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 Moroccocharacterized by low water availability and high evapotranspirative demand. Despitethe adaptation of the argan tree to drought stress, the extent of the argan forest hasdeclined markedly due to increased aridity, land use changes and the expansion of olivecultivation. The oil of the argan seed is used for cooking and as the basis for numerouscosmetics. The identification of argan tree varieties with enhanced drought tolerancemay minimize the economic losses associated with the decline of the argan forestand constrain the spread of desertification. In this study we collected argan ecotypesfrom four contrasting habitats and grew them under identical controlled environmentconditions to investigate their response to drought. Leaf gas exchange analysisindicated 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-ordinatingphotosynthesis, stomatal behavior and metabolism. The stomata of the argan trees werehighly sensitive to increased leaf to air vapor pressure deficit, representing an adaptationto growth in an arid environment where potential evapotranspiration is high. However,despite originating in contrasting environments, the four argan ecotypes exhibited similargas exchange characteristics under both fully irrigated and water deficit conditions.Population genetic analyses using microsatellite markers indicated a high degree ofrelatedness between the four ecotypes; indicative of both artificial selection and thetransport of ecotypes between different provinces throughout centuries of managementof 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 inidentifying individuals and then developing varieties with enhanced drought tolerance toenable the maintenance of argan production as climate change results in more frequentand severe drought events in Northern Africa.