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Effect of seed priming on emergence and yield of soybean
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Muhammad Arif1, Mohammad Tariq Jan1, Philip Hollington2,3 and David Harris2
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Running head: Seed priming in soybean
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Address for correspondence: Dr PA Hollington, CAZS Natural Resources, Thoday Building,
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College of Natural Sciences, Bangor University, Deiniol Road, Bangor LL57 2UW, Wales,
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UK
Department of Agronomy, NWFP Agricultural University, Peshawar, Pakistan. CAZS Natural Resources, College of Natural Sciences, Bangor University, UK Corresponding author
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Telephone:
+44 (0)1248 382285
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Fax:
+44(0)1248 364747
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Email:
[email protected]
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Additional keywords: osmopriming, hydropriming, establishment
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Abstract
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The aim of this work was to determine, using water and solutions of polyethylene glycol
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(PEG), the optimum conditions for seed priming soybean (Glycine max) cv. William-82 and
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so improve its establishment and production in Pakistan. After a laboratory experiment to
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determine the optimum combination of priming duration and PEG 8000 concentration, we
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conducted a field experiment in 2003 and 2004 with three priming durations (6, 12 and 18 h)
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and five molarities of PEG 8000 solution (0, 0.013, 0.025, 0.038 and 0.050 moles l-1, 1
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equivalent to OP of 0, -0.2, -0.5, -1.1, -1.8 , -3.0 and -4.2 MPa), together with a dry seed (non
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primed) control. Seeds were primed, air dried and sown in the field on 3rd May, 2003 and 5th
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May, 2004. On average over the two years, plants from primed seed emerged, flowered and
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matured faster than plants from non-primed seed. Primed seed established better, and gave
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taller and higher yielding plants. Taking the results as a whole, priming for 6 hours, or with a
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solution of OP -1.1 MPa, were the most beneficial treatments. However, priming with PEG is
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not practical for farmers in the field, and, although priming with water was not so effective it
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was still better than not priming.
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Introduction
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Many developing countries, including Pakistan, face serious deficiencies in protein and
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vegetable oil, and must spend a considerable amount of foreign exchange on the import of
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vegetable oil in particular to meet local demand (MINFAL 2001-02). Soybean (Glycine max
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(L.) Merrill) has the potential to become the leading oil seed crop in the country. Increased
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local production would decrease Pakistan’s dependence upon costly imports, as well as
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having other benefits such as fulfilling the increasing demand for this oil by diet-conscious
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consumers (Hatam and Abbasi 1994), and for soybean meal for the developing poultry
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industry and livestock sectors.
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However, soybean yield is low in Pakistan, with poor germination and low seed viability
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among the problems (Khan et al. 2004). The use of high quality seed is essential, with rapid
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and uniform germination, and which can withstand adverse environmental conditions after
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sowing. However, most farmers use poor quality seed, leading to inadequate plant
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populations, and crop production is widely limited by poor stand establishment and nutrient
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deficiencies (Arif et al. 2005). Particularly in drought prone environments, germination tends 2
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to be irregular and can extend over long periods (Harris 2006). The resulting poor stand leaves
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gaps in the canopy, which are rapidly filled by vigorously growing weeds at the onset of the
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short rainy season. Accelerating and homogenizing germination is a prerequisite for good crop
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establishment, efficient resource use, and high yields (Harris 1996).
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Seed priming aims to synchronize and improve germination by submitting the seeds to a
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period of imbibition, and a wide variety of treatments (reviewed in Ashraf and Foolad 2006)
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have been used for this, including soaking in water (Harris et al. 1999) or osmotic solution
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(Knypl and Khan 1981). The degree of growth and germination enhancement depends upon
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the temperature, water potential, duration and other conditions (Heydecker and Coolbear
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1977), and is apparent in many species (Bradford 1986), although some long duration, low
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water potential or high temperature priming treatments can have a negative effect on
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subsequent germination response (Hardegree and Emmerich 1992). Enhancement is
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particularly valuable under conditions of abiotic stress such as drought (Harris 2006) or
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salinity (Rashid et al. 2006).
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The technique of hydropriming consists of soaking seeds in water, which may be aerated
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(Thornton et al. 1993), and re-drying them back to storage weight before they complete
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germination. Harris et al. (2001a) advocated a similar technique, “on-farm seed priming,” as a
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low-cost approach for farmers in dryland areas – the seeds are imbibed and pre-germination
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metabolic activities take place, although later stages of germination are inhibited (Pill and
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Necker 2001). On-farm seed priming has been successful in a range of crops and in many
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environments (Harris 2006). Direct benefits include faster emergence, better and more
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uniform stands with less need to re-sow, more vigorous plants, better drought tolerance,
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earlier flowering, earlier harvest and higher grain yield. A number of indirect benefits have
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also been noted in some situations, included earlier sowing of following crops, earlier 3
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harvesting of those crops and increased willingness to use fertilizers because of reduced risk
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of crop failure (Harris et al. 2001b)
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The technique known as osmopriming or osmoconditioning (Heydecker et al. 1975) has given
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promising results for many legumes (Bradford 1986), and a few studies on soybean are
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encouraging, although more information is required before its use as a routine practice (Knypl
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and Khan 1981). It involves soaking seeds in inert osmotic agents such as Polyethylene
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Glycol (PEG) or sugar solutions, usually in an aeration system, followed by drying back to
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storage weight. The low water potential of the solution partially hydrates the seed, which
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allows pre-germination metabolism to start, but inhibits germination (Pill and Necker 2001).
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The result is usually rapid and uniform germination (Ashraf and Foolad 2006).
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Several physiological and biochemical changes take place in seeds during priming or as a
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consequence of osmotic conditioning. These include increase in germinating power and
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vigour, and overcoming of dormancy (Mayer and Poljakoff-Mayber 1989; Jie et al. 2002),
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and have been noted in soybean (GongPing et al. 2000) and maize (Finch-Savage et al. 2004)
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using PEG-6000. However, almost all of the work on osmopriming has been conducted under
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laboratory conditions, and few detailed studies have been reported on the performance of
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osmotically treated seed in the field. Helsel et al. (1986), working in cool field environments
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in North America, found osmopriming with PEG promoted rapid and uniform emergence with
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early planting, but with later planting in warmer soil there was less benefit. Park et al. (1999)
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reported that priming old soybean seed resulted in good germination and stand establishment
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in field trials.
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The objective of this work was to assess the effects of hydropriming and osmopriming on the
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yield and yield components of field-grown soybean, compared with non-primed controls, as a
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preliminary to the development of protocols for the use of the technique in Pakistan. 4
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Materials and methods
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An initial laboratory experiment was conducted to determine optimum polyethylene glycol
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8000 (PEG) concentration and seed priming duration for maximum germination. Osmotic
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potentials (OP) were determined according to Michel (1983). Seeds of soybean cv. Williams-
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82 were primed for 6, 12 and 18 h using PEG 8000 solutions with concentrations of 0, 100,
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200, 300, 400, 500 and 600 g PEG l-1 water. These were equivalent to molarities of 0, 0.013,
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0.025, 0.038, 0.050, 0.0625 and 0.075 moles l-1 respectively (OP 0, -0.2, -0.5, -1.1, -1.8, -3.0
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and -4.2 MPa) at 25°C, oxygenated with an aquarium pump in order to prevent damage to the
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seed. The control treatment was dry seeds (non primed). After priming, seeds were rinsed with
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tap water for two minutes to facilitate handling. Twenty seeds of each treatment were placed
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in 14 cm diameter Petri dishes, covered with 2 cm of river silt (particle size 0.002 – 0.02 mm)
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as the growing medium, replicated six times. The dishes were placed in an incubator at a
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constant temperature of 25°C to simulate temperatures in the area in April and May, the usual
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sowing time for soybean. Germination counts were made daily for 7 days, after which no
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further germination occurred.
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Field experiments were conducted at the Agricultural Research Farm of NWFP Agricultural
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University, Peshawar during 2003 and 2004. The experimental site was located at 34°0’ N,
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71° 35’ E and an altitude of 450 meters above sea level. The soil was a silty clay loam with a
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clay type montmorillonite, low in nitrogen (0.03-0.04%) and organic matter (0.7-0.9%), and
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alkaline in reaction (pH 8.0-8.2).
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Seeds of the same variety were primed in the laboratory for 6, 12 and 18 h using water, or
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100, 200, 300 and 400 g PEG l-1 water (OP 0, -0.2, -0.5, -1.1 and 1.8 MPa respectively). They
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were aerated as before, air dried for 30 m under a fan to moisture content of 10 – 12% to 5
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facilitate sowing, and sown in the field on 3rd May 2003 and 5th May 2004. The control
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treatment was dry seed (non-primed). The experiment was a randomized complete block
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(RCB) design with four replications, plus an additional unprimed control plot in each
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replicate: there were a total of 4 x ((4 x 3) + 1) = 52 plots each year. Plot size was 2 x 3 m,
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with row to row distance of 50 cm and plant to plant distance 10 cm. One hundred and twenty
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seeds were sown in each plot, and were not thinned. Plots were hoed twice by hand to control
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weeds, and were watered when needed. A basic fertilizer dose of 30 kg nitrogen (N) and 90 kg
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phosphorus (P) per hectare was applied as urea and triple superphosphate, broadcast before
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sowing. The experiments were harvested in the second week of September in each year, when
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the majority of the leaves had dropped, the lowest pods had turned yellow, and seed moisture
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content was estimated to be 12 – 13%. Data for emergence, height, yield and yield
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components were recorded.
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Days to 50% emergence were calculated from the date of sowing by counting seedling
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emergence in each plot daily. Emergence was recorded in a one meter row length of three
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randomly selected rows in each plot, and converted to emergence per square metre. Plant
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height was recorded at maturity on five plants randomly selected from each treatment, as was
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number of pods per plant. A random sample of one thousand grains was taken from each plot
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to record thousand grain weight. The number of grains per pod was recorded in five randomly
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selected pods in each plot. Each plot was harvested using a sickle, dried in the field for one
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week to an estimated seed moisture content of 10%, and weighed to record total biological
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yield. After threshing the harvested material, the grains were cleaned and weighed.
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Statistical Analysis
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Data were analysed using Genstat v 8.1. The laboratory trial was analysed as a general
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analysis of variance taking days after sowing, duration of soaking, and PEG concentration as 6
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factors, with an additional level of duration to take account of the unprimed control. The
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germination percentages were transformed using the angular transformation in order to
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account for non-normality of the data. The analysis for the field experiments was also a
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general analysis of variance, taking years, duration of soaking and PEG concentration as
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factors, and again with an additional level in duration of soaking to take account of the
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unprimed control. Due to the complexity, the structure of the analysis of variance for the field
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experiment is shown in Table 1, that for the laboratory trial was similar. Standard errors of
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differences were used to assess the significance of differences between treatment and
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interaction means. Orthogonal contrasts were not used due to the complex design. Most of the
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data from the field experiments are presented as the means of two years, as there were few
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significant differences either between years, or interactions involving them.
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Table 1 near here
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Results
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Laboratory experiments
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Seed priming with either water or PEG significantly increased germination (Table 2)
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compared with not priming. Priming with solutions of -1.1 or -1.8 MPa was most effective,
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averaged over the germination period, and soaking for 8 hours was optimum. Soaking for 18 h
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had no effect compared with not priming, and was detrimental at high OPs. As a result, we
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selected water, and PEG concentrations of 100, 200, 300 and 400 g l-1 water (OPs of 0, -0.2, -
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0.5, -1.1 and -1.8 respectively), for the subsequent experiments.
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Table 2 near here
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Field experiments
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On average (Table 3), priming had significant effects on days to emergence, days to maturity,
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height (P
