Therefore, the observed differences in pollen germination and pollen tube length in the present study were a reflection of cultivar variability. Cultivar differences for cardinal temperatures were recorded in the current study Tables 2 and 3. However, the differences in cardinal temperatures did not reflect the tolerance or susceptibility of a cultivar to high temperatures because the cultivars which had a higher optimum temperature did not always have a higher temperature maximum or vice versa.
Cultivars that had higher T opt also had a higher pollen germination percentage and maintained a higher pollen germination even at high temperatures. Similar pollen behaviour was observed in snake melon Matlob and Kelly, , corn Binelli et al.
Recent studies with Brassica napus have suggested that reduced pollen germination rather than pollen viability under high temperature is the major cause of low pollen fertility Young et al. Prasad et al. Further studies will be required to determine the minimum number of germinated pollen grains required to have effective fertilization. Recently, ur Rahman et al.
In cotton, heat tolerance does not correlate with degree of cell membrane lipid saturation Rikin et al. However, the genotypic differences for pollen germination and pollen tube growth identified in this study could be due to the variation in their pollen carbohydrate concentration. Studies have shown that carbohydrates are responsible for pollen development and, especially, pollen cytoplasmic carbohydrates and sucrose are involved in protecting pollen viability during exposure and dispersal Pacini et al.
This was attributed to a decrease in sucrose utilization by pollen grains under high temperature, even though the pollen grains accumulated more starch and sugars than under normal temperature conditions. Therefore, under-utilization or unavailability of carbohydrates hinders pollen germination on exposure to high temperatures. Future studies need to study the genotypic differences or pollen carbohydrate concentration and its role in determining the temperature tolerance of cotton pollen.
The PCA is perhaps the most useful statistical tool for screening multivariate data with significantly high correlations Johnson, The cluster analysis applied to the principal components divided the cultivars into four distinct groups Fig.
The PC1 vectors indicated that cultivars with high optimum temperature do not necessarily have high pollen germination or long pollen tubes. But, tolerance to high temperatures will result only from successful fertilization of the megagametophyte that requires both pollen germination and pollen tube elongation.
However, for accurate yield predictions, future studies should quantify boll retention under high temperature and investigate the relationship between pollen germination, boll number and air temperatures under controlled conditions with high levels of solar radiation. Studies will also be required to validate the performance of high temperature-tolerant cultivars identified by these in vitro methods in high-temperature environments.
We thank Drs Harry F. Hodges, Jack C. McCarthy and Donald T. Krizek for their critical comments and suggestions on the manuscript. We thank D. Brand and K. Gourley for technical support. This is contribution no. The effect of high temperature and high atmospheric CO 2 on carbohydrate changes in bell pepper Capsicum annuum pollen in relation to its germination.
Physiologia Plantarum : — Tolerance to high temperature in cotton Gossypium hirsutum L. Environmental and Experimental Botany 34 : — Barrow JR. Comparisons among pollen viability measurement methods in cotton. Crop Science 23 : — Temperature effects on pollen germination and pollen tube growth in maize.
Genetica Agraria 39 : — Burke JJ. Sprinkler-induced flower losses and yield reductions in cotton Gossypium hirsutum L. Agronomy Journal 95 : — In vitro analysis of cotton pollen germination. Agronomy Journal 96 : — Cotton Farming.
Cotton farming's guide to seed. Heat tolerance in groundnut. Field Crops Research 80 : 63 — Heat-stress effects on reproduction and seed set in Linum usitatissimum L. Plant, Cell and Environment 26 : — Climates of the 20th and 21st centuries simulated by the NCAR climate system model. Journal of Climate 14 : — Economic Research Service.
Agricultural resources and environmental indicators, Agriculture Handbook, No. Hall AE. Breeding for heat tolerance. Plant Breeding Reviews 10 : — High temperature stress and pollen viability of maize. Crop Science 20 : — Climate change the scientific basis. Johnson DE. Applied multivariate methods for data analysis. New York: Duxbury Press. Response of in vitro pollen germination and pollen tube growth of groundnut Arachis hypogaea L. Plant, Cell and Environment 25 : — Effect of high temperature on pollen grain germination, pollen tube growth and seed yield in Chinese cabbage.
HortScience 16 : 67 — Maestro MC, Alvarez J. The effects of temperature on pollination and pollen tube growth in muskmelon Cucumis melo L.
The other sperm cell is fused with the central cell. The central cell contains two haploid polar nuclei. Hence, the resulting cells are triploid, which are divided by mitosis , forming the endosperm. Endosperm is a nutrient-rich tissue, found inside the seed. Different stages of double fertilization in flowering plants are shown in figure 1. Figure 1: Double Fertilization. The ovary of an angiosperm is developed into a fruit after the fertilization.
Some plants like avocados contain a single ovule in the ovary per a flower. These plants develop a single seed per fruit. Some plants like kiwi fruit contain several ovules in the ovary of a flower. They produce multiple seeds per fruit. In fruits with multi-seeds, multiple pollen grains are involved in the fertilization of several ovules. The two types of fertilizations in animals are internal fertilization and external fertilization. Internal fertilization shows high survival rates of the embryo than the external fertilization.
Internal fertilization takes place inside the female organism. Oviparity , viviparity , and ovoviparity are the three methods of internal fertilization. Internal fertilization occurs in mammals, reptiles, some birds, and some fish. External fertilization takes place in damp environments outside the female organism.
The eggs and sperms are called spawn in the external fertilization. Both male and female gametes should be released to the environment at the same time. Shown here is a bee orchid Ophrys apifera. After pollen is deposited on the stigma, it must germinate and grow through the style to reach the ovule. The microspores, or the pollen, contain two cells: the pollen tube cell and the generative cell.
The pollen tube cell grows into a pollen tube through which the generative cell travels. The germination of the pollen tube requires water, oxygen, and certain chemical signals. During this process, if the generative cell has not already split into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by the synergids present in the embryo sac; it enters the ovule sac through the micropyle.
Of the two sperm cells, one sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm. Together, these two fertilization events in angiosperms are known as double fertilization. After fertilization is complete, no other sperm can enter. The fertilized ovule forms the seed, whereas the tissues of the ovary become the fruit, usually enveloping the seed.
Double fertilization : In angiosperms, one sperm fertilizes the egg to form the 2n zygote, while the other sperm fuses with two polar nuclei to form the 3n endosperm. This is called a double fertilization. After fertilization, embryonic development begins. The zygote divides to form two cells: the upper cell terminal cell and the lower cell basal cell. The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal tissue.
The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The terminal cell also divides, giving rise to a globular-shaped proembryo. In dicots eudicots , the developing embryo has a heart shape due to the presence of the two rudimentary cotyledons.
In non-endospermic dicots, such as Capsella bursa , the endosperm develops initially, but is then digested. In this case, the food reserves are moved into the two cotyledons. As the embryo and cotyledons enlarge, they become crowded inside the developing seed and are forced to bend. Ultimately, the embryo and cotyledons fill the seed, at which point, the seed is ready for dispersal.
Embryonic development is suspended after some time; growth resumes only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.
After fertilization, the zygote divides to form an upper terminal cell and a lower basal cell. The basal cell also divides, giving rise to the suspensor. Monocot and dicot seeds develop in differing ways, but both contain seeds with a seed coat, cotyledons, endosperm, and a single embryo. The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat, known as the testa, and inner coat, known as the tegmen.
The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl. The embryonic axis terminates in a radicle, which is the region from which the root will develop. In angiosperms, the process of seed development begins with double fertilization and involves the fusion of the egg and sperm nuclei into a zygote. The second part of this process is the fusion of the polar nuclei with a second sperm cell nucleus, thus forming a primary endosperm.
Right after fertilization, the zygote is mostly inactive, but the primary endosperm divides rapidly to form the endosperm tissue. This tissue becomes the food the young plant will consume until the roots have developed after germination. The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant: the inner integument forms the tegmen and the outer forms the testa.
When the seed coat forms from only one layer, it is also called the testa, though not all such testae are homologous from one species to the next. In gymnosperms, the two sperm cells transferred from the pollen do not develop seed by double fertilization, but one sperm nucleus unites with the egg nucleus and the other sperm is not used.
Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed during early development. The seed is composed of the embryo and tissue from the mother plant, which also form a cone around the seed in coniferous plants such as pine and spruce. The ovules after fertilization develop into the seeds. The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, the single cotyledon is called a scutellum; it is connected directly to the embryo via vascular tissue.
Food reserves are stored in the large endosperm. Upon germination, enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the endosperm and embryo.
The enzymes degrade the stored carbohydrates, proteins, and lipids. These products are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Monocots and dicots : The structures of dicot and monocot seeds are shown. Dicots left have two cotyledons. Monocots, such as corn right , have one cotyledon, called the scutellum, which channels nutrition to the growing embryo. Both monocot and dicot embryos have a plumule that forms the leaves, a hypocotyl that forms the stem, and a radicle that forms the root.
The embryonic axis comprises everything between the plumule and the radicle, not including the cotyledon s. In endospermic dicots, the food reserves are stored in the endosperm. During germination, the two cotyledons act as absorptive organs to take up the enzymatically-released food reserves, similar to the process in monocots.
In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized, moving into the developing cotyledon for storage. Upon germination in dicot seeds, the epicotyl is shaped like a hook with the plumule pointing downwards; this plumule hook persists as long as germination proceeds in the dark.
Therefore, as the epicotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Upon exposure to light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl continues to elongate.
During this time, the radicle is also growing and producing the primary root. As it grows downward to form the tap root, lateral roots branch off to all sides, producing the typical dicot tap root system. Monocot seeds : As this monocot grass seed germinates, the primary root, or radicle, emerges first, followed by the primary shoot, or coleoptile, and the adventitious roots.
In monocot seeds, the testa and tegmen of the seed coat are fused. As the seed germinates, the primary root emerges, protected by the root-tip covering: the coleorhiza. Next, the primary shoot emerges, protected by the coleoptile: the covering of the shoot tip. Upon exposure to light, elongation of the coleoptile ceases and the leaves expand and unfold.
At the other end of the embryonic axis, the primary root soon dies, while other, adventitious roots emerge from the base of the stem. This produces the fibrous root system of the monocot. Depending on seed size, the time it takes a seedling to emerge may vary. Recalcitrant seeds usually do not show this transition period between maturation and germination.
P ollination is the transfer of pollen from the anther to the stigma after it germinates. The pollen grain has two cells, the tube nuclei and the generative cell.
The generative cell divides mitotically and produces two sperm nuclei. When the female part of the flower is matures, the stigma secretes a sugary solution. This promotes the germination of the pollen grain, if viable, compatible pollen comes in contact with this moist stigma. Sexual reproduction in plants is centered in flowers.
Co-evolution of flowering plants and their pollinators started about million years ago. In this part of the cycle reduction division of the chromosomes occurs to produce the haploid n chromosome number i.
The germinated pollen grain on the stigma produces a tube pollen tube that carries two sperm nuclei n down the style into the ovary until it reaches the embryo sac.
In the embryo sac, fertilization takes place and hence one sperm fertilizes the egg to form a zygote 2n. The other sperm fuses with the large central cell of the embryo sac to produce a triploid 3n cell, a process called triple fusion.
The next step after fertilization is the development of the ovule containing the zygote and the 3n central cell into a seed. The nucleus is enveloped by one gymnosperms or two angiosperms covering layers diploid maternal tissue , called the integuments. An ovule is therefore, in a developmental sense, an unfertilized, immature seed precursor 9 and, in a morphological and evolutionary sense, a mega sporangium surrounded by integuments.
These integuments develop into the test a seed coat , of which in mature seeds the outer cell layers of the outer integument usually forms a dead covering layer, while inner cell layers may remain alive. The mega gametophytes of gymnosperms and angiosperms differ considerably. In most angiosperm species, the mega gametophyte, in its mature state also called the embryo sac, is seven-celled and eight-nucleate, referred to as the Polygonum-type. In a typical mature gymnosperm seed, the embryo has two covering layers: the haploid maternal mega gametophyte with stored nutrients and the diploid integument tissue that develops into the test a.
In contrast to the gymnosperms, the angiosperm ovules and seeds are covered; they are enclosed inside the ovary. A mature ovary contains one or more mature seeds and is called a fruit; a pericarp fruit coat develops from the ovary wall and can contain additional flower parts.
Both seeds and fruits can be the dispersal units of angiosperms. Since the central cell of most angiosperm species has either one or two nuclei, the resulting fertilized endosperm is either diploid or triploid. During the s and s, there was a significant effort by seed technologists to clarify the maturation process and to define the primary changes occurring during seed development.
The following changes occur during seed development:. That value decreases during maturation although it remains relatively high throughout most of the maturation period because water is the vehicle for transferring nutrients from the parent plant to the developing seeds. This decrease in moisture content proceeds until hygroscopic equilibrium is attained. From that point on, moisture content changes are associated with variations in relative humidity. However, seeds produced in fleshy fruits have a lower decrease in moisture content than seeds produced in dry fruits.
Seed size: The fertilized ovule is a small structure with respect to final seed size. Plant species with large seeds have an advantage under low light conditions, when their greater protein and lipid reserves, or their more advanced development, can facilitate growth. Seed dry weight: After sexual fusion, the developing seeds begin to increase in weight as a result of nutrient accumulation and water uptake.
Seed fill is initially slow because cell division and elongation are occurring during this stage. Soon after, dry mass accumulation increases until seeds reach their maximum dry weight. Germination: Seeds of various cultivated species are able to germinate a few days after ovule fertilization.
In this case, germination refers to protrusion of the primary root, not the formation of a normal seedling because histo-differentiation has not been completed and reserve accumulation is still incipient at this phase. Therefore, this germination does not lead to the production of vigorous seedlings. Vigor: Seed vigor changes are usually parallel to nutrient reserve transfer from the parent plant. This means that the proportion of vigorous seeds increases during maturation, reaching a maximum near to or at the same time as seed maximum dry weight.
The identification of the time of physiological maturity has been a controversial subject among different authors studying seed maturation. Among the differing physiological maturity concepts, three are dominant:. According to the prevailing concept, seed development ceases when physiological maturity is achieved, but this idea remains controversial because this expression is frequently used with different meanings. For example, the first concept of physiological maturity was proposed by Shaw and Loomis in as the stage in which the seed possesses maximum dry weight and yield.
Consequently, physiological maturity and harvest time are distinct events. The determination of physiological maturity usually requires differing measurements of seed dry weight during the seed filling period which is often affected by sampling variation. This makes it difficult to determine the precise time of physiological maturity, i. Seed dry weight is usually determined at different intervals during maturation. The most frequent method is drying seeds in an oven immediately after harvest followed by weighing the dried seeds for a moisture content determination.
Probably the most accurate method of determining physiological maturity is the measurement of 14C assimilate uptake by the developing seed. This was demonstrated by TeKrony et al. Changes in seed color or other visual changes in seed or fruit structure are also excellent morphological indicators of physiological maturity. For example, physiological maturity was closely associated with yellow color in Dovyalis caffra fruit. Harrington proposed that these three parameters occurred at the same time and could be considered together as markers of physiological maturity.
As a result, the existence of these differing studies about physiological maturity create confusion in the literature that can be attributed primarily to the wide variation in experimental designs and varying concepts of physiological maturity.
Finally, the definition of physiological maturity based on seed maximum dry weight should be considered a reference point to characterize the end of seed development and the physiological independence of the seed from the parent plant. Physiological maturity identifies the moment seeds possess or are close to their maximum physiological potential.
As a result, the decision to identify this time as the optimum time for harvesting a seed production area should be logical. In the latter collection of fruit of Dovyalis caffra may not be convenient, and also fungal attack may interfere with the germination process, particularly under laboratory conditions.
Components of the environment factors that influence seed performance include soil fertility, water, temperature, light, and seed position on the plant. Soil fertility: In general, plants that have been fertilized with the three major elements N, P, and K produce larger seeds than those which have not been fertilized. The increase in seed size is due to a enhanced seed development rate during the seed filling period as a consequence of increased nutrient availability.
According to Copeland and McDonald 17 when the effects of individual fertilization elements on seed development are considered, nitrogen has the greatest influence on seed size, seed germination, and vigor. Water: Water deficits reduce plant metabolism and seed development. Research has reported decreases in leaf area, photosynthetic rate, and other effects that promote flower abortion and negatively influence assimilate production and translocation to developing seeds; one of the most important effects is the decrease in carbohydrate supply caused by a reduction in photosynthesis rate.
Prolonged droughts and reduced soil water availability cause decreases in seed size, particularly when these effects occur during seed filling. If water deficits occur during flowering, its primary effect is on a reduction in seed number. Temperature: High temperatures during seed development produce smaller seeds, while low temperatures retard seed growth. Seed germination and vigor are also adversely affected by exposure to low temperatures during development. This phenomenon is also caused by water deficits or desiccant application at inappropriate times during maturation.
The occurrence of greenish seeds is undesirable because this abnormality results in decreases in seed germination and vigor. Light: The seasonal distribution of solar radiation is a fundamental factor in assuring adequate plant development.
In general, reduced light to the parent plant results in smaller seeds. The position in the inflorescence can affect seed development rate. For example, distal seeds in a wheat spike have slower growth rates and shorter seed filling periods than proximal seeds. Corn seeds at the tip of the ear are smaller than those at the base which has been attributed to inadequate photosynthetic supply.
Soybean pods located in lower plant branches are produced before those located in the upper nodes and are affected by different environmental conditions during development and this causes differences in seed performance.
Smaller seeds are also produced from smaller fruits or those that mature later in the growing season or are exposed to unfavorable environmental conditions. The usual consequence is decreased seed germination and vigor. Sexual propagation involves the union of pollen from a male flower part with the egg of a female ovary to produce a seed. Seed can be recalcitrant, inter mediator orthodox seed. The longevity of orthodox seeds is increased over a wide range of environmental conditions by decreasing storage temperature and seed water content without causing significant damage to seed metabolism.
Orthodox seeds are, therefore, considered desiccation tolerant. Recalcitrant seeds, however, decrease in viability when their water content is reduced below a relatively high value.
They are, therefore, considered desiccation intolerant. The seed is made up of three parts: the outer seed coat, which protects the seed; the endosperm , which is a food reserve; and the embryo, which is the young plant itself. Figure 4 Seed germination is a critically important juncture in the plant life cycle and the decision made by an imbibing seed to initiate germination can be considered to be a critical regulatory step in plant development.
Following initial water uptake, this phase of development is characterized by relatively little change in seed water content until it is terminated by the initiation of embryo growth.
After the seeds have germinated, they require favorable environment for their establishment. All these factors become available if the seeds have chance to germinate in their original ecology or if the original ecology is not seriously disturbed.
Germination potential of seeds of many plants can be influenced by various environmental and the seed internal factors.
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