higher maximum CO(2) assimilation rates, maximum stomatal conductance and maximum rates of carboxylation than shade foliage. Compared with shade leaves, sun leaves had significantly higher nocturnal stomatal conductances. In all species, shade foliage tended to have higher induction states 60 s after leaf illumination than sun foliage. Sun and shade foliage did not differ in the rate of disappearance of the transient biochemical limitation during the induction phase. Longer time periods were required to reach 90% photosynthetic induction and 90% stomatal induction in sun foliage than in shade foliage of the less shade-tolerant F. sylvatica and A. pseudoplatanus and in sun-demanding P. abies; however, in sun foliage of the strongly shade-tolerant species T. cordata and A. alba, the time needed for photosynthetic induction was similar to, or less than, that for shade foliage. Shade but not sun needles of P. abies and A. alba had significantly slower induction kinetics than the broadleaf tree species. Among species, the sun-demanding P. abies exhibited the shortest stomatal induction times in both sun and shade leaves. Independently of shade tolerance ranking, the transient stomatal and total limitations that characterize photosynthetic induction were relieved significantly earlier in shade foliage than in sun foliage. Sun foliage generally exhibited a hyperbolic photosynthetic induction response, whereas a sigmoidal induction response was more frequent in shade foliage. The different relative proportions of transient biochemical and stomatal limitations during photosynthetic induction in sun and shade foliage indicate an essential role of stomata in photosynthetic limitation during induction, mainly in shade foliage, with a consequent influence on the shape of the photosynthetic induction curve.

Differences in photosynthetic activity, chlorophyll and carotenoid levels, and in chlorophyll fluorescence parameters in green sun and shade leaves of Ginkgo and Fagus.

The differences in pigment levels and photosynthetic activity of green sun and shade leaves of ginkgo (Ginkgo biloba L.) and beech (Fagus sylvatica L.) are described. Sun leaves of both tree species possessed higher levels in chlorophylls (Chl) and carotenoids on a leaf area basis, higher values for the ratio Chl a/b and lower values for the ratio Chl/carotenoids (a+b)/(x+c) in comparison to shade leaves. The higher photosynthetic rates P(N) of sun leaves (ginkgo 5.4+/-0.9 and beech 8.5+/-2.1 micromol m(-2)s(-1)) were also reflected by higher values for the Chl fluorescence decrease ratios R(F)(d) 690 and R(F)(d) 735. In contrast, the shade leaves had lower P(N) rates (ginkgo 2.4+/-0.3 and beech 1.8+/-1.2 micromol m(-2)s(-1)). In both tree species the stomatal conductance G(s) was significantly higher in sun (range: 70-19 1 mmol m(-2)s(-1)) as compared to shade leaves (range: 5-55 mmol m(-2)s(-1)). In fact, at saturating light conditions there existed a close correlation between G(s) values and P(N) rates. Differences between sun and shade leaves also existed in several other Chl fluorescence ratios (F(v)/F(m), F(v)/F(o), and the stress adaptation index Ap). The results clearly demonstrate that the fan-shaped gymnosperm ginkgo leaves show the same high and low irradiance adaptation response as the angiosperm beech leaves.

Characterization of the laser-induced blue, green and red fluorescence signatures of leaves of wheat and soybean grown under different irradiance.

The blue, green and red fluorescence emission of green wheat (Triticum aestivum L. var. Rector) and soybean leaves (Glycine max L. var. Maple Arrow) as induced by UV light (nitrogen laser: 337 nm) was determined in a phytochamber and in plants grown in the field. The fluorescence emission spectra show a blue maximum near 450 nm, a green shoulder near 530 nm and the two red chlorophyll fluorescence maxima near 690 and 735 nm. The ratio of blue to red fluorescence, F450/F690, exhibited a clear correlation to the irradiance applied during the growth of the plants. In contrast, the chlorophyll fluorescence ratio, F690/F735, and the ratio of blue to green fluorescence, F450/F530, seem not to be or are only slightly influenced by the irradiance applied during plant growth. The blue fluorescence F450 only slightly decreased, whereas the red chlorophyll fluorescence decreased with increasing irradiance applied during growth of the plants. This, in turn, resulted in greatly increased values of the ratio, F450/F690, from 0.5 - 1.5 to 6.4 - 8.0. The decrease in the chlorophyll fluorescence with increasing irradiance seems to be caused by the accumulation of UV light absorbing substances in the epidermal layer which considerably reduces the UV laser light which passes through the epidermis and excites the chlorophyll fluorescence of the chloroplasts in the subepidermal mesophyll cells.

Winter photosynthetic activity of twenty temperate semi-desert sand grassland species.

The winter photosynthetic activity (quantified by net CO(2) assimilation rates and chlorophyll (Chl) a fluorescence parameters) of 20 plant species (including two lichens and two mosses) of a Hungarian temperate semi-desert sand grassland was determined on one occasion per year in 1984, 1989 and 1994. Throughout winter, the overwintering green shoots, leaves or thalli were regularly exposed to below zero temperatures at night and daytime temperatures of 0-5 degrees C. In situ tissue temperature varied between -2.1 and +6.9 degrees C and the photosynthetic photon flux density (PPFD) between 137 and 351 micromol m(-2)s(-1). Under these conditions 18 of the grassland species exhibited photosynthetic CO(2) uptake (range: vascular plants ca. 0.2-3.8 micromol m(-2)s(-1), cryptogams 0.3-2.79 micromol kg(-1)s(-1)) and values of 0.9-5.1 of the Chl fluorescence decrease ratio R(Fd). In 1984, Festuca vaginata and Sedum sexangulare had net CO(2) assimilation at leaf temperatures of -0.85 to -1.2 degrees C. In 1989, all species except Cladonia furcata showed net CO(2) assimilation at tissue temperatures of 0 to +3.3 degrees C, with the highest rates observed in Poa bulbosa and F. vaginata. The latter showed a net CO(2) assimilation saturation at a PPFD of 600 micromol m(-2)s(-1) and a temperature optimum between +5 and +18 degrees C. At the 1994 measurements, the photosynthetic rates were higher at higher tissue water contents. The two mosses and lichens had a net photosynthesis (range: 1.1-2.79 micromol CO(2)kg(-1)s(-1)) at 2 degrees C tissue temperature and at 4-5 degrees C air temperature. Ca. 80% of the vascular grassland plant species maintained a positive C-balance during the coldest periods of winter, with photosynthetic rates of 1.5-3.8 micromol CO(2)m(-2)s(-1). In an extremely warm beginning March of the relatively warm winter of 2006/2007, the dicotyledonous plants had much higher CO(2) assimilation rates on a Chl (range 6-14.9 micromol g(-1)Chl s(-1)) and on a dry weight basis (9-48 micromol kg(-1)dw s(-1)) than in the cold winter of 1994. However, the assimilation rates of the three investigated cryptogams (Tortula and two Cladonia) and the two grasses Festuca and Poa were not affected by this increase. The results indicate that the photosynthetic activity of temperate semi-desert sand grassland species can help somewhat in slowing the general CO(2) rise in winter and function as a potential carbon sink of the investigated semi-desert Hungarian grassland species.

Biosynthesis, accumulation and emission of carotenoids, alpha-tocopherol, plastoquinone, and isoprene in leaves under high photosynthetic irradiance.

The localization of isoprenoid lipids in chloroplasts, the accumulation of particular isoprenoids under high irradiance conditions, and channelling of photosynthetically fixed carbon into plastidic thylakoid isoprenoids, volatile isoprenoids, and cytosolic sterols are reviewed. During leaf and chloroplast development in spring plastidic isoprenoid biosynthesis provides primarily thylakoid carotenoids, the phytyl side-chain of chlorophylls and the electron carriers phylloquinone K1, alpha-tocoquinone and alpha-tocopherol, as well as the nona-prenyl side-chain of plastoquinone-9. Under high irradiance, plants develop sun leaves and high light (HL) leaves with sun-type chloroplasts that possess, besides higher photosynthetic CO2 assimilation rates, different quantitative levels of pigments and prenylquinones as compared to shade leaves and low light (LL) leaves. After completion of chloroplast thylakoid synthesis plastidic isoprenoid biosynthesis continues at high irradiance conditions, constantly accumulating alpha-tocopherol (alpha-T) and the reduced form of plastoquinone-9 (PQ-9H2) deposited in the steadily enlarging osmiophilic plastoglobuli, the lipid reservoir of the chloroplast stroma. In sun leaves of beech (Fagus) and in 3-year-old sunlit Ficus leaves the level of alpha-T and PQ-9 can exceed that of chlorophyll b. Most plants respond to HL conditions (sun leaves, leaves suddenly lit by the sun) with a 1.4-2-fold increase of xanthophyll cycle carotenoids (violaxanthin, zeaxanthin, neoxanthin), an enhanced operation of the xanthophyll cycle and an increase of beta-carotene levels. This is documented by significantly lower values for the weight ratio chlorophylls to carotenoids (range: 3.6-4.6) as compared to shade and LL leaves (range: 4.8-7.0). Many plant leaves emit under HL and high temperature conditions at high rates the volatile compounds isoprene (broadleaf trees) or methylbutenol (American ponderosa pines), both of which are formed via the plastidic 1-deoxy-D: -xylulose-phosphate/2-C-methylerythritol 5-phosphate (DOXP/MEP) pathway. Other plants by contrast, accumulate particular mono- and diterpenes. Under adequate photosynthetic conditions the chloroplastidic DOXP/MEP isoprenoid pathway essentially contributes, with its C5 isoprenoid precusors, to cytosolic sterol biosynthesis. The possible cross-talk between the two cellular isoprenoid pathways, the acetate/MVA and the DOXP/MEP pathways, that preferentially proceeds in a plastid-to-cytosol direction, is shortly discussed.

Multispectral fluorescence and reflectance imaging at the leaf level and its possible applications.

Images taken at different spectral bands are increasingly used for characterizing plants and their health status. In contrast to conventional point measurements, imaging detects the distribution and quantity of signals and thus improves the interpretation of fluorescence and reflectance signatures. In multispectral fluorescence and reflectance set-ups, images are separately acquired for the fluorescence in the blue, green, red, and far red, as well as for the reflectance in the green and in the near infrared regions. In addition, 'reference' colour images are taken with an RGB (red, green, blue) camera. Examples of imaging for the detection of photosynthetic activity, UV screening caused by UV-absorbing substances, fruit quality, leaf tissue structure, and disease symptoms are introduced. Subsequently, the different instrumentations used for multispectral fluorescence and reflectance imaging of leaves and fruits are discussed. Various types of irradiation and excitation light sources, detectors, and components for image acquisition and image processing are outlined. The acquired images (or image sequences) can be analysed either directly for each spectral range (wherein they were captured) or after calculating ratios of the different spectral bands. This analysis can be carried out for different regions of interest selected manually or (semi)-automatically. Fluorescence and reflectance imaging in different spectral bands represents a promising tool for non-destructive plant monitoring and a 'road' to a broad range of identification tasks.

THE 1-DEOXY-D-XYLULOSE-5-PHOSPHATE PATHWAY OF ISOPRENOID BIOSYNTHESIS IN PLANTS.

In plants the biosynthesis of prenyllipids and isoprenoids proceeds via two independent pathways: (a) the cytosolic classical acetate/mevalonate pathway for the biosynthesis of sterols, sesquiterpenes, triterpenoids; and (b) the alternative, non-mevalonate 1-deoxy-d-xylulose-5-phosphate (DOXP) pathway for the biosynthesis of plastidic isoprenoids, such as carotenoids, phytol (a side-chain of chlorophylls), plastoquinone-9, isoprene, mono-, and diterpenes. Both pathways form the active C5-unit isopentenyl diphosphate (IPP) as the precursor from which all other isoprenoids are formed via head-to-tail addition. This review summarizes current knowledge of the novel 1-deoxy-d-xylulose-5-phosphate (DOXP) pathway for isopentenyl diphosphate biosynthesis, apparently located in plastids. The DOXP pathway of IPP formation starts from D-glyceraldehyde-3-phosphate (GA-3-P) and pyruvate, with DOXP-synthase as the starting enzyme. This pathway provides new insight into the regulation of chloroplast metabolism.