Abortive Ovules Research Articles

Ovule abortion accompanied by programmed cell death in Castanea mollissima

Chinese chestnut (Castanea mollissima Blume.) ovary contains 12-18 ovules, but only one can develop and grow normally, which indicates that the abortive rate of ovules can go as high as 94%. In this study, we observed the developmental characteristics and cytological changes of abortive ovules and fertile ovules during development. Morphological analysis showed it would take about 15∼20 days from globular to cotyledon embryo in fertile ovules, which is accompanied with the formation and apoptosis of endosperm. But there were stark differences in the nutrient transport, microstructure and ultrastructure of fertile and abortive ovules. The fluorescence disodium fluorescein and FDA indicated that all ovules were viable at early stages, but some ovules became inactive at different times as the ovary developed. Fluorescence labelling and ultrastructure showed cell nuclei of abortive ovules were disintegrated at 25 days after anthesis (DAA). Compared with the fertile ovules, starch grains were synthesized at earlier periods, but disappeared immediately, and the cells were usually irregular in shape, and had folded cell membranes in the abortive ovules. Furthermore, the Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) was carried out to assess programmed cell death in fertile and abortive ovules, results showed that DNA fragmentation was occurred in the cells of abortive ovules. In short, these results provide new insights into ovule abortion in the angiosperm.

Genome-Wide Characterization and Functional Validation of the ACS Gene Family in the Chestnut Reveals Its Regulatory Role in Ovule Development.

Ovule abortion significantly contributes to a reduction in chestnut yield. Therefore, an examination of the mechanisms underlying ovule abortion is crucial for increasing chestnut yield. In our previous study, we conducted a comprehensive multiomic analysis of fertile and abortive ovules and found that ACS genes in chestnuts (CmACS) play a crucial role in ovule development. Therefore, to further study the function of ACS genes, a total of seven CmACS members were identified, their gene structures, conserved structural domains, evolutionary trees, chromosomal localization, and promoter cis-acting elements were analyzed, and their subcellular localization was predicted and verified. The spatiotemporal specificity of the expression of the seven CmACS genes was confirmed via qRT-PCR analysis. Notably, CmACS7 was exclusively expressed in the floral organs, and its expression peaked during fertilization and decreased after fertilization. The ACC levels remained consistently greater in fertile ovules than in abortive ovules. The ACSase activity of CmACS7 was identified using the genetic transformation of chestnut healing tissue. Micro Solanum lycopersicum plants overexpressing CmACS7 had a significantly greater rate of seed failure than did wild-type plants. Our results suggest that ovule fertilization activates CmACS7 and increases ACC levels, whereas an overexpression of CmACS7 leads to an increase in ACC content in the ovule prior to fertilization, which can lead to abortion. In conclusion, the present study demonstrated that chestnut ovule abortion is caused by poor fertilization and not by nutritional competition. Optimization of the pollination and fertilization of female flowers is essential for increasing chestnut yield and reducing ovule abortion.

Transcriptomic and Metabolic Profiling Reveal the Mechanism of Ovule Development in Castanea mollissima.

Ovule abortion, which is the main cause of empty burs in the Chinese chestnut, affects the formation of embryos and further reduces yield; therefore, it is important to study the mechanism of ovule abortion. In this study, we analyzed the transcriptomic and metabolomic data of ovules at critical developmental stages to explore the key regulatory networks affecting ovule development. The metabolites were enriched mainly in pathways involved in phytohormone signaling, energy metabolism, and amino acid synthesis in the endoplasmic reticulum. Analysis of the differentially expressed genes (DEGs) revealed that the HSP genes were significantly down-regulated during fertilization, indicating that this process is extremely sensitive to temperature. The hormone and sucrose contents of ovules before and after fertilization and of fertile and abortive ovules at different developmental stages showed significant differences, and it is hypothesized that that abnormal temperature may disrupt hormone synthesis, affecting the synthesis and catabolism of sucrose and ultimately resulting in the abortive development of Chinese chestnut ovules. At the pollination and fertilization stage of chestnuts, spraying with ethylene, ACC, and AIB significantly increased the number of developing fruit in each prickly pod compared to CK (water) treatment. These results indicated that both ethylene and ACC increased the rate of ovule development. This study provides an important theoretical molecular basis for the subsequent regulation of ovule development and nut yield in the Chinese chestnut.

Comparative analysis of the transcriptome during single-seed formation of Castanea henryi: regulation of starch metabolism and endogenous hormones

BackgroundIn seed plants, the ovule is the precursor to the seed. The process of ovule development and differentiation is regulated by multiple factors, including starch metabolism and endogenous hormones. Castanea henryi produces nuts with high nutritional value. However, the high proportion of empty buds restricts the commercial use of the tree. Previous studies have shown that the empty bud phenotype is closely related to ovule abortion. If none of the ovules in the ovary expand rapidly and develop in 7–8 weeks after pollination, an empty bud will form. Therefore, we studied the development and molecular mechanisms underlying single seed formation in C. henryi.ResultsWe found that 49 days after pollination (DAP) is a critical period for the formation of fertile and abortive ovules. The morphology and starch distribution of the fertile and abortive ovules differed significantly at 49 DAP. The fertile ovules were smooth and round in appearance, with a large amount of starch. In contrast, abortive ovules were smaller with only a small amount of starch. The embryo sac of the abortive ovule proceeded to develop abnormally, and the entire ovule lacked starch. We identified 37 candidate genes involved in metabolism with potential roles in the regulation of starch levels. Three ADP-glucose pyrophosphorylase (AGPase) genes, one granule-bound starch synthase (GBSS) gene, and two beta-amylase genes could affect starch accumulation. The levels of auxin, cytokinins, gibberellins, and jasmonic acid in fertile ovules were higher than those in abortive ovules. In addition, the levels of endogenous abscisic acid and salicylic acid in abortive ovules were higher than those in fertile ovules of the same age, consistent with the expression patterns of genes related to the synthesis of abscisic and salicylic acid and signal transduction. We identified and mapped the differentially expressed genes associated with hormone synthesis and signal transduction.ConclusionsThese results improve our general understanding of the molecular mechanisms underlying single seed development in C. henryi and the phenomenon of empty buds, providing directions for future research.

Morphological observation and protein expression of fertile and abortive ovules in Castanea mollissima.

Chinese chestnuts (Castanea mollissima Blume.) contain 12–18 ovules in one ovary, but only one ovule develops into a seed, indicating a high ovule abortion rate. In this study, the Chinese chestnut ‘Huaihuang’ was used to explore the possible mechanisms of ovule abortion with respect to morphology and proteomics. The morphology and microstructure of abortive ovules were found to be considerably different from those of fertile ovules at 20 days after anthesis (20 DAA). The fertile ovules had completely formed tissues, such as the embryo sac, embryo and endosperm. By contrast, in the abortive ovules, there were no embryo sacs, and wide spaces between the integuments were observed, with few nucelli. Fluorescence labelling of the nuclei and transmission electron microscopy (TEM) observations showed that cells of abortive ovules were abnormally shaped and had thickened cell walls, folded cell membranes, condensed cytoplasm, ruptured nuclear membranes, degraded nucleoli and reduced mitochondria. The iTRAQ (isobaric tag for relative and absolute quantitation) results showed that in the abortive ovules, low levels of soluble protein with small molecular weights were found, and most of differently expressed proteins (DEPs) were related to protein synthesis, accumulation of active oxygen free radical, energy synthesis and so on. These DEPs might be associated with abnormal ovules formation.

Role of ethylene in the regulatory mechanism underlying the abortion of ovules after fertilization in Xanthoceras sorbifolium.

Genes related to the MAPK cascade, ethylene signaling pathway, Pi starvation response, and NAC TFs were differentially expressed between normal and abortive ovules. Receptor-mediated ethylene signal perception and transmission play an important role in regulating fruit and ovule development. Xanthoceras sorbifolium, a small to medium-sized tree endemic to northern China, is an emerging dedicated oilseed crop designed for applications in advanced biofuel, engine oil, and functional food, as well as for pharmaceutical and cosmetic applications. Despite the importance of Xanthoceras seed oil, low seed productivity has constricted commercial exploitation of the species. The abortion of developing seeds (ovules after fertilization) is a major factor limiting fruit and seed production in the plant. To increase fruit and seed yields, a better understanding of the mechanisms underlying the abortion of fertilized ovules is critical. This study revealed differences in nucellus degeneration, endosperm development, and starch grain content between normally and abnormally developing ovules after fertilization. We constructed 6 RNA-sequencing (RNA-seq) libraries from normally and abnormally developing ovules at the onset of their abortion process. Comparative transcriptome analysis between the normal and abnormal ovules identified 818 differentially expressed genes (DEGs). Among DEGs, many genes involved in mitogen-activated protein kinase (MAPK) cascades, ethylene signaling pathway, and NAC transcription factor genes showed up-regulated expression in abnormal ovules. The RNA-seq data were validated using quantitative reverse-transcription PCR. Using virus-induced gene silencing (VIGS) methods, evaluation of an ethylene receptor gene (XsERS) function indicated that the gene was closely related to early development of fruits and seeds. Based on the data presented here, we propose a model for a MAPK-ethylene signaling-NAC2 gene regulatory cascade that plays an important role in the regulation of the ovule abortion process in X. sorbifolium. The present study is imperative for understanding the mechanisms of ovule abortion after fertilization and identifying the critical genes and gene networks involved in determining the fate of ovule development.

RNA-seq Analysis Reveals Gene Expression Profiling of Female Fertile and Sterile Ovules of PinusTabulaeformis Carr. during Free Nuclear Mitosis of the Female Gametophyte.

The development of the female gametophyte (FG) is one of the key processes of life cycle alteration between the haploid gametophyte and the diploid sporophytes in plants and it is required for successful seed development after fertilization. It is well demonstrated that free nuclear mitosis (FNM) of FG is crucial for the development of the ovule. However, studies of the molecular mechanism of ovule and FG development focused mainly on angiosperms, such as Arabidopsis thaliana and further investigation of gymnosperms remains to be completed. Here, Illumina sequencing of six transcriptomic libraries obtained from developing and abortive ovules at different stages during free nuclear mitosis of magagametophyte (FNMM) was used to acquire transcriptome data and gene expression profiles of Pinus tabulaeformis. Six cDNA libraries generated a total of 71.0 million high-quality clean reads that aligned with 63,449 unigenes and the comparison between developing and abortive ovules identified 7174 differentially expressed genes (DEGs). From the functional annotation results, DEGs involved in the cell cycle and phytohormone regulation were highlighted to reveal their biological importance in ovule development. Furthermore, validation of DEGs from the phytohormone signal transduction pathway was performed using quantitative real-time PCR analysis, revealing the dynamics of transcriptional networks and potential key components in the regulation of FG development in P. tabulaeformis were identified. These findings provide new insights into the regulatory mechanisms of ovule development in woody gymnosperms.

Anatomy, microstructure, and endogenous hormone changes in Gnetum parvifolium during anthesis

AbstractThe study of the sexual systems of plants is very important for understanding their evolutionary process. Gnetum is a pivotal species in comprehending the seed plant phylogeny. However, the sexual systems of this genus are not fully understood. To investigate the sexual system of Gnetum, the anatomical structure, microstructure, and endogenous hormones were investigated from the differentiation of flower buds to fertilization in the floral organs of female and male of G. parvifolium (Warb.) C.Y. Cheng, widely distributed in the north of China. After 2 years of field investigations, the staminate strobili were observed on female plants for the first time and the abortive ovules were also found on male plants. In addition, high levels of gibberellin A3 and zeatin riboside led to masculinization, whereas a high level of indole‐3‐acetic acid favored feminization. A comprehensive analysis of the results indicates that G. parvifolium may not be completely dioecious on morphology, but female and male plants can only perform their own gender function, which would provide a better understanding of the diversity of sexual systems in Gnetum, and reproductive structures of seed plants.

REPRODUCTIVE BIOLOGY OF GNETUM LATIFOLIUM BLUME (GNETALES) IN TAMIL NADU

The reproductive biology of Gnetum latifolium Blume studied in four different forest sites of Tamil Nadu. G. latifolium reported to have dioecious plants with less number male population in all the selected areas. Entamophilous cues observed with common anaemophily and pollinator may be flies. Pollination dropsare also reported in young ovules. The seed maturation and germination frequency observed with very low frequency in all the selected population whereas in high number of abortive ovules reported in Western Ghats.

Comparison of phytohormone biosynthesis and signal transduction pathways in developing and abortive hazelnut ovules

In hazelnuts, ovule abortion is associated with the formation of empty nuts, resulting in serious yield losses. Phytohormones play important roles in plant growth and development; however, the mechanisms through which they regulate ovule abortion in hazelnut are largely unknown. RNA-seq identified 1209 differentially expressed genes with Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotation between developing and abortive ovules in hazelnut. Hormone signal transduction (ko04075), and several phytohormone biosynthesis pathways, were found to be significantly enriched after pathway enrichment analysis. Compared with developing ovules, the levels of indole-3-acetic acid (IAA), gibberellins (GA), cytokinin (CTK), and salicylic acid (SA) were found to be significantly lower in abortive ovules, while levels of abscisic acid (ABA), ethylene (ETH), and jasmonic acid (JA) were found to be significantly higher. Consistent with these results, some important rate-limiting enzymes involved in the biosynthesis of phytohormones, including 9-cis-epoxycarotenoid dioxygenase, ACC synthase, and phospholipase A1 were significantly up-regulated in abortive ovules. Some key genes in the ETH, JA, and ABA signal transduction pathways were highly upregulated, including ethylene-responsive transcription factor 1 (ERF1/2) and the serine/threonine-protein kinase CTR1 in the ETH signal pathway, jasmonate ZIM domain-containing protein (JAZ) and the transcription factor MYC2 in the JA signal pathway, and ABA responsive element binding factor (ABF) in the JA signal pathway. Thus, ABA, ETH, and JA signaling were more active in abortive ovules than in developing ovules. Expression levels of NCED, ABF, ACS, ERF1/2, PLA1, and MYC2 revealed by q-RT-PCR were consistent with the results obtained by differentially expressed genes (DEG) analysis. Our data suggest that multiple phytohormone signaling pathways are involved in the development of hazelnut ovules. Inhibition of IAA, GA, CTK, and SA biosynthesis and signaling pathways, and activation of ABA, ETH, and JA biosynthesis and signaling pathways may contribute to the formation of abortive ovules in hazelnut.

Biosynthesis and Contents of Gibberellins in Seeded and Seedless Sweet Orange (Citrus sinensis L. Osbeck) Cultivars

In this work, we study the capacity to biosynthesize gibberellins (GA) of ovules (either fertilised or unfertilised), developing seeds and pericarp from fruitlets and their relation with fruit set capacity. Experiments were performed in adult, 12-year-old trees of seeded (Pineapple) and seedless parthenocarpic (Washington navel) sweet orange [Citrus sinensis L. Osbeck] cultivars. The activity of GA20-, GA3- and GA2-oxidases and gibberellin levels were measured in the ovules and pericarp of fruitlets in different development states. The results indicate that ovules are the main sites of gibberellin synthesis in fruitlets during the post-anthesis period. The most intense GA1 synthesis—coincident with the highest expression of GA20ox2, GA3ox1 and GA2ox1—was detected in the ovules of the seeded cultivar, probably induced by fecundation and associated with low early fruitlet abscission rates. By contrast, the low activity detected in the sterile cultivar appears to be rather developmentally or constitutively regulated. As a fruitlet develops, the GA1 concentration is augmented in the pericarp in comparison to ovules or developing seeds, and levels therein did not exhibit noticeable differences between varieties. Furthermore, developing seeds from pineapple had higher GA1 content than the unfertilised abortive ovules from Washington navel. Taken together, data suggest a main role for this hormone in the control of fruitlet abscission, and also demonstrate a function in seed development.

Seedless x seedless grape progeny: Technique, results and perspectives

Recent refinements in the in vitro embryo rescue technique employed to raise progenies from abortive ovules of seedless x seedless cultivars include addition of 0.2 ppm NAA (naphtaleneacetic acid) to the medium, inducing highly improved root systems and faster plant development. Selling seedless Vitis vinifera yielded seedless progeny only. Open pollinated Perlette and Flame Seedless gave rise to a high percentage (75 and 87%, respectively) of seedless progeny. Progeny from various crosses between seedless cultivars segregated into 65 normal seeded and 204 seedless. From the totality of 204 seedless progeny 192 bore fruit with very slight seed traces. Progeny from crosses between seeded and seedless segregated only 7.5-8% individuals with comparably slight seed traces, amounting to 1/3 of the progeny rated as seedless. Fresh weight determinations of aborted seeds per berry showed a pronouncedly lower weight in progenies from seedless x seedless crosses. Reduction in average berry size in the seedless fraction of seedless x seedless progenies compared to midparent values was of a similar order of magnitude as that obtained in seedless progeny derived from seeded x seedless crosses.

Sporogenesis and gametogenesis in Chinese chinquapin (Castanea henryi (Skam) Rehder & Wilson) and their systematic implications

We provide new embryological information regarding the Fagaceae family. Abortive ovules may be a result of mate choice and be necessary to maximize reproductive success in Chinese chinquapin ( Castanea henryi ). In contrast to most angiosperms, in which fertilization occurs 1–2 days after pollination, fertilization in Fagales is delayed from 4 days to more than 1 year. However, the phenomenon of delayed fertilization in Castanea Miller species (Fagaceae), which are the economically important species of Fagaceae and widely distributed or cultured in many countries of the world, has been consistently neglected, raising questions regarding what the delayed fertilization process is and why fertilization is delayed. To answer these questions, we systematically investigated the micro- and megasporogenesis in addition to male and female gametogenesis in Castanea henryi (Castanea Miller species). Our results show that the ovules primordia are immature at the time of pollination and require 6 weeks to become fully developed. During this 6-week period, 32.30 % of ovules abort because of the inability to form an embryo sac (ES). Approximately 15.79 % of ovules abort because of abnormal development of ES. From 7 to 8 weeks after pollination in which double fertilization occurs, most mature ovules are also aborted because of cell degeneration in gametophytes. Only one ovule can develop into a ripe seed. Thus, the delayed fertilization in Castanea henryi may be necessary to increase the time for mate choice and selective fertilization. A certain number of ovule abortions may be the result of delayed fertilization to maximize reproductive success.

Transcriptome analysis and gene expression profiling of abortive and developing ovules during fruit development in hazelnut.

BackgroundA high ratio of blank fruit in hazelnut (Corylus heterophylla Fisch) is a very common phenomenon that causes serious yield losses in northeast China. The development of blank fruit in the Corylus genus is known to be associated with embryo abortion. However, little is known about the molecular mechanisms responsible for embryo abortion during the nut development stage. Genomic information for C. heterophylla Fisch is not available; therefore, data related to transcriptome and gene expression profiling of developing and abortive ovules are needed.Methodology/Principal FindingsIn this study, de novo transcriptome sequencing and RNA-seq analysis were conducted using short-read sequencing technology (Illumina HiSeq 2000). The results of the transcriptome assembly analysis revealed genetic information that was associated with the fruit development stage. Two digital gene expression libraries were constructed, one for a full (normally developing) ovule and one for an empty (abortive) ovule. Transcriptome sequencing and assembly results revealed 55,353 unigenes, including 18,751 clusters and 36,602 singletons. These results were annotated using the public databases NR, NT, Swiss-Prot, KEGG, COG, and GO. Using digital gene expression profiling, gene expression differences in developing and abortive ovules were identified. A total of 1,637 and 715 unigenes were significantly upregulated and downregulated, respectively, in abortive ovules, compared with developing ovules. Quantitative real-time polymerase chain reaction analysis was used in order to verify the differential expression of some genes.Conclusions/SignificanceThe transcriptome and digital gene expression profiling data of normally developing and abortive ovules in hazelnut provide exhaustive information that will improve our understanding of the molecular mechanisms of abortive ovule formation in hazelnut.

Self-sterility in Camellia oleifera may be due to the prezygotic late-acting self-incompatibility.

In this report, self-sterility in Camellia oleifera was explored by comparing structural and statistical characteristics following self-pollination (SP) and cross-pollination (CP). Although slightly delayed pollen germination and pollen tube growth in selfed ovaries compared to crossed ovaries was observed, there was no significant difference in the percentages of pollen that germinated and pollen tubes that grew to the base of the style. There was also no difference in morphological structure after the two pollination treatments. However, the proportions of ovule penetration and double fertilization in selfed ovules were significantly lower than in crossed ovules, indicating that a prezygotic late-acting self-incompatible mechanism may exist in C. oleifera. Callose deposition was observed in selfed abortive ovules, but not in normal. Ovules did not show differences in anatomic structure during embryonic development, whereas significant differences were observed in the final fruit and seed set. In addition, aborted ovules in selfed ovaries occurred within 35 days after SP and prior to zygote division. However, this process did not occur continuously throughout the life cycle, and no zygotes were observed in the selfed abortive ovules. These results indicated that the self-sterility in C. oleifera may be caused by prezygotic late-acting self-incompatibility (LSI).

Anatomical Study on the Multi-Ovule Development and Abortion of Hanfu Apple

The terminal flower buds of 6-yr-old Hanfu apple were used to study the ovule development, ovular characteristics, cell death of abortive ovules, and dynamic change of starch grain quantity in the embryo sac with paraffin slices and terminal deoxynucleotidyl transferase-mediated fluorescein deoxyuridine triphosphate nick-end labeling (TUNEL) system. Four anatropous ovules in each ventricle could be observed before flowering. With the developing of floral organ, the bulk of normal ovules enlarged in each ventricle, the mature embryo sac differentiated into nucellus, and the egg cell developed into zygote by double fertilization. A large number of starch grains were observed during pollen tube growth and double fertilization, which guaranteed basic nutrient supply in the normal development of ovules. Moreover, abortion phenomenon of runtish ovules emerged at the stages of mature embryo sac, double fertilization and zygote development. The abortion characteristics included deformity of ovule development, degradation of nucellus tissue, separation between funiculus and ovule, abnormality of four-nucleate embryo sac, as well as development interruption of mature embryo sac. TUNEL analysis proved that ovule abortion was programmed cell death.

Strobilus and Seed Production ofDioon Edule(Zamiaceae) in a Population with low Seedling Density in San Luis Potosí, Mexico

We describe strobilus and seed development in a Dioon edule (chamal, palma, dameu') population characterized by low seedling and high adult tree density, in order to improve conservation decisions for this endangered cycad species. Female strobili required 16-17 months and male 4-5 months to develop. During this period 80% female and 100% male strobili were not damaged by herbivores. The method of cone analysis used to evaluate seed production of pines was modified for D. edule, providing actual yield of individual strobili compared to potential seed yield, explaining productivity in terms of seed efficiency, and identifying and quantifying the types of seed failures. Larger strobili have higher seed potential calculated as the number of seeds potentially produced in the fertile megasporophylls of each strobilus, total seeds actually produced, and seeds with an embryo (filled). Calculated average seed potential per strobilus was of 230.4 seeds. Using the X-ray technique, average seed traits were 100.2 full, 1.8 insect damaged, and 0.6 malformed. Seed efficiency (number of filled seeds as a percentage of the seed potential of each strobilus) of 42.5% indicated that major seed loss was attributable to abortive ovules and seeds, possibly due to ineffective pollination. Seedling survival in greenhouse conditions was 100% after one year. Low seed production affects the population structure and hinders its conservation.

Morphology and Anatomy of the Flowers of Trichilia catigua A. Juss., T. elegans A. Juss. and T. pallida Sw. (Meliaceae)

Morphological and structural features of flowers of Trichilia catigua A. Juss., T. elegans A. Juss. and T. pallida Sw. (Meliaceae) are presented here with the purpose to stimulate future reproductive and preservation studies of these brazilian native species. Anthesis occurred from April to August in T. catigua, January to May in T. pallida and September to January in T. elegans. Sepals and petals presented a papillose and pilose epidermis and a parenchymatous mesophyll. Male flowers presented pistillodes with abortive ovules and a staminal tube with tetrasporangiate anthers. Anther wall had a papilose epidermis, a fibrous endothecium, two middle layers and a secretory tapetum. Female flowers presented antherodes and bi - or tricarpellate syncarpous pistils. Ovary had a simple structure with anatropous, bitegmic ovules. Style was hollow or solid and the stigma was constituted by uni - or bicelullar trichomes. Strictly dioecious specimens were not found in the study area.

ABORTION OF OVULES AND SEEDS OF Spondias purpurea L. (Anacardiaceae)

The ovules of Spondias purpurea are slightly campylotropous, bitegmic, crassinucellate, pendulous, and with a special funicular pro-tuberance on a well developed funicle. During anthesis the ovules accumulated callose and lignin in a group of nucellar cells con-tiguous to the chalazal end of the embryo sac. In the field, pollination is not a limiting factor for setting fruit, and abortion of ovules and seeds is not caused by a lack of pollen. The main stage of ovule abortion occurs in preanthesis, and these abortive ovules do not develop a nucellus nor an embryo sac. A second stage of abortion occurs four to five weeks after anthesis. The total percentage of abortion of ovules varies from 55% to 84%. The mature embryo is straight with a short axis (5% to 10% of the total length of the embryo).

Reproductive phenology and seed development of Gentianella caucasea in different habitats in the Central Caucasus

Flowering phenology, seed development and reproductive characteristics influencing seed set were investigated in four populations of Gentianella caucasea, a therophyte growing in different habitats (subalpine meadows, pastures and alpine grassland) along an altitudinal gradient between 1900 and 2700m a.s.l. in the Central Caucasus. During the growing season phenological stages were recorded and samples were collected at short intervals. The embryological stages and the time course of seed formation were determined microscopically using the interference contrast technique.Considerable differences in phenological dates, seed development and seed set were observed between the population growing in the alpine grassland, the population in a hay meadow which is regularly mown during the first decade of August, and the population on an intensively grazed sheep pasture. The extremely low seed set connected with a high percentage of anomalous and abortive ovules as observed in the latter population can be regarded as a criterion for severe stress due to habitat disturbance. Very little variation in reproductive characteristics was found among the population growing in a short-grass vegetation extending from 2200 to 2750m a.s.l. Along such an altitudinal transect the developmental processes would be expected to be retarded in view of the decrease in temperature with increasing altitude. However it appears that, for herbaceous plants, it is not the adiabatic lapse rate which is important, but the specific microclimatic conditions, which on southward slopes are more favourable even at higher altitudes.