Nuovi approcci in Tamarix spp. per l'identificazione tassonomica e la caratterizzazione funzionale in condizioni di stress salino
Author(s)
Abbruzzese, Grazia
Date Issued
March 10, 2011
Type
Doctoral Thesis
Abstract
In a recent period, many researchers have tried to elucidate the complex physiological and
biochemical mechanisms, combined with morphological adaptations, involved in plant salt
tolerance (Greenway e Munns, 1980; Hasegawa et al., 2000; Ashraf e Harris 2004; Munns et al.,
2006; Flowers e Colmer, 2008). Ecologically, halophytes are the native flora of saline soils and
have developed several physiological and biochemical strategies of adaptations including the
filtration of salts by roots and the compartmentation and extrusion of salts from leaf tissues.
Frequently observed morpho-anatomical adaptations of plants from saline and dry habitats are an
increase in leaf succulence and thickness of leaf cuticle, some variations of stomata morphology, an
early lignification of cell walls and an increase in cutin deposition on the walls of epidermal cells
(Waisel 1972; Robinson et al. 1997; Wahid, 2003; Kim e Park, 2010). These xeromorphic
adaptations are common or shared between desert plants and other species commonly found in
physiologically dry habitats. These features play a crucial role in conserving the water balance in
plants inhabiting salty soils characterized by extremely negative water potentials. Because of the
complexity and variability of salt tolerance trait, it is difficult to develop early morpho-functional
indicators for the selection of plant tolerant material. The main focus of this study is the research of
new approaches for the identification of morpho-functional indicators to salt tolerance in Tamarix.
This genus is represented by 54 species and consists of mostly halophytic shrubs and dwarf trees
native to arid and semiarid regions of Eurasia and Africa (Baum, 1978). Tamarisks show a wide
ecological plasticity and can tolerate different stress factors, such as drought, elevated temperatures,
high solar radiation (Cleverly et al., 1997; Zhuang e Chen, 2006), and high salinity (Glenn et al.,
1998; Vandersande et al., 2001). Tamarisks are characterized by the presence of salt glands, which
are specialized structures for the extrusion of excess salts from leaf tissues; as a result, these species
belong to recretohalophytes group. Such adaptive features play an important role in regulating ionic
balance (Ramadan, 1998), maintaining or stabilizing osmotic and turgor pressure under high salinity
(Ding et al., 2009).
Therefore a better understanding of salt secretion and physiological role of salts accumulated in
Tamarix shoots is needed. For this purpose, a salt experiment was carried out with three species of
Tamarix coming from Israel (T. aphyl la, T. jordani s and T. tetragyna). These are considered
model species for salt tolerance studies because they commonly inhabit extreme environments like
the desert areas of North Africa and Middle East and along the shores of the Dead Sea (Waisel,
1961; Thomson et al., 1969; Bar-Nun e Poljakoff-Mayber, 1977; Hagemeyer e Waisel, 1988;
Solomon et al., 1994; Jones et al., 2006). The plants were cultivated in 8 L-pots and distributed in
four randomized blocks (control and three salt treatments: 150, 300 and 450 mM NaCl solution).
After 41 days, all the species studied were able to tolerate and photosintetize actively also under the
highest salt stress level. These data confirm the higher salt tolerance of these species that can be
categorized, according to increasing productivity, in the following order: T. aphyl la, T.
tetragyna and T. jordanis. Concerning the physiology of ions, the main useful traits to characterize
the degree of salt tolerance in these species are: the ability to limit sodium content in leaf tissues
and the level of salt extrusion. Anatomically, the main traits with the highest discriminatory power
are the stomatal size and density, the lignification of walls of xylem vessels and suberification of
endodermis in roots. Compared to the control, at 450 mM T. jordanis is the only one that increased
the thickness of palisade and epidermis as well as the thickness of root endodermis. By a combined
analysis of physiological and morphological traits, T. jordanis and T. aphylla are the most and the
least salt tolerant species, respectively.
Taxonomically, Tamarix L. is a critical genus among the Angiosperms because its members show
few distinctive external features mainly based on floral characters. So the identification of tamarisk
species is more difficult without flowers and so far the specific composition of Italian natural
populations has been little studied. Thus, further approaches and tools for Tamarix species
identification are needed.
A first database of identified tamarisk plants, growing in the Southern Italian sites, was created. The
identification of Tamarix species was based mainly on the analysis of some flower traits as reported
by Baum (1978). Due to the difficulties in producing paradermal sections in species with
xeromorphic habitus like tamarisk, a new method for creating high quality leaf imprints was
performed to study the leaf epidermis morphology. From these imprints, three variables were
identified useful for taxonomic identification of the species mostly represented in Italian
populations, T. africana and T. gallica: stomatal density, guard cell length and salt glands density.
Multivariate statistical analyses were performed in order to provide adding characters for tamarisks
classification in the future and to analyse the influence of different environments on epidermal
morphology. The guard cell length is the micromorphological trait more influenced by ecological
factors such as the distance from the river and the type of vegetation (dune or riparian vegetation).
Some of the developed morpho-functional indicators provided both useful tools for the
characterization of the most salt and drought tolerant species and new elements for better
understanding of Tamarix taxonomy.
Additional information
Dottorato di ricerca in Ecologia forestale
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