Said Wahbi

Changes in antioxidant activities and phenol content in tomato plants subjected to partial root drying and regulated deficit irrigation

Abstract Partial rootzone drying (PRD) and regulated deficit irrigation (RDI) are water saving irrigation systems that have been developed to increase water use efficiency (WUE) without significant yield reduction. To examine whether tomato responded differently to RDI and PRD, we compared the changes in antioxidative defenses in tomato plants using a split-root system. Tomato plants were grown for 21 days under controlled conditions with their roots separated equally between two soil compartments. Three irrigation treatments were imposed: Control, receiving an amount of water equivalent to 100% of plant transpiration; PRD in which one compartment was watered with 50% of the amount of water supplied to the controls, allowing one-half of the root system to be exposed to dry soil, and switching irrigation between sides weekly; RDI in which 50% of the amount of water given to the controls was supplied, half to each side of the root system. Relative water content (RWC), midday leaf Ψ and chlorophyll content decreased largely in RDI-treated plants, whereas the PRD plants exhibited relatively higher Ψ and RWC values. An enhanced level of lipid peroxidation in both roots and leaves indicated that PRD and RDI caused oxidative stress in tomato plants. In leaves, superoxide dismutase (SOD), soluble peroxidase (POX) and polyphenol oxidase (PPO) activities showed an increase in the early phase of water deficit, and then decreased in the remaining phase of the drying cycle. However, the increase was more pronounced under RDI. Catalase (CAT) activity declined continuously from the onset of PRD and RDI treatments to below the control level, and the reduction was less under PRD than RDI. POX cell-wall associated activities exceeded the control level by 450% and 230%, respectively, under RDI and PRD. At the root level, while CAT activity also decreased under both PRD and RDI, the activities of SOD, POX and PPO significantly increased and their activities showed an alternating increase/decrease paralleling the alternating irrigation in PRD-treated roots. As a result of the difference in POX and PPO activities between the two water treatments applied, PRD-treated plants accumulated more soluble and cell-wall bound phenolic compounds.

Leaf water status, osmoregulation and secondary metabolism as a model for depicting drought tolerance in Argania spinosa

The present investigation was undertaken to characterise and to distinguish four contrasting Argania spinosa ecotypes in terms of drought tolerance by exploring the changes of leaf water status, osmoregulation and secondary metabolism. A. spinosa plants corresponding to four contrasting ecotypes (Lks, Alz, Rab and Adm) were subjected to drought stress. The results exhibited that there was a significant decrease in predawn leaf water potential (Ψpd), stomatal conductance (gs) and leaf relative water content under the influence of the intensity and duration of drought stress. Negative and significant correlations were recorded between epicuticular wax load (EWL) and residual transpiration rate. Electrolyte leakage (EL) increased significantly in leaves of plants under drought stress treatment compared to control plants. Furthermore, our data revealed that drought stress can induce shikimate and phenylpropanoid pathways in A. spinosa. A significant induction of phenylalanine ammonium lyase (PAL), shikimate dehydrogenase (SKDH) and cinnamate 4-hydroxylase (C4H) enzymes and an increase in polyphenol content were recorded, of which Lks showed the highest induction and accumulation among ecotypes. Accumulation of polyphenols was positively correlated with the SKDH, PAL and C4H activities. The strong induction of secondary metabolism in Lks might be linked to its better ability of drought tolerance. The proline and soluble sugar content in leaves of all ecotypes increased substantially in parallel with the severity of stress-induced. According to canonical discriminant analysis of our data, the four ecotypes were separated by the following physiological and biochemical parameters: EL, gs, EWL, soluble sugars and polyphenols.

Physiological and biochemical traits of drought tolerance in Argania spinosa

The objective of this study was to understand and characterize the physiological and biochemical tolerance mechanisms of Argania spinosa under drought stress for selection tolerant ecotypes. Significant differences were observed among ecotypes in indices of leaf water status studied: stomatal conductance (gs), predawn leaf water potential (Ψpd) and leaf relative water content. There was a significant decrease in these physiological traits with increasing degree of drought stress in all ecotypes. Drought stress significantly increased endogenous H2O2 and lipid peroxidation. Moderate and severe drought stress increased significantly the catalase, superoxide dismutase, peroxidase, polyphenoloxidase and lipoxygenase activities, depending on time. Their constitutive activities were higher in inland ecotypes than in coastal ecotypes. According to canonical discriminant analysis, the inland ecotypes were essentially distinguished from the coastal ecotypes by the following physiological and biochemical traits: Ψpd, gs, polyphenol oxidase, superoxide dismutase and malonyldialdehyde. Inland ecotypes seem to be more tolerant to drought stress than coastal ecotypes.

Differential physiological and antioxidative responses to drought stress and recovery among four contrasting Argania spinosa ecotypes

ABSTRACT Our study was undertaken to ascertain whether the change of the water status and the activation of superoxide dismutase and their isoenzymes in Argan tree can support edaphic drought tolerance and its recovery under rehydration. An experiment was conducted on four contrasting ecotypes of Argania spinosa plants: two contrasting coastal ecotypes (Admine (Adm) and Rabia (Rab)) and two contrasting inland ecotypes (Aoulouz (Alz) and Lakhssas (Lks)). Drought stress significantly decreased the leaf water potential and stomatal conductance in the four contrasted ecotypes. In terms of biochemical responses, significant accumulation of carbonyl groups, hydrogen peroxide and superoxide radical has been recorded in the leaves of stressed plants reflecting oxidative stress. In parallel, the activities of total superoxide dismutase (SOD) and their isoenzymes Cu/Zn-SOD, Cu/Zn-SOD and Fe-SOD were also found to have increased to scavenging ROS and protecting the cell against induced oxidative stress. The recovery kinetics of A. spinosa, as a response to rehydration, were significant and rapid. According to the traits having the most discriminating power, both inland ecotypes (Lks and Alz) showed a better upregulation of its protective mechanisms compared to coastal ecotypes (Rab and Adm). All these adaptive traits make the inland ecotypes as an elite resource of drought tolerance and might become the new focus of domestication research of argan tree in arid and semi-arid environments.

An Assessment of Genetic Diversity and Drought Tolerance in Argan Tree (Argania spinosa) Populations: Potential for the Development of Improved Drought Tolerance

The argan tree (Argania spinosa) occurs in a restricted area of Southwestern Morocco characterized by low water availability and high evapotranspirative demand. Despite the adaptation of the argan tree to drought stress, the extent of the argan forest has declined markedly due to increased aridity, land use changes and the expansion of olive cultivation. The oil of the argan seed is used for cooking and as the basis for numerous cosmetics. The identification of argan tree varieties with enhanced drought tolerance may minimize the economic losses associated with the decline of the argan forest and constrain the spread of desertification. In this study we collected argan ecotypes from four contrasting habitats and grew them under identical controlled environment conditions to investigate their response to drought. Leaf gas exchange analysis indicated that the argan ecotypes showed a high degree of adaptation to drought stress, maintaining photosynthetic activity at low levels of foliar water content and co-ordinating photosynthesis, stomatal behavior and metabolism. The stomata of the argan trees were highly sensitive to increased leaf to air vapor pressure deficit, representing an adaptation to growth in an arid environment where potential evapotranspiration is high. However, despite originating in contrasting environments, the four argan ecotypes exhibited similar gas exchange characteristics under both fully irrigated and water deficit conditions. Population genetic analyses using microsatellite markers indicated a high degree of relatedness between the four ecotypes; indicative of both artificial selection and the transport of ecotypes between different provinces throughout centuries of management of the argan forest. The majority of genetic variation across the four populations (71%) was observed between individuals, suggesting that improvement of argan is possible. Phenotypic screening of physiological responses to drought may prove effective in identifying individuals and then developing varieties with enhanced drought tolerance to enable the maintenance of argan production as climate change results in more frequent and severe drought events in Northern Africa.

Electrolyte ions and glutathione enzymes as stress markers in Argania spinosa subjected to drought stress and recovery

Understanding the mechanisms underlying Argania spinosa responses to drought stress is essential for its regeneration and domestication. Toward that end, an integrative study of tolerance responses to drought stress in four A. spinosa ecotypes (2 contrasting coastal ecotypes (Adm and Rab) and 2 contrasting inland ecotypes (Alz and Lks)) have been conducted. Responses to soil drying and re -watering were measured at physiological and biochemical levels. Soil drying resulted in significant increase in leaf concentrations of potassium (K + ), calcium (Ca 2+ ) and magnesium (Mg 2+ ) with differential responses between ecotypes. The glutathione-related enzymes: glutathione peroxidase (GP), glutathione reductase (GR) and glutathione S-transferase (GST) showed a significant increase in their enzymatic activity in A. spinosa plants subjected to drought stress. Additionally, a significant increase in thiol protein content in the four ecotypes was recorded, during drought stress. These antioxidant traits responded differently depending on ecotype. However, rapid and significant changes in the studied physiological and biochemical traits were observed during recovery from drought, only after four days. According to the traits having the most discriminating power, the both inland ecotypes, especially Lks ecotype, seem to be potential candidates for regeneration of argan forest and their domestication in arid and semi-arid environments. Key words : Argania spinosa, drought stress, glutathione enzymes, thiol compounds, recovery.

Using chlorophyll fluorescence, photosynthetic enzymes and pigment composition to discriminate drought-tolerant ecotypes of Argania spinosa

Abstract The objective of our present study was undertaken to identify and to discriminate the tolerant ecotypes of Argania spinosa in terms of drought tolerance by studying the change of chlorophyll fluorescence traits, chlorophyllase and Ferredoxin-NADP+-oxidoreductase (FNR) activities and pigment composition. A. spinosa plants corresponding to four contrasting ecotypes (Rabia, Admine, Aoulouz and Lakhssas) were exposed to drought stress. Under severe drought conditions (25% of field capacity), we recorded a significant decrease in the maximum efficiency of PSII, the intrinsic efficiency of open PSII (), the coefficient of photochemical quenching (qp) and the quantum yield of PSII electron transport (ΦPSII) (p < 0.001). However, drought stress induced a significant increase in the coefficient of non-photochemical quenching. Positive and significant correlations were found between the ratio Chl a/b and chlorophyllase activity during the period of stress. Anthocyanin content and percentage of pheophytinization increased substantially in parallel with the severity of induced stress. Highest constitutive activity of FNR was recorded in the inland ecotypes (Lakhssas and Aoulouz). According to the canonical discriminant analysis, the four ecotypes have been separated mainly by the traits having the most discriminating power: ΦPSII, qp, and anthocyanin content. Both inland ecotypes seem to be more drought tolerant and very promising for the regeneration of the Moroccan Arganeraie.

Anti-oxidant activity in Argania spinosa callus selected under water stress conditions

Summary This study was carried out to develop an efficient in vitro-selection system for water stress tolerance in Argania spinosa (L.). Calli were induced from nodal explants on Murashige and Skoog medium containing half-strength salts, supplemented with 1.0 mg l-1 _-naphthaleneacetic acid and 1.0 mg l-1 2,4-dichlorophenoxyacetic acid. Increasing concentrations (0, 5, 10, 20, 30, 40, 50, 60, 70, or 80 g l-1) of polyethylene glycol 6000 (PEG 6000) were used to study the effect of simulated water stress on the survival ability of A. spinosa calli. There was a significant reduction in callus growth with increasing concentrations of PEG 6000. Selection for water stress tolerance was achieved by exposing 1- month-old calli to 37.5 g l-1 PEG 6000 (-0.019 MPa) for 3 months. The mechanism(s) of tolerance in the selected calli were studied biochemically. After 8 d of water stress, K+ and Na+ ion concentrations were unaffected in PEG 6000- selected clones (n = 6). The selected calli also showed a significant increase in H2O2 concentration with increased tolerance to water stress compared to non-PEG 6000-selected calli. The activities of several important anti-oxidant enzymes were measured. The results showed important variations between the PEG 6000-selected and non-selected calli. PEG 6000-selected calli show significant increases in the activities of three important anti-oxidant enzymes involved in oxygen metabolism (catalase, peroxidase, and polyphenol oxidase), while superoxide dismutase activity was unchanged. Insignificant increases in malondialdehyde concentration were recorded in the PEG 6000-selected lines compared to the non-PEG 6000-selected calli.The in vitro system described here may be used for in vitro screening for A. spinosa calli that are more tolerant to water stress. Further studies on the performance of plants regenerated from the PEG 6000-stress-selected calli remain to be undertaken to verify the genetic stability of the tolerance induced.