Drought Stress Responses of Four Contrasting Provenances of Argania spinosa ^†

Abstract

The Argane tree (Argania spinosa (L.) Skeels) is an endemic species of Morocco, widely adapted to the arid and semi-arid climate of the southwest. The Argane may serve as an oilseed crop in desert areas threatened by drought and salinity. Its domestication will open up important economic opportunities for Morocco in the face of the growing demand for Argane oil and will help reduce the pressure on the wild Argane forest. In this research, we aimed to study and characterize some physiological and biochemical traits of Argane tolerance to drought stress. We measured physiological parameters related to the water content (foliar water potential and relative water content of the leaves), biochemical parameters involved in osmoregulation (proline and total sugars), and photosynthesis (chlorophylls) in Argane seedlings from four contrasting provenances (Bouizakarne, Agadir, Essaouira and Berkane) cultivated under drought stress induced by cessation of irrigation. The results showed that the basic and minimal foliar water potential, relative water content as well as chlorophyll content significantly decreased in seedlings under severe drought stress compared to control ones, whereas a significant accumulation of proline and total soluble sugars was noted in stressed seedlings. Nonetheless, inter-provenance differences were recorded for some parameters studied. Provenance effect was determinant for variation in drought stress responses of A. spinosa. The study of drought stress-adaptive traits in Argane tree can help to understand the tolerance mechanisms and discriminate between the most drought tolerant provenances in order to rehabilitate degraded Argane forests. It is also relevant for domestication and conservation programs in others abiotic stress conditions resulting from climate change.

1. Introduction

The Argane tree [Argania spinosa (L.) Skeels] is a horticultural forestry species endemic to Morocco that has multiple uses. It is the only tree representing the tropical family of Sapotaceae listed in the flora of Morocco [1]. Argane tree plays an important role, mainly for the local population, in terms of its botanical, ecological, and economic interest as well as its social value [2]. It contributes to the preservation of the ecosystem and provides an environment conducive to maintaining floristic and faunistic biodiversity [2]. However, the sustainability of this ecosystem is threatened by the effect of global climatic changes such as the successive chronic drought which considerably reduces its natural spread. Tree species cannot escape drought like annuals but they have developed several mechanisms of tolerance against this abiotic constraint reflecting different types of adaptations [3]. Biogeographical distribution of Argane tree in a restricted area in southwestern Morocco is characterized by low water availability and high evapotranspiration demand [2]. Nonetheless, this species can withstand drought by developing adaptive mechanisms through various biochemical and physiological processes [4,5,6,7]. In this perspective, we studied some physiological and biochemical mechanisms of drought tolerance in four contrasting provenances of the Argane tree in order to discriminate the most tolerant provenances with a view to rehabilitate the degraded Argane forests.

2. Materials and Methods

Argane seedlings (6-month-old) from the germination of seeds of four contrasting provenances (Essaouira (Ess), Agadir (Agd), Bouizakarne (Bzk) and Berkane (Brk)), were subjected to severe water stress by cessation of irrigation for 25 days under greenhouse conditions. Control was irrigated every two days during the planned experimental period. Physiological and biochemical measurements were collected every five days interval from 3 replications per treatment/provenance. For physiological parameters, we determined basic (Ψ_b) and minimum (Ψ_m) leaf water potential and the relative leaf water content (RWC). For the biochemical parameters, we determined proline, total sugars and total proteins as well as chlorophyll a and b contents.

All the results obtained were statistically analyzed using analysis of variance (ANOVA). The Tukey post-hoc test was used to compare the means at the 5% significance level. A canonical discriminant analysis (CDA) was achieved on the four contrasting A. spinosa provenances considering all traits studied. All these statistical analyzes were carried out using SPSS software, version 25.

3. Results and Discussion

The results of the physiological and biochemical parameters studied in this work showed significant differences across observation time and provenance (

Table 1. Analysis of variance (ANOVA) showing the effect of provenance, observation time, and drought stress and their interaction on the physiological parameters studied in the four contrasting provenances of the A. spinosa (Berkane, Essaouira, Agadir, and Bouizakarne).

	Ψb	Ψm	RWC
Berkane (Brk)	−0.50 b	−1.41 b	62.82 a
Essaouira (Ess)	−0.41 a	−1.19 a	56.54 b
Agadir (Agd)	−0.38 a	−1.42 b	65.26 a
Bouizakarne (Bzk)	−0.47 b	−1.11 a	50.70 c
ANOVA (p ≤ 0.05)
Provenance (P)	≤0.05	≤0.05	≤0.05
Time (T)	≤0.05	≤0.05	0.15
Stress (S)	≤0.05	≤0.05	≤0.05
P × S	≤0.05	≤0.05	0.18
P × T	≤0.05	≤0.05	≤0.05
T × S	≤0.05	≤0.05	0.78
P × S × T	≤0.05	≤0.05	≤0.05

(Means within a column flanked by the same letter (a–c) are not significantly different at 5% (Tukey test)).

and

Table 2. Analysis of variance (ANOVA) showing the effect of provenance, observation time, and drought stress and their interaction (ANOVA) on the biochemical parameters studied in the four contrasting provenances of the A. spinosa (Berkane, Essaouira, Agadir, and Bouizakarne).

	Proline Content	Sugars Content	Proteins Content	Chl a	Chl b	Chl ab
Berkane (Brk)	32.41 b	103.29 a	214.37 b	11.19 a	4.49 a	15.68 a
Essaouira (Ess)	28.76 c	97.89 ab	266.61 a	9.07 c	3.29 c	12.36 a
Agadir (Agd)	42.42 a	96.43 c	229.89 b	9.30 bc	3.40 bc	14.70 a
Bouizakarne (Bzk)	34.05 b	101.44 ab	273.25 a	10.36 ab	4.04 ab	14.40 a
ANOVA (p ≤ 0.05)
Provenance (P)	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05	0.17
Observation time (T)	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05
Stress (S)	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05
P × S	≤0.05	≤0.05	0.74	≤0.05	≤0.05	≤0.05
P × T	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05
T × S	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05
P × S × T	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05	≤0.05

(Means within a column flanked by the same letter (a–c) are not significantly different at 5% (Tukey test)).

). At the end of the experimental period, stopping irrigation significantly reduced leaf RWC by 13.6%, 11.1%, 9.2% and 14.6% in Ess, Agd, Bzk and Brk, respectively. Similarly, stress decreased Ψ_b about 2 to 4.6-fold and Ψ_m about 1.5 to 4.6-fold in Argane seedlings compared to the control ones. Significant decrease in RWC and leaf water potential in Argane tree under severe water stress (25 days unirrigated) has been reported in different provenances [6,8]. This reduction in leaf water status has been considered a clear response to drought stress to prevent water loss in A. spinosa [6].

At the end of the experimental period, the cessation of irrigation also induced a significant decrease of chlorophylls a (29 to 78%) and b (11 to 86%) contents in all A. spinosa provenances. This decrease in chlorophylls under drought stress is mainly attributed to the damage of chloroplasts caused by active oxygen species [5]. However, stressed Argane seedlings showed a significant increase in free proline (46 to 167%), total sugars (60 to 166%) and total protein (17 to 67%) contents compared to control ones. The marked accumulation of osmolytes (proline and sugars) in the leaves of the Argane tree under drought stress conditions has also been reported by other authors [6,7]. These osmolytes are considered as one of the adaptive strategies triggered by the plant in response to environmental constraints. Agd and Brk showed the high accumulation of proline (42.42 mg L⁻¹) and total sugars (103.29 mg L⁻¹), respectively, compared to other provenances (Table 2). Furthermore, protein accumulation in stressed Argane seedlings may be due to the synthesis of new proteins, including antioxidant enzymes, to ensure growth and/or survival under drought stress conditions [6]. The highest protein accumulation was recorded in Bzk (273.25 mg L⁻¹) and Ess (266.61 mg L⁻¹).

Provenance × water (stress) × observation time interaction was significant for all physiological and biochemical parameters studied (Table 1 and Table 2). According to CDA, Agd and Brk provenances were mainly separated from Ess and Bzk by a good conservation of the water (high values of RWC and high negative values of Ψ_b). The physiological and biochemical differences noted in our study between the four provenances indicated adaptive genetic diversity of Argane tree for the conservation of its forests.

4. Conclusions

The present study highlighted the inter-provenance variation in the adaptation mechanism of A. spinosa to drought stress. The four contrasting provenances of Argane tree showed, under drought conditions, a significant decrease in water potential, as a strategy of water conservation, as well as a significant accumulation of certain osmolytes related to osmoregulation and homeostasis maintenance. These physiological and biochemical responses varied significantly according the provenance of the Argane.