Effect of seed priming techniques on germination and seedling vigor in rice (Oryza sativa L.)

Introduction

What determines successful crop establishment in rice paddies across diverse agroclimatic zones? The answer lies substantially in the quality and vigor of seeds used for sowing. Rice (Oryza sativa L.) remains the staple food for more than half of the global population, and India stands as one of the largest producers contributing approximately 23% of world rice production [1] . Seed quality parameters including germination percentage, vigor index, and seedling growth characteristics directly influence crop stand and subsequent yield potential [2] . Seed priming represents a controlled hydration technique that initiates the pre-germinative metabolic processes without allowing radicle emergence [3] . This physiological intervention triggers various biochemical changes including enzyme activation, membrane reorganization, and metabolic preparation that collectively enhance seed performance upon sowing [4] . The priming process allows seeds to reach an advanced physiological state, and when subsequently dried back to original moisture content, seeds can be stored or immediately used for planting [5] . Several priming techniques have been developed and evaluated over the past decades. Hydropriming, the simplest method, involves soaking seeds in water for a defined duration followed by surface drying [6] . Osmopriming utilizes osmotic solutions such as polyethylene glycol (PEG) or mannitol to control water uptake during the priming process [7] . The osmotic potential of the solution regulates the rate and extent of seed hydration, allowing better synchronization of germination events. Halopriming employs salt solutions like potassium chloride or sodium chloride, which besides controlling water uptake may also provide ionic benefits [8] .

Research conducted globally has demonstrated the positive effects of seed priming on germination and seedling characteristics. A comprehensive investigation by Harris and colleagues [9] showed that primed rice seeds germinated 2-3 days earlier than non-primed seeds. Similarly, work conducted at various agricultural research stations has confirmed improvements in germination percentage ranging from 8-15% in primed seeds compared to untreated controls [10] . The vigor enhancement translates to better field emergence, particularly under suboptimal conditions of temperature or moisture stress [11] . The biochemical basis of priming-induced vigor improvement involves several mechanisms. Activation of hydrolytic enzymes including amylases and proteases mobilizes seed reserves more efficiently [12] . Membrane repair mechanisms restore cellular integrity during the controlled hydration phase [13] . Additionally, antioxidant enzyme systems become more active in primed seeds, providing better protection against oxidative stress during germination [14] . These physiological changes collectively contribute to faster and more uniform germination. Despite extensive research on seed priming globally, locationspecific optimization of priming protocols remains necessary given the varietal differences in response to priming treatments [15] . The present research was therefore undertaken to evaluate the comparative effectiveness of hydropriming, osmopriming, and halopriming techniques on germination behavior and seedling vigor characteristics of rice variety Pusa Basmati 1121, a popular aromatic rice widely cultivated in northern India [16] .

Research Area Description

The experimental research was conducted at the Seed Testing Laboratory situated within the agricultural research complex. The laboratory facility is equipped with controlled environment chambers capable of maintaining temperature at 25±1°C with relative humidity of 85±5%, conditions optimal for rice seed germination as per international testing protocols. The laboratory maintains accreditation for seed quality testing and follows standard operating procedures aligned with ISTA guidelines. Seed samples of rice variety Pusa Basmati 1121 were obtained from the nucleus seed production plot harvested during the preceding kharif season. The variety is characterized by extralong slender grains, exceptional aroma, and cooking quality. Seeds were stored under controlled conditions (15°C, 45% RH) prior to the initiation of priming treatments to maintain initial seed quality. The initial seed lot showed a germination percentage of 96% and moisture content of 12.4%. All priming solutions were prepared using analytical grade chemicals and double-distilled water. The laboratory maintains a well-equipped seed processing unit with facilities for seed cleaning, grading, and treatment application. Temperature during priming was maintained at 25°C using thermostatically controlled water baths.

Material and Methods

Material: Foundation seed of rice (Oryza sativa L.) variety Pusa Basmati 1121 harvested during kharif 2018 was used as experimental material [1] . Seeds were cleaned to remove broken, shriveled, and discolored seeds. Initial seed quality parameters were assessed before treatment application. Polyethylene glycol 6000 (PEG-6000, molecular weight 6000) was procured from Merck India Ltd. and used for preparing osmotic solutions [7] . Potassium chloride (KCl) of analytical grade was obtained from Qualigens Fine Chemicals. The following priming treatments were evaluated: T1 -Control (un-primed), T2 -Hydropriming (distilled water, 12 h), T3 -Osmopriming (PEG-6000, 10%, 24 h), T4 -Osmopriming (PEG-6000, 15%, 24 h), T5 -Halopriming (KCl, 2%, 18 h) [8] . All chemicals used met the specifications required for seed physiological research [12] .

Methods

Seeds were surface sterilized with 0.1% mercuric chloride solution for 2 minutes followed by three washes with sterile distilled water [3] . For hydropriming, seeds were soaked in distilled water at 25°C for 12 hours with a seed to water ratio of 1:5 (w/v). Osmopriming treatments involved soaking seeds in PEG-6000 solutions of -0.8 MPa (10%) and -1.2 MPa (15%) osmotic potential for 24 hours at 25°C [4] . Halopriming was performed by soaking seeds in 2% KCl solution for 18 hours. After priming, seeds were removed, washed briefly with distilled water, surface-dried on blotting paper for 30 minutes, and then shade-dried at room temperature for 24 hours to restore original moisture content [5] . Germination test was conducted using the between-paper method as per ISTA Rules [6] . Four replicates of 100 seeds each from every treatment were placed on moist germination paper in germination trays. Trays were incubated in a seed germinator maintained at 25±1°C with 85±5% relative humidity. First count was taken on day 5 and final count on day 14 after sowing. A seed was considered germinated when both plumule and radicle had emerged to at least 2 mm length.

Experimental Design

The experiment was laid out in a Completely Randomized Design (CRD) with five treatments and four replications, making a total of 20 experimental units. Each replication consisted of 100 seeds, thus 400 seeds per treatment were used for germination evaluation. The randomization of treatments was done using random number tables to eliminate any positional bias within the germinator. The experimental period extended from October 2019 to December 2019. Germination parameters were recorded as per standard protocols: germination percentage calculated as the number of normal seedlings divided by total seeds tested multiplied by 100; mean germination time computed using the formula MGT = Σ(n×d)/Σn where n is the number of seeds germinated on day d; germination index calculated as GI = Σ(Gt/Tt) where Gt is germination count on day t and Tt is time in days; seedling vigor index-I computed as germination percentage multiplied by seedling length (root + shoot); seedling vigor index-II calculated as germination percentage multiplied by seedling dry weight. Seedling growth measurements were taken from 10 randomly selected normal seedlings from each replication on the 14th day. Root length was measured from the collar region to the tip of the primary root. Shoot length was measured from the collar region to the tip of the primary leaf. Seedling dry weight was determined after drying at 80°C for 24 hours to constant weight.

Results

Comprehensive Interpretation

The data analysis reveals that osmopriming with 10% PEG-6000 emerged as the most effective treatment across all parameters evaluated. The germination enhancement can be attributed to controlled hydration that allows optimal metabolic preparation without triggering radicle emergence. The 24-hour osmopriming duration appeared sufficient for achieving the desired physiological conditioning in rice seeds.

Comparing the two osmotic concentrations tested, 10% PEG-6000 outperformed 15% PEG-6000, suggesting that the lower osmotic potential (-0.8 MPa) provided more favorable conditions for priming-induced changes than the higher osmotic stress (-1.2 MPa). This observation aligns with the principle that excessive osmotic stress during priming may have inhibitory effects on seed metabolism.

Discussion: The significant improvement in germination percentage and seedling vigor observed in primed seeds can be explained through several physiological mechanisms. Seed priming initiates phase II of water uptake where metabolic processes become activated while the seed remains in a state of controlled hydration [3] . During this phase, repair of damaged DNA, synthesis of proteins required for germination, and activation of pre-formed enzymes occur [12] . These processes prepare the seed for rapid and uniform germination upon subsequent rehydration during sowing.

The superiority of osmopriming over hydropriming observed in the present research can be attributed to better control of water uptake rate in osmotic solutions. When seeds are soaked in water (hydropriming), rapid uncontrolled imbibition may cause imbibitional damage to cellular membranes [4] . Osmotic priming solutions regulate the rate of water absorption, allowing gradual rehydration and reducing membrane damage [7] . The PEG molecules being large cannot penetrate the seed coat, thus exerting osmotic control from outside while allowing seeds to hydrate slowly. Our findings regarding the effectiveness of 10% PEG-6000 are consistent with earlier reports in rice and wheat [9] . The optimal osmotic potential for priming varies with crop species and seed lot quality. For rice, osmotic potentials ranging from -0.8 to -1.0 MPa have been reported as most effective [10] . Higher osmotic stress may limit water availability to the extent that metabolic activation becomes suboptimal. The reduction in mean germination time by approximately 1.8 days in osmoprimed seeds has practical implications for crop establishment. Faster germination leads to earlier seedling emergence, which can be advantageous in terms of competition with weeds and escape from soil-borne pathogens during the vulnerable germination phase [11] . The enhanced vigor index indicates that primed seeds not only germinate better but also produce more robust seedlings capable of establishing well under field conditions.

Halopriming with potassium chloride also showed significant improvement over control, though not matching osmopriming performance. The beneficial effects of halopriming involve both osmotic regulation and ionic effects. Potassium ions play important roles in enzyme activation and cellular osmotic adjustment [8] . However, at higher concentrations, salt priming may induce ionic toxicity, necessitating optimization of concentration for each crop-variety combination.

The biochemical basis of enhanced vigor in primed seeds involves multiple pathways. Activation of antioxidant enzymes including superoxide dismutase, catalase, and peroxidase during priming provides protection against reactive oxygen species that accumulate during seed deterioration and germination stress [13] . Enhanced alpha-amylase activity in primed seeds ensures efficient starch hydrolysis and energy supply for germinating embryo [14] . These enzymatic improvements translate to faster and more uniform emergence.

Conclusions

Acknowledgement

Acknowledgements Funding SourcesThis research was conducted as part of the institutional research program. The authors acknowledge the financial support provided for consumables and laboratory facilities.

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