Stomatal Abundance and Distribution in Prosopis Strombulifera Plants Growing Under Different Iso-Osmotic Salt Treatments*
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CONICET Digital American Journal of Plant Sciences, 2013, 4, 80-90 Published Online December 2013 (http://www.scirp.org/journal/ajps) http://dx.doi.org/10.4236/ajps.2013.4.12A3010 Stomatal Abundance and Distribution in Prosopis strombulifera Plants Growing under Different Iso-Osmotic * Salt Treatments Mariana Andrea Reginato1, Herminda Reinoso2, Analía Susana Llanes1, María Virginia Luna1# 1Laboratorio de Fisiología Vegetal, Departamento de Ciencias Naturales, Universidad Nacional de Río Cuarto, Río Cuarto, Argentina; 2Laboratorio de Morfología Vegetal, Departamento de Ciencias Naturales, Universidad Nacional de Río Cuarto, Río Cuarto, Argentina. Email: #[email protected] Received October 29th, 2013; revised December 2nd, 2013; accepted December 13th, 2013 Copyright © 2013 Mariana Andrea Reginato et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Changes in several environmental parameters are thought to affect stomatal development. Under salt stress, plants can regulate their transpiration flux through a better control of the stomatal opening (as a short-term response) and through modifications of leaf anatomy (as a long-term response). We investigate how leaf micromorphology (stomatal abun- dance and distribution) of the halophyte Prosopis strombulifera (a spiny shrub particularly abundant in high-salinity areas of central Argentina) responds to different water status when plants are subjected to different salt treatments (NaCl, Na2SO4 and their iso-osmotic mixture). Different salt treatments on P. strombulifera plants influenced leaf mi- cromorphological traits differently. In this study, Na2SO4-treated plants showed an increase in stomatal density (SD) and epidermal cell density (ECD) (with smaller stomata) at moderate and high salinity (−1.9 and −2.6 MPa), whereas in NaCl and NaCl + Na2SO4 treated plants, a decrease in these variables was observed. In Na2SO4-treated plants, transpi- ration was the highest at moderate and high salinity, with the highest content of ABA registered. A possible explanation is that, despite of these high ABA levels, there is no inhibition in stomatal opening, resulting in increased water loss, growth inhibition, and acceleration of senescence processes. We demonstrate that P. strombulifera responds to progres- sive salt stress by different salts changing the leaf development, particularly in Na2SO4-treated plants, leading to struc- tural modifications in leaf size and micro-morphology of leaf cells. Keywords: Halophytes; NaCl; Na2SO4; Stomatal Characteristics 1. Introduction province). In these high-salinity soils, proportions of NaCl and Na SO salts are generally similar, although Soil salinization is a severe problem that limits plant 2 4 Na2SO4 was up to three times more abundant in certain productivity. According to [1], salinity affects about 20% samples [5]. Most studies concerning salt tolerance of of the irrigated soils in the world, and sodic soils alone plant species have been based on experiments in which represent 3.4% of all emerged lands. NaCl is the predominant salt, and injury symptoms are The genus Prosopis includes many important arboreal often ascribed to the toxicity of Na+ and Cl− ions. Rela- and shrub-like species present in high-salinity areas of tively few studies have focused on the effects of Na SO North and South America [2,3], having the unique ability 2 4 on plant growth and physiology, even if Na2SO4 is pre- to fix nitrogen and grow in such habitats [4]. The spiny sent at higher concentrations than NaCl in the soils and shrub Prosopis strombulifera (Lam.) Benth. [2] ranges groundwater in many areas of the world [6-9]. For that from the Arizona desert (USA) to Patagonia (Argentina), reason, it is important to compare the effects of these two and is particularly abundant in high-salinity areas of cen- salts on plant growth, in order to better understand the tral Argentina (Córdoba and southwestern San Luis physiological responses of plants to soil salinity. *Stomatal characteristics in P. strombulifera under salt stress. Our previous studies have shown that P. strombulifera #Corresponding author. has considerable variability in the salinity responses, Open Access AJPS Stomatal Abundance and Distribution in Prosopis strombulifera Plants Growing 81 under Different Iso-Osmotic Salt Treatments depending on the type of salt(s) used, and osmotic poten- stomatal epidermal cells, while SI depends solely on cell- tial (Ψo) in the culture medium. Stimulation of shoot type proportion, regardless of cell size, and therefore growth at Ψo values up to −1.9 MPa (500 mM) NaCl is both traits provide complementary information on final an interesting halophytic response. These plants grown in stomatal abundance and pattern [23]. an increasing gradient of NaCl (250 - 700 mM·L−1) do not The aim of this work was to investigate stomatal abun- develop salt glands in the leaves. Some tissues display dance and distribution of P. strombulifera plants in re- vacuolization, and the root system undergoes precocious sponse to different water status when the plants are sub- lignification and/or suberisation of endodermal cells, jected to different salt treatments (NaCl, Na2SO4 and with Casparian strips found much closer to the root tip their iso-osmotic mixture). Knowledge of these morpho- than in glycophytes. These plants can therefore more physiological aspects will enable to understand the adap- efficiently filter soil solution to prevent passage of excess tive and survival strategies developed by this halophyte ions into the xylem [10]. In contrast, P. strombulifera is in relation to the environment in which it develops, pro- much less tolerant of Na2SO4. Plants grown in the pres- viding important information for future work in biotech- ence of this salt showed immediate and sustained reduc- nological areas. tion of shoot height and leaf number per plant, accompa- nied by senescence symptoms such as chlorosis, necrosis, 2. Materials and Methods and leaf abscission. Total and individual leaf areas were 2.1. Plant Material and Growing Conditions strongly affected by Na2SO4 treatment, with a reduction that reached 60% and 30% respectively, at high salinity Prosopis strombulifera seeds were collected from an area [11]. Na2SO4 treatment also induced structural alterations in southwestern San Luis province, Argentina, located at in cells and tissues, with consequent changes in growth 33˚43'S, 66˚37'W, altitude 400 - 500 m, with a temperate patterns at various levels of organization, and anatomical climate (average annual temperature 15˚C - 20˚C) (Fig- and histological differences in roots, stems and leaflets, ure 1). This is predominantly a Prosopis alba forest lo- compared to control plants or plants grown in high NaCl cated in a saline depression between annual 300 to 400 [12]. These results demonstrate that plant growth re- mm isohyets in the Monte phytogeographic region [26]. sponses may vary depending on the anion associated The soil was saline-sodic with abundant calcareous ma- with sodium. terial and moderate salinity, has a sandy-loam texture and Plants adaptation to environmental constraints such as pH 7.5 [27]. Chemical composition was determined ac- drought or salinity requires transitory and long-term re- cording to [28] Peña Zubiate et al. (1998). Profile from 0 ductions of transpirational water fluxes, a physiological to 35 cm depth is presented in Table 1. response that is mediated by both constitutive and induced Pods were collected randomly from 100 plants in the genetic determinants [13]. Stomata plays a fundamental population, and peeled. Seeds were visually selected on role in the regulation of transpiration because, based on the basis of uniform size and generally good health, their density at the leaf surface, and their closing mecha- scarified with 98% H2SO4 (sulfuric acid) for 10 min, nism in response to environmental stimuli, they can exert washed overnight under running water, rinsed in distilled a tight control on plant water loss [14]. However, the water, and germinated in Petri dishes with two layers of relationship between stomatal functionality and plant water-saturated filter paper at 37˚C for 24 h [10]. Ger- water status is very complex and several factors are in- minated seedlings with roots ~20 mm long were grown volved [15]. Many studies have shown that water deficit in hydroponic conditions in black trays (28 × 22 × 10 cm; leads to an increase in stomatal density [16-19] and a 200 seedlings per tray) with 10% full-strength Hoagland’s decrease in stomatal size [17,20,21], indicating this may solution. Seedlings were self-supported in small holes on enhance the adaptation of plant to drought [22]. Plant the tray cover until the end of the experiment. The trays survival also depends on the compromise between pho- were placed in a growth chamber (Conviron E15, Con- tosynthesis and transpiration, and seedling stomatal abundance and distribution are probably under selective pressure in natural environments [23]. The traits currently used to score stomatal abundance are stomatal index (SI), which measures the proportion of epidermal cells that are stomata and stomatal density (SD) or number of stomata per area unit. SI and SD are the results of cell division patterns and of cell differentiation Figure 1. (A) Sampling site (near El Bebedero stream); (B) and expansion during organ growth [24,25]. SD depends General view of salty soils in the sampling site; (C) P. on stomatal number and on the size and number of non- strombulifera native plant in their habitat. Open Access AJPS 82 Stomatal Abundance and Distribution in Prosopis strombulifera Plants Growing under Different Iso-Osmotic Salt Treatments Table 1. Chemical composition of soil profile in the sam- Table 2. Increasing salt concentrations obtained by sequen- pling area. tial addition of pulses every 48 h. i.e. 4 pulses means 4 × 37.9 −1 ml aliquot of Na2SO4 L Hoagland solution.