Carotenoid composition in sun and shade leaves of plants with different life forms

The carotenoid composition of sun leaves of nine species of annual crop plants (some with several varieties) was compared with sun and shade leaves of several other groups of plants, among those sun and shade leaves of several species of perennial shrubs and vines and deep-shade leaves of seven rainforest species. All sun leaves contained considerably greater amounts of the components of the xanthophyll cycle violaxanthin, antheraxanthin and zeaxanthin as well as of β-carotene than the shade leaves, as had previously been reported for a variety of other species by Thayer & Björkman (Photosynthesis Research, 1990, 23, 331–343). Therefore, high light specifically stimulated β,β-carotenoid synthesis. The sun leaves of these crop species did not contain α-carotene which was, however, present in large amounts in all shade leaves and in smaller amounts in sun leaves of three of the four species of perennial shrubs and vines. There was no difference in neoxanthin content on a chlorophyll basis between sun and shade leaves, and there was no consistent general difference in the lutein content between all sun and all shade leaves. The zeaxanthin (and antheraxanthin) content at peak irradiance and the xanthophyll cycle pool size were compared for sun leaves from the different groups of plants with different life forms and different metabolic activities. When growing in full sunlight the annual crop species and a perennial mesophyte had high rates of photosynthesis whereas the perennial shrubs and vines had relatively low photosynthesis rates. More zeaxanthin (and antheraxanthin) were accumulated at noon in full sunlight in those species with the lower photosynthesis rates. However, it was not such that those species also possessed the larger pools of violaxanthin + antheraxanthin + zeaxanthin. Instead, the xanthophyll cycle pools of sun leaves of the annual crop species and the perennial mesophyte were not smaller, and were even possibly larger, than those of sun leaves of the perennial shrubs and vines with low photosynthesis rates. This was so in spite of the fact that the crop species experienced much lesser degrees of excessive light at full sun than the shrubs and vines. Thus, many of the crop species converted only about 30–50% of their xanthophyll cycle pool to zeaxanthin at noon, whereas the shrubs and vines typically converted more than 80% of their pool into zeaxanthin. The crop species also had larger pools of β-carotene than the shrubs and vines but smaller pools of lutein than the majority of the latter species.     

Xanthophyll cycle components and capacity for non-radiative energy dissipation in sun and shade leaves ofLigustrum ovalifolium exposed to conditions limiting photosynthesis

The relationships between photosynthetic efficiency, non-radiative energy dissipation and carotenoid composition were studied in leaves ofLigustrum ovalifolium developed either under full sunlight or in the shade. Sun leaves contained a much greater pool of xanthophyll cycle components than shade leaves. The rate of non-radiative energy dissipation, measured as non-photochemical fluorescence quenching (NPQ), was strictly related to the deepoxidation state (DPS) of xanthophyll cycle components in both sun and shade leaves, indicating that zeaxanthin (Z) and antheraxanthin (A) are involved in the development of NPQ. Under extreme conditions of excessive energy, sun leaves showed higher maximum DPS than shade leaves. Therefore, sun leaves contained not only a greater pool of xanthophyll cycle components but also a higher proportion of violaxanthin (V) actually photoconvertible to A and Z, compared to shade leaves. Both these effects contributed to the higher NPQ in sun versus shade leaves. The amount of photoconvertible V was strongly related to chla/b ratio and inversely to leaf neoxanthin content. This evidence indicates that the amount of photoconvertible V may be dependent on the degree of thylakoid membrane appression and on the organization of chlorophyll-protein complexes, and possible explanations are discussed. Exposure to chilling temperatures caused a strong decline in the photon yield of photosynthesis and in the intrinsic efficiency of PS II photochemistry in sun leaves, but little effects in shade leaves. These effects were accompanied by increases in the pool of xanthophyll cycle components and in DPS, more pronounced in sun than in shade leaves. This corroborates the view that Z and A may play a photoprotective role under unfavorable conditions. In addition to the xanthophyll-related non-radiative energy dissipation, a slow relaxing component of NPQ, independent from A and Z concentrations, has been found in leaves exposed to low temperature and high light. This quenching component may be attributed either to other regulatory mechanism of PS II efficiency or to photoinactivation.Research supported by National Research Council of Italy, Special Project RAISA, Sub-Project 2, Paper N. 1587.     

植物叶黄素循环的组成、功能和调节(综述)

对近几年来植物体内叶黄素循环的组成、功能、堇菜黄素脱环氧化酶和玉米黄素环氧化酶的结构、生化性质和调节,以及叶黄素的可转变性、定位等方面的研究进展作了综述。     

叶黄素循环及其在光保护中的分子机理研究

植物的生命活动离不开充足的光照,但是当光照过强时,叶片吸收的光能超过了光合电子传递所需,过剩的光能便会对光合器官产生潜在的危害,引起光合作用的光抑制或光破坏.依赖于叶黄素循环的热耗散被认为是光保护的主要途径.本文着重介绍近年来有关植物叶黄素循环在酶学方面的分子调控、它的主要功能以及依赖于叶黄素循环的热耗散在光保护中的分子机理等,并对需进一步研究的问题作了探讨.     

叶黄素循环及其在光保护中的分子机理研究

植物的生命活动离不开充足的光照 ,但是当光照过强时 ,叶片吸收的光能超过了光合电子传递所需 ,过剩的光能便会对光合器官产生潜在的危害 ,引起光合作用的光抑制或光破坏。依赖于叶黄素循环的热耗散被认为是光保护的主要途径。本文着重介绍近年来有关植物叶黄素循环在酶学方面的分子调控、它的主要功能以及依赖于叶黄素循环的热耗散在光保护中的分子机理等 ,并对需进一步研究的问题作了探讨     

Morpho‐anatomical and physiological differences between sun and shade leaves in Abies alba Mill. (Pinaceae,Coniferales): a combined approach

Morphology, anatomy and physiology of sun and shade leaves of Abies alba were investigated and major differences were identified, such as sun leaves being larger, containing a hypodermis and palisade parenchyma as well as possessing more stomata, while shade leaves exhibit a distinct leaf dimorphism. The large size of sun leaves and their arrangement crowded on the upper side of a plagiotropic shoot leads to self‐shading which is explainable as protection from high solar radiation and to reduce the transpiration via the lamina. Sun leaves furthermore contain a higher xanthophyll cycle pigment amount and Non‐Photochemical Quenching (NPQ) capacity, a lower amount of chlorophyll b and a total lower chlorophyll amount per leaf, as well as an increased electron transport rate and an increased photosynthesis light saturation intensity. However, sun leaves switch on their NPQ capacity at rather low light intensities, as exemplified by several parameters newly measured for conifers. Our holistic approach extends previous findings about sun and shade leaves in conifers and demonstrates that both leaf types of A. alba show structural and physiological remarkable similarities to their respective counterparts in angiosperms, but also possess unique characteristics allowing them to cope efficiently with their environmental constraints.     

Involvement of gibberellins in the stem elongation of sun and shade ecotypes of Stellaria longipes that is induced by low light irradiance

Plants from two ecotypes of Stellaria longipes, alpine (an open, sunny habitat) and prairie (where adjacent plants provide a shaded habitat), were grown under normal and reduced levels of photosynthetically active radiation (PAR). Growth under low PAR is significantly promoted in both ecotypes. When quantified by the stable isotope dilution method, endogenous gibberellins (GAs) (GA1, GA8, GA20, GA19) were significantly elevated under low PAR in both 'sun' and 'shade' ecotypes, as was GA53 in the shade ecotype. Changes in endogenous GA1 levels were significantly correlated with stem growth during a 28 d growth cycle and with relative growth rate (RGR) for height under low PAR for both ecotypes. Interestingly, under low irradiance PAR, changes (both increases and decreases) in GA8, the 2beta-hydroxylated 'inactive' catabolite of GA1, closely parallel bidaily stem growth changes for both ecotypes. Because the significantly greater stem elongation of both ecotypes in response to low irradiance PAR is associated with significant increases in the endogenous levels of five GAs (GA53, GA19, GA1, GA8) in the early 13-hydroxylation GA biosynthesis pathway (measured at days 7,14 and 21), we conclude that the low irradiance PAR has very likely induced an overall increase in GA biosynthesis.     

The spatial distribution of chloroplast water in Acer platanoides sun and shade leaves

We evaluated a new, two-dimensional (2-D) nuclear magnetic resonance (NMR) imaging technique as a method for measuring the distribution of chloroplasts in leaves. NMR images that showed the distribution of chloroplast water and of total water as a function of depth into Acer platanoides sun and shade leaves were compared with the distribution of chlorophyll in the same leaf types (as measured by fluorescence microscopy), with the cellular structure (by scanning electron microscopy), and with published information. Results showed that the volume fraction of chloroplast water was much larger in shade than in sun leaves, and that it averaged about one-third larger in the palisade than in the spongy parenchyma region of both leaf types. Chlorophyll fluorescence was more intense in shade than in sun leaves. In sun leaves, fluorescence was maximal in the palisade region near the junction with the spongy parenchyma, while in shade leaves, fluorescence was maximal in the upper part of the spongy layer. We concluded that 2-D NMR imaging reliably indicates the location of chloroplast water.     

The xanthophyll cycle - molecular mechanism and physiological significance

The light-dependent, cyclic changes of xanthophyll pigments: violaxanthin, antheraxanthin and zeaxanthin, called the xanthophyll cycle, have been known for about fifty years. This process was characterised for higher plants, several fern and moss species and in some algal groups. Two enzymes, violaxanthin de-epoxidase (VDE) and zeaxanthin epoxidase (ZE), belonging to the lipocalin protein family, are engaged in the xanthophyll cycle. VDE requires for its activity ascorbic acid and reversed hexagonal structure formed by monogalactosyldiacylglycerol. ZE, postulated to be a flavoprotein, has not been purified yet and it is known from its gene sequence only. Zeaxanthin epoxidation is dependent on the reducing power of NADPH and presence of additional proteins. The xanthophyll cycle is postulated to play a role in many important physiological processes. Zeaxanthin, formed from violaxanthin under high light conditions, is thought to be a main photoprotector in autotrophic cells due to its ability to dissipate excess of absorbed light energy that can be measured as a non-photochemical quenching. In addition the zeaxanthin formation is important in protection of the thylakoid membranes against lipid peroxidation. Other postulated functions of the xanthophyll cycle, which include regulation of membrane physical properties, blue light reception and regulation of abscisic acid synthesis, are also discussed.     

Relationships between carotenoid composition and growth habit in British plant species

The pigment composition of leaves from a number of different plant species collected from field sites in the region of Sheffield, UK, have been compared using high-performance liquid chromatography. Expression of pigment content per unit leaf area was dominated by variation in the total leaf chlorophyll. Neither chlorophyll per unit area nor the chlorophyll a/b ratio were found to be correlated with the habitat from which the plants originated. When the amounts of different carotenoids were expressed relative to the total carotenoid pool, it was found that whilst neither total carotene (α- +β-carotene) nor neoxanthin correlated with ability to grow in shade, the leaf content of both lutein and the total xanthophyll cycle carotenoids (zeaxanthin, anther-axanthin and violaxanthin) did, with lutein content being high in shade species and xanthophyll cycle intermediates low. There was a strong negative correlation between the relative amounts of each of these groups of carotenoids. The ratio of lutein to xanthophyll cycle carotenoids was strongly correlated to an index of shade tolerance.     

The xanthophyll cycle activity in kidney bean and cabbage leaves under salinity stress

Seven-day-old kidney bean and cabbage seedlings were treated with 0.1–0.3 M NaCl solutions for 3 days. Chlorophyll content decreased in NaCl-treated Phaseolus seedlings, but did not significantly decrease in Brassica seedlings. Photochemical efficiency of photosystem II at dark-adapted state was similar in both Phaseolus and Brassica. The de-epoxidation state of violaxanthin increased more than sixfold in Phaseolus but showed no significant change in Brassica seedlings during NaCl treatment under low light. Maximum de-epoxidation state of violaxanthin in vivo tested in high light (2000 μmol quanta/(m² s) increased in salt-stressed Phaseolus but decreased in Brassica seedlings. The nonphotochemical quenching (NPQ) also increased in Phaseolus but decreased in Brassica. This suggests that xanthophyll cycle pigments influence the NPQ in both Phaseolus and Brassica, but in an opposite way. The increase in the de-epoxidation state of violaxanthin in salt-stressed Phaseolus even under low light may be considered an early light signal to protect the pigment-protein complexes from salt-stress induced photodamage. It is proposed that in salt-stressed Brassica, the de-epoxidation is retarded and/or the epoxidation is accelerated leading to the accumulation of violaxanthin and a lower de-epoxidation state. Thus, light-induced violoxanthin cycle operation largely controls the photoprotection of photosynthetic apparatus in kidney bean leaves. Published in Russian in Fiziologiya Rastenii, 2006, Vol. 53, No. 1, pp. 113–121. The text was submitted by the authors in English.     

High carbon dioxide and sun/shade effects on isoprene emission from oak and aspen tree leaves

Abstract. Isoprene (2-methyl 1, 3-butadiene) is emitted from many plants, especially trees. We tested the effect of growth at high CO₂ partial pressure and sun versus shade conditions on the capacity of Quercus rubra L. (red oak) and Populus tremuloides Michx. (quaking aspen) leaves to make isoprene. Oak leaves grown at high CO₂ partial pressure (65 Pa) had twice the rate of isoprene emission as leaves grown at 40Pa CO₂. However, aspen leaves behaved oppositely, with high CO₂-grown leaves having just 60-70% the rate of isoprene emission as leaves grown in 40 Pa CO₂. Similar responses were observed from 25 to 35 °C leaf temperature during assay. The stimulation of isoprene emission by growth at high CO₂ and the stimulation in high temperature resulted in isoprene emission consuming over 15% of the carbon fixed during photosynthesis in high-CO₂ grown oak leaves assayed at 35 °C. Leaves from the south (sunny) sides of trees growing in natural conditions had rates of isoprene emission double those of leaves growing in shaded locations on the same trees. This effect was similar in both aspen and oak. The leaves used for these experiments had significantly different chlorophyll a/b ratios indicating they were functionally sun (from the sunny locations) or shade leaves (from the protected locations). Because the metabolic pathway of isoprene synthesis is unknown, we are unable to speculate about how or why these effects occur. However, these effects are more consistent with metabolic control of isoprene release rather than a metabolic leak of isoprene from metabolism. The results are also important for large scale modelling of isoprene emission and for predicting the effect of future increases in atmospheric CO₂ level on isoprene emission from vegetation.     

Chlorophyll and light gradients in sun and shade leaves of Spinacia oleracea

Abstract. Light gradients were measured and correlated with chlorophyll concentration and anatomy of leaves in spinach (Spinacia oleracea L.). Light gradients were measured at 450, 550 and 680 nm within thin (455 μm) and thick (630 μm) leaves of spinach grown under sun and shade conditions. The light gradients were relatively steep in both types of leaves and 90% of the light at 450 and 680 nm was absorbed by the initial 140 μm of the palisade. In general, blue light was depleted faster than red light which, in turn was depleted faster than green light. Light penetrated further into the thicker palisade of sun leaves in comparison to the shade leaves. The distance that blue light at 450 nm travelled before it became 90% depleted was 120 μm in sun leaves versus 76 μm in shade leaves. Red light at 680 nm and green light at 550 nm travelled further but the trends were similar to that measured at 450nm. The steeper light gradients within the palisade-of shade leaves were caused by increased scattering of light within the intercellular air spaces and/or cells which were less compact than those in sun leaves. The decline in the amount of light within the leaf appeared to be balanced by a gradient in chlorophyll concentration measured in paradermal sections. Progressing from the adaxial epidermis, chlorophyll content increased through the palisade and then declined through the spongy mesophyll. Chlorophyll content was similar in the palisade of both sun and shade leaves. Chloroplast distribution within both sun and shade leaves was relatively uniform so that the chlorophyll gradient appeared to be caused by greater amounts of chlorophyll within chloroplasts located deeper within the leaf. These results indicate that the anatomy of the palisade may be of special importance for controlling the penetration of photo-synthetically active radiation into the leaf. Changing the structural characteristics of individual palisade cells or their arrangement may be an adaptation that maximizes the absorption of light in leaves with varying mesophyll thickness due to different ambient light regimes.     

Photoinhibition of Photosynthesis: Role of Carotenoids in Photoprotection of Chloroplast Constituents

Exposure of plants to irradiation, in excess to saturate photosynthesis, leads to reduction in photosynthetic capacity without any change in bulk pigment content. This effect is known as photoinhibition. Photoinhibition is followed by destruction of carotenoids (Cars), bleaching of chlorophylls (Chls), and increased lipid peroxidation due to formation of reactive oxygen species if the excess irradiance exposure continues. Photoinhibition of photosystem 2 (PS2) in vivo is often a photoprotective strategy rather than a damaging process. For sustainable maintenance of chloroplast function under high irradiance, the plants develop various photoprotective strategies. Cars perform essential photoprotective roles in chloroplasts by quenching the triplet Chl and scavenging singlet oxygen and other reactive oxygen species. Recently photoprotective role of xanthophylls (zeaxanthin) for dissipation of excess excitation energy under irradiance stress has been emphasised. The inter-conversion of violaxanthin (Vx) into zeaxanthin (Zx) in the light-harvesting complexes (LHC) serves to regulate photon harvesting and subsequent energy dissipation. De-epoxidation of Vx to Zx leads to changes in structure and properties of these xanthophylls which brings about significant structural changes in the LHC complex. This ultimately results in (1) direct quenching of Chl fluorescence by singlet-singlet energy transfer from Chl to Zx, (2) trans-thylakoid membrane mediated, pH-dependent indirect quenching of Chl fluorescence. Apart from these, other processes such as early light-inducible proteins, D1 turnover, and several enzymatic defence mechanisms, operate in the chloroplasts, either for tolerance or to neutralise the harmful effect of high irradiance.     

Responses of photosystem I compared with photosystem II to high-light stress in tropical shade and sun leaves

Sun and shade leaves of several plant species from a neotropical forest were exposed to excessive light to evaluate the responses of photosystem I in comparison to those of photosystem II. Potential photosystem I activity was determined by means of the maximum P700 absorbance change around 810 nm (ΔA_810max) in saturating far-red light. Leaf absorbance changes in dependence of increasing far-red light fluence rates were used to calculate a ‘saturation constant’, K_s, representing the far-red irradiance at which half of the maximal absorbance change (ΔA_810max/2) was reached in the steady state. Photosystem II efficiency was assessed by measuring the ratio of variable to maximum chlorophyll fluorescence, F_v/F_m, in dark-adapted leaf samples. Strong illumination caused a high degree of photo-inhibition of photosystem II in all leaves, particularly in shade leaves. Exposure to 1800–2000 μ mol photons m⁻² s⁻¹ for 75 min did not substantially affect the potential activity of photosystem I in all species tested, but caused a more than 40-fold increase of K_s in shade leaves, and a three-fold increase of K_s in sun leaves. The increase in K_s was reversible during recovery under low light, and the recovery process was much faster in sun than in shade leaves. The novel effect of high-light stress on the light saturation of P700 oxidation described here may represent a complex reversible mechanism within photosystem I that regulates light-energy dissipation and thus protects photosystem I from photo-oxidative damage. Moreover, we show that under high-light stress a high proportion of P700 accumulates in the oxidized state, P700⁺. Presumably, conversion of excitation energy to heat by this cation radical may efficiently contribute to photoprotection.     

Instantaneous canopy photosynthesis: analytical expressions for sun and shade leaves based on exponential light decay down the canopy and an acclimated non-rectangular hyperbola for leaf photosynthesis

Analytical expressions for the contributions of sun and shade leaves to instantaneous canopy photosynthesis are derived. The analysis is based on four assumptions. First, that the canopy is closed in the sense that it is horizontally uniform. Secondly, that there is an exponential profile of light down the canopy with the same decay constant for light from different parts of the sky. Thirdly, that the leaf photosynthetic response to incident irradiance can be described by a three-parameter non-rectangular hyperbola (NRH). And lastly, that light acclimation at the leaf level occurs in only one parameter of the NRH, that describing the light-saturated photosynthetic rate, which is assumed to be proportional to the local averaged leaf irradiance. These assumptions have been extensively researched empirically and theoretically and their limitations are quite well understood. They have been widely used when appropriate. Combining these four assumptions permits the derivation of algebraic expressions for instantaneous canopy photosynthesis which are computationally efficient because they avoid the necessity for numerical integration down the canopy. These are valuable for modelling plant and crop ecosystems, for which canopy photosynthesis is the primary driver. Ignoring the sun/shade dichotomy can result in overestimates of canopy photosynthesis of up to 20 %, but using a rectangular hyperbola instead of a non-rectangular hyperbola to estimate canopy photosynthesis taking account of sun and shade leaves can lead to a similarly sized underestimate.     

Regulation of violaxanthin de-epoxidase activity by pH and ascorbate concentration

The activity of violaxanthin de-epoxidase has been studied both in isolated thylakoids and after partial purification, as a function of pH and ascorbate concentration. We demonstrate that violaxanthin de-epoxidase has a K_m for ascorbate that is strongly dependent on pH, with values of 10, 2.5, 1.0 and 0.3 mM at pH 6.0, 5.5, 5.0 and 4.5, respectively. These values can be expressed as a single K_m±0.1±0.02 mM for the acid form of ascorbate. Release of the protein from the thylakoids by sonication was also found to be strongly pH dependent with a cooperativity of 4 with respect to protons and with an inflexion point at pH 6.7. These results can explain some of the discrepancies reported in the literature and provide a more consistent view of zeaxanthin formation in vivo.     

Characteristics of Photosynthetic Apparatus in Mn-Starved Maize Leaves

The effects of Mn-deficiency on CO₂ assimilation and excitation energy distribution were studied using Mn-starved maize leaves. Mn-deficiency caused about 70 % loss in the photon-saturated net photosynthetic rate (P _N) compared to control leaves. The loss of P _N was associated with a strong decrease in the activity of oxygen evolution complex (OEC) and the linear electron transport driven by photosystem 2 (PS2) in Mn-deficienct leaves. The photochemical quenching of PS2 (q_P) and the maximum efficiency of PS2 photochemistry (F_v/F_m) decreased significantly in Mn-starved leaves under high irradiance, implicating that serious photoinhibition took place. However, the high-energy fluorescence quenching (q_E) decreased, which was associated with xanthophyll cycle. The results showed that the pool of de-epoxidation components of the xanthophyll cycle was lowered markedly owing to Mn deficiency. Linear electron transport driven by PS2 de-creased significantly and was approximately 70 % lower in Mn-deficient leaves than that in control, indicating less trans-thylakoid pH gradient was built in Mn deficient leaves. We suggest that the decrease of non-radiative dissipation depending on xanthophyll cycle in Mn-starved leaves is a result of the deficiency of trans-thylakoid pH gradient.