International Journal of Agriculture and Crop Sciences. Available online at www.ijagcs.com IJACS/2013/6-20/1373-1381 ISSN 2227-670X ©2013 IJACS Journal
Seed Priming A Technique Javid Nawaz1, Muhammad Hussain*1, Abdul Jabbar1, Ghulam Abbas Nadeem1, Muhammad Sajid2,MashoodUl Subtain1 and Imran Shabbir1 1. Department of Agronomy, University of Agriculture, Faislabad-38040 Pakistan. 2. Institute of Soil and Environmental Sciences, University of Agriculture, Faislabad-38040 Pakistan. Corresponding author email:
[email protected] ABSTRACT: Insufficient seedling emergence and inappropriate stand establishment are the main constraints in areas receiving less rainfall. Management of soil texture and soil structure is a big challenge in semi-arid regions. Poor farmers do not have sufficient resources to meet the requirements of fine seedbed preparation for sowing and they are at more risk as compared to progressive farmers. On the other hand, good establishment increases competitiveness against weeds, increases tolerance to drought periods, increases yields and avoids the time-consuming need for re-sowing that is costly too. It is well accepted fact that priming improves germination, reduces seedling emergence time and improves stand establishment. It is a simple, low-cost, easily performable; on-farm seed priming can, if refined and developed by ensuring the farmer participation, make a good impression on farmers livelihoods enhancing the crop emergence rate, thus increasing rates of crop development, decreasing the total crop duration and getting higher productivity. A lot of work has been done on seed priming and results of these studies indicate well the importance of priming to get a good crop stand and final emergence. On-farm seed priming (Hydro-priming) can significantly be helpful in order to obtain good crop establishment in many crops of tropical region such as sorghum, rice, maize and pigeon pea. Inducing resistance against stresses like drought stress, heat stress etc. is one of the prominent advantages of seed priming in many important field crops. Key words: Seed priming, physiological and biochemical aspects, germination, stand establishment, types INTRODUCTION Priming allows some of the metabolic processes necessary for germination to occur without germination take place. In priming, seeds are soaked in different solutions with high osmotic potential. This prevents the seeds from absorbing in enough water for radicle protrusion, thus suspending the seeds in the lag phase (Taylor et al., 1998). Seed priming has been commonly used to reduce the time between seed sowing and seedling emergence and to synchronize emergence (Parera and Cantliffe, 1994). In seed priming, the osmotic pressure and the period for which the seeds are maintained in contact with the membrane are sufficient to allow pre-germinative metabolic processes to take place within the seeds up to a level limited to that immediately preceding radicle emergence. Methods for germinating seed and inducing desiccation tolerance in seed are also provided. Preferably the semipermeable membrane is provided in the form of a tube of circular or polygonal cross-section which is rotated with the seeds on its inner surface and the solution retained between its outer surface and a further body to which the membrane is sealed in a watertight manner. Seed priming have important role in increasing the yield of different crops in relation to enhance 37, 40, 70, 22, 31, 56, 50 and 20.6% in wheat, barley, upland rice, maize, sorghum, pearl millet, and chick pea respectively (Harris et al., 2005). Seed priming technique has been practiced in many countries including Pakistan, China and Australia and more than thousand trials had been conducted to evaluate the performance of priming in a variety of crops. Fifty three farmers tested maize seeds priming in the kharif season in 1996 in tribal areas of Rajasthan, Gujarat and Madhya Pradesh; India (Harris et al., 1999). Almost all farmers thought that primed crops grew more vigorously, flowered and matured earlier and produced bigger cobs and higher yield. Independent measurements on a subset of 35 trials showed a mean increase in cob weight of 6% (Harris et al., 2001). Farmers in the project area of
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priming reported that primed crops grew more vigorously, tolerated dry spells better, flowered earlier (typically 7-10 days) and matured earlier (8-10 days) (Harris et al., 1999). Across different sites in farmer managed trials, priming led to a significant increase in maize grain yields by -1 -1 105 kg ha and 182 kg ha higher than those from un-primed maize. An increase of 14% and 18% was recorded during 1999-2000 and 2000-01, respectively in economic yield (Clark et al., 2001). The plants grown from primed maize seed were consistently larger and also flowered and matured earlier than non-primed seed (Murunguet al., -1 -1 -1 2004a). In six farmer implemented trials, total biomass (10.81 t ha ), straw yield (7.49 t ha ), cob yield (3.32 t ha ), -1 grain yield (2.74 t ha ) of maize were significantly increased by priming with water compared to not primed treatment (Harris et al., 2007). Forty farmers primed sorghum seed in Zimbabwe during the 1997 and 1998 season and most of the farmers agreed that priming accelerated emergence and plants flowered and matured earlier relative to non-primed crops (Harris et al., 2001). Priming technique is the need of present time to get the enhanced germination and establishment in maize in order to utilize the soil moisture and solar radiation to a maximum extent. In this way plants would be able to complete their growth before the stresses arrive (Subedi and Ma, 2005). Osmopriming is commercially used technique for improving seed germination and vigour. It involves controlled imbibition of seeds to start the initial events of germination followed by seed drying up to its original weight. Osmopriming has many advantages including rapid and uniform emergence, improved seedling growth and better stand establishment under any environmental and soil conditions (Chiu et al., 2002). Polyethylene glycol and KNO3 solutions increased the fresh and dry weight of roots in maize at 2% and 5% concentration primed for 12 h and 18 h. In addition they also increased the vigour index (Abdnadani and Ramezani, 2012). Grain yield was significantly increased in many crops subjected to priming as compared to non-primed crops. The increase in yield was 13% in case of hydro-priming alone and 26% when primed with ZnSO4 solution. So, it can be concluded that both the treatments have contributed well to the total increase in yield. Final emergence, emergence index, plant height, leaf area, stover dry weight, total dry weight, individual cob weight, cob yield, cob number and number of grains per cob were observed to indicate almost same kind of response to priming treatments in increasing the final yield (Harris et al., 2007). Research on priming has proved that crop seeds primed with water germinated early, root and shoot development started rapidly, grew more vigorously and seedling length was also significantly greater than nonprimed seeds. It could also improve the performance of crop by alleviating the effect of salts under saline soil conditions (Mohammadiet al., 2008). Soaking seed in water overnight before sowing can increase the rate of germination and emergence even in soil conditions where moisture content is very low (Clark et al., 2001). Inhibition of fungal contaminants were checked by using the leaf and bark water extracts of Jatrophacurcas and Moringaoleifera. Physiological and biochemical aspects of priming Priming also has been shown to induce nuclear DNA synthesis in the radical tip cells in tomato (Liu et al., 1997) and several other plant species, including maize (Zea mays L.) (Garcia et al., 1995). Osmopriming has been shown to activate processes related to cell cycle. In wild rye (Leymuschinensis) seeds, for example, priming with 30% PEG for 24 h resulted in increase in the activity of superoxide dismutase (SOD) and peroxidase (POD) and a rapid increase in the respiratory intensity, which were associated with an increase in germination vigor (Jieet al., 2002). Osmopriming may also contribute to rapid seed germination by reducing the mechanical restraint of endosperm on developing embryo (Mayer and Mayber, 1989). It was determined that osmotic priming of tomato seed increased the endo-beta mannanase activity in the endosperm cap and decreased its mechanical restraint on the germinating embryo (Tooropet al., 1998). A strong correlation was observed between lowering of the mechanical restraint and the activity of endo-beta-mannanase. Primed seeds significantly showed the increased emergence percentage, rate of emergence, root length and seed vigor in all amaranth cultivars. Trigin cultivar showed the best performance among cultivars. Total seed protein, POD and PPO were also increased significantly by seed priming. Almont and Plainsman cultivars exhibited high protein content and POD activity. PPO activity increased by seed priming comparing to controls for Amont, Plainsman and Mercado cultivars, but for Trigin cultivar, no increase was detected. The highest increase in PPO activity was observed in Mercado cultivar (Moosaviet al., 2009). Seed germination Germination is an important stage of seedling establishment and therefore it plays a key role in crop production. Crop establishment however depends on the interaction between seedbed environment and seed quality (Perry, 1984) where salinity has been identified as one of the major seedbed factors influencing 1374
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establishment (Barbour, 1970). Priming is one of the most important physiological method which improves the seed performance and provides faster and synchronized germination. The primed seeds give earlier, more uniform and sometime greater germination and seedling establishment and growth (Bradford, 1986). Enzyme activation Enzymes such as amylases, proteases, and in some cases, lipases, play vital roles in the early growth and development of embryo. Any increase in the activity of these enzymes may result in early vigorous growth and good crop establishment. It has been demonstrated that osmopriming affects the activity of these enzymes in the germinating seed of different plant species. For example, in muskmelon (Cucumismelo), seed osmoconditioned with PEG-6000 showed enhanced activity of dehydrogenase and amylase and improved germination under nonsaline conditions (Singh et al., 1999). In oilseed crops, the glyoxylate pathway, which converts lipids into sugars, plays an important role in the early development of embryo (Taiz and Zeiger, 2002). Osmoconditioning also enhanced the activity of ATPase in the germinating seed of peanut primed with PEG. Furthermore, acid phosphatase and RNA syntheses were significantly higher in embryonic axes and cotyledons of osmo-conditioned seed compared to control seed. Thus, osmopriming may contribute to improved germination rate in part by increasing various enzyme activities. Osmopriming has been shown to activate processes related to cell cycle. In wild rye (Leymuschinensis) seed, priming with 30 % PEG for 24 h resulted in increase in the activity of superoxide dismutase (SOD) and peroxidase (POD) and a rapid increase in the respiratory intensity, which were associated with an increase in germination vigor (Jieet al., 2002). Seed priming techniques Seed priming have various techniques for improving the performance of the growth, emergence, and yield of the crop. There are some techniques which are used i. e. hydro-priming, halopriming, osmopriming and hormonal priming Hydro-priming Hydro-priming involves soaking the seeds in water before sowing (Pill and Necker, 2001) and may or may not be followed by air-drying of the seeds. In many agricultural areas, a major cause of poor stand establishment and low crop yield is unfavorable environmental conditions for seed germination and seedling emergence. However, rapidly germinating seedlings could emerge and produce deep roots before the upper layers of the soil are dried out and crusted, which may result in good crop establishment and higher crop yield. Rafiq et al. (2006) reported that seed priming reduces the effect of salinity on the morphological parameter of the plants. Any factor that facilitates rapid germination may contribute to establishment of a successful crop. A low cost approach, designated as on farm seed priming was proposed by Harris, (1992) and involve soaking of seed in water before sowing. This pre sowing seed treatment, known as hydro-priming, allows the seed to imbibe water and go through the first phase of germination in which pre-germination metabolic activities are started while the latter two phases of germination are inhibited (Pill and Necker, 2001). Although soaking seed in water and drying before sowing is the easiest way to achieve hydration, a major disadvantage is that it may result in uneven hydration and non-uniform germination. Soaking is not suitable for some plant species, as rapid hydration may cause leakage of essential nutrients out of the seed, resulting in seed damage. To overcome these potential problems, various methods have been devised to deliver appropriate hydration to the seed. One method is seed humidification a pre sowing treatment in which seed are treated under conditions of high humidity (Suzuki and Khan, 2001). For example, in partially-aged mustard (Brassica juncea) seeds, humidification resulted in a significant improvement in germination and seedling vigor and a decrease in leakage of electrolytes from germinating seed (Srinivasan et al., 1999). A second method of pre-sowing seed hydration is aerated hydration (AH), in which seed are hydrated in a column of aerated water to moisture content close to that required for radicle protrusion (Thornton and Powell, 1992). Seed are held inside the column at this moisture content, and subsequently are removed and dried before radicle protrusion occurs. Thornton and Powell, (1992) determined that for seeds of cauliflower (Brassica oleracea), for 8 h o AH treatment at 25 C was the most effective for improving the rate and uniformity of germination, root growth, seed vigor and additional improvements in seed vigor were obtained by AH for up to 32 h at 20°C. The overall research results suggest that activation of metabolic repair phenomena is a major contributor to improvements achieved by AH. Hydro-priming involves soaking seeds in water before sowing (Pill and Necker, 2001) and may or may not be followed by air-drying of the seeds. Hydro-priming may enhance seed germination and seedling emergence under both saline and non-saline conditions. Roy and Srivastava, (1999) found that soaking wheat kernels in water improved their germination rate under saline conditions. Improvement in salt tolerance of maize (Zea mays L.) (Ashraf and Rauf, 2001), pigeon pea (Cajanuscajan) (Jyotsna and Srivastava, 1998), and Acacia seeds (Rehmanet 1375
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al., 1998) was also observed following hydro-priming. The precise mechanisms by which application of this simple technique can achieve sometimes quite dramatic improvements in plant growth and seed yield in saline or nonsaline conditions remain unclear. Some researchers have considered hydro-priming a key technology that is simple and cost effective, the impact of which is very high in terms of enhanced yield (Ashraf and Foolad, 2005). Organic and inorganic substances have major role in the plant growth and development of storages bodies of plants. Presoaking of seeds in water may alter the mobilization of both inorganic and organic substances from the storage organs to the developing embryo in some species. In sugar beet and pigeon pea, it was determined that the effect of hydro-priming on improving seed germination was closely related to the solubilization of P-subunit of 11-S globulin storage protein and were very effective in the mobilization of compounds such as proteins, free amino acids, and soluble sugars from storage organs to growing embryonic tissues under salt stress (Capron et al., 2000: Jyotsna and Srivastava, 1998). In contrast, lipid composition of Korean black soybean (Glycine max) seed, including percentages of neutral fats, glycolipids, and phospholipids, was determined to remain unchanged after soaking in water (Oh et al., 1992). Also, while soaking did not affect major components of neutral fats and glycolipids, a slight change was observed in some components of phospholipids. However, in view of these contrasting reports, it is not possible to draw any strict parallels between mobilization of different inorganic or organic substances due to hydro-priming and improved germination. It is likely that the extent of mobilization of these substances depends on plant species and time period for which seed are subjected to water redrying and sowing. Hydro-priming of seed has growth-promoting effects on plants at the initial and later developmental stages. Promoting effects at later stages can be due to alteration in various metabolic phenomena responsible for enhanced yield. In field experiments, hydro-priming of safflower (Carthamustinctorius) seed for 12 h resulted in 2 higher number of plants m , capitula per plant, grains per capitulum, 1000 seed weight, grain yield, and oil content compared to untreated seed (Bastia et al., 1999). Similar improvements were observed in maize, rice, chickpea (Harris et al., 1999), and pearl millet (Kumar et al., 2002) grown under dry-land conditions. Hydro-priming had pronounced effect on field emergence, its rate and early seedling growth of maize crop and it improved the field stand and plant growth, both at vegetative stage and at maturity of maize (Nagar et al., 1998). Harris et al.(1999) in India and Chivasaet al.(2000) in Zimbabwe noted significantly faster emergence, taller and heavier seedlings and more leaves per plant (14 DAS) from maize seeds primed for longer than 8 h. The number of root axes per plant was not increased relative to the non-primed treatment, until seeds had been primed for at least 14 h to 20 h. Plant height and shoot dry weight of maize were increased by priming (without drying) (AlSoqueer, 2004). The treatment effects of priming on field emergence were more pronounced at early stage (7 DAS) (10.6 to 28.9%) than at later stage of crop growth (14 and 21 DAS). The hydro-primed maize seed produced consistently and significantly longer shoots after 5 days than the untreated control (Matthews and Hosseini, 2007). Hydro-priming plays an important role in the seed germination and radical and plumule emergence in different crop species. Similar to other priming techniques, hydro-priming generally enhances seed germination and seedling emergence under both saline and non-saline conditions, although there are exceptions. Roy and Srivastava, (1999) reported that soaking wheat kernels in water improved their germination rate under all conditions; no such improvement was obtained in similar research conducted by Ashraf and Iram, (2002). Yet hydro-priming improved salt tolerance of maize (Ashraf and Rauf, 2001) and pigeon pea seeds (Jyotsna and Srivastava, 1998). However, more research is needed to determine the value of hydro-priming in different plant species. Hydro-priming plays an important role in the enzymatic activities of the wheat, maize, rice, and other vegetable seeds. In seed of some plant species, trypsin-like proteolytic enzymes, which are produced during seed development, are important during germination. The activity of such enzymes, however, is often prevented by trypsin inhibitors, which may be present in the seed and play regulatory roles in protein mobilization during germination (Bewley and Black, 1994). Priming, however, may reduce the inhibitory activities of such enzymes and promote germination. For example, in sorghum, soaking seed in distilled water or salt solution reduced inhibitory activities of trypsin and chymotrypsin, although the effect of the latter treatment was greater (Mulimani and Vadiraj, 1994). Similar results were obtained when seed of red gram (Cajanuscajan) were pre-soaked in distilled water or salt solution (Mulimani and Paramjyothi, 1995). Amylases are key enzymes that play a vital role in hydrolyzing the seed's starch reserve, thereby supplying sugars to the developing embryo. Effects of hydro-priming on water potential, the driving force for water uptake during imbibition, and the activity of α-amylase were examined in wheat and rice kernels (Andoh and Kobata, 2002). At the time of sowing, while water and osmotic potentials of hydroprimed wheat seed were -7.2 and -12.3 MPa, respectively, they were -4.8 and -9.9 MPa in the non-primed seed. In rice seed, hydro-priming did not change either water or osmotic potential. In primed seeds of both wheat and barley (Hordeurnvulgare), however, the activity of α-amylase at 12 h after sowing was, respectively, 2.7 and 2.8 times greater than that in non-primed seeds. The primed seeds also exhibited a faster rate of germination and seedling emergence. The improvements in seed germination and seedling emergence were due to enhanced supply of 1376
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soluble carbohydrates to the growing embryo, which was caused by an increase in α-amylase activity. Re-drying of seed following hydro-priming did maintain activity of other enzymes at the levels required for occurance of germination. In a different study, it was determined that hydro-priming mitigated the adverse effect of salinity on amylase activity in wheat kernel (Roy and Srivastava, 1999). Thus, hydro-priming may have significant beneficial effects on enzyme activity required for rapid seed germination. Halo priming Halo priming refers to soaking of seeds in solution of inorganic salts i.e. NaCl, KNO3 CaCl2, CaSO4, etc. A number of studies have shown a significant improvement in seed germination, seedling emergence and establishment, and final crop yield in salt affected soils in response to halopriming. Khan et al. (2009) evaluated the -1 response of seeds primed with NaCl solution (1 mM) at different salinity levels 0, 3, 6 and 9 dSm in relation to early growth stage and concluded that seed priming with NaCl has been found to be better treatment as compared to non-primed seeds in case of hot pepper for improving the seedling vigour and stand establishment under saltstressed conditions. Priming with NaCl and KCl was helpful in removing the deleterious effects of salts (Iqbal et al., 2006). Rice seed treated with a mixed salt solution germinated more speedily than unprimed seed under salt-stress conditions (Chang-Zhenget al., 2002). Seed germination is promoted by halopriming but also stimulate subsequent growth, thereby enhancing final crop yield (Eleiwa, 1989; Sallam, 1999). Sedghiet al. (2010) results indicated that with increasing salinity, germination traits such as germination percent, rate and plumule length decreased, but seed priming with GA3 and NaCl showed lower decrease. In all of the salinity levels, primed seeds possessed more germination rate and plumule length than control. The highest -1 radicle fresh and dry weight in pot marigold was seen at 7.5 dSm salinity stress level. It seems that higher germination rate in pot marigold shows higher tolerance to salinity than sweet fennel. The results of the experiment under undesirable conditions such as salinity stress, priming with GA3 and NaCl can prepare a suitable metabolic reaction in seeds and can improve seed germination performance and seedling establishment (Khan et al., 2009). Bajehbaj, (2010) evaluated the effects of NaCl priming with KNO3 on the germination traits and seedling growth of four Helianthus annuus L. cultivars under salinity conditions and reported that germination percentage of primed seeds was greater than that of un-primed seeds. Seedling establishment is an important for the better growth and development of the plant either treated with priming agent or not. Halopriming have very important role in germination, seedling emergence, and plant growth at all developmental stage of the plants. It was determined that rice seed treated with mixed salt solution germinated significantly more rapidly than unprimed seed under salt-stress conditions (Chang- Zhenget al., 2002). Pretreatment of seed of a salt-tolerant and a salt sensitive cultivar of spring wheat (Triticumaestivum L.) with either distilled water for 12 h or varying concentrations of KC1, KNO3, CaC12.2H2O or Ca(NO3)2.4H2O did not improve germination rate in saline medium (Ashraf and Iram, 2002). and cotton (Gossypiumhirsutum) seed primed with 1060 mmol/L CaC12 exhibited decreased germination and seedling emergence under NaCl treatment, with the degree of inhibition increasing with increased concentration of the treatment chemical (Xiao-Fang et al., 2000). The overall evidence indicates that while halopriming can improve seed germination and seedling emergence in some plant species, it does not do so in all species. Further investigation is needed to determine the effects of halopriming on seed germination and seedling emergence in a larger array of plant species. Relatively few and small chemical changes are observed in dry seed; however, many changes occur in the seed immediately after rehydration (Bewley and Black, 1994). Of these, changes in activity of enzymes involved in hydrolysis of storage products play important roles in the initial growth and development of the embryo. Priming seed with inorganic salts may significantly alter activity of enzymes in germinating seed. For example, seed of muskmelon soaked with KNO3 solution showed enhanced activity of dehydrogenase and α-amylase under low temperature (Singh et al., 1999). In untreated wheat seed germinating under salt stress, amylase activity decreased with increasing salinity; in wheat seed pretreated with CaC12, the negative effect of salinity was diminished (Roy and Srivastava, 1999). Similarly, in Pennisetumamericanum and Sorghum bicolor seeds soaked in CaC12 or KNO3 solution, activity of total amylase, α -amylase and proteases was increased in germinating seeds under salt stress (Kadiri and Hussaini, 1999). And in rice, seed primed with mixed-salt solution resulted in significant increase in activity of α-amylase, p-amylase, and root dehydrogenase, and moderate increase in the activity of shoot catalase under salt stress (Chang-Zhenget al., 2002). Such increases in enzyme activity have direct or indirect effects on subsequent seed germination and seedling growth and development. Salinity interacts not only with organic substances but also with inorganic nutrients in the plant. Mobilization of seed food reserves to the developing embryo during germination is a ubiquitous process. Storage products such as carbohydrates, amino acids, fatty acids, and inorganic nutrients are mobilized in germinating seed at varying rates in different species (Bewley and Black, 1994). However, such mobilization may be perturbed when germinating seed are subjected to unfavorable 1377
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environmental conditions such as soil salinity (Ashraf et al., 2003). The extent of this perturbation is determined by the level of activity of enzymes involved in hydrolysis of storage chemicals. However, as indicated in the previous section, priming seed with inorganic salts would increase activity of most of the enzymes involved in seed germination, and thus contribute to mobilization of organic substances to different parts of the embryo. In pigeon pea (Cajanuscajan), seed treated with KNO3 or CaC12 generally exhibited improvement in proteins, free amino acids, and soluble sugars during germinating under salt stress (Jyotsna and Srivastava, 1998). Osmopriming This is also known as osmo-conditioning or osmotic conditioning. In this technique, seeds are soaked for a certain period in solutions of sugar, polyethylene glycol (PEG), glycerol, sorbitol, or mannitol followed by air drying before sowing. Osmopriming not only improves seed germination but also enhances general crop performance under nonsaline or saline conditions, osmo-conditioning of Italian ryegrass (Loliummultiflorum) and sorghum o (Sorghum bicolor) seeds with 20% PEG-8000 for 2 days at 10 C enhanced germination rate, water-stressed, waterlogged, cold-stress, or saline conditions (Hur, 1991). The low water potential of the treatment solution allows partial seed hydration so that pre-germination metabolic processes begin but germination is inhibited (Bennett et al., 1992; McDonald, 2000; Pill and Necker, 2001). When the primed seed are planted in the field, they usually exhibit rapid and uniform germination. Osmopriming not only improves seed germination but also enhances general crop performance under non saline or saline conditions. Salehzadeet al. (2009) conducted to enhance the germination and seeding growth of wheat (TriticumaestivumL.) cvZarin seeds using different Osmopriming treatments. Seeds were osmoprimed with polyethylene Glycol (PEG-8000), solution for 12 h. The osmotic potential of the all solutions were -0.3,-0.6, -0.9 MPa. During Osmopriming operation all solutions aerated with aquarium pump. The control seeds were not treated. Osmopriming treatments improved germination and seedling vigor than that control. Priming with boric acid using lower concentrations significantly improved the seedling stand establishment parameters (Rehmanet al., 2012). Shorrocks, (1997) reported that priming with boric acid showed both stimulatory and inhibitory effects on different crop plants. In pappya species the priming with boron increased the growth of all plants (Deb et al., 2010). Numerous biochemical changes have been reported in osmo-primed seeds of different plant species. In tomato, for instance, a space is developed in the primed seed that facilitates water uptake, thereby accelerating the speed of germination (Argerich, 1989). Also, during priming, the embryo expands considerably and compresses the endosperm, deforming the tissues that have lost flexibility due to dehydration (Liptay and Zariffa, 1993). It has been proposed that priming causes considerable invigoration of the dry seed (Heydecker and Coolbear, 1978). Priming also has been shown to induce nuclear DNA synthesis in the radical tip cells in tomato (Liu et al., 1997) and several other plant species, including corn (Zea mays L.) (Garcia et al., 1995), and leek (Ashraf and Bray, 1993) .Osmopriming has been shown to activate processes related to cell cycle. In wild rye (LeymuschinensisL.) seed, for example, priming with 30% PEG for 24 h resulted in increases in the activity of superoxide dismutase (SOD) and peroxidase (POD) and a rapid increase in the respiratory intensity, which were associated with an increase in germination vigor (Jieet al., 2002). Osmopriming may also contribute to rapid seed germination by reducing the mechanical restraint of endosperm on developing embryo (Mayer and Poljakoff-Mayber, 1989). It was determined that osmotic priming of tomato seed increased the endo-beta-mannanase activity in the endosperm cap and decreased its mechanical restraint on the germinating embryo (Tooropet al., 1998). A strong correlation was observed between lowering of the mechanical restraint and the activity of endo-beta-mannanase. Enzymes such as amylases, proteases, and in some cases, lipases, play vital roles in the early growth and development of embryo. Any increase in the activity of these enzymes may result in early vigorous growth and good crop establishment. It has been demonstrated that osmo-priming affects the activity of these enzymes in the germinating seed of different plant species. In muskmelon (CucumismeloL.) seed osmo-conditioned with PEG-6000 showed enhanced activity of dehydrogenase and amylase and improved germination under non-saline conditions (Singh et al., 1999). In oilseed crops, the glyoxylate pathway, which converts lipids into sugars, plays an important role in the early development of embryo (Taiz and Zeiger, 2002). Up or down regulation of any of the enzymes involved in this pathway may affect embryo growth. Osmo-conditioning also enhanced the activity of ATPase in the germinating seed of peanut primed with PEG. Furthermore, acid phosphatase and RNA syntheses were significantly higher in embryonic axes and cotyledons of osmo-conditioned seed compared to control seed. Thus, osmopriming may contribute to improved germination rate in part by increasing various enzyme activities. Hormonal priming Hormonal priming is the pre seed treatment with different hormones i.e. salicylic acid, ascorbate, kinetin, etc. which promote the growth and development of the seedlings. The interactive effects of salinity stress (40, 80, 1378
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120 and 160 mMNaCl) by soaking wheat seeds in ascorbic acid and thiamin (0.3 mM) or sodium salicaylate (0.6 mM) (Hamada and al-Hakimi, 2001). The contents of cellulose, lignin of either shoots or roots, pectin of roots and soluble sugars were lowered with increase of NaCl concentration. On the other hand, contents of hemi-cellulose and soluble sugars of roots, starch and soluble proteins of either shoots or roots and amino acids of roots were increased. Seed soaking in ascorbic acid, thiamin or sodium salicyclate could counteract the adverse effects of NaCl salinity on the seedlings of wheat by suppression of salt stress included accumulation of praline (Al-Hakimi and Hamada, 2001). Afzal et al., (2006) reported that wheat cultivar Auqab-2000 was treated with different priming agents i.e. Abscisic acid (ABA), Salicylic acid (SA) and ascorbic acid and were sown under normal and saline -1 condition (15 dSm ), and showed that under saline conditions these treatment reduces the time for 50% germination, increase final germination count, and significantly increased the fresh and dry weight but ascorbic acid does not showed such results. Hormonal priming has reduced the severity of the effect of salinity but the amelioration was better due to 50 ppm SA and 50 ppm ascorbic acid treatments as these showed best results on seedling growth, fresh and dry weights under non-saline and saline conditions whereas hormonal priming with ABA as not effective under present experimental material and conditions (Afzal et al., 2006). Ashraf et al. (2002) found that GA3 treatment enhanced the vegetative growth of two wheat cultivars under + but caused a slight reduction in their grain yield. GA3 treatment enhanced the deposition of Na and Cl in both root and shoots of wheat plants under prevailing field conditions. It also caused a significant increase in photosynthetic activity in both lines at the vegetative stage of the crop. Hussein et al. (2007) evaluated the effect of salinity and salicylic acid on growth of maize plants. The beneficial aspects of SA are that it could be used for the improvement of salt bearing capacity of many crops. CONCLUSION Based on previous reviewed literature it is concluded that spring planted maize has to face extremely low temperature at early developmental stages and extremely high temperature at later reproductive stages. Therefore, it is the need of time to study the potential of such techniques that can ensure the successful emergence and early development of spring maize at low temperature. Seed priming is a popular and commercially used technique developed mainly to accelerate the process of germination so the effects of low and high temperatures can be minimized. It is the best solution of germination related problems especially when crops are grown under unfavorable conditions. Many priming techniques have been evolved which are being utilized in many crops now-adays. Among these hydro-priming, halopriming and osmopriming are most common and popular techniques. REFERENCES Abandani RR, Ramezani M. 2012.Thephysiological effects on some traits of osmopriming germination of maize (Zea mays L.), rice (Oryza sativa L.) and cucumber (Cucumissativus L). Int. J. Agron. 4132-148. Afzal I, Basra SMA, Farooq M, Nawaz A. 2006. Alleviation of Salinity Stress in Spring Al-Hakimi AMA, Hamada AM. 2001. Counteraction of salinity stress on wheat plants by grain soaking in ascorbic acid, thiamin or sodium salicylate. Biologia. Plantarum. 44253–261. Al-Soqueer AA .2004. 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