Effect of Seasonal Environmental Changes on Leaf Anatomical Responses of Limoniastrum guyonianum in Sabkha Biotope ^†

Abstract

Climate change conditions can strongly influence the kinetics of morphogenetic processes. Our study showed that the total thickness of leaf lamina, adaxial palisade parenchyma, abaxial palisade parenchyma and spongy parenchyma increased significantly during the dry period, especially in August (31.4%, 52.1%, 37.6%, 27.69%, respectively). Moreover, the adaxial and abaxial epidermis becomes thicker during the most dry months (July and August). Likewise, the adaxial cuticle thickness increased during the dry period. The stomata density in the adaxial and abaxial leaf sides is 1.36- and 1.4-fold higher than those recorded during the wet periods. However, the salt glands’ density showed a much greater increase in the abaxial face (+2.4-fold). The bundle sheath size was unchanged under the seasonal environmental fluctuation in the sabkha. The xylem vessels diameter showed a maximum reduction in August (–63.8%). Likewise, the xylem vessels density increased significantly during the dry period. The closer relationship between the anatomical proprieties with soil salinity allows us to conclude that salt stress is one of the most limiting factors for Limoniastrum guyonianum in its natural biotope.

1. Introduction

Limoniastrum guyonianum Boiss is a wild herb (Plumbaginaceae) growing in the deserts of North Africa, especially in Northern Sahara (Algeria, Tunisia) in the salty soils of the great chotts [1]. This halophyte is characterized by the presence of salt glands that contribute to salt excess excretion [2]. Commonly used for dune stabilization and landscaping [3] also played an important role in folk medicine as an anti-dysenteric, antibacterial and antidiabitic [4]. Photochemical studies have shown that this species presents anti-inflammatory and antitumor proprieties [5]. However, no study has been carried out on the impacts of environmental condition changes on L. guyonianum grown in their natural habitats yet. Saline habitats could be subjected, in addition to salinity, to high temperature; drought; flooding; active deflation depending on site and seasons [6]. Consequently, this study aimed to evaluate the monthly anatomical behavior of L. guyonianum in relation to the environmental conditions of the Sabkha of Aïn Maïder.

2. Materials and Methods

The plant was harvested from the shott of the Sebkha of Aïn Maider–Boughrara, a south-eastern coastal area, located at 33°27′52″ N, 10°43′31″ E, which is 35 km from the city of Medenine, Tunisia. Meteorological data were obtained monthly from the nearest synoptic weather station to the study site. The soil electrical conductivity (EC) was measured using a conductivity meter. Sodium content was determined using an atomic absorption spectrophotometer. The Anatomical observations were performed under a light microscope.

3. Results and Discussion

The soil EC showed significant monthly variation, and the highest value was recorded in August which was about 4.68-fold greater than those measured during the wet period (1.27 in January) (Figure 1).The Na⁺ value significantly increased during the dry period with a maximum of 769.1 mmol·g⁻¹ soil in August, reflecting an increase of about 4.68-fold as compared to those obtained during the rainy period.

Leaf anatomical modifications under limited moisture availability can play an important role under salt stress, and they are an indicator of the degree of tolerance.The increased salinity in Ain Maider’s Sabkha significantly increased the leaf lamina thickness, palisade and spongy parenchyma (

Table 1. Anatomical parameters changes in the aerial part of L. guyonianum under the variation of climatic conditions in the Sabkha of Ain Maider.

Characters	Treatments
	January	February	March	April	May	June	July	August
Leaf thickness (µm)	485.4 ± 3.98 F	484.1 ± 7.66 F	491.7 ± 6.98 F	518.8 ± 7.18 E	554.6 ± 9.66 D	581.7 ± 6.45 C	608.2 ± 3.29 B	637.9 ± 6.98 A
Adaxial epidermis (µm)	24.1 ± 0.35 B	23.9 ± 0.40 B	24.0 ± 0.32 B	24.2 ± 0.38 B	24.1 ± 0.42 B	24.3 ± 0.30 B	25.1 ± 0.71 B	26.1 ± 0.26 A
Adaxial stomatal density (nb·mm−2)	62.2 ± 1.01 C	60.3 ± 1.52 C	63.1 ± 1.05 C	63.0 ± 2.64 C	63.3 ± 1.55 C	71.3 ± 4.02 B	82.0 ± 1.18 A	84.3 ± 0.69 A
Adaxial salt glandsdensity (nb·mm−2)	8.23 ± 0.25 CD	7.73 ± 0.61 D	8.16 ± 0.15 CD	8.32 ± 0.20 CD	8.90 ± 0.36 C	10.56 ± 0.40 B	11.66 ± 0.76 A	11.85 ± 0.33 A
Abaxial epidermis (µm)	23.9 ± 0.15 BC	23.2 ± 0.41 BC	23.1 ± 0.30 C	23.7 ± 0.35 BC	23.9 ± 0.65 BC	23.7 ± 0.40 BC	24.3 ± 0.20 B	25.1 ± 0.61 A
Abaxial stomatal density (nb·mm−2)	58.7 ± 1.12 D	57.9 ± 2.83 D	58.8 ± 2.02 D	60.4 ± 2.15 D	58.5 ± 2.50 D	66.7 ± 1.30 C	75.6 ± 1.35 B	82.1 ± 4.37 A
Abaxial salt glandsdensity (nb·mm−2)	10.4 ± 0.64 D	10.5 ± 0.71 D	11.7 ± 0.27 D	12.3 ± 0.36 D	13.0 ± 0.30 D	15.7 ± 0.37 C	23.8 ± 2.41 B	25.7 ± 2.83 A
Adaxial palisadeparenchyma (µm)	95.3 ± 3.51 F	92.6 ± 2.08 F	96.3 ± 1.52 F	102.7 ± 2.10 E	113.4 ± 4.49 D	127.1 ± 2.68 C	138.0 ± 3.10 B	144.9 ± 2.57 A
Abaxial palisadeparenchyma (µm)	62.0 ± 1.01 CD	60.3 ± 1.52 D	63.7 ± 2.05 CD	64.6 ± 1.50 CD	68.0 ± 2.04 C	75.3 ± 4.85 B	82.8 ± 1.25 A	85.3 ± 3.51 A
Spongyparenchyma (µm)	269.3 ± 6.94 D	273.0 ± 9.58 D	273.7 ± 8.61 D	292.8 ± 6.25 C	313.8 ± 6.27 B	319.4 ± 3.87 B	326.5 ± 4.11 B	344.6 ± 3.81 A
Xylem vessel diameter (µm)	14.7 ± 0.21 A	14.9 ± 0.15 A	14.1 ± 0.10 B	12.0 ± 0.10 C	11.4 ± 0.25 D	11.0 ± 0.05 D	9.4 ± 0.10 F	9.4 ± 0.06 F
Xylem density (nb·mm−2)	1934.3 ± 15.1 E	1909.0 ± 10.6 E	1930.6 ± 24.1 E	2149.5 ± 54.3 D	2662.1 ± 53.8 C	2852.0 ± 50.1 B	3246.2 ± 77.4 A	3317.0 ± 43.8 A

The different letters (A–F) in the same row indicate significantly different values at p < 0.05 as described by Duncan’s test.

and Figure 2). The necessity to conserve water renders the leaves succulent, thus increasing leaf thickness. These anatomical features may help in storing ions inside the plant body due to increased vacuolar volume [7], thus permitting the plant to cope with higher salt amounts.

Concerning epidermis size, it is common to observe a thickening of the skin under salt stress, which can be related to salt tolerance. In our results, epidermis thickness was significantly thicker only during the highest dry month (August). A positive correlation between the thickening of epidermal cells and salt tolerance has been demonstrated [8]. This characteristic is critical in conditions where the availability of water is limited; in fact, a thick epidermis can better control the loss of water [9].

Regarding the vascular system, our results exhibited that high salinity (dry period) reduced the xylem vessel diameter, while the xylem density increased (Table 1). Stiller et al. [10] reported that cavitation occurs when the flow of water in the xylem vessels cannot keep the sweat rate. Thus, selection for narrow vessels in response to improved water use efficiency would reduce the risk of xylem embolisms in saline habitats.

This study also revealed that stomata were evenly distributed on both leaf surfaces of L. guyonianum. Numerous works linked drought and or salt stress plant adaptation to the increase in stomatal density [11,12]. In the present study, leaf stomatal density increased during the dry season (June–August).

Salt glands play an important role in the regulation of ionic balance, contributing to salt tolerance [13]. In our study, higher salt gland densities were observed in L. guyonianum plants subjected to summer climatic conditions as these epidermal structures are important for the plant’s survival when grown under stressful conditions. In Limoniastrum monopetalum the salt gland is organized as an embedded cup of multiple cells [14].

4. Conclusions

Our study indicated that Limoniastrum guyonianum used many anatomical mechanisms to tolerate salinity, especially a decrease of the mesophyll cells area, the promotion of xylem production and the accumulation and excretion of the excess of salt by salt glands.