Effects of Priming Techniques on Seed Germination and Early Growth Characteristics of Bromus tomentellus L. and Bromus inermis L.

Seed priming is known as a seed treatment which improves seed performance under environmental conditions. Objective of this study was to evaluate the effects of different priming treatments on seed germination behaviour of two genotypes of Bromus under laboratory conditions to find out the most effective priming treatment. Seeds were treated with the following seed-soaking media: (i) unsoaked seed (control); (ii) hydropriming with distilled water for 4, 8, 12 and 16 hours, (iii) osmopriming treatments with PEG 6000 for Bromus tomentellus were -0.6, -0.8, -1.0 and -1.2 MPa osmotic potentials with duration of 12, 24 and 36 hours and for Bromus inermis osmoticpotentials were the same as B. tomentellus but duration of priming treatments was soaking seeds for 12, 24, 36 and 48 hours. Seeds of both genotypes were placed in liquid priming media at 25ºC. Priming treatments significantly affected germination vigour of both genotypes. The response of both genotypes to different priming techniques approximately was similar. Osmopriming treatment (-0.6 MPa and 12 h) increased final germination percentage of B. tomentellus. Priming treatments increased coleoptile length significantly comparing to control but hydropriming for 4 h exhibited longer coleoptile than others. Higher vigour index was observed in hydropriming for 12 h but this difference was not significant with osmoprime treatment (12 h-0.6 Mpa). B. inermis produced more germinated seeds and vigourous seedlings than B. tomentellus but germination rate was higher in B. tomentellus .


Introduction
Seed priming is known as seed treatment wich improves seed performance under environmental conditions.In fact seed priming is a procedure that partially hydrates the seed, then seeds are dried, so that germination processes begin, but radicle emergence does not occur.Methods of seed priming have been described comprehensively by Bradford (1986) and Khan (1992) which include soaking seed in water or osmotic solution, and intermixture with porous matrix material.
Seed priming is commonly used to reduce the time between seed sowing and seedling emergence (Parera and Cantliffe, 1994).Earlier works showed that the success of seed priming is influenced by the complex interaction of factors including plant species, water potentiality of the priming agent, duration of priming, temperature, seed vigour and dehydration, and storage conditions of the primed seed (Parera and Cantliffe, 1994).Although, the previous studies indicate that some benefits are associated with pre-sowing treatments for seed vigour enhancement, but there is dearth of information about the germination performance of primed seeds of Bromus spp.Therefore, the present study was carried out with the objective of evaluating the effects of different priming treatments on seed germination behaviour of two genotypes of Bromus under laboratory conditions to find out the most effective priming treatment.

Materials and methods
The study was conducted in the seed laboratory of Natural Resources Faculty, University of Tehran, Iran.Seeds of two Bromus genotypes including B. tomentellus and at 5% level of probability.The vigour index was calculated according to the following formula: Vigour index (VI) = [seedling length (cm) × germination percentage] Mean germination time (MGT) was calculated based on the Ellis and Roberts equation (1981).

Results
Priming treatments significantly affected germination vigour of both genotypes.The response of both genotypes to different priming techniques approximately was similar.Speed of germination was recorded for hydro and osmoprimed seeds as indicated by lower value of MGT or by higher germination rate (Tab. 1 and 2).
Osmopriming treatment (-0.6 Mpa and 12 h) increased final germination percentage of B. tomentellus by 22% while hydropriming (4 h) increased germination percentage about 12% compared to control.So, B. tomentollus germination percentage exhibited posetive response to osmopriming than hydroprimig (Tab.1).But for germination rate, osmopriming treatment could not exhibit significant effect while hydropriming treatment (4 and 8 h) had significant effect and hydroprimed seed for 4 h germiated faster than non-primed treatment.All priming treatments increased coleoptile length significantly compared to control but hydropriming for 4 h exhibited longer coleoptile than the others.
Radicle length of osmoprimed seeds increased due to priming treatment of (-0.6 Mpa, 12 h) significantly while hydropriming showed no significant result.All priming treatments increased seedling length of B. tomentellus B. inermis subjected to seed priming.Seeds were treated with the following seed-soaking media: (i) unsoaked seed (control); (ii) hydropriming with distilled water for 4, 8, 12 and 16 hour; (iii) osmopriming treatments with PEG 6000 for B. tomentellus were -0.6, -0.8, -1.0 and -1.2 MPa osmotic potentials with duration of 12, 24 and 36 hours while for B. inermis osmotic potentials were the same as B. tomentellus but duration of priming treatments was soaking seeds for 12, 24, 36 and 48 hours.Seeds of both genotypes were placed in liquid priming media at 25ºC.Seeds were covered with plastic bags to refuse moisture loss.
After soaking, seeds were washed with distilled water, then redried in the incubator at 25ºC in the dark.Germinaton test was conducted by placing 25 seeds from each of the treatments in 90 mm diameter Petri dishes on Whatman filter paper that was moistened with 5 ml distilled water.Seeds were kept in germinator at 25ºC in dark condition.A completely randomized design with three replications was used.Radicle protrusion of 2 mm was scored as germination (Kaya et al., 2006).Germination was counted in 24 hours intervals and continued until no further germination occurred.The seedlings were evaluated as described in Seedling Evaluation Handbook (AOSA, 1991).
Final germination percentage (%), coleoptile and radicle length (cm) and seedling length (cm) was recorded after 14 days of planting on filter paper.For statistical analysis, the data of germinating percentage was transformed to arcsin√(100/X).Experimental data was analyzed by a statistical packet SAS, version 6.12.Treatments means were compared using Duncan's multiple comparsion test but it was more clear in hydropiriming for 4 h and osmopriming (-0.6 Mpa, 12 h).Finally, osmoprimed seeds of B. tomentellus exhibited higher vigour index than hydro primed seeds.Germination percentage of B. inermis was increased due to seed priming, and hydropriming seeds for 8 h exhibited higher germination percentage than the others (Tab.2).Germination rate was also affected by seed priming and osmopriming (12 h and -0.6 Mpa) exhibited higher speed of germination compared to control and other priming treatments.No significant increse was obtained for radicle length of B. inermis due to seed priming.All osmopriming treatments for 12 h increased coleoptile length significantly but there was no significant difference between all concentrations of PEG in this duration for seed priming.Higher seedling length was observed in osmopriming treatment (12 h, -0.6 Mpa) compared to other treatments including control.Higher vigour index was observed in hydropriming for 12 h but this difference was not significant with osmoprime treatment (12 h, -0.6 Mpa) (Tab.2).B. inermis produced more germinated seeds and vigourous seedlings than B. tomentellus but germination rate was higher in B. tomentellus (Fig. 1).

Discussion and conclusions
Seed germination and seedling growth are critical for seedling at the first life stages and often subject to high mortality rates.The three early phases of germination are: (i) imbibition, (ii) lag phase, and (iii) protrusion of the radical through the testa (Simon, 1984).Priming is a procedure that partially hydrates seed, followed by drying of seed, therefore germination processes begin, but radicle emergence does not occur.There are reports that hydration of seed up to, but not exceeding, the lag phase with priming permits early DNA replication (Bray et al., 1989), increased RNA and protein synthesis (Fu et al., 1988;Ibrahim et al., 1983), greater ATP availability (Mazor et al., 1984), faster embryo growth (Dahal et al., 1990), repair of deteriorated seed parts (Karssen et al., 1989;Saha et al., 1990).These help radicle protrusion through the seed coat and shorten the time to seed germination.
In this research, seed priming increased germination characteristics of under study Bromus geynotyps.This might be due to faster water uptake by primed seed comparing to the control treatment.Similarly, Khan (1993) reported that osmo-conditioning with seed hydration treatments, seed hardening and moisturizing on vermiculite improved the performance of sweet corn seed.The pre- soaking of seeds allows the hydration of membranes and proteins, and the initiation of various metabolic systems.These are arrested when the seeds are dried or moisture is withheld, but recommence when the seeds imbibe water for the second time (Bewley and Black, 1982).Ashraf and Rauf (2001) reported that final germination percentage, fresh and dry weight of corn seed increased by seed priming significantly.So due to all this information and present study it is clear that most effects of seed priming is due to seed hydration.So, optimization of priming technique is very important to achive the best time and concentration combination.Higher and faster germination will increase uniformity and final yield.According to B. tomentellus and B. inermis values in view point of livestocks forage in Iranian rangelands, also considering their natural habitats soil condition (relatively high moisture and low salinity), findings of this research help us to prepare Bromus seeds through priming tech-niques when their sowing, as high quality forage, is necessary in arid and semi arid areas of Iran.Additionally, it is suggested to work on how could seed priming of Bromus seeds affects tolerance to environmental stress.

Fig. 1 .
Fig. 1.Germination characteristics of two Bromus species under different priming treatments: a) Germination percentage; b) Germination rate; c) Vigour index (seedling length × germination percentage) Tab. 1.Effect of priming treatments on the germination and seedling characteristics of Bromus tomentellus * Figures not sharing the same letters in the same column differ significantly at p< 0.05; 1: Mean germination time Tab. 2. Effect of priming treatments on the germination and seedling characteristics of Bromus inermis *Figures not sharing the same letters in the same column differ significantly at p< 0.05