FACTORS AFFECTING SEED GERMINATION

INTRODUCTION:

     SEED AND SEED GERMINATION:-

 Seed is the fertilized mature ovule having viable embryo and seed coat and living endosperm for nourishment of embryo. Germination is the process by which a plant grows from a seed. The most common example of germination is the emergence of a seedling  from a seed of a angiosperm or gymnosperm. In addition, the growth of a sporeling from a spore, such as the spores of hyphae from   spores, is also germination. Thus, in a general sense, germination can be thought of as anything expanding into greater being from a small existence or germ

 SEED GERMINATION FACTORS

           Several factors influence if, and how, seeds germinate. The most important factors are 

• water availability, 
• Temperature 
• Sunlight.

 Water is crucial to seed germination. The seed must go through imbibition to activate root growth. However, too much water can be a bad thing, as most gardeners know. When a plant is still growing underground, during root formation, it cannot use the sun to make food, like most grown plants do. It must rely on the stored food inside the seed, and oxygen from the environment to make energy. If the soil is too soggy, there will not be enough oxygen and the plant will not thrive. Think about a person being kept underwater. We wouldn't last too long!

Temperature is also an important factor. Some seeds germinate when it is cold, such as plants in northern environments. Other seeds only germinate when the weather reaches spring temperatures, which is why we see so much plant growth in the spring in temperate climates. Other seeds only germinate after extreme temperatures, such as after a fire in the grasslands.The humidity is also important factor that effect germination of seed.

 ASCORBIC ACID:-

      Ascorbic acid is a naturally occurring organic compound with antioxidant properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbate is a major metabolite in plants. It is an antioxidant and, in association with other components of the antioxidant system, protects plants against oxidative damage resulting from aerobic metabolism, photosynthesis and a range of pollutants. Recent approaches, using mutants and transgenic plants, are providing evidence for a key role for the ascorbate–glutathione cycle in protecting plants against oxidative stress. Ascorbate is also a cofactor for some hydroxylase enzymes (e.g. prolyl hydroxylase) and violaxanthin de-epoxidase. The latter enzyme links ascorbate to the photoprotective xanthophyll cycle. A role in regulating photosynthetic electron transport has been proposed. The biosynthetic pathway of ascorbate in plants has not been identified and evidence for the proposed pathways is reviewed. Ascorbate occurs in the cell wall where it is a first line of defence against ozone. Cell wall ascorbate and cell wall-localized ascorbate oxidase (AO) have been implicated in control of growth. High AO activity is associated with rapidly expanding cells and a model which links wall ascorbate and ascorbate oxidase to cell wall extensibility is presented. Ascorbate has also been implicated in regulation of cell division by influencing progression from G1 to S phase of the cell cycle. There is a need to increase our understanding of this enigmatic molecule since it could be involved in a wide range of important functions from antioxidant defence and photosynthesis to growth regulation.

              

 SALINITY:-

Salinity is one of the environmental factors that has a critical influence on the germination of halophyte seeds and plant establishment. Salinity affects imbibition, germination and root elongation. However, the way in which NaCl exerts its influence on these vital processes, whether it is through an osmotic effect or a specific ion toxicity, is still not resolved. Dimorphic seeds of the halophytesAtriplex prostrataandA. patulawere treated with various iso-osmotic solutions of NaCl and polyethylene glycol (PEG). For each treatment, imbibition, germination rate, percent germination, germination recovery and nuclear area of root tip cells were compared. Higher concentrations of NaCl (-1.0 MPa) were more inhibitory to imbibition, germination and seedling root elongation than iso-osmotic PEG solutions. All seeds recovered from a pre-treatment with -2.0 MPa NaCl and PEG solutions, except large seeds ofA. prostratawhich failed to germinate following transfer from -2.0 MPa NaCl. NaCl caused a greater increase in nuclear volume than iso-osmotic PEG solutions. 

                                

salinity reduces substrate water potential, thereby restricting water and nutrient uptake by plants; salinity may also cause ionic imbalance and toxicity. Because substrate salinity fluctuates through the growing season, a plant may be exposed to different salinity levels, at various stages of development, with potentially significant consequences on population dynamics. Here, we present the results of a study of the effect of substrate salinity on seed germination, seedling emergence, and growth of Aster laurentianus, an annual marsh plant, endemic to the Gulf of St. Lawrence and potentially threatened. Seed germination was reduced in low salt concentration (10 g sea salt/L) and completely inhibited by salinity levels >/=20 g sea salt/L. However, this inhibiting effect was reversible: seeds from the salt treatments germinated readily after being washed in distilled water. Though seedling emergence was diminished at low salinity levels, postemergence survival was little affected. Plant growth was reduced, but net carbon assimilation rate was not affected by high salinity levels. Increased root respiration and respiratory costs associated with salt tolerance might have contributed to lower C accumulation at higher salinity levels. All developmental processes considered are thus negatively affected by substrate salinity, with potentially significant consequences on population abundance and distribution in salt marshes. Yet, the tolerance of this species to high salinity levels after seedling emergence is remarkable. Seed germination represents a major bottleneck in the species life cycle, potentially controlling local distribution and abundance in the natural habitat.

   SALT TOLERANCE:-

                             Plants exposed to salt stress undergo changes in their environment. The ability of plants to tolerate salt is determined by multiple biochemical pathways that facilitate retention and/or acquisition of water, protect chloroplast functions, and maintain ion homeostasis. Essential pathways include those that lead to synthesis of osmotically active metabolites, specific proteins, and certain free radical scavenging enzymes that control ion and water flux and support scavenging of oxygen radicals or chaperones. The ability of plants to detoxify radicals under conditions of salt stress is probably the most critical requirement. Many salt-tolerant species accumulate methylated metabolites, which play crucial dual roles as osmoprotectants and as radical scavengers. Their synthesis is correlated with stress-induced enhancement of photorespiration. In this paper, plant responses to salinity stress are reviewed with emphasis on physiological, biochemical, and molecular mechanisms of salt tolerance. This review may help in interdisciplinary studies to assess the ecological significance of salt stress.

SEED PRIMING

                 Many different vegetables are primed before they are planted. Priming is a water-based process that is performed on seeds to increase uniformity of germination and emergence from the soil, and thus enhance vegetable stand establishment. Priming decreases the time span between the emergence of the first and the last seedlings. Priming also increases the rate of emergence so the stand establishes itself faster. A uniform plant stand helps to ensure maximum cartons per acre at harvest. Wide ranges in seedling emergence decrease the amount of harvestable plants per acre, an undesirable situation. Lettuce is especially vulnerable to this particular field problem because what is harvested is what initially emerges, multiplied by the effects of photosynthesis. To attain the same size, seedlings must spend an equal amount of time in the sun. Seedlings that emerge 2 or 3 days later than the main crop never catch up in size because of the competitive effects of their bigger neighbors.

       Priming is an important mechanism of various induced resistance phenomena in plants against biotic stresses , whereas an analogy exists for vaccinated animals for an adaptive immunity to a disease and the ultimate prevention or amelioration of the pathogens infection effects. Proposed priming mechanisms include the accumulation of signaling proteins or transcription factors in an inactive form or the occurrence of epigenetic changes that are modulated upon exposure to stress and developed rapidly resulting in a more efficient defense mechanism . Over the past few years, it has become apparent that priming phenomena are also involved in the context of environmental stress . Several studies have examined priming events against environmental stimuli in various plant systems. For example, this was evidenced in the case of NaCl pre-treatment on Glycine max seedlings in order to induce acclimation to subsequent salt stress , acclimation of Deschampsia antarctica to cold stress or application of low levels of Cd to Triticum aestivum for consequent Cd toxicity acclimation . Similar findings were shown for polyethylene glycol pre-treatment ofElaeagnus oxycarpa seedlings in order to induce acclimation to salinity, drought preconditioning of Lolium perenne for cold acclimation , and application of low levels of Zn for subsequent Cd toxicity acclimation in wheat . More interestingly, a primed state could also be induced in plants following an initial exposure to a priming agent, such as natural or synthetic compounds including nitric oxide , hydrogen peroxide (H2O2; hydrogen sulfide (H2S; , β-aminobutyric acid (BABA, and polyamines ; . This chemical-based priming against abiotic stresses somewhat resembles the systemic acquired resistance phenomenon in the case of biotic stresses . Additionally, in analogy to the priming treatments on the vegetative parts of plants, seed water-based priming with controlled imbibition for seed invigoration and advance in germination, in which long lasting effects occur after germination as well, has been widely characterized . This experimental evidence indicates that priming against environmental stress represents a fruitful area for future research in terms of both basic and applied agriculture science in order to promote the advent of, the more environmentally friendly, sustainable agriculture.