ABSTRACT
ABSTRACT
The argane tree is a remarkable essence by its botanical interest and its socioeconomic value. It is endemic species in the southwest of Morocco, where prolonged drought stress may occur. Although its tolerance has been commonly attributed to various mechanisms at the whole plant, the root system has a main role in the whole process of adaptation. We studied in argane tree plants the change in hydraulic conductivity, electrolyte leakage in root as well as root growth under drought stress and recovery. Our findings showed that the root hydraulic conductivity (Lpr) value significantly decreased under drought stress treatment. This was associated with an increase of root electrolyte leakage, signaling the occurrence of an injury to root cell membranes. At root growth level, stressed plants managed to maintain their root elongation despite decreased root mass. After short period of rehydration, the argane tree plants exhibited a tendency of increased hydraulic conductivity during recovery after drought stress, suggesting that this root physiological response may be intimately linked to drought stress tolerance strategies. These results also could be important to contribute to selection of tolerant genotypes and develop argane tree regeneration programs in regions that suffer from lack of water.
Introduction
Drought stress is a major problem in arid and semi-arid regions around the world, as it affects plant development and productivity. Plants respond to the decreased water availability in the soil by diverse mechanisms of tolerance and through certain vegetative organs. Besides the physical role, the roots of plants play an essential physiological role in the internal regulation of plant water state in relation to the external environment. These organs are in direct contact with the soil and therefore they are considered as the first biological sensors of the plant that can sense and predict drought stress.1,2 The water transport in roots takes place through radial pathway, which consists of apoplastic and cell-to-cell water pathways (symplastic and the transmembrane).3 So, the root system plays a vital role in maintaining the water balance and the soil-plant-atmosphere continuum. It contributes up to 50% in the hydraulic resistance of the whole plant.4 Indeed, the roots possess a high plasticity of regulation of these hydraulic properties, which depends on the species and external factors5,6. In response to drought stress conditions, the plant reacts by adjusting the root hydraulic conductivity,7-9 through regulating the anatomy and morphology of roots, or gene expression.1,6,7Nonetheless, the root system is quite sensitive to environmental constraints that might even impair root vitality.2,10
Argane tree [Argania spinosa (L.)] Skeels is mainly grown in the southwest of Morocco. This species is known by its endemicity and adaptive behavior to arid and semi-arid regions where they grow naturally in vast forests, called “arganeraie”. One such environment is Mediterranean-type ecosystems. Argane tree has developed a series of physiological and biochemical mechanisms to tolerate and grow under drought stress conditions.11-15 As dicotyledonous plants, the argan tree has taproots that can transport water from deeper soil horizons. In adult argnae trees, this root system is gradually and slightly being substituted by an adventitious root system that is through to trap water in the surface soil. Presence of these both types of roots, of right shaped and revolving on itself (taproots) as well as roots that develop horizontally (adventitious roots), explains its uniqueness. With its amazing roots network, the argane tree could survive under extreme drought conditions (desert in southern Morocco).
In the present work, we studied changes in the root hydraulic conductivity of A. spinosa plant under drought stress and recovery and investigated the effect of these water treatments on the morphology of roots and their cell state. The aim of the present study is to characterize the root hydraulic conductivity as one of the key mechanisms of tolerance of the argane tree to its arid and semi-arid environment.
Material and methods
Plant material and experimental design
The experiment was carried out in a growth chamber at the faculty of Science and Technology of Marrakech. Plant materials were obtained from germination using seeds collected from Essaouira provenance of known maternal origin.14,15 Two-year-old seedlings of A. spinosa were grown into large plastic pots (30 cm high and 12 cm diameter) containing a mixture of soil and peat 1/1 (v/v). Seedlings were maintained in the growth chamber with a day/night cycle of 26 ± 2/22 ± 2°C in a 16:8 photoperiod, relative humidity ranged between 60 and 70% and an average maximum photosynthetically active radiation (PAR) of about 400 μmol m−2s−1 ensured by fluorescent lamps. The drought treatment was applied on the seedlings in the same environmental conditions as above, by withholding the irrigation for 40 days followed by rehydration for 7 days.
Soil moisture content
From five pots per treatment, soil samples were taken 5 cm deep without plant residues, weighed (fresh weight) and dried in an oven at 80°C for 72 h before measuring the dry weight. Moisture content of the soil sample was expressed in percent according to the formula: Soil moisture content (%) = [(wet weight – dry weight)/dry weight] × 100. Four independant replicates were performed.
Morphological traits
Five A. spinosa plants per treatment were harvested and root length, root dry weight and root diameter were measured by using a millimeter tape, high precision balance and vernier caliper, respectively. The root/shoot ratio was also estimated. The measurements were recoded thrice during the experiment: at the beginning and at the end of the drought stress period and after rehydration. There were 4 replicates per treatment (one plant per replicate).
Electrolyte leakage
Root cell membrane injury was determined by recording of electrolyte leakage (REL) as described by McKay,16 with some modifications. 3 cm of mid-section of the root system were cut into small fragments (2 mm diameter) per replicate from A. spinosa roots (five plants par treatment) were washed in deionized water to eliminate soil and surface electrolytes and placed in stoppered vials containing 15 ml of distilled water. After 2 h under stirring at 25°C, the initial conductivity was determined by a conductivity meter (Hach, model. sensION+ EC7). The same samples were placed in boiling water bath for 15 min and their electrical conductivity rerecorded. REL was calculated as the percentage of the initial conductivity of the total conductivity. There were 4 replicates per treatment (one plant per replicate).
Root hydraulic conductivity (LPr)
The root hydraulic conductivity was measured according to Rodriguez et al.,17 with slight modifications. The stem was cut with 3 cm above the soil surface and the soil was carefully washed away from the roots. Then the root system was submerged in a container of water and placed in the pressure chamber with the cut stump exposed to the outside. The air pressure, after a good seal was obtained, was increased at an approximate rate of 0.1 MPa up to pressure of 0.5 MPa. A small piece of cotton, of known weight, was fitted to the stump for 60 s and then the exudate was collected and its weight measured. The hydraulic root conductivity was calculated using the formula: Lpr = J/(P × L), where Lpr is expressed in mg m−1 s−1 MPa−1, J is the water flow rate through the entire root system in mg s−1, P is the applied hydrostatic pressure in MPa and L is the root length in m. Five replicates per treatment were performed (one plant per replicate).
Statistical analysis
All data were subjected to the analysis of variance (ANOVA) to test the effect of watering regime. Means were compared using the Tukey’s post hoc test. Statistical tests were considered significant at p < 0.05. All statistical analyses were performed with SPSS 10.0 for Windows.
Results
The moisture soil content was significantly decreased after withholding watering for 40 days; this physiological-edaphic trait reduced of about 74% compared with the well-watered conditions (P ≤ 0.05) (Figure 1). Indeed, this significant decrease indicates the occurrence of severe drought stress. However, the initial level of moisture soil content was significantly and quickly restored in the pots containing argane plants after rehydration period (P ≤ 0.05) (Figure 1).
Published online:
23 July 2018Figure 1. Effect of drought stress and recovery on soil moisture content. C – Control, S – Drought stress and R- Rehydration. Values with different letters are significantly different at the 5% level Tukey’s test.
![]({"type":"image","src":"/na101/home/literatum/publisher/tandf/journals/content/kpsb20/2018/kpsb20.v013.i07/15592324.2018.1489669/20180830/images/medium/kpsb_a_1489669_f0001_b.gif"})
Figure 1. Effect of drought stress and recovery on soil moisture content. C – Control, S – Drought stress and R- Rehydration. Values with different letters are significantly different at the 5% level Tukey’s test.
The drought stress has affected differently the studied morphological traits (Figure 2). We noted a significant decrease of root weight of about 34% compared with control plants (P ≤ 0.05) (Figure 2a). However, both length and diameter traits of root (Figure 2b, c), as well as the root/shoot ratio were not changed significantly after 40 days of drought stress. These morphological traits were not significantly changed in argane plants under rehydration treatment, in comparison with the stressed plants.
Published online:
23 July 2018Figure 2. Effect of drought stress and recovery on (a) root weight, (b) root length and (c) root diameter. C – Control, S – Drought stress and R- Rehydration. Values with different letters are significantly different at the 5% level Tukey’s test.
![]({"type":"image","src":"/na101/home/literatum/publisher/tandf/journals/content/kpsb20/2018/kpsb20.v013.i07/15592324.2018.1489669/20180830/images/medium/kpsb_a_1489669_f0002_b.gif"})
Figure 2. Effect of drought stress and recovery on (a) root weight, (b) root length and (c) root diameter. C – Control, S – Drought stress and R- Rehydration. Values with different letters are significantly different at the 5% level Tukey’s test.
The electrolyte leakage values increased significantly in roots of stressed A. spinosa plants of about 48% compared with the control plants (P ≤ 0.05) (Figure 3). This cell membranes injury was not repaired after 7 days of rehydration. Indeed, the electrolyte leakage level remained high in the roots of rehydrated plants, about 44% compared with the well-watered argane plants (P ≤ 0.05) (Figure 3).
Published online:
23 July 2018Figure 3. Effect of drought stress and recovery on root electrolyte leakage. C – Control, S – Drought stress and R- Rehydration. Values with different letters are significantly different at the 5% level Tukey’s test.
![]({"type":"image","src":"/na101/home/literatum/publisher/tandf/journals/content/kpsb20/2018/kpsb20.v013.i07/15592324.2018.1489669/20180830/images/medium/kpsb_a_1489669_f0003_b.gif"})
Figure 3. Effect of drought stress and recovery on root electrolyte leakage. C – Control, S – Drought stress and R- Rehydration. Values with different letters are significantly different at the 5% level Tukey’s test.
The root hydraulic conductivity values significantly decreased in A. spinosa plants under drought stress conditions (P ≤ 0.05) (Figure 4). The reduction of this physiological trait was also recorded in rehydrated plants. Compared to the control plants, we noted a decrease of Lpr values of about 44 and 35% in stressed and rehydrated argane plants, respectively. During recovery, the values of Lpr became intermediate between the control and stressed plants. The rehydrated plants showed a tendency of increased hydraulic conductivity (17%) compared with the plants grown under drought stress conditions (Figure 4).
Published online:
23 July 2018Figure 4. Effect of drought stress and recovery on root hydraulic conductivity. C – Control, S – Drought stress and R- Rehydration. Values with different letters are significantly different at the 5% level Tukey’s test.
![]({"type":"image","src":"/na101/home/literatum/publisher/tandf/journals/content/kpsb20/2018/kpsb20.v013.i07/15592324.2018.1489669/20180830/images/medium/kpsb_a_1489669_f0004_b.gif"})
Figure 4. Effect of drought stress and recovery on root hydraulic conductivity. C – Control, S – Drought stress and R- Rehydration. Values with different letters are significantly different at the 5% level Tukey’s test.
Discussion
The significant decrease of moisture soil content under drought stress conditions is due primarily to the withholding watering and secondly to the plant which continuously absorbs water from the soil for transpiration and growth. This physiological-edaphic trait highlights the interaction between soil properties and plant root indices.18 Under drought conditions, the photosynthesis and transpiration responses of plants strongly depend on the water content stored in the soil. The ability of studied argane plant to maintain deep roots under drought stress treatment (Figure 2b) and thus access soil moisture is decisive for its survival. Plants manage drought stress mainly through their root system. Our results showed that drought stress treatment affected significantly the root weight, by inducing a decrease of about 34% (P ≤ 0.05). This significant decrease can be considered as an adaptive response of the root system of A. spinosa to drought stress by restricting the growth of lateral roots while maintaining the growth of the primary root.1 The absence of the changes in length and diameter of root demonstrate the capacity of argane tree plants to maintain root elongation despite the decline in root mass under drought stress. As an alternative physiological strategy of adaptation, we recorded in previous experiments that stomatal conductance decreased in argane tree under drought stress,14,12,15, and thus suggested that this response enables maintenance of leaf hydration in spite of the decline in capacity of roots to conduct water detected in the present experiments. The recourse to this strategy allows the argane tree to conserve and maintain the growth and functioning of its roots under drought stress conditions. As water-use strategy, argane root system has the ability to maintain its growth in length to explore deeper soil horizons.19,20 The roots are in direct contact with the soil, so the root electrolyte leakage can be a better indicator of membrane stability than that of leaves. The significant increase in REL in argane plant grown under drought stress indicates either that the trans-membrane electrolyte transport was impaired or that an injury to root cell membranes was occurred. Knowing that they are among the most fragile parts of plants, the roots are sensitive to many environmental stresses and, hence, the quantification of the ions leakage from damaged membranes can provide an indicator of root viability.9
The root hydraulic conductivity levels were decreased significantly in the stressed A. spinosa plants. This reduction of Lpr could be considered as a biophysical response to minimize water loss from A. spinosa roots through water channels6 and to maintain leaf hydration.7 Moreover, Tataranni et al.6 reported that Lpr in olive roots was strongly influenced by drought stress and it was also modulated by structural root modifications, especially by apoplastic barriers. After rehydration, the plants exhibited a tendency of increased Lpr about 17% of that measured in stressed plants. The root hydraulic conductivity recovery, in short time, could be explained by the effect of sugars and hormones accumulation which have a deep effect on the recovery of plants.21 Sugars in roots can interact positively with abscisic acid signalling and thence modulate aquaporin abundance and further adjust the root hydraulic conductivity.8,21 The regulation of Lpr has been mainly attributed, on one hand, to the formation of apoplastic barriers which regulate the rate of the apoplastic water flow; and on the other hand, to cell-to-cell pathway and especially to the expression and abundance of transmembrane aquaporins.21,8,5,6 Veselov et al.,8demonstrated importance of increased root hydraulic conductivity for maintenance of water flow from the roots as a way to increase leaf hydration in response to increased evaporative demand. This increase of Lpr has various implications for rhizosphere processes and plant nutrient acquisition and growth.7 In dry soil, increased root hydraulic conductivity may provide moisture that facilitates favorable biogeochemical conditions for enhancing mineral nutrient availability, microbial processes and solute uptake by roots.2 Also, increased Lpr may prolong or enhance fine-root activity by keeping them hydrated.2,7 This biophysical trait can affect the exchange between the root and the shoot, the transpiration rate and stomatal conductance.5 Lpr changes in A. spinosa plants in response to water treatments seem to represent an internal mechanism of tolerance and to constitute an integral part of the plant hydraulic responses.
Conclusion
Our study has shown that the drought stress affected the health, growth and functioning of the root system of A. spinosa plant, by causing an increase in the REL and a decrease in the root weight and the Lpr. After rehydration, the plants revealed a tendency of increased hydraulic conductivity in a reduced time, signaling that this species is able to adapt to environmental stresses, particularly drought. Considering the ecological, and socio-economic interest of agrane tree at the regional, national and international levels, our finding could be important to better understand its adaptive behavior in the arid and semi-arid climate that characterizes their forests and therefore promote its domestication as a part of the Arganiculture (argane culture).
Disclosure of potential conflicts of interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
Acknowledgments
I thank all my co-authors who have contributed effectively towards the realization of this paper.
References
- Xiong LM, Wang RG, Mao GH, Koczan JM. Identification of drought tolerance determinants by genetic analysis of root response to drought stress and abscisic acid. Plant Physiol 2006;142:1065–1074. doi:10.1104/pp.106.084632. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Brunner I, Herzog C, Dawes MA, Arend M, Sperisen C. How tree roots respond to drought. Front Plant Sci 2015;6:547. doi:10.3389/fpls.2015.00547. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Steudle E, Peterson CA. How does water get through roots? J Exp Bot 1998;49(322):775–788. doi:10.1093/jxb/49.322.775. [Crossref], [Web of Science ®], [Google Scholar]
- Martre P, Cochard H, Durand JL. Hydraulic architecture and water flow in growing grass tillers (Festuca arundinacea Scherb). Plant Cell Environ 2001;24:65–76. doi:10.1046/j.1365-3040.2001.00657.x. [Crossref], [Web of Science ®], [Google Scholar]
- Meng D, Fricke W. Changes in root hydraulic conductivity facilitate the overall hydraulic response of rice (Oryza sativa L.) cultivars to salt and osmotic stress. Plant Physiol Biochem 2017;113:64–77. doi:10.1016/j.plaphy.2017.02.001. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Tataranni G, Santarcangelo M, Sofo A, Xiloyannis C, Tyerman SD, Dichio B. Correlations between morpho-anatomical changes and radial hydraulic conductivity in roots of olive trees under water deficit and rewatering. Tree Physiol. 2015; 35(12):1356–65. https://doi:10.1093/treephys/tpv074 [Crossref], [Google Scholar]
- Kaneko T, Horie T, Nakahara Y, Tsuji N, Shibasaka M, Katsuhara M. Dynamic regulation of the root hydraulic conductivity of barley plants in response to salinity/osmotic stress. Plant Cell Physiol 2015;56(5):875–882. doi:10.1093/pcp/pcv013. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Caldwell MM, Dawson TE, Richards JH. Hydraulic lift: consequences of water efflux from the roots of plants. Oecologia 1998;113(2):151–161. doi:10.1007/s004420050363. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Veselov DS, Sharipova GV, Veselov SY, Dodd IC, Ivanov I, Kudoyarova GR. Rapid changes in root HvPIP2;2 aquaporins abundance and ABA concentration are required to enhance root hydraulic conductivity and maintain leaf water potential in response to increased evaporative demand. Funct Plant Biol 2016;45(2):143–149. doi:10.1071/FP16242. [Crossref], [Web of Science ®], [Google Scholar]
- Radoglou K, Cabral R, Repo T, Hasanagas N, Sutinen ML, Waisel Y. Appraisal of root leakage as a method for estimation of root viability. Plant Biosyst 2007;141(3):443–459. doi:10.1080/11263500701626143. [Taylor & Francis Online], [Web of Science ®], [Google Scholar]
- Chakhchar A, Lamaoui M, Aissam S, Ferradous A, Wahbi S, El Mousadik A, Ibnsouda-Koraichi S, Filali-Maltouf A, C.E. Modafar. Electrolyte ions and glutathione enzymes as stress markers in Argania spinosa subjected to drought stress and recovery. Afr J Biotechnol 2017a;16(1):10–21. doi:10.5897/AJB2016.15234. [Crossref], [Google Scholar]
- Chakhchar A, Lamaoui M, Aissam S, Ferradous A, Wahbi S, El Mousadik A, Ibnsouda-Koraichi S, Filali-Maltouf A, El Modafar C. Using chlorophyll fluorescence, photosynthetic enzymes and pigment composition to discriminate drought tolerant ecotypes of Argania spinosa. Plant Biosyst 2017b;152(3):356–367. doi:10.1080/11263504.2017.1297334. [Taylor & Francis Online], [Web of Science ®], [Google Scholar]
- Chakhchar A, Wahbi S, Lamaoui M, Ferradous A, El Mousadik A, Ibnsouda-Koraichi S, Filali-Maltouf A, C.E Modafar. Physiological and biochemical traits of drought tolerance in Argania spinosa. J Plant Interact 2015b;10(1):252–261. doi:10.1080/17429145.2015.1068386. [Taylor & Francis Online], [Web of Science ®], [Google Scholar]
- Chakhchar A, Lamaoui M, Aissam S, Ferradous A, Wahbi S, El Mousadik A, Ibnsouda-Koraichi S, Filali-Maltouf A, C.E. Modafar. Differential physiological and antioxidative responses to drought stress and recovery among four contrasting Argania spinosa ecotypes. J Plant Interact 2016;11(1):30–40. doi:10.1080/17429145.2016.1148204. [Taylor & Francis Online], [Web of Science ®], [Google Scholar]
- Chakhchar A, Lamaoui M, Wahbi S, Ferradous A, El Mousadik A, Ibnsouda-Koraichi S, Filali-Maltouf A, C.E. Modafar. Leaf water status, osmoregulation and secondary metabolism as a model for depicting drought tolerance in Argania spinosa. Acta Physiol Plant 2015a;37:1–16. doi:10.1007/s11738-015-1833-8. [Crossref], [Web of Science ®], [Google Scholar]
- Chakhchar A, Haworth M, C.E. Modafar, M, Mattioni C, Wahbi S, Centritto M. An assessment of genetic diversity and drought tolerance in argan tree (Argania spinosa) populations: potential for the development of improved drought tolerance. Front Plant Sci 2017c;8:276. doi:10.3389/fpls.2017.00276. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- McKay HH. Root electrolyte leakage and root growth potential as indicators of spruce and larch establishment. Silva Fennica 1998;32:241–252. doi:10.14214/sf.684. [Crossref], [Google Scholar]
- Rodriguez P, Dell’Amico J, Morales D, Sanchez Blanco MJ, Alarcon JJ. Effects of salinity on growth, shoot water relations and root hydraulic conductivity in tomato plants. J Agric Sci 1997;128(4):439–444. doi:10.1017/S0021859697004309. [Crossref], [Google Scholar]
- Wang YQ, Shao MA, Shao HB. A preliminary investigation of the dynamic characteristics of dried soil layers on the Loess Plateau of China. J Hydrol 2010;381:9–17. doi:10.1016/j.jhydrol.2009.09.042. [Crossref], [Web of Science ®], [Google Scholar]
- Zunzunegui M, Boutaleb S, Díaz Barradas MC, Esquivias MP, Valera J, Jáuregui J, Tagma T, Ain-Lhout F. Reliance on deep soil water in the tree species Argania spinosa. Tree Physiol 2017. doi:10.1093/treephys/tpx152. [Crossref], [Web of Science ®], [Google Scholar]
- Ain-Lhout F, Boutaleb S, Diaz-Barradas MC, Jauregui J, Zunzunegui M. Monitoring the evolution of soil moisture in root zone system of Argania spinosa using electrical resistivity imaging. Agric Water Manag 2016;164:158–166. doi:10.1016/j.agwat.2015.08.007. [Crossref], [Web of Science ®], [Google Scholar]
- Calvo-Polanco M, Sánchez-Romera B, Aroca R. Mild salt stress conditions induce different responses in root hydraulic conductivity of Phaseolus vulgaris over-time. PLoS ONE 2014;9(3):e90631. doi:10.1371/journal.pone.0090631. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]