Germination responses of Diplotaxis harra to temperature and salinity

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Abstract

Diplotaxis harra (Forssk.) Boiss, an annual herb in the family of Brassicaceae, is widely distributed in many sandy and gypseous areas in southern Tunisia. Laboratory experiments were carried out to assess the effects of temperature and salinity on seed germination and recovery responses after seed transfer to distilled water. The germination responses of the seeds in complete darkness were determined over a wide range of temperatures (5, 10, 15, 20, 25 and 30 °C) and salinities (0, 50, 100, 150 and 200 mM NaCl). Germination was inhibited by either an increase or decrease in temperature from the optimal temperature (15 °C). Highest germination percentages were obtained under non-saline conditions and an increase in NaCl concentrations progressively inhibited seed germination. Rate of germination decreased with an increase in salinity at all temperatures but comparatively higher rates were obtained at 15 °C. Salt stress decreased both the percentage and the rate of germination. An interaction between salinity and temperature yielded no germination at 200 mM NaCl. Seeds were transferred from salt solution to distilled water after 20 days, and those from low salinities recovered at all temperatures. At NaCl concentration of 200 mM, the recovery of germination was completely inhibited.

Introduction

Diplotaxis harra (Forssk.) Boiss is a common annual species distributed in many regions in arid Tunisia. It is a ramified stem herb which occurs in both sandy and gypseous soils reaching a height of 50–60 cm (Chaieb and Boukhris, 1998; Pottier-Alapetite, 1979), and grows best in soils where NaCl levels are around 580 ppm. When dried, this herb is browsed and appreciated by animals but its abuse seems to have certain toxicity; however, the fresh plants are generally refused by the animals (Le Floc’h, 1983). It is also a medicinal plant used against the scabies of the animals. The decoction of the leaves was used against constipation (Boulos, 1983). D. harra plants contain high percentages of antimicrobial components (Hashem and Saleh, 1999). The reproductive behaviour of D. harra varies according to site variation and amount of rainfall (Hegazy, 2001). Seedling and juvenile growth of this herb occur during late winter and early spring. Three ecological life-cycles are distinguished for the species (i) ephemeral behaviour is where plants possess un-branched stems with a single inflorescence, (ii) modular monocarpic behaviour is where the plants maintain the potential for continued growth and reproduction until the environmental factors surpass the plant's range of tolerance; plants possess branched stems with two or more inflorescences, and (iii) coppiced polycarpic behaviour occurs when plants maintain a persistent vegetative stump (Hegazy, 2001). Fruiting and seed dispersal of D. harra start during February and extend to June. Seedling and juvenile growth of all plants occur during late winter and early spring.

Successful establishment of plants largely depends on successful germination. Germination is a crucial stage in the life cycle of plants and tends to be highly unpredictable over space and time. Several environmental factors such as temperature, salinity, light, and soil moisture simultaneously influence germination (El-Keblawy and Al-Rawai, 2005, El-Keblawy and Al-Rawai, 2006; Huang et al., 2003; Ungar, 1995; Zia and Khan, 2004). Seed germination behaviour in relation to thermal and salt stress is very important to determine the colonization capacity of a species (Ungar, 1982, Ungar, 1995). Temperature is a determining factor for seed germination (Probert, 1992). The establishment of plants in arid regions is often limited by the temperature even though the conditions of humidity are favourable (Evans and Etherington, 1990; Jordan and Haferkamp, 1989; Oberbauer and Miller, 1982). Knowledge of temperature effects on germination may be useful to evaluate the germination characteristics or the establishment potential among range species (Jordan and Haferkamp, 1989). Temperature changes may affect a number of processes controlling seed germinability, including membrane permeability and the activity of membrane-bound and cytosolic enzymes (Bewley and Black, 1994; Gul and Weber, 1999). Tolerance to salinity during germination is critical for the establishment of plants growing in saline soil of arid regions (Khan and Gulzar, 2003; Ungar, 1995). Increased salinity leads to a reduction and/or delay in germination of both halophyte and glycophyte seeds. Germination failures in saline soils are often a result of high salt concentrations in the seed-planting zone because of the upward movement of soil solution and subsequent evaporation at the soil surface. This has been attributed to both osmotic and toxic effects (Khan and Rizvi, 1994; Khan and Ungar, 1998; Song et al., 2005). Seed germination under saline conditions occurs after high precipitation where soil salinity is usually reduced due to leaching (El-Keblawy, 2004; Huang et al., 2003; Khan and Ungar, 1996; Redondo et al., 2004). Recovery germination of seeds from hypersaline conditions is affected by temperatures to which seeds are exposed.

Temperature can interact with salinity affecting germination in saline dry areas (Al-Khateeb, 2006; Badger and Ungar, 1989; El-Keblawy and Al-Rawai, 2005; Esechie, 1993; Gorai and Neffati, 2007; Gulzar et al., 2001; Huang et al., 2003; Khan and Gulzar, 2003; Khan and Ungar, 1996; Khan et al., 2000). Initial establishment of species in salt deserts is related to germination response of seeds to salinity and temperature and early establishment usually determines if a population will survive to maturity (Huang et al., 2003; Song et al., 2005; Tobe et al., 2000). Although higher salinity decreases germination, the detrimental effect of salinity is generally less severe at optimum germination temperature (Al-Khateeb, 2006; Esechie, 1993; Gorai and Neffati, 2007; Gulzar et al., 2001; Khan and Gulzar, 2003; Khan et al., 2000).

This study was conducted to better understand the seed germination requirements of D. harra. The effects of a wide range of salinity levels and temperature on percentage of germination, rate of germination, and recovery responses of D. harra seeds were studied to determine their individual effect and the interaction between these factors on germination.

Section snippets

Seed collection site

Seeds of D. harra were obtained from plants, which were collected from a location near El Fjé, Médenine (10°39′N, 33°30′E; South-East Tunisia) in June 2006. This area is arid to semi-arid with a typical Mediterranean climate, characterized by irregular rainfall events and a harsh dry summer period. Annual rainfall is around 144 mm and annual mean evapotranspiration 1096 mm. Mean annual temperature is 20.5 °C with a minimum temperature 6.2 °C in January and 36.8 °C maximum in August.

Germination experiments

Seeds were

Effects on final germination

Temperature, salinity and their interaction significantly (p<0.0001) affected the final percentage of germination of D. harra (Table 1). Seed germination was highest in distilled water and germination percentages decreased with increase in salinity (Fig. 1). Seeds of D. harra were able to germinate at temperatures between 5 and 30 °C and the optimal temperature corresponds to 15 °C (Fig. 1). Seed germination decreased with an increase in NaCl concentrations at all temperatures (Fig. 2). Seeds

Discussion

Deserts are regions of unpredictable rainfall in which potential evapotranspiration exceeds precipitation. In certain areas the water table is high and a high rate of evaporation causes an accumulation of salts on the surface of the soil. These harsh conditions have led to differential life history strategies in desert plants maximizing their fitness (Gutterman, 2002; Kigel, 1995). The establishment of species may differ in their life cycle (annual/perennial), life form (shrubs/herbaceous),

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    Authors have participated equally to this work.

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