Interspecific Variation in Salt Tolerance of Some Acacia Species at Seed Germination Stage
Abstract
Germination of seeds from six Acacia species was evaluated under sal t stresses using five treatment levels: 0, 100, 200, 300, and 400mM of NaCl. Corrected germination rate (GC), germination rate index (GRI) and mean germination time (MGT) were recorded during 10 days. The results indicate that germination was significantly reduced in all species with the increase in NaCl concentrations. However, significant interspecific variation for salt tolerance was observed. The greatest variability in tolerance was observed at moderate salt stress (200 mM of NaCl) and the decrease in germination seems to be more accentuated in A. cyanophylla and A. cyclops. Although, A. raddiana, remains the most interesting, it preserved the highest percentage (GC = 80%) and velocity of germination in all species studied in this work, even in the hig h salt levels. This species exhibits a particular adaptability to salt environment, at least at this stage in the life cycle, a nd could be recommended for plantation establishment in salt affected areas. On the other hand, when ungerminated seeds were transferred from NaCl treatments to distilled water, they recovered largely their germination without a lag period, and with high speed. This indicated that the germination inhibition was related to a reversible osmotic stress that induced dormancy rather than specific ion toxicity.
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Introduction
Salinity of soils is one of the most environmental factors limiting agricultural production and has significant effects on crop productivity and biodiversity. It has more severe impact in arid and semi-arid environments and combined with the water constraint presents a serious threat to food stability in these areas [1,2,3].Indeed, salinization already has affected more than 800 million hectares throughout the world or 6% of total land [4,5]. In the North Africa and the Middle East, salinity affects 15 million hectares of arable lands and this area is in continuous progression [6,7]. In Morocco, more than 5% of areas are already affected by salinity to various degrees [8]. They correspond especially to arid and semi-arid regions where 80% of available water for irrigation contains salinity higher or equal to 5 g/l [9]. These wide geographical areas are not exploited to a great extent, except when they occasionally constitute a poor pasture land or irrigated domain with low yield.
The best economic approach for exploitation and rehabilitation of these marginal regions is selection of salt tolerant species and varieties capable of sustaining a reasonable yield within salt-affected soils [10,11,12]. The effectiveness of such approach depends on the availability of genetic variation in relation with salt tolerance and its exploitation by screening and selection of the powerful plants under saline stress [13,14,15]. For many crop species, intraspecific variability for salt tolerance has been identified among cultivars and wild species [16,17,18,19]. These rustic resources are well adapted and constitute a potential reservoir useful to provide interesting materials in order to diversify and increase the productivity, particularly in pasture land affected by salinity.
Acacia species have the ability to survive in a diverse range of habitat and environments. They are well adapted to the arid and semi-arid regions and are known for them tolerance to high pH and salinity as well [20,21]. These species are able to stabilize and fertilize soils via nitrogen fixing and mycorrhizal symbioses [22,23] and constitute sources of wood, fodder, medicine and gum [24]. Acacia trees may thus include ideal candidates for enabling saline land reclamation with a potential for financial returns because of their combined production and soil improvement [25,26,27].
In Morocco, the genus Acacia is represented by four spontaneous species including one Moroccan endemic (Acacia gummifera Wild.); the other three are Acacia raddiana Savi, Acacia ehrenbergiana Hayne., Acacia albida Del. Otherwise, about ten species are introduced for ornamentation, reforestation or the fight against desertification [28].
Successful establishment of plants often depends on germination success, especially those that grow in salt affected areas. Thus, seeds must remain viable for long period in high salinity conditions and germinate when salinity decreases [29]. Various halophytic or glycophytic species, show an important variability with their ability to germinate under different salt concentration [30,31,32]. The effects on germination depend on the concentration of NaCl and varied among the plant species [33]. To overcome salt stress effect, plants have evolved various mechanisms that help them to adapt the osmotic and ionic stress caused by high salinity [26]. Otherwise, salts can affect germination of seeds either by restricting the supply of water (osmotic effect) or by causing specific injury through ions to the metabolic machinery (ionic effect) [34].
The purpose of this study was to assess and compare the seed germination response of six Acacia species (Acacia gummifera, Acacia raddiana, Acacia eburnea, Acacia cyanophylla, Acacia cyclops and Acacia horrida) under different NaCl concentrations in order to explore opportunities for selection and breeding salt tolerant genotypes that can be utilized in future land reclamation projects. The study will also assess to determine whether salt stress is induced by osmotic constraints or by toxic effect of NaCl.
Conclusion
The present study, indicate that salinity decreased rate and speed of germination. It reveals also significant interspecific variation in the potential for salt tolerance at germination among the six species. The Moroccan species Acacia raddiana remains the most interesting at this stage. It preserves a high rate of germination until a concentration 400 mM of NaCl, with a germination rate that exceeds 40%. Germination recovery after removing the saline stress indicates that sodium chloride caused a reversible osmotic effect of germination rather than ion specific toxicity and exerts a temporary inhibition of germination which is eliminated with the removal of the constraint. This ability to germinate after exposure to higher concentrations of NaCl suggest that these species, especially the most tolerant, could be able to germinate under the salt affect ed soils and could be utilized for the rehabilitation of damaged arid zones. Nevertheless, germination in field conditions is more difficult due to other environmental factors such as, drought, light, and temperature. A more ambitious program, including Acacia species, is necessary, not only at germination but also at the other stages of the life cycle. This opens the possibility to continue this work to verify correlation between salt tolerance during seed germination and early stage of plant development which will be most useful in a breeding program for selecting salt -tolerant genotypes in Acacia species.