Dynamics of growth and phytomass allocation in seedlings of Pistacia atlantica

Introduction

The problem of salinity becomes more and more extensive in the majority of the countries in the process of development, mainly due to water scarcity (Rengasamy, 2010). Approximately 20% of the world's cultivated land and nearly half of all irrigated lands are affected by salinity (Zhu, 2001; Yamaguchi and Blumwald, 2005; Sun et al., 2009; Wu et al., 2010). In Tunisia, the saline lands are relatively frequent, occupying a surface of 1.5 million hectares, about 25% of the total cultural soils of the country (Jbira et al., 2001; Ben Ahmed et al., 2008). Salt stress affects plant growth through a double osmotic and ionic constraint followed by nutriment imbalance. Firstly, high concentrations of salts in the soil solution impose an osmotic stress on cell water relations which leads to reduction in water availability to plants and cellular dehydration (Sairam et al., 2002; Chartzoulakis, 2005). Secondly, an ionic toxicity caused by ions accumulation (mainly Na+ and Cl-) at high levels (Tavakkoli et al., 2011). Besides, nutriment deficiencies occurred due to the interaction of these ions with other nutritive elements (Tejera et al., 2006; Rejili, 2007; Evelin et al., 2009). The disturbance of potassium (K+) nutrition resulting from potassium–sodium interaction is often reported (Cramer et al., 1987).The salinity of soil and water of irrigation counts among the principal factors which limit plant growth and productivity (Zahran, 1999; Flowers, 2004; Parida and Das, 2005). Species develop morphological and physiological changes in the dynamics of growth, such as leaf number, plant height and leaf area reductions, increases in root growth and changes in dry phytomass allocation, as increases in leaf senescence, a decrease in specific leaf area and greater biomass allocation to roots. Plant growth reduction is often accompanied by inducing the senescence of leaves, mechanism adopted by the pistachio tree and other Mediterranean species (Colla et al., 2006).The leaf expansion decreases immediately following an increase in the salt concentration and the duration of the treatment (Munns, 2002; Bartels and Sunkar, 2005). The seedlings of pistachio tree reduce their growth when they are subjected to high salinity (Chelli-Chaabouni et al., 2010). In spite of the direct exposure of the root to salinity, its growth is less affected by salt than the shoot (Parida and Das, 2005). It was reported that species surviving under optimal conditions present high specific leaf area (SLA), high relative growth rate (RGR) and high allocation of biomass to shoots, while species with low SLA and low RGR, allocate more biomass to roots in unfavourable habitat (Poorter and Garnier, 1999). The early stage of seedling growth is a critical phase in arid and semi-arid lands where plants are exposed to a series of abiotic and biotic stress. Plants exposed to salt stress are negatively affected with reduction in establishment and growth.

Pistacia vera and Pistacia atlantica are two pistachio species, cultivated and spontaneous respectively, belonging to the Anacardiaceae family. They are species of economic, ecological and medicinal interests (Sari et al., 2010; Tomaino et al., 2010). They are characteristic of the arid and semi-arid area. In Tunisia, according to Chaieb and Boukhris (1998), they are cultivated on all the territory, with prevalence in the arid and semi-arid areas. P. vera, which produces edible fruits, can survive in areas where annual rainfall is lower than 200 mm (Chaieb and Boukhris, 1998). This species supports the dryness better than the olive, the almond and the fig tree, considered as a typical xerophytes tree, with the advantage to resist to salinity (Behboudian et al., 1986; Rieger, 1995) and to tolerate extreme temperature, until -30°C in winters and 50°C during the summer. But for others, it is classified as a medium tolerant to salinity (Ebert, 2000).

P. atlantica is a very powerful woody and spontaneous species, adapted to the constraining pedo-climatic conditions of the arid and semi-arid areas. However, it is observed only on the degraded forests as aged and dispersed individuals, not exceeding 1500 individuals in Tunisia (Ghorbel et al., 1998), being threatened with extinction. Their rusticities in front of edaphic and climatic constraining factors and their low natural regeneration rates made very interesting to study these species. The reintroduction of tolerant species in the degraded lands could limit the extension of the marginal zones (Melgar et al., 2006; Tattini et al., 2009). P. atlantica is also useful to receive the graft of fruit pistachio (Gijón et al., 2010), improving vigor and production. However, the mechanism of improvement still not understood. In woody crops, rootstocks are mainly used in order to improve some of the tree characteristics as tree vigor, crop production or production quality (Olien and Lakso, 1986; Cohen and Naor, 2002; Weibel et al., 2003), thus, the graft may influence the movement of substances in the xylem such as ions (Jones, 1971; Tavallali and Rahemi, 2007), water status (Olien and Lakso, 1986) and plant-growth regulating hormones (Kamboj et al., 1999). In this context, Atkinson et al. (2003) and Solari et al. (2006) showed that the tree hydraulic conductance associated with specific rootstocks are determinant of growth potential in grafted trees. The used pistachio rootstocks are Pistacia vera L, Pistacia integerrima L., Pistacia terebinthus L. and Pistacia atlantica Desf (Karimi and Kafkas, 2011). The most used rootstock in Tunisia are P. atlantica and P. vera, but Germana (1997) reported that P. atlantica is more susceptible to drought stress than P. terebinthus. The present study was aimed to: a) assess comparatively the effect of increasing concentrations of NaCl on the dynamics of growth and phytomass production in Pistacia vera and Pistacia atlantica at the seedling stage, b) identify the principal physiological mechanisms of salt stress tolerance for the endangered Pistacia atlantica versus Pistacia vera and c) find a potential relationship between the improvement of P. vera vigor since grafted on P. atlantica and salinity tolerance.