Involvement of Abscisic Acid in Transition of Pea (Pisum sativum L.) Seeds from Germination to Post-Germination Stages.

The transition from seed to seedling represents a critical developmental step in the life cycle of higher plants, dramatically affecting plant ontogenesis and stress tolerance. The release from dormancy to acquiring germination ability is defined by a balance of phytohormones, with the substantial contribution of abscisic acid (ABA), which inhibits germination. We studied the embryonic axis of Pisum sativum L. before and after radicle protrusion. Our previous work compared RNA sequencing-based transcriptomics in the embryonic axis isolated before and after radicle protrusion. The current study aims to analyze ABA-dependent gene regulation during the transition of the embryonic axis from the germination to post-germination stages. First, we determined the levels of abscisates (ABA, phaseic acid, dihydrophaseic acid, and neo-phaseic acid) using ultra-high-performance liquid chromatography-tandem mass spectrometry. Second, we made a detailed annotation of ABA-associated genes using RNA sequencing-based transcriptome profiling. Finally, we analyzed the DNA methylation patterns in the promoters of the PsABI3, PsABI4, and PsABI5 genes. We showed that changes in the abscisate profile are characterized by the accumulation of ABA catabolites, and the ABA-related gene profile is accompanied by the upregulation of genes controlling seedling development and the downregulation of genes controlling water deprivation. The expression of ABI3, ABI4, and ABI5, which encode crucial transcription factors during late maturation, was downregulated by more than 20-fold, and their promoters exhibited high levels of methylation already at the late germination stage. Thus, although ABA remains important, other regulators seems to be involved in the transition from seed to seedling.

Red light-induced inhibition of maize (Zea mays) mesocotyl elongation: evaluation of apoplastic metabolites.

Light is a crucial factor affecting plant growth and development. Besides providing the energy for photosynthesis, light serves as a sensory cue to control the adaptation of plants to environmental changes. We used the etiolated maize (Zea mays ) seedlings as a model system to study the red light-regulated growth. Exposure of the maize seedlings to red light resulted in growth inhibition of mesocotyls. We demonstrate for the first time (to the best our knowledge) that red light affected the patterns of apoplastic fluid (AF) metabolites extracted from the mesocotyl segments. By means of the untargeted gas chromatography-mass spectrometry (GC-MS)-based metabolomics approach, we identified 44 metabolites in the AF of maize mesocotyls and characterised the dynamics of their relative tissue abundances. The characteristic metabolite patterns of mesocotyls dominated with mono- and disaccharides, organic acids, amino acids, and other nitrogen-containing compounds. Upon red light irradiation, the contents of ß -alanine, putrescine and trans -aconitate significantly increased (P -value<0.05). In contrast, there was a significant decrease in the total ascorbate content in the AF of maize mesocotyls. The regulatory role of apoplastic metabolites in the red light-induced inhibition of maize mesocotyl elongation is discussed.

Seed-to-Seedling Transition: Novel Aspects.

Transition from seed to seedling represents a critical stage in plants' life cycles. This process includes three significant events in the seeds: (i) tissue hydration, (ii) the mobilization of reserve nutrients, and (iii) the activation of metabolic activity. Global metabolic rearrangements lead to the initiation of radicle growth and the resumption of vegetative development. It requires massive reprogramming of the transcriptome, proteome, metabolome, and attendant signaling pathways, resulting in the silencing of seed-maturation genes and the activation of vegetative growth genes. This Special Issue discusses the mechanisms of genetic, epigenetic, and hormonal switches during seed-to-seedling transitions. Detailed information has also been covered regarding the influence of germination features on seedling establishment.

Seed-to-Seedling Transition in Pisum sativum L.: A Transcriptomic Approach.

The seed-to-seedling transition is a crucial step in the plant life cycle. The transition occurs at the end of seed germination and corresponds to the initiation of embryonic root growth. To improve our understanding of how a seed transforms into a seedling, we germinated the Pisum sativum L. seeds for 72 h and divided them into samples before and after radicle protrusion. Before radicle protrusion, seeds survived after drying and formed normally developed seedlings upon rehydration. Radicle protrusion increased the moisture content level in seed axes, and the accumulation of ROS first generated in the embryonic root and plumule. The water and oxidative status shift correlated with the desiccation tolerance loss. Then, we compared RNA sequencing-based transcriptomics in the embryonic axes isolated from pea seeds before and after radicle protrusion. We identified 24,184 differentially expressed genes during the transition to the post-germination stage. Among them, 2101 genes showed more prominent expression. They were related to primary and secondary metabolism, photosynthesis, biosynthesis of cell wall components, redox status, and responses to biotic stress. On the other hand, 415 genes showed significantly decreased expression, including the groups related to water deprivation (eight genes) and response to the ABA stimulus (fifteen genes). We assume that the water deprivation group, especially three genes also belonging to ABA stimulus (LTI65, LTP4, and HVA22E), may be crucial for the desiccation tolerance loss during a metabolic switch from seed to seedling. The latter is also accompanied by the suppression of ABA-related transcription factors ABI3, ABI4, and ABI5. Among them, HVA22E, ABI4, and ABI5 were highly conservative in functional domains and showed homologous sequences in different drought-tolerant species. These findings elaborate on the critical biochemical pathways and genes regulating seed-to-seedling transition.

Transition from Seeds to Seedlings: Hormonal and Epigenetic Aspects.

Transition from seed to seedling is one of the critical developmental steps, dramatically affecting plant growth and viability. Before plants enter the vegetative phase of their ontogenesis, massive rearrangements of signaling pathways and switching of gene expression programs are required. This results in suppression of the genes controlling seed maturation and activation of those involved in regulation of vegetative growth. At the level of hormonal regulation, these events are controlled by the balance of abscisic acid and gibberellins, although ethylene, auxins, brassinosteroids, cytokinins, and jasmonates are also involved. The key players include the members of the LAFL network-the transcription factors LEAFY COTYLEDON1 and 2 (LEC 1 and 2), ABSCISIC ACID INSENSITIVE3 (ABI3), and FUSCA3 (FUS3), as well as DELAY OF GERMINATION1 (DOG1). They are the negative regulators of seed germination and need to be suppressed before seedling development can be initiated. This repressive signal is mediated by chromatin remodeling complexes-POLYCOMB REPRESSIVE COMPLEX 1 and 2 (PRC1 and PRC2), as well as PICKLE (PKL) and PICKLE-RELATED2 (PKR2) proteins. Finally, epigenetic methylation of cytosine residues in DNA, histone post-translational modifications, and post-transcriptional downregulation of seed maturation genes with miRNA are discussed. Here, we summarize recent updates in the study of hormonal and epigenetic switches involved in regulation of the transition from seed germination to the post-germination stage.

Microgravity modelling by two-axial clinorotation leads to scattered organisation of cytoskeleton in Arabidopsis seedlings.

Proper plant development in a closed ecosystem under weightlessness will be crucial for the success of future space missions. To supplement spaceflight experiments, such conditions of microgravity are modelled on Earth using a two-axial (2A) clinorotation, and in several fundamental studies resulted in the data on proteome and metabolome adjustments, embryo development, cell cycle regulation, etc. Nevertheless, our understanding of the cytoskeleton responses to the microgravity is still limited. In the present work, we study the adjustment of actin microfilaments (MFs) and microtubules (MTs) in Arabidopsis thaliana (L.) Heynh. seedlings under 2A clinorotation. Modelled microgravity resulted in not only the alteration of seedlings phenotype, but also a transient increase of the hydrogen peroxide level and in the cytoskeleton adjustment. Using GFP-fABD2 and Lifeact-Venus transgenic lines, we demonstrate that MFs became 'scattered' in elongating root and hypocotyl cells under 2A clinorotation. In addition, in GFP-MAP4 and GFP-TUA6 lines the tubulin cytoskeleton had higher fractions of transverse MTs under 2A clinorotation. Remarkably, the first static gravistimulation of continuously clinorotated seedlings reverted MF organisation to a longitudinal one in roots within 30 min. Our data suggest that the 'scattered' organisation of MFs in microgravity can serve as a good basis for the rapid cytoskeleton conversion to a 'longitudinal' structure under the gravity force.

Desiccation Tolerance as the Basis of Long-Term Seed Viability.

Desiccation tolerance appeared as the key adaptation feature of photoautotrophic organisms for survival in terrestrial habitats. During the further evolution, vascular plants developed complex anatomy structures and molecular mechanisms to maintain the hydrated state of cell environment and sustain dehydration. However, the role of the genes encoding the mechanisms behind this adaptive feature of terrestrial plants changed with their evolution. Thus, in higher vascular plants it is restricted to protection of spores, seeds and pollen from dehydration, whereas the mature vegetative stages became sensitive to desiccation. During maturation, orthodox seeds lose up to 95% of water and successfully enter dormancy. This feature allows seeds maintaining their viability even under strongly fluctuating environmental conditions. The mechanisms behind the desiccation tolerance are activated at the late seed maturation stage and are associated with the accumulation of late embryogenesis abundant (LEA) proteins, small heat shock proteins (sHSP), non-reducing oligosaccharides, and antioxidants of different chemical nature. The main regulators of maturation and desiccation tolerance are abscisic acid and protein DOG1, which control the network of transcription factors, represented by LEC1, LEC2, FUS3, ABI3, ABI5, AGL67, PLATZ1, PLATZ2. This network is complemented by epigenetic regulation of gene expression via methylation of DNA, post-translational modifications of histones and chromatin remodeling. These fine regulatory mechanisms allow orthodox seeds maintaining desiccation tolerance during the whole period of germination up to the stage of radicle protrusion. This time point, in which seeds lose desiccation tolerance, is critical for the whole process of seed development.

Bringing New Methods to the Seed Proteomics Platform: Challenges and Perspectives.

For centuries, crop plants have represented the basis of the daily human diet. Among them, cereals and legumes, accumulating oils, proteins, and carbohydrates in their seeds, distinctly dominate modern agriculture, thus play an essential role in food industry and fuel production. Therefore, seeds of crop plants are intensively studied by food chemists, biologists, biochemists, and nutritional physiologists. Accordingly, seed development and germination as well as age- and stress-related alterations in seed vigor, longevity, nutritional value, and safety can be addressed by a broad panel of analytical, biochemical, and physiological methods. Currently, functional genomics is one of the most powerful tools, giving direct access to characteristic metabolic changes accompanying plant development, senescence, and response to biotic or abiotic stress. Among individual post-genomic methodological platforms, proteomics represents one of the most effective ones, giving access to cellular metabolism at the level of proteins. During the recent decades, multiple methodological advances were introduced in different branches of life science, although only some of them were established in seed proteomics so far. Therefore, here we discuss main methodological approaches already employed in seed proteomics, as well as those still waiting for implementation in this field of plant research, with a special emphasis on sample preparation, data acquisition, processing, and post-processing. Thereby, the overall goal of this review is to bring new methodologies emerging in different areas of proteomics research (clinical, food, ecological, microbial, and plant proteomics) to the broad society of seed biologists.

Harrison's rule scales up to entire parasite assemblages but is determined by environmental factors.

Harrison's rule states that parasite body size and the body size of their hosts tend to be positively correlated. After it was proposed a century ago, a number of studies have investigated this trend, but the support level has varied greatly between parasite/host associations. Moreover, while the rule has been tested at the individual species level, we still lack knowledge on whether Harrison's rule holds at the scale of parasite and host communities. Here, we mapped flea (parasites) and rodent (hosts) body sizes across Mongolia and asked whether Harrison's rule holds for parasite/host assemblages (i.e. whether a parasite's average body size in a locality is positively correlated with its host's average body size). In addition, we attempted to disentangle complex relationships between flea size, host size and environmental factors by testing alternative hypotheses for the determinants of fleas' body size variation. We gathered occurrence data for fleas and rodents from 2,370 sites across Mongolia, constructed incidence matrices for both taxa and calculated the average body sizes of fleas and their hosts over half-degree cells. Then, we applied a path analysis, accounting for spatial autocorrelation, trying to disentangle the drivers of the correlation between parasite and host body sizes. We found a strong positive correlation between average flea and host size across assemblages. Surprisingly though, we found that environmental factors simultaneously affected the body sizes of both fleas and hosts in the same direction, leading to a most likely deceptive correlation between parasite and host size across assemblages. We suggest that environmental factors may, to a great extent, reflect the environmental conditions inside the hosts' burrows where fleas develop and attain their adult body size, thus influencing their larval growth. Similarly, rodent body size is strongly influenced by air temperature, in the direction predicted by Bergmann's rule. If our findings are valid in other host-parasite associations, this may explain the dissenting results of both support and lack thereof for Harrison's rule.

Intraspecific variation of body size in fleas: effects of host sex and flea phenology.

We investigated the effects of host sex and flea phenology (estimated as periods of high versus low abundance) on individual body size in four fleas of small mammals. Amalaraeus penicilliger and Ctenophthalmus uncinatus are ectoparasites of the bank vole Myodes glareolus, whereas Doratopsylla dasycnema and Palaeopsylla soricis are ectoparasites of the common shrew Sorex araneus. We found significant effects of host sex and phenology on the body size of all flea species, although there was no general trend in the directions of these effects. Larger A. penicilliger were found on female hosts, whereas larger P. soricis were found on male hosts. In the remaining species, larger fleas were collected from male hosts during periods of high abundance (male C. uncinatus and female D. dasycnema) and from female hosts during periods of low abundance (male C. uncinatus). Regarding phenology, larger fleas were recorded during periods of either high (A. penicilliger, C. uncinatus, D. dasycnema) or low (C. uncinatus, P. soricis) abundance, but this depended on flea and/or host sex. We conclude that the directions of the host sex and phenology effects varied between flea species. Furthermore, the direction of the host sex effect was mediated by the effect of phenology and vice versa.

Comparative analysis of the plastid conversion, photochemical activity and chlorophyll degradation in developing embryos of green-seeded and yellow-seeded pea (Pisum sativum) cultivars.

Developing seeds of some higher plants are photosynthetically active and contain chlorophylls (Chl), which are typically destroyed at the late stages of seed maturation. However, in some crop plant cultivars, degradation of embryonic Chl remains incomplete, and mature seeds preserve green colour, as it is known for green-seeded cultivars of pea (Pisum sativum L.). The residual Chl compromise seed quality and represent a severe challenge for farmers. Hence, comprehensive understanding of the molecular mechanisms, underlying incomplete Chl degradation is required for maintaining sustainable agriculture. Therefore, here we address dynamics of plastid conversion and photochemical activity alterations, accompanying degradation of Chl in embryos of yellow- and green-seeded cultivars Frisson and Rondo respectively. The yellow-seeded cultivar demonstrated higher rate of Chl degradation at later maturation stage, accompanied with termination of photochemical activity, seed dehydration and conversion of green plastids into amyloplasts. In agreement with this, expression of genes encoding enzymes of Chl degradation was lower in the green seeded cultivar, with the major differences in the levels of Chl b reductase (NYC1) and pheophytinase (PPH) transcripts. Thus, the difference between yellow and green seeds can be attributed to incomplete Chl degradation in the latter at the end of maturation period.

Does Protein Glycation Impact on the Drought-Related Changes in Metabolism and Nutritional Properties of Mature Pea (Pisum sativum L.) Seeds?

Protein glycation is usually referred to as an array of non-enzymatic post-translational modifications formed by reducing sugars and carbonyl products of their degradation. The resulting advanced glycation end products (AGEs) represent a heterogeneous group of covalent adducts, known for their pro-inflammatory effects in mammals, and impacting on pathogenesis of metabolic diseases and ageing. In plants, AGEs are the markers of tissue ageing and response to environmental stressors, the most prominent of which is drought. Although water deficit enhances protein glycation in leaves, its effect on seed glycation profiles is still unknown. Moreover, the effect of drought on biological activities of seed protein in mammalian systems is still unstudied with respect to glycation. Therefore, here we address the effects of a short-term drought on the patterns of seed protein-bound AGEs and accompanying alterations in pro-inflammatory properties of seed protein in the context of seed metabolome dynamics. A short-term drought, simulated as polyethylene glycol-induced osmotic stress and applied at the stage of seed filling, resulted in the dramatic suppression of primary seed metabolism, although the secondary metabolome was minimally affected. This was accompanied with significant suppression of NF-kB activation in human SH-SY5Y neuroblastoma cells after a treatment with protein hydrolyzates, isolated from the mature seeds of drought-treated plants. This effect could not be attributed to formation of known AGEs. Most likely, the prospective anti-inflammatory effect of short-term drought is related to antioxidant effect of unknown secondary metabolite protein adducts, or down-regulation of unknown plant-specific AGEs due to suppression of energy metabolism during seed filling.

Analysis of Chemically Labile Glycation Adducts in Seed Proteins: Case Study of Methylglyoxal-Derived Hydroimidazolone 1 (MG-H1).

Seeds represent the major source of food protein, impacting on both human nutrition and animal feeding. Therefore, seed quality needs to be appropriately addressed in the context of viability and food safety. Indeed, long-term and inappropriate storage of seeds might result in enhancement of protein glycation, which might affect their quality and longevity. Glycation of seed proteins can be probed by exhaustive acid hydrolysis and quantification of the glycation adduct Nɛ-(carboxymethyl)lysine (CML) by liquid chromatography-mass spectrometry (LC-MS). This approach, however, does not allow analysis of thermally and chemically labile glycation adducts, like glyoxal-, methylglyoxal- and 3-deoxyglucosone-derived hydroimidazolones. Although enzymatic hydrolysis might be a good solution in this context, it requires aqueous conditions, which cannot ensure reconstitution of seed protein isolates. Because of this, the complete profiles of seed advanced glycation end products (AGEs) are not characterized so far. Therefore, here we propose the approach, giving access to quantitative solubilization of seed proteins in presence of sodium dodecyl sulfate (SDS) and their quantitative enzymatic hydrolysis prior to removal of SDS by reversed phase solid phase extraction (RP-SPE). Using methylglyoxal-derived hydroimidazolone 1 (MG-H1) as a case example, we demonstrate the applicability of this method for reliable and sensitive LC-MS-based quantification of chemically labile AGEs and its compatibility with bioassays.

Glycation of Plant Proteins: Regulatory Roles and Interplay with Sugar Signalling?

Glycation can be defined as an array of non-enzymatic post-translational modifications of proteins formed by their interaction with reducing carbohydrates and carbonyl products of their degradation. Initial steps of this process rely on reducing sugars and result in the formation of early glycation products-Amadori and Heyns compounds via Schiff base intermediates, whereas their oxidative degradation or reactions of proteins with α-dicarbonyl compounds yield a heterogeneous group of advanced glycation end products (AGEs). These compounds accompany thermal processing of protein-containing foods and are known to impact on ageing, pathogenesis of diabetes mellitus and Alzheimer's disease in mammals. Surprisingly, despite high tissue carbohydrate contents, glycation of plant proteins was addressed only recently and its physiological role in plants is still not understood. Therefore, here we summarize and critically discuss the first steps done in the field of plant protein glycation during the last decade. We consider the main features of plant glycated proteome and discuss them in the context of characteristic metabolic background. Further, we address the possible role of protein glycation in plants and consider its probable contribution to protein degradation, methylglyoxal and sugar signalling, as well as interplay with antioxidant defense.

Phosphatidic acids mediate transport of Ca2+ and H+ through plant cell membranes.

Phosphatidic acids (PAs) are a key intermediate in phospholipid biosynthesis, and a central element in numerous signalling pathways. Functions of PAs are related to their fundamental role in molecular interactions within cell membranes modifying membrane bending, budding, fission and fusion. Here we tested the hypothesis that PAs are capable of direct transport of ions across bio-membranes. We have demonstrated that PAs added to the maize plasma membrane vesicles induced ionophore-like transmembrane transport of Ca2+, H+ and Mg2+. PA-induced Ca2+ fluxes increased with an increasing PAs acyl chain unsaturation. For all the PAs analysed, the effect on Ca2+ permeability increased with increasing pH (pH 8.0>pH 7.2>pH 6.0). The PA-induced Ca2+, Mg2+ and H+ permeability was also more pronounced in the endomembrane vesicles as compared with the plasma membrane vesicles. Addition of PA to protoplasts from Arabidopsis thaliana (L.) Heynh. roots constitutively expressing aequorin triggered elevation of the cytosolic Ca2+ activity, indicating that the observed PA-dependent Ca2+ transport occurs in intact plants.

Species and site contributions to ß-diversity in fleas parasitic on the Palearctic small mammals: ecology, geography and host species composition matter the most.

The ß-diversity of fleas parasitic on small mammals in 45 regions of the Palearctic was partitioned into species [species contributions to ß-diversity (SCBD)] and site ( = assemblage) contributions [local contributions to ß-diversity (LCBD)]. We asked what are the factors affecting SCBD and LCBD and tested whether (a) variation in ecological, morphological, life history and geographic traits of fleas can predict SCBD and (b) variation in flea and host community metrics, off-host environmental factors, host species composition of flea assemblages can predict LCBD. We used spatial variables to describe geographic distribution of flea assemblages with various LCBD values. SCBD significantly increased with an increase in abundance and a decrease in phylogenetic host specificity of a flea as well as with size and latitude of its geographic range, but was not associated with any morphological/life history trait. LCBD of flea assemblages did not depend on either flea or host species richness or environmental predictors, but was significantly affected by compositional uniqueness ( = LCBD) of regional host assemblages and variables describing their species composition. In addition, variation in LCBD was also explained by broad-to-moderate-scale spatial variables. We conclude that SCBD of fleas could be predicted via their ecological and geographic traits, whereas LCBD of their assemblages could be predicted via host composition.

Proteome Map of Pea (Pisum sativum L.) Embryos Containing Different Amounts of Residual Chlorophylls.

Due to low culturing costs and high seed protein contents, legumes represent the main global source of food protein. Pea (Pisum sativum L.) is one of the major legume crops, impacting both animal feed and human nutrition. Therefore, the quality of pea seeds needs to be ensured in the context of sustainable crop production and nutritional efficiency. Apparently, changes in seed protein patterns might directly affect both of these aspects. Thus, here, we address the pea seed proteome in detail and provide, to the best of our knowledge, the most comprehensive annotation of the functions and intracellular localization of pea seed proteins. To address possible intercultivar differences, we compared seed proteomes of yellow- and green-seeded pea cultivars in a comprehensive case study. The analysis revealed totally 1938 and 1989 nonredundant proteins, respectively. Only 35 and 44 proteins, respectively, could be additionally identified after protamine sulfate precipitation (PSP), potentially indicating the high efficiency of our experimental workflow. Totally 981 protein groups were assigned to 34 functional classes, which were to a large extent differentially represented in yellow and green seeds. Closer analysis of these differences by processing of the data in KEGG and String databases revealed their possible relation to a higher metabolic status and reduced longevity of green seeds.

Effects of exogenously-applied L-ascorbic acid on root expansive growth and viability of the border-like cells.

Functions of exogenous L-ascorbic acid in plant roots are poorly understood. Recent study by Makavitskaya et al. (doi.org/10.1093/jxb/ery056) has demonstrated that exogenous ascorbate can be released from roots in response to salt stress, and can trigger elevation in the cytosolic free Ca2+. Here, we report that exogenous ascorbate significantly modifies root elongation in Arabidopsis thaliana. Using a medium exchange technique, we have shown that 10-100 µM ascorbate induces small but significant increase in root elongation while higher levels cause its dramatic decrease. Root border cells of Pisum sativum have been losing viability twice faster in the presence of ascorbate that under control conditions, as tested by the confocal microscopy and a combined staining with propidium iodide and fluorescein diacetate.

The effect of simulated microgravity on the Brassica napus seedling proteome.

The magnitude and the direction of the gravitational field represent an important environmental factor affecting plant development. In this context, the absence or frequent alterations of the gravity field (i.e. microgravity conditions) might compromise extraterrestrial agriculture and hence space inhabitation by humans. To overcome the deleterious effects of microgravity, a complete understanding of the underlying changes on the macromolecular level is necessary. However, although microgravity-related changes in gene expression are well characterised on the transcriptome level, proteomic data are limited. Moreover, information about the microgravity-induced changes in the seedling proteome during seed germination and the first steps of seedling development is completely missing. One of the valuable tools to assess gravity-related issues is 3D clinorotation (i.e. rotation in two axes). Therefore, here we address the effects of microgravity, simulated by a two-axial clinostat, on the proteome of 24- and 48-h-old seedlings of oilseed rape (Brassica napus L.). The liquid chromatography-MS-based proteomic analysis and database search revealed 95 up- and 38 downregulated proteins in the tryptic digests obtained from the seedlings subjected to simulated microgravity, with 42 and 52 annotations detected as being unique for 24- and 48-h treatment times, respectively. The polypeptides involved in protein metabolism, transport and signalling were annotated as the functional groups most strongly affected by 3-D clinorotation.

Photochemical activity changes accompanying the embryogenesis of pea (Pisum sativum) with yellow and green cotyledons.

The pea seeds are photosynthetically active until the end of the maturation phase, when the embryonic chlorophylls degrade. However, in some cultivars, the underlying mechanisms are compromised, and the mature seeds preserve green colour. The residual chlorophylls can enhance oxidative degradation of reserve biomolecules, and affect thereby the quality, shelf life and nutritive value of seeds. Despite this, the formation, degradation, and physical properties of the seed chlorophylls are still not completely characterised. So here we address the dynamics of seed photochemical activity in the yellow- and green-seeded pea cultivars by the pulse amplitude modulation (PAM) fluorometric analysis. The experiments revealed the maximal photochemical activity at the early- and mid-cotyledon stages. Thereby, the active centres of PSII were saturated at the light intensity of 15-20µmol photons m-2 s-1. Despite of their shielding from the light by the pod wall and seed coat, photochemical reactions can be registered in the seeds with green embryo. Importantly, even at the low light intensities, the photochemical activity in the coats and cotyledons could be detected. The fast transients of the chlorophyll a fluorescence revealed a higher photochemical activity in the coat of yellow-seeded cultivars in comparison to those with the green-seeded ones. However, it declined rapidly in all seeds at the late cotyledon stage, and was accompanied with the decrease of the seed water content. Thus, the termination of photosynthetic activity in seeds is triggered by their dehydration.