Chloroembryos: A unique photosynthesis system

The embryos of some angiosperm taxa contain chlorophyll and this chlorophyllous stage is persisting until the embryo matures (further referred as chloroembryos). Besides being chlorophyllous, these embryos seem to have the ability to photosynthesize. This suggests that the chlorophyllous state of the embryo has an important role in seed development. The photosynthesis of chloroembryos is highly shade adaptive in nature as it is embedded within the supporting tissues (several layers of pod wall, seed coat and endosperm). Moreover, these chloroembryos are developing in a highly osmotic environment, and contain various components of the photosynthetic machinery. Detailed studies were performed in these chloroembryos in order to elucidate the structure of the chloroplasts, pigment composition, the photochemical activities, the rate of carbon assimilation and also the shade adaptive features. It has been shown that the respired CO₂ within these chloroembryos is recycled by the efficient photosynthetic components of the chloroembryos and thus potentially influences the seed's carbon economy. Thus, the major role of embryonic photosynthesis is to produce both energy-rich molecules and oxygen, of which the former can be directly used for biosynthesis. During embryogenesis oxygen production is especially important, in a situation wherein the oxygen is limited within the enclosed seed. As these chloroembryos grow in an environment of a sugar rich endosperm, it requires some adaptive mechanisms in this high osmotic environment. The additional polypeptides found in the thylakoids of chloroembryo chloroplasts in comparison to the thylakoids of leaf chloroplast have been suggested to have a role in protecting the photosynthetic components in the chloroembryos in an environment of high osmotic strength. An attempt to understand osmotic stress tolerance existing in these chloroembryos may lead to a better understanding of tolerance of photosynthesis to osmotic stress.     

Chlorophylls <Emphasis Type="Italic">d</Emphasis> and <Emphasis Type="Italic">f</Emphasis> and their role in primary photosynthetic processes of cyanobacteria

The finding of unique Chl d- and Chl f-containing cyanobacteria in the last decade was a discovery in the area of biology of oxygenic photosynthetic organisms. Chl b, Chl c, and Chl f are considered to be accessory pigments found in antennae systems of photosynthetic organisms. They absorb energy and transfer it to the photosynthetic reaction center (RC), but do not participate in electron transport by the photosynthetic electron transport chain. However, Chl d as well as Chl a can operate not only in the light-harvesting complex, but also in the photosynthetic RC. The long-wavelength (Q_y) Chl d and Chl f absorption band is shifted to longer wavelength (to 750 nm) compared to Chl a, which suggests the possibility for oxygenic photosynthesis in this spectral range. Such expansion of the photosynthetically active light range is important for the survival of cyanobacteria when the intensity of light not exceeding 700 nm is attenuated due to absorption by Chl a and other pigments. At the same time, energy storage efficiency in photosystem 2 for cyanobacteria containing Chl d and Chl f is not lower than that of cyanobacteria containing Chl a. Despite great interest in these unique chlorophylls, many questions related to functioning of such pigments in primary photosynthetic processes are still not elucidated. This review describes the latest advances in the field of Chl d and Chl f research and their role in primary photosynthetic processes of cyanobacteria.     

Limitations for finding a linear relationship between chlorophyll content and photosynthetic activity

In experiments with fodder cabbage and sugar beet the influences were tested which restrict the finding of a uniform linear dependence between the chlorophyll content and photosynthetic rate. The main factor is most probably the irradiation during the growth of plants and the measuring of photosynthetic rate. Changes in the density and length of irradiation with the proceeding spring season are reflected in the decrease of unit chlorophyll activity in all leaves of the plant. Such differences were not observed in plants grown in winter months at a low greenhouse irradiation density. High irradiation density during the measurement of photosynthesis ensures the finding of high correlation of photosynthetic rate with the chlorophyll amount per unit area of leaf. With a low irradiation density, or in thick leaves, the lower layers of chloroplasts are not saturated with radiation, and hence a higher correlation coefficient is found by relating photosynthetic rate to the percentage of chlorophyll in dry weight than to the quantity of chlorophyll per unit area of leaf blade. In some experiments higher activity of chlorophylls was found in young leaves before they have reached the state of “photosynthetic maturity” than after this turning point. This difference diminished by relating the photosynthetic rate to chlorophylla content instead of the total amount of chlorophylls (a+b). High activity of chlorophylls in very young leaves and great individual variability within the experimental plant groups caused by uneven cultivation conditions are the basis of apparent sigmoid character of average curves for the chlorophyll-photosynthesis relation.     

Activity of the System for Chlorophyll Biosynthesis and Structural and Functional Organization of Chloroplasts in a Plastome <Emphasis Type="Italic">en:chlorina-5</Emphasis> Sunflower Mutant

Chlorophyll (Chl) deficiency in leaves of a plastome sunflower (Helianthus annuus L.) en:chlorina-5 mutant is due to the formation of smaller chloroplasts with a markedly reduced membrane system, as compared to the parent 3629 line. Abnormalities in the structure of the photosynthetic apparatus in the mutant can be mainly attributed to changes in the formation of photosystem I and its light-harvesting complexes. Chl deficiency in en:chlorina-5 correlated with its lower capability of synthesizing the first specific Chl precursor, 5-aminolevulinic acid (ALA) in the light. Light-independent stages of Chl biosynthesis in the mutant had the same efficiency as in leaves of the parent line. ALA formation in darkness and its conversion into protochlorophyllide did not depend on the extent of photosynthetic membrane development and photosynthetic activity.     

Fruit Spray of 24-Epibrassinolide and Fruit Shade Alter Pericarp Photosynthesis Activity and Seed Lipid Accumulation in <Emphasis Type="Italic">Styrax tonkinensis</Emphasis>

As demand for biofuel feedstock production increases, the substitutable woody oilseed benzoin (Styrax tonkinensis (Pierre) Craib ex Hartwich) has received increased attention among Chinese researchers. However, the mechanism of seed oil regulation has not been revealed in this species. Maternal control of seed lipid content has been observed in many plant species. In this study, in planta treatments were applied to create different pericarp photosynthesis activation levels in benzoin. Compared with fruit shading treatment, superior photosynthetic physiological characteristics were found in the brassinosteroid hormone 24-epibrassinolide-treated pericarps during maturation (56–112 days after flowering); specifically, these superior characteristics were reflected by higher average and peak values of the Chl a/b ratio, NADPH/NADP⁺ ratio, NADP-dependent malate dehydrogenase activity, and ribulose 1,5-bisphosphate carboxylase/oxygenase activity. Next, we found that the pericarp-encapsulated seed yield and oil content were closely connected to the maternal-specific photosynthetic events. The enhanced pericarp photosynthesis activity (treated with 10 μM brassinosteroid) was accompanied by higher fatty acid synthase activity and a faster lipid deposition rate, which was verified by transmission electron microscopy. These changes led to a final increase in the total fatty acid (FA) content of mature seeds of 19%, which could be largely accounted for by the elevated saturated FA (mostly long-chain FAs) percentage in benzoin oil. When benzoin fruits grew in low light (bagged in three layers of black non-woven fabrics), lipid synthesis capability was markedly impaired. The dynamics of cellular storage reserves were further analyzed, suggesting that the pericarp-seed carbohydrate translocation efficiency in fruits and the carbon partitioning among seed reserves were directly responsible for the seed oil diversification under the treatments. Taken together, our results highlighted the maternal control, via pericarp photosynthesis, over the regulation of the seed oil content in benzoin.     

Living off the Sun: chlorophylls,bacteriochlorophylls and rhodopsins

Pigments absorbing 350–1,050 nm radiation have had an important role on the Earth for at least 3.5 billion years. The ion pumping rhodopsins absorb blue and green photons using retinal and pump ions across cell membranes. Bacteriochlorophylls (BChl), absorbing in the violet/blue and near infra red (NIR), power anoxygenic photosynthesis, with one photoreaction centre; and chlorophylls (Chl), absorbing in the violet/blue and red (occasionally NIR) power oxygenic photosynthesis, with two photoreaction centres. The accessory (bacterio)chlorophylls add to the spectral range (bandwidth) of photon absorption, e.g., in algae living at depth in clear oceanic water and in algae and photosynthetic (PS) bacteria in microbial mats. Organism size, via the package effect, determines the photon absorption benefit of the costs of synthesis of the pigment–protein complexes. There are unresolved issues as to the evolution of Chls vs. BChls and the role of violet/blue and NIR radiation in PS bacteria.     

Important photosynthetic contribution of silique wall to seed yield-related traits in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

In plants, green non-foliar organs are able to perform photosynthesis just as leaves do, and the seed-enclosing pod acts as an essential photosynthetic organ in legume and Brassica species. To date, the contribution of pod photosynthesis to seed yield and related components still remains largely unexplored, and in Arabidopsis thaliana, the photosynthetic activity of the silique (pod) is unknown. In this study, an Arabidopsis glk1/glk2 mutant defective in both leaf and silique photosynthesis was used to create tissue-specific functional complementation lines. These lines were used to analyze the contribution of silique wall photosynthesis to seed yield and related traits, and to permit the comparison of this contribution with that of leaf photosynthesis. Our results showed that, together with leaves, the photosynthetic assimilation of the silique wall greatly contributed to total seed yield per plant. As for individual components of yield traits, leaf photosynthesis alone contributed to the seed number per silique and silique length, while silique wall photosynthesis alone contributed to thousand-seed weight. In addition, enhancing the photosynthetic capacity of the silique wall by overexpressing the photosynthesis-related RCA gene in this tissue resulted in significantly increased seed weight and oil content in the wild-type (WT) background. These results reveal that silique wall photosynthesis plays an important role in seed-related traits, and that enhancing silique photosynthesis in WT plants can further improve seed yield-related traits and oil production. This finding may have significant implications for improving the seed yield and oil production of oilseed crops and other species with pod-like organs.     

Gradients of lipid storage, photosynthesis and plastid differentiation in developing soybean seeds

This study establishes a topographical framework for functional investigations on the regulation of lipid biosynthesis and its interaction with embryo photosynthesis in developing soybean seed. Structural observations, combined with molecular and functional parameters, revealed the gradual transformation of chloroplasts into storage organelles, starting from inner regions going outwards. This is evidenced by electron microscopy, confocal laser scanning microscopy, in situ hybridization and histochemical/biochemical data. As a consequence of plastid differentiation, photosynthesis becomes distributed along a gradient within the developing embryo. Electron transport rate, effective quantum yield and O2 production rate are maximal in the embryo periphery, as documented by imaging pulse-amplitude-modulated fluorescence and O2 release via microsensors. The gradual loss of photosynthetic capacity was accompanied by a similarly gradual accumulation of starch and lipids. Noninvasive nuclear magnetic resonance spectroscopy of mature seeds revealed steep gradients in lipid deposition, with the highest concentrations in inner regions. The inverse relationship between photosynthesis and lipid biosynthesis argues against a direct metabolic involvement of photosynthesis in lipid biosynthesis during the late storage stage, but points to a role for photosynthetic oxygen release. This hypothesis is verified in a companion paper.     

Gradients of seed photosynthesis and its role for oxygen balancing

Seeds are generally viewed in the context of plant reproduction and the supply of food and feed, but only seldom as a site of photosynthesis. However, the seeds of many plant species are green, at least during their early development, which raises the issue of the significance of this greening for seed development. Here we describe the two contrasting modes of photosynthesis in the developing seed. The dicotyledonous pea seed has a green embryo, while the monocotyledonous barley caryopsis has a chlorenchymatic layer surrounding its non-green endosperm (storage organ). We have employed pulse-amplitude-modulated fluorescence and oxygen-sensitive microsensors to localize and describe gradient distributions of photosynthetic activity across the seed/caryopsis, and have discussed its role in maintaining the endogenous O₂ balance. We also report the lack of photosynthetic activity in the stay-green embryo axis of the sacred lotus (Nelumbo nucifera) seed following imbibition. The observations are discussed with respect to in vivo light supply and contrasted with the characteristics of leaf photosynthesis.     

Adaptive temperature regulation in the little bird in winter: predictions from a stochastic dynamic programming model

Stomatal CO₂ responsiveness and photosynthetic capacity vary greatly among plant species, but the factors controlling these physiological leaf traits are often poorly understood. To explore if these traits are linked to taxonomic group identity and/or to other plant functional traits, we investigated the short-term stomatal CO₂ responses and the maximum rates of photosynthetic carboxylation (V _cmax) and electron transport (J _max) in an evolutionary broad range of tropical woody plant species. The study included 21 species representing four major seed plant taxa: gymnosperms, monocots, rosids and asterids. We found that stomatal closure responses to increased CO₂ were stronger in angiosperms than in gymnosperms, and in monocots compared to dicots. Stomatal CO₂ responsiveness was not significantly related to any of the other functional traits investigated, while a parameter describing the relationship between photosynthesis and stomatal conductance in combined leaf gas exchange models (g ₁) was related to leaf area-specific plant hydraulic conductance. For photosynthesis, we found that the interspecific variation in V _cmax and J _max was related to within leaf nitrogen (N) allocation rather than to area-based total leaf N content. Within-leaf N allocation and water use were strongly co-ordinated (r ² = 0.67), such that species with high fractional N investments into compounds maximizing photosynthetic capacity also had high stomatal conductance. We conclude that while stomatal CO₂ responsiveness of tropical woody species seems poorly related to other plant functional traits, photosynthetic capacity is linked to fractional within-leaf N allocation rather than total leaf N content and is closely co-ordinated with leaf water use.     

Use of the atLEAF+ chlorophyll meter for a nondestructive estimate of chlorophyll content

At present, chlorophyll meters are widely used for a quick and nondestructive estimate of chlorophyll (Chl) contents in plant leaves. Chl meters allow to estimate the Chl content in relative units - the Chl index (CI). However, using such meters, one can face a problem of converting CI into absolute values of the pigment content and comparing data acquired with different devices and for different plant species. Many Chl meters (SPAD-502, CL-01, CCM-200) demonstrated a high degree of correlation between the CI and the absolute pigment content. A number of formulas have been deduced for different plant species to convert the CI into the absolute value of the photosynthetic pigment content. However, such data have not been yet acquired for the atLEAF+ Chl meter. The purpose of the present study was to assess the applicability of the atLEAF+ Chl meter for estimating the Chl content. A significant species-specific exponential relationships between the atLEAF value (corresponding to CI) and extractable Chl a, Chl b, Chl (a+b) for Calamus dioicus and Cleistanthus sp. were shown. The correlations between the atLEAF values and the content of Chl a, Chl b, and Chl (a+b) per unit of leaf area was stronger than that per unit of dry leaf mass. The atLEAF value- Chl b correlation was weaker than that of atLEAF value-Chl a and atLEAF value-Chl (a+b) correlations. The influence of light conditions (Chl a/b ratio) on the atLEAF value has been also shown. The obtained results indicated that the atLEAF+ Chl meter is a cheap and convenient tool for a quick nondestructive estimate of the Chl content, if properly calibrated, and can be used for this purpose along with other Chl meters.     

Solute patterns and diurnal variation of photosynthesis and chlorophyll fluorescence in Korean coastal sand dune plants

Four plant species, Elymus mollis Trin., Carex kobomugi Ohwi, Glehnia littoralis F. Schmidt ex Miq., and Vitex rotundifolia L.f., are dominant perennial species in coastal sand dunes of Korea. We examined a physiological adaptation of these species by measurements of diurnal variation in photosynthesis and chlorophyll (Chl) fluorescence and solute patterns in leaves during one season (June), which is favorable for plant growth of all four species. All four species adopted different strategies in order to utilize radiation and to maintain water status under a fluctuating microclimate. Although the lowest water contents among four plant species was found, E. mollis with a high Chl and K⁺ content showed better photosynthetic performance, with high stomatal conductance (g _s), net photosynthetic rate (P _N), instantaneous carboxylation efficiency (CE), and water-use efficiency. Midday depression of P _N in E. mollis and G. littoralis, without a reduction of gs, was associated with a reduction in CE and maximum photochemical efficiency of PSII, indicating nonstomatal limitation. Photosynthesis depression in both C. kobomugi and V. rotundifolia, with relatively low g _s values, could be attributed to both stomatal and nonstomatal limitations. The high storage capacity for inorganic ions in E. molli, C. kobomugi, and G. littoralis may play an efficient role in regulating photosynthesis and maintaining leaf water status through stomatal control, and can also play an important role in osmotic adjustment.     

The effects of lead on photosynthetic performance of waxberry seedlings (<Emphasis Type="Italic">Myrica rubra</Emphasis>)

The photosynthesis was investigated 30 d after Pb treatment in Myrica rubra seedlings. The Pb treatment resulted in significantly increased Pb concentrations in shoots. Low Pb concentration exposure (≤2 mM) reduced the net photosynthetic rate (P_N), transpiration rate (E), and stomatal conductance (gs) without affecting the intercellular CO₂ concentration (C_i), chlorophyll (Chl) content, and Chl fluorescence parameters. At 10 d after severe Pb treatment (≥4 mM), P_N was inhibited and accompanied by Chl damage, while at 30 d, the inhibition of P_N was followed by an increase of C_i and a decrease of gs, E, Chl content, and Chl fluorescence parameters. M. rubra showed a promising prospect for use in the soil phytoremediation, when Pb concentration is low, but the remediation efficiency of M. rubra is limited if Pb exceeds 2 mM.     

Precocious floral initiation and identification of exact timing of embryo physiological maturity facilitate germination of immature seeds to truncate the lifecycle of pea

We propose herein a novel single seed descent protocol that has application across a broad phenotypic range of pea genotypes. Manipulation of key in vivo growing conditions, including light, photoperiod and temperature, combined with precocious in vitro germination of the embryo at full physiological maturity substantially shortened the pea lifecycle. We define full embryo physiological maturity as the earliest point in seed development when precocious in vitro germination and robust seedling growth can be reliably achieved without supply of exogenous hormones. Under our optimised conditions for accelerated plant growth, embryo physiological maturity was attained at c. 18 days after pollination, when seed moisture content was below 60?% and sucrose level under 100 mg g^?1 DW. No delay penalty in terms of time to flowering and plant development was caused by the culture of immature seeds 18 days after pollination compared to the used of mature ones. Determining the role embryo maturity plays in the fitness of the germinated plant has facilitated the truncation of the lifecycle across pea genotypes. The accelerated single seed descent system proposed within this research will benefit complex genetic studies via the rapid development of recombinant inbred lines (RIL) and multi-parental advanced generation intercrosses (MAGIC) populations.     

Why is chlorophyll <Emphasis Type="Italic">b</Emphasis> only used in light-harvesting systems?

Chlorophylls (Chl) are important pigments in plants that are used to absorb photons and release electrons. There are several types of Chls but terrestrial plants only possess two of these: Chls a and b. The two pigments form light-harvesting Chl a/b-binding protein complexes (LHC), which absorb most of the light. The peak wavelengths of the absorption spectra of Chls a and b differ by c. 20 nm, and the ratio between them (the a/b ratio) is an important determinant of the light absorption efficiency of photosynthesis (i.e., the antenna size). Here, we investigated why Chl b is used in LHCs rather than other light-absorbing pigments that can be used for photosynthesis by considering the solar radiation spectrum under field conditions. We found that direct and diffuse solar radiation (PAR_dir and PAR_diff, respectively) have different spectral distributions, showing maximum spectral photon flux densities (SPFD) at c. 680 and 460 nm, respectively, during the daytime. The spectral absorbance spectra of Chls a and b functioned complementary to each other, and the absorbance peaks of Chl b were nested within those of Chl a. The absorption peak in the short wavelength region of Chl b in the proteinaceous environment occurred at c. 460 nm, making it suitable for absorbing the PAR_diff, but not suitable for avoiding the high spectral irradiance (SIR) waveband of PAR_dir. In contrast, Chl a effectively avoided the high SPFD and/or high SIR waveband. The absorption spectra of photosynthetic complexes were negatively correlated with SPFD spectra, but LHCs with low a/b ratios were more positively correlated with SIR spectra. These findings indicate that the spectra of the photosynthetic pigments and constructed photosystems and antenna proteins significantly align with the terrestrial solar spectra to allow the safe and efficient use of solar radiation.     

Role of chlorophylls and carotenoids in seed tolerance to abiotic stressors

It is found that chlorophylls are not fully destructed during seed ripening and can be detected in appreciable quantities in physiologically mature seeds. The elevated content of residual chlorophylls reduces seed tolerance to abiotic stresses. The seed carotenoids were represented mainly by lutein and, in much smaller quantities, by β-carotene. Carotenoids were found to accumulate in seeds during accelerated aging and during seed germination at high temperatures. The ratio of carotenoid to chlorophyll content (Car/Chl) is proposed to be a measure of seed tolerance to stress factors. The seeds with elevated Car/Chl ratio were characterized by higher tolerance to stress treatments. It is supposed that the presence of chlorophylls in seeds enhances oxidative stress induced by abiotic stress factors. Carotenoids are considered as antioxidants protecting the seeds against oxidative stress.     

Photosynthetic activity of chloroembryos of a few selected tropical plants

Abstract The role of chlorophyll in the mature embryos of several tropical plants (Phthirusa pyrifolia [H.B.K.] Eichl. [Loranthaceae]. Murraya koenigia Kurz. [Rutaceae], Murraya paniculata Jack. [Rutaceae], Syzygium cuminii [L.] Skeels [Myrtaceae]) was investigated. Extracted chloroembryos of all species do photosynthesize when illuminated. Whole mature fruits of M. koeningii, M. paniculata and Syzygium cuminii exhibited some photosynthetic activity, but pericarps of none of the fruits photosynthesized when exposed to light. Thus the photosynthetic activity of fruits may be ascribed to CO₂ uptake by chloroembryos embedded in the fruits. A specific aspect of plant physiology, namely the re-utilization of respired CO₂ in the process of photosynthesis is emphasized. It is postulated that within embedded chloroembryos, conditions such as high CO₂ concentration, high light intensity, and low oxygen concentration are favourable for conducting intensive photosynthesis. Photosynthesis within enclosed organs has an additional advantage in that is does not expose the plant to any risk of water loss usually associated with photosynthesis.