Germination ecology of wild onion: a rainfed crop weed

Tanveer, A.; Sibtain, M.; Javaid, M.M.; Ali, H.H.

doi:10.1590/S0100-83582014000100008

Abstracts

Asphodelus tenuifolius is becoming a more common weed in rain-fed area in Pakistan. Laboratory and greenhouse experiments were conducted to determine the effect of different environmental factors on germination and emergence of A.tenuifolius. Results showed that A.tenuifolius can tolerate a wide range of varying environmental factors. Greatest percentage of germination (80%) was recorded at 15 ºC constant temperature; however, considerable germination occurred at 20 and 25 ºC. Light for 10 h photoperiod stimulate germination of Asphodelus tenuifolius compared with complete darkness. Germination was totally inhibited at osmotic stress higher than -0.8 MPa. There was no significant difference in germination at pH 6 to 8; however, there was a slightly decrease at pH 9, compared with distilled water. Asphodelus tenuifolius was very sensitive to salinity; however, a few seeds of A.tenuifolius were able to germinate even at 150 mM NaCl concentration. Greatest emergence occurred with seed placed at soil surface and emergence decreased with increase in burial depth. No emergence occurred from 4 cm or greater. This information may aid in developing tools and strategies for management.

Asphodelus tenuifolius; temperature; water stress; salt stress; pH; seedling emergence; seeding depth

Asphodelus tenuifolius está se tornando uma planta daninha cada vez mais comum em plantio de sequeiro no Paquistão. Foram realizados experimentos no laboratório e na estufa para avaliar a influência de diferentes fatores ambientais na germinação e emergência da A. tenuifolius. Os resultados mostraram que a A. tenuifolius é capaz de tolerar uma ampla gama de fatores ambientais diferentes. A porcentagem máxima de germinação (80%) foi registrada a 15ºC de temperatura constante. No entanto, houve germinação considerável em 20ºC e 25ºC. A inserção de luz em 10h de fotoperíodo estimula a germinação da Asphodelus tenuifolius, o que não ocorre com escuridão completa. A germinação foi totalmente inibida quando houve estresse osmótico maior que -0,8MPa. Não houve diferença significativa na germinação com pH de 6 a 8, porém, houve uma pequena redução com pH 9, tendo como referência a água destilada. Asphodelus tenuifolius se mostrou muito sensível à salinidade, mas, ainda assim, algumas sementes germinaram com concentração de 150mM de NaCl. Ocorreu maior emergência quando as sementes foram plantadas na superfície do solo. E, na medida em que o enterramento se tornava mais profundo, sua emergência reduzia. Não foram observadas emergências a partir de 4 centímetros de profundidade. Estas informações podem auxiliar no desenvolvimento de ferramentas e estratégias para seu manejo.

Asphodelus tenuifolius; temperatura; estresse hídrico; estresse salino; pH; emergência de plântulas; profundidade de semeadura

ARTICLES

Tanveer, A.^I; Sibtain, M.^I; Javaid, M.M.^II; Ali, H.H.^III

^IUniversity of Agriculture Faisalabad, Pakistan

^IIDepartment of Agronomy, University College of Agriculture, University of Sargodha, Pakistan, <mmjavaid@uos.edu.pk>

^IIICollege of Agriculture and Environmental Sciences, The Islamia University of Bahawalpur, Pakistan

ABSTRACT

Asphodelus tenuifolius is becoming a more common weed in rain-fed area in Pakistan. Laboratory and greenhouse experiments were conducted to determine the effect of different environmental factors on germination and emergence of A.tenuifolius. Results showed that A.tenuifolius can tolerate a wide range of varying environmental factors. Greatest percentage of germination (80%) was recorded at 15 ^oC constant temperature; however, considerable germination occurred at 20 and 25 ^oC. Light for 10 h photoperiod stimulate germination of Asphodelus tenuifolius compared with complete darkness. Germination was totally inhibited at osmotic stress higher than -0.8 MPa. There was no significant difference in germination at pH 6 to 8; however, there was a slightly decrease at pH 9, compared with distilled water. Asphodelus tenuifolius was very sensitive to salinity; however, a few seeds of A.tenuifolius were able to germinate even at 150 mM NaCl concentration. Greatest emergence occurred with seed placed at soil surface and emergence decreased with increase in burial depth. No emergence occurred from 4 cm or greater. This information may aid in developing tools and strategies for management.

Keywords:Asphodelus tenuifolius, temperature, water stress, salt stress, pH, seedling emergence, seeding depth.

RESUMO

Asphodelus tenuifolius está se tornando uma planta daninha cada vez mais comum em plantio de sequeiro no Paquistão. Foram realizados experimentos no laboratório e na estufa para avaliar a influência de diferentes fatores ambientais na germinação e emergência da A. tenuifolius. Os resultados mostraram que a A. tenuifolius é capaz de tolerar uma ampla gama de fatores ambientais diferentes. A porcentagem máxima de germinação (80%) foi registrada a 15^oC de temperatura constante. No entanto, houve germinação considerável em 20^oC e 25^oC. A inserção de luz em 10h de fotoperíodo estimula a germinação da Asphodelus tenuifolius, o que não ocorre com escuridão completa. A germinação foi totalmente inibida quando houve estresse osmótico maior que -0,8MPa. Não houve diferença significativa na germinação com pH de 6 a 8, porém, houve uma pequena redução com pH 9, tendo como referência a água destilada. Asphodelus tenuifolius se mostrou muito sensível à salinidade, mas, ainda assim, algumas sementes germinaram com concentração de 150mM de NaCl. Ocorreu maior emergência quando as sementes foram plantadas na superfície do solo. E, na medida em que o enterramento se tornava mais profundo, sua emergência reduzia. Não foram observadas emergências a partir de 4 centímetros de profundidade. Estas informações podem auxiliar no desenvolvimento de ferramentas e estratégias para seu manejo.

Palavras-chave:Asphodelus tenuifolius, temperatura, estresse hídrico, estresse salino, pH, emergência de plântulas, profundidade de semeadura.

INTRODUCTION

Wild onion (Asphodelus tenuifolius) is a very harmful and noxious weed to chickpea, wheat, mustard, linseed and lentil in Pakistan and India (Ponia et al., 2001; Tiwari et al., 2001; Nasir & Sultan, 2004), due to its high competitive ability, greater production of seeds and reproduction potential. It is native to the Mediterranean region, but it is widespread, extending from the Mediterranean region east through the Arabian Peninsula to almost 17 countries around the world (Holm et al., 1977). It is an annual herb, 15-50 cm tall with linear, acute, sheathing at the base, with fine puberulous leaves. The plant is eaten raw as well as in cooked form, particularly during famine. The seeds are sharply trigons, and black in colour.

Seed germination, an important factor in determining weed success, is regulated by many environmental factors, e.g. light, temperature, soil pH, soil salinity, weed seed size and available moisture (Koger et al., 2004; Harrison et al., 2007; Chauhan & Johnson, 2008a). Factors like acidity, water and salinity are not limiting; temperature is the major key for successful weed seed germination (Martinkova et al., 2006), and within same species it may be different for varied genotypes (Chauhan & Johnson, 2008a; Harrington, 2009; Wang et al., 2009; Malik et al., 2010; Masin et al., 2010; Gorai et al., 2011). Seeds of some weeds germinate well in the dark and with light; and broad leaf weeds having small seeds are photoblastic (Widderick et al., 2004; Zhou et al., 2005).

Water stress is always a very important attribute to inhibit seed germination (Norsworthy & Oliveira, 2006). Various species behave differently during water uptake in seed imbibition on their germination process (Chauhan & Johnson, 2008b). Ability to germinate under condition of moisture stress may enable weed to take advantage of the condition that limits the growth of other species. The impact of soil moisture on germination varies among weed species; and field condition may differ spatially and temporally, depending on rainfall, temperature, and type of soil. Soil pH is another important environmental factor affecting seed germination of various weeds. Seeds of some weeds can germinate over a broad pH range (Chachalis et al., 2008; Chejara et al., 2008), while other weeds can germinate well in acidic soil conditions (Fried et al., 2008).

Salt stress is a major constraint in crop or weed production worldwide. Physiological processes in some weeds are negatively affected by soil salinity (DiTommaso, 2004; Chauhan & Johnson, 2008b). High salt concentration inhibits the germination of many weed seeds and it ultimately leads towards death (Koger et al., 2004; Pahlevani et al., 2008; Nemati et al., 2011), with some exceptions as a few types of weed seeds show tolerance to high salt concentrations (Chauhan & Johnson, 2008c). Seed burial depth also affects its germination and seedling emergence (Koger et al., 2004; Mennan & Ngouajio, 2006). Weed seed location in the soil seed bank influences germination, emergence and vertical seed distribution in the soil (Norsworthy & Oliveira, 2006).

Biological and ecological information, specifically germination ecology of a specific weed, is necessary to optimize weed control and maximize the efficiency of management tactics (Bhowmik, 1997). In order to understand the expansion of the geographic range of A.tenuifolius in Pakistan, we need to learn how their seeds respond to varied climatic factors. To date, no specific research on the germination ecology of these weeds has been conducted. The objectives of this study are to determine the effects of temperature, light, salt and osmotic stress, pH and burial depth on A.tenuifolius seed germination and seedling emergence.

MATERIALS AND METHODS

Seed collection

Experiments were conducted at the Department of Agronomy, University of Agriculture Faisalabad, Pakistan under laboratory and greenhouse conditions during 2009. In March-April 2009 mature wild onion (A. tenuifolius) plants were collected from several farmer fields in district Bhakkar (31^o N, latitude and 71^o E, longitude), Pakistan and a composite sample was prepared. A working sample was drawn from this composite sample. Then seeds were removed and cleaned manually and dried at 25 ^oC (room temperature) for 20 days and stored in small cloth bags until used in all experiments.

Experimental methodology

Germination was determined by placing 25 seed evenly in a 9 cm diameter Petri plate containing filter paper Whatman No. 10, moistened with 5 mL distilled water or a treatment solution. Seed of A. tenuifolius were surface sterilized by soaking in 10% sodium hypochlorite (NaOCl) for 5 min, followed by five rinses with distilled water before the start of each germination trial. Petri plates were sealed with Para film to reduce the water loss. All the experiments (except for the temperature experiment) were conducted at 15 ^oC temperature with 10 h photoperiod (except for the light experiment). Fluorescent lamps were used to produce a photosynthetic photon flux density of 200 µmol m^-2 s^-1 for all the experiments, except for the dark treatment in the light experiment. Germinated seed with a radicle at least 2 mm long were counted and removed daily for a period of 3 wk. Petri plates assigned to complete darkness treatment were opened in a dark room with a safe green light.

Effect of temperature

Seeds were incubated at 15, 20 and 25 ^oC for 3 wk to determine the optimum temperature for germination. Petri plates were kept in a germinator (Seedburo Equipment Company, Chicago, IL, USA) and the temperatures were maintained constant.

Effect of light

To evaluate the effect of light on A.tenuifolius germination, seeds were placed in Petri plates and immediately wrapped with aluminum foil to ensure no light penetration. Other treatment was left uncovered to allow 10 h light exposure. Light was provided by fluorescent over-head bulbs set for 10-h light, 14-h dark regime with a photosynthetic photon flux density of 200 µm m^-2 s^-1.

Effect of drought stress

Seed germination of A.tenuifolius was checked against drought stress using aqueous solutions of different osmotic potential 0 (Control) -0.2, -0.4,-0.6, -0.8,-1.0 MPa. These aqueous solutions were prepared with PEG 6000 (Polyethylene glycol) by using the Michel & Kaufman (1973) equation as under:

Water potential = - (1.18 × 10^-2) C - (1.18 × 10^-4) C² + (2.67 × 10^-4) 18CT + (8.39 × 10^-7) C²T

where C is the concentration of PEG (g kg^-1 distilled water) and T is temperature (^oC).

After preparing the solutions osmotic potential was confirmed by osmometer (vapour pressure type), model 5520; Wescor, Logan, UT, USA. Control treatment was carried out with distilled water.

Effect of pH

To examine the effect of pH on germination, buffer solutions of pH 6 to 9 were prepared according to the method described by Chacahlis & Reddy (2002). A 2-mM solution of MES [2-(N-morpholino) ethanesulfonic acid] was adjusted to pH 6 with 1 N hydrogen chloride (HCl) or sodium hydroxide (NaOH). A 2-mM solution of HEPES [N-(2-hydroxy-methyl) piperazine-N-(2-ethanesulfonic acid)] was adjusted to pH 7 or 8 with 1 N NaOH. A pH 9 buffer was prepared with 2-mM TRICINE [NTris (hydroxymethyl) methylglycine] and adjusted with 1 N NaOH. Unbuffered deionized water (pH 6.2) was used as a control.

Effect of salt stress

Germination of A.tenuifolius seeds was checked against different levels of sodium chloride 50, 100, 150, 200 mM. While in control treatment distilled water was used.

Effect of seeding depth

This experiment was carried out in green house. Pots having 25 cm diameter were filled with sand (0.5 kg per pot). Twenty five seeds of A.tenuifolius per pot were placed at soil surface or within the soil from 0 cm (surface placement) 1, 2, 3, 4, 5, 6 cm burial depths. Every pot was initially irrigated with 100 ml of distilled water. Greenhouse temperatures were 15±2 ^oC during the day and 10±2 ^oC at night with a 10-h photoperiod (200 to 500 µmol photons m^-2 s^-1). Pots were left opened and watered as needed to maintained adequate soil moisture. Each pot has a hole at the bottom for drainage. Seedlings were considered emerged when a cotyledon was visible at the soil surface. Seedling emergence was recorded daily for 21 days.

Germination/emergence index (GI/EI) was calculated by following formula used by Association of Official Seed Analysis (AOSA, 1990).

It was noted when the germination/emergence of a treatment started, i.e. when first seed of A.tenuifolius was germinated or emerged. Coolbear et al. (1984) formula was used to determine time to 50% germination/emergence.

where N is the final number of germinated or emerged seed and n_j and n_i are the cumulative number of seed germinated by adjacent counts at times t_j (day) and t_i, (day) respectively, when n_i < N/2 < n_j.

Mean germination or emergence time (MGT or MET) was calculated according to the equation of Ellis & Roberts (1981):

where n is the number of seed that had germinated on day D and D is the number of days counted from the beginning of germination experiment. The germination or emergence index (GI or EI) was calculated as described by the Association of Official Seed Analysis (AOSA, 1983) using the following formula:

Statistical analysis

A completely randomized design with four replications was used in all experiments. Data were subjected to analysis of variance (ANOVA) with the use of SAS (2002). The model structure of ANOVA was Y_ij = µ + T_i+∈ij where Y_ij is the observed response variable, µ is an overall mean, T_i is the explanatory variable and ∈_ij is the error. The significant difference among treatment means were identified by using Fisher, LSD at P<0.05 (Steel et al., 1997). A square-root arcsine transformation was used to stabilize the variances for percentage data before analysis (Bartlett, 1947). Nonlinear regression analysis was used to determine how NaCl, osmotic stress or burial depth affected percentage germination or emergence. Germination (%) values at different concentrations of NaCl and osmotic potential were fitted to a functional three-parameter logistic model using Sigma Plot 2008 (version 11.0). The model fitted was G (%) = G_max/[1+(x/x₅₀)^g], where G is the total germination (%) at concentration x, G_max represents maximum germination (%), x₅₀ is the NaCl concentration or osmotic potential for 50% inhibition of the maximum germination and g indicates the slope. A three parameter logistic model {E (%) = E_max /[1+(x/x₅₀)^e]} was fitted to A. tenuifolius seedling emergence (%) obtained at different burial depth of 0 to 6 cm. where E is the total seedling emergence (%) at burial depth x, E_max is the greatest seedling emergence (%), x₅₀ is the burial depth for 50% inhibition of the greatest seedling emergence and e indicates the slope. The t-test was used to assess significant difference between light and dark treatment in the light experiment.

RESULTS AND DISCUSSION

Effect of temperature

Asphodelus tenuifolius germination was tested against varying temperature regimes from 15 to 25 ^oC with an interval of 5 ^oC. Asphodelus. tenuifolius germination was significantly affected by different temperatures; and greatest A.tenuifolius germination (80%) was recorded at 15 ^oC (Figure 1). However germination of A.tenuifolius decreased with increasing temperature, and lowest (32%) was noted at 25 ^oC (Figure 1). The starting time for A.tenuifolius germination was lowest at 15 ^oC (2.75 days). However it increased with increasing temperature, i.e. A.tenuifolius took 3.00 and 4.00 days at 20 ^oC and 25 ^oC, respectively, to start its germination (Table 1). Asphodelus tenuifolius took statistically similar days to its 50% germination at 15 ^oC (5.06 days) and at 20 ^oC (4.91 days), however A. tenuifolius took greatest (7.75 days) at 25 ^oC. Mean germination time of A. tenuifolius was highest (7.40 days) at 25 ^oC and it was followed by 5.84 days at 15 ^oC, however lowest (5.57 days) A.tenuifolius mean germination time was noted at 20 ^oC. Maximum (3.86) A.tenuifolius germination index was noted at 15 ^oC and it decreased with increasing temperature i.e. 2.62 at 20 ^oC and 1.22 at 25 ^oC (Table 1).

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Our findings show that 25 ^oC temperature and above have negatively affected A.tenuifolius germination; and that chances of heavy A.tenuifolius infestation during chickpea crop emergence in October are less likely because temperature is about 25.9 ^oC in October. However, as temperatures decrease in November, December and January (20 to 13 ^oC), A.tenuifolius germination in chickpea may increase. Many researchers observed a significant effect of temperature on weed seeds germination (Kabat et al., 2007; Guma et al., 2010). Seed germination of Leptochloa chinensis and Cuscuta campestris was highest at 25 ^oC and 30 ^oC, respectively (Benvenuti et al., 2004, 2005), however C.campestris seed germination was completely inhibited at 10 ^oC. Similarly, Ingolia et al. (2008) observed 60 to 80% seed germination of Rorippa subumbellata at 24 ^oC. Seeds of coolatai grass were reported to germinate over a wide range of temperature range from 5 to 45 ^oC; however germination percentage was very low above 40 ^oC (Chejara et al., 2008).

Effect of light

Light in 10 h photoperiod resulted in highest germination percentage (87%) of A.tenuifolius compared with complete darkness (0 hours light) that resulted in 78% germination. Table 2 showed a non-significant effect of dark and light on A.tenuifolius time to start germination, time to 50% germination and mean germination time. However statistically significant and highest germination index (5.21) of A.tenuifolius was observed under light and lowest (4.54) was observed under darkness. The results of our experiments show that darkness did not significantly reduce A.tenuifolius germination and 78% germination under darkness shows that A. tenuifolius is less sensitive to darkness (0 hours of light) for its germination. It suggests that A.tenuifolius seeds can germinate well under dense crop canopy or under shade conditions, and this ability of A.tenuifolius seeds can pose a major threat to crop production. In contrast, light sensitivity of Borreria ocymoides (purple leaf button) (Chauhan & Johnson, 2008e), Amaranthus spinosus (spiny amaranth) (Chauhan & Johnson, 2009c) and Synedrella nodiflora (Chauhan & Johnson, 2009b) was reported in different studies. Similar to our results Rao et al. (2008) and Wang et al. (2009) reported that germination of Beckmannia syzigachne (American sloughgrass) and Urena lobata (cadillo) seeds was not affected by darkness or light. Seeds of Eluesine indica (goosegrass) germinated well (68.72%) under 12 hours of light and dark, compared with complete darkness (17-25%) (Chauhan & Johnson, 2008d).

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Effect of drought stress

Asphodelus tenuifolius seed germination decreased from 87 to 8% as drought stress increased from 0 to 0.6 MPa. No germination of A. tenuifolius seeds was observed at 0.8 MPa osmotic potential. Logistic model showed 50% inhibition in germination of A.tenuifolius seeds at -0.38 MPa osmotic potential (Figure 3). Increase in time to start A.tenuifolius germination was noted with increased osmotic stress (From 0 to -0.6 MPa) and A.tenuifolius seeds took most days (6.75 days) to start its germination at 0.6 MPa osmotic stress (Table 3). Similar trend in time to obtain 50% germination and MGT was observed and A.tenuifolius took most days (9.94 and 8.81 days) at -0.6 MPa, respectively (Table 3). Table 3 showed that (GI) decreased with increased levels of drought stress. The highest G.I was recorded with control (3.74) and lowest was recorded at -0.6 MPa drought stress (0.27).

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Our results showed that moisture availability or rainfall is very important for A.tenuifolius germination, and that drought can adversely decreased A.tenuifolius germination. We further conclude that during the drier chickpea growing months A.tenuifolius germination would be restricted to low laying areas, where more moisture of summer rainfall is preserved in August-September. Similar response for seed germination of Eupatorium adenophorum (crofton weed); (Lu et al., 2006), Synedrella nodiflora (Chauhan & Johnson, 2008b) and Eluesine indica (goosegrass) (Chauhan & Johnson, 2008d) was recorded, i.e the seeds of these weeds are sensitive to different levels of drought stress.

Contrary to the above findings, seed germination of Hibiscus trionum (venice mallow); (Chachalis et al., 2008), Rapistrum rugosum (turnip weed); (Chauhan et al., 2006c) and Solanum sarrachoides (hairy night shade) (Zhou et al., 2005) was recorded under drought stress (low water potential). Similarly, in the studies of Chauhan & Johnson (2008b), seed germination of indian crabgrass was inhibited at higher levels (-1.0 MPa) of water stress, however southern crabgrass seeds germinated to some extent.

Effect of pH

Different levels of pH had a significant effect on A. tenuifolius seed germination and other germination related parameters. Asphodelus tenuifolius seeds exhibited highest (98%) germination at pH 7. Asphodelus tenuifolius seeds showed similar germination of 96% at pH levels of 6.00, 6.20, 6.50 and 7.50. Lowest germination of A. tenuifolius (82%) was recorded at pH 9.00 (Figure 4). Starting time of A.tenuifolius germination increased with increasing pH levels from 6.00 to 7.5 (2.00 to 5.00 days), however pH increase from 7.5 to 9.00 did not affect the starting time of A.tenuifolius germination (5.00 days). Asphodelus tenuifolius seeds took the lowest time to 50% germination (3.91 days) and MGT (5.36 days) with control (pH 6.2); however maximum time was taken at pH levels of 7.5 and 8.00. The highest GI (5.87) was measured with control and the lowest was measured at pH level 9.00 (2.82). However, pH levels of 7.5, 8.00, 8.5 and 9.00 exhibited statistically similar GI of A.tenuifolius seeds (Table 4).

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Our findings regarding the effect of pH on A.tenuifolius germination showed that A.tenuifolius can germinate under a wide range of pH, under various soil conditions. This behavior of weed plants is very helpful for their successful invasion. Watanabe et al. (2002) reported similar behavior for invasive weed species. Seed germination of many weed species remained unaffected (from pH 4 to 9) in various studies (Thomas et al., 2006; Chachalis et al., 2008; Wang et al., 2009). Germination of coolatai grass was 92% at pH level of 7.00, however at moderate level of pH (5.00) or (9.00) germination was reduced by around 10%. Germination of coolatai grass was reduced at pH level of 4.00 or 10.00 (Chejara et al., 2008). Similar results to those of Chauhan et al. (2006f) were found in Lolium rigidum (rigid ryegrass). In contrast, 92 to 95% germination of Eleusine indica (goosegrass) was measured at a pH range between 4 and 10 (Chauhan & Johnson, 2008d).

Effect of salt stress

Different salt stress levels exhibited significant effects on A. tenuifolius seed germination. It is clear from three parameter logistic model (Figure 4 and 5), that maximum (88%) germination of A.tenuifolius was noted with 0 mM (control) and it decreased significantly with a progressive increase in NaCl stress (from 50 to 150 mM) and the lowest (8%) was recorded with 150 mM salt stress. At 200 mM of NaCl stress, germination of A.tenuifolius was completely inhibited. A 50% inhibition of A.tenuifolius maximum germination was noted at 80 mM of salt stress (Figure 5).

Starting time of A. tenuifolius germination was statistically similar in 0 and 50 mM of NaCl. Greatest starting time of A.tenuifolius germination (10 days) was noted at 150 mM of NaCl stress (Table 5). Asphodelus tenuifolius also took more days to get 50% germination (11.69 days) and mean germination time (11.42 days) with a NaCl stress level of 150 mM. However, 50 and 100 mM of NaCl stress resulted statistically similar time to get 50% germination and MGT. Lowest time to get 50% germination (5.88 days) and MGT (6.58 days) were recorded with control (0 mM NaCl).

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However, with increase in salt stress from 0 to 150 mM, GI of A.tenuifolius decreased from 4.37 to 0.17 (Table 5). Our findings suggested that A. tenuifolius is very sensitive to salt stress, however a few seeds of A.tenuifolius were able to germinate even at high salt stress (150 mM), which enables successful A.tenuifolius spread in saline areas around the country. Similar to A. tenuifolius, seeds of Mimosa invisa (Chauhan & Johnson, 2008a) and Brassica tournefortii (Chauhan et al., 2006e) exhibited germination even at very high salt concentration.

Salts decreased water imbibition by seeds (Bae et al., 2006), inhibiting the germination of many species (Chauhan & Johnson, 2008b). Nandula et al. (2006) found that germination of horse weed decreased with increasing NaCl concentration, and that only 4% germination of horse weed was found at 160 mM NaCl. Similarly, an inverse or negative relationship in seed germination of american sloughgrass (Beckmannia syzigachne) (Rao et al., 2008) and Atriplex verrucifera (Peng-Peng & Gui-Ling, 2008) was recorded with increased salt concentration.

Effect of seeding depth

Effect of varying seeding depths was significant on A.tenuifolius emergence. Greatest emergence of A. tenuifolius (79%) was recorded with the seeds placed on soil surface and its emergence was gradually decreased with increase in seeding depth upto 4 cm (only 3% emergence). Emergence of A.tenuifolius was zero at 5/6 cm after 28 days of sowing (Figure 6). Starting time of A.tenuifolius emergence was non-significant among all the treatments. Greatest time to get 50% A. tenuifolius emergence (9.56 days) and MET (9.52 days) was noted with the seeds placed at 3 cm depth and the lowest time to obtain 50% germination and MET was recorded with A.tenuifolius placed at 2 and 1 cm depth (5.06 and 6.20 days, respectively) (Table 6).

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Asphodelus tenuifolius showed greatest emergence index with the seeds placed at 1 cm (3.70) and it was statistically similar with soil surface placement (3.64). Emergence index of A.tenuifolius decreased progressively after 2 cm depth and lowest was recorded at 4 cm depth (0.07). A 50% inhibition of A.tenuifolius greatest emergence occurred at 2.28 cm. Greatest A. tenuifolius emergence at soil surface enables its successful distribution and increases its density to economic injury level throughout chickpea producing areas, because practice of no tillage is common in these areas. However, reasonable emergence of A.tenuifolius also occurred at 1, 2 and 3 cm burial depths. Studies by Thomas et al. (2006) showed that seeds of Amaranthus viridis (Slender Amaranth) emerged in between 0.5 and 2 cm. Contrary to the above findings, the emergence of threehorn bedstraw was negligible at soil surface and it increased with increasing burial depths up to 5 cm (Chauhan et al., 2006d). In contrast, seedling emergence of Sonchus oleraceus decreased with increasing seeding depth (Chauhan et al., 2006a).

Results of our laboratory experiments suggested that A.tenuifolius can tolerate a wide range of varying environmental factors which is a plus point of A.tenuifolius against its management. At 15 ^oC, germination of A.tenuifolius was at its highest; however at 20 and 25 ^oC considerable A.tenuifolius germination was measured. A.tenuifolius seeds exhibited greatest germination under light as compared with complete darkness. Increase in water stress significantly decreased A.tenuifolius germination. A considerable germination of A.tenuifolius occurred at all pH levels from 6.00 to 9.00, which the pH level of soils in almost all areas of Pakistan. Asphodelus tenuifolius is very sensitive to salinity; however a few seeds of A.tenuifolius were able to germinate even at 150 mM of salt stress. Seeds of A.tenuifolius showed greatest emergence when planted at soil surface, however considerable number of seeds emerged at 1, 2 and 3 cm burial depths. This fact suggests that for a successful management of A.tenuifolius in chickpea, a deep burial of its seeds at 5 cm or below is helpful. No tillage practice is common in chickpea growing areas, and that may explain the successful outcome of A.tenuifolius.

LITERATURE CITED

Recebido para publicação em 23.7.2013

Aprovado em 23.1.2014.

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