Jalal Kassout1,2,3, Mohammed Ater1,3, Sarah Ivorra2,3, Hicham Barbara1,2,3, Bertrand Limier2,3, Jérôme Ros2,3, Vincent Girard2,3, Laure Paradis2,3, Jean-Frédéric Terral2,3,*
1 Laboratoire Botanique Appliquée, Equipe bio-Agrodiversité, Faculté des Sciences, Université Abdelmalek Essaâdi. BP 2060, Tétouan 93 030, Morocco.
2 ISEM, Université de Montpellier, CNRS, IRD, EPHE. Equipe DBA, place Eugène Bataillon, CC65, 34095 Cedex, France.
3 Associated International Laboratory EVOLEA, INEE-CNRS (France – Morocco). *Corresponding author: jean-frederic.terral@umontpellier.fr
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Abstract
This study questions and quantifies the wood anatomical plasticity in Olea europaea L. through two subspecies distributed in Morocco (Olea europaea subsp. europaea var. sylvestris and O.
e. subsp. maroccana) along a biogeographical, climatic and vegetation gradient in order to
assess sap conduction strategies developed by the olive to resist to increasing stressful environmental conditions, particularly aridity.
Wood material was sampled from trees of different populations along a North-South transect of changing environmental conditions, from humid/subhumid to semi-arid bioclimatic conditions. From transversal thin sections of wood prepared with a sliding microtome, characters involved in sap conduction were measured (vessel surface area (µm²)) and counted (vessel density (N/mm²)), number of vessels joined in radial files (N)) using an image analysis system attached to a light transmission microscope. In order to rely branch diameter, anatomical features and environmental parameters, Principal Component Analysis, linear and SMA (standardized major axis) regression analyses were carried out.
Quantitative eco-anatomical analysis highlighted for each olive population a positive linear relationship between the branch diameter and the surface area of vessels. Moreover, we showed that the terms of the simple linear regressions vary according to aridity, water resources and vegetation coverage. From humid/subhumid to semi-arid populations, the intercept of the linear function tends to increase while the slope seems to decrease.
This study sheds light on the pattern of the olive tree functioning, on the adaptation of its sap performance according to ecological conditions and thus, on its strategy to resist to stressful conditions. The established reaction norms show how the cambium functioning modulates and controls sap conduction according to available water resources.
Finally, this study opens new and interesting perspectives for studying at the Mediterranean scale, in wild forms and cultivated varieties, the resistance and the vulnerability of the olive tree to heterogeneous and changing environmental conditions.
Keywords aridity, cambium functioning, functional and ecological xylem anatomy, sap
116
Introduction
In the Mediterranean basin, the ongoing landscape aridification caused by an increase of temperatures, frequency of drought periods and human pressures is one of the major consequence of global change (Quintana-Seguí et al., 2016; Cramer et al., 2018). Moreover, drought stress is considered as one of the most important factor limiting plant development and reproduction. Then, drought stress tolerance and ability to resist to increasing aridity determine the survival and distribution of plants.
This study addresses the question of resistance and vulnerability to drought and aridification of one of the most of iconic trees of the Mediterranean Basin, the olive tree, represented in Morocco by two subspecies. The olive tree belongs to the Olea europaea L. complex comprising 6 subspecies distributed from Asia, Africa to Europe (Médail et al., 2001; Green, 2002; Besnard et al., 2008; Garcia-Verdugo et al., 2009). Four subspecies are diploids: Olea
europaea subsp. europaea distributed around the Mediterranean Sea, O. e. subsp. laperrinei
limited to the Hoggar and Aïr Saharan mountains, O. e. subsp. guanchica (Vargas et al., 2001) in Canary Islands and O. e. subsp. cuspidata broadly distributed from China to South-Africa. Two subspecies located in restricted geographical areas are polyploids, O. e. subsp.
cerasiformis (4x) in Madeira (Portugal) and O. e. subsp. maroccana (6x) in South-western
Morocco.
Among these 6 subspecies, only one of them, Olea europaea subsp. europaea has been domesticated. It is nowadays one of the most important species of the Mediterranean Basin from climatic, ecological, cultural, religious and economic points of view (Kaniewski et al., 2012). The wild olive or ‘oleaster’, Olea europaea subsp. europaea var. sylvestris, is the ancestor of the whole cultivated varieties grouped under the O. e. subsp. e. var. sativa denomination. Wild olive is very widespread around the Mediterranean Basin where it is a fundamental structuring element of many woody plant communities (matorral) that are most often scrublands where it is currently associated with other sclerophyllous species. However, its geographical distribution is very fragmented due to climatic changes, aridification and anthropogenic disturbances on natural habitats. Moreover, some of its populations seem to be very strongly admixed by the cultivated olive tree as the two varieties have overlapping distributions, and finally constitute a complex of populations ranging from true to wild to feral forms (Besnard et al., 2018).
Morocco may be considered as a representative snapshot of olive resistance and vulnerability in the Mediterranean Basin confronted with dramatic environmental issues, climatic changes
117 and increasing human disturbances of natural habitats (FAO and Plan Bleu report, 2018). Economically, the development of competitive and sustainable oleiculture that contributes to food security and local livelihoods, is at the heart of the national policies aiming to meet the needs of national and international markets and to standards determining the extension of olive growing areas (development of irrigated and most often monovarietal orchards) (Ilbert and Lamani, 2016). The choice and the mass spread of allochthonous varieties (for example the Spanish ‘Arbequina’ an ‘Picual’ introduced both in Morocco and Israel) are likely to threaten the local traditional varieties, agroecosystems and practices, as well as the ancestral wild olive plant communities.
In Morocco, under various edaphic contexts, the wild olive and feral populations are broadly distributed from humid and subhumid meso-thermomediterranean bioclimatic conditions in northern regions, to inframediterranean and semi-arid bioclimatic conditions in southern areas where its populations become scarce (Benabid, 1984) (Fig. 1). Wild olive is omnipresent in numerous vegetation types and phytoecological units, and mainly linked with thermophilous Mediterranean plant communities (Benabid and Fennane, 1994). In Northern Morocco, oleaster is the keystone agro-ecosystems where it is preserved and used for rootstock, oil, shade, fodder and for religious reasons (Hmimsa and Ater, 2008; Haouane, 2012; Aummeruddy-Thomas et al., 2014; Ater et al., 2016).
In southern areas, oleaster may be found in sympatry with the endemic Moroccan olive, Olea
europaea subsp. maroccana, associated with other emblematic species such as the Argan tree
(Argania spinosa, Sapotaceae) and Arar tree (Tetraclinis articulata, Cupressaceae) (Fig. 1). As its distribution area is extremely restricted (western part of the High Atlas and in the western Anti-Atlas), its populations are reduced and often isolated, gene flow and subsequent introgression from cultivated olive may be possible, the long-term survival of the Moroccan olive may be questioned.
118 Figure 1. Biogeographical and bioclimatic context of Olea europaea populations (wild and Moroccan olive) from which wood samples were collected and analysed.
119 As in the entire Mediterranean basin, Moroccan wild olive plant communities seem to be greatly affected by global changes (worsening climatic conditions and increasing human pressure). In the current state of research, few studies directly carried out on wild forms (Terral et al., 2004; Kassout et al., 2019) have focused on the understanding of tree functioning in relation to changing and heterogeneous ecological conditions and on the assessment of tree resistance and vulnerability to global warming. However, such studies appear to be essential and justified at least for two fundamental aspects: (1) preservation and conservation of the local and regional biodiversity heritage (Besnard et al., 2018), (2) breeding for new varieties, particularly resistant to pathogen and climatic changes (Bosso et al., 2016; Gabaldón-Leal et al., 2017; De Ollas et al., in press).
The present study deals with the exploration in the wild olive tree through the two subspecies distributed in Morocco, O. e. subsp. europaea var. sylvestris and O. e. subsp. maroccana, of its ecological plasticity by the appraisal of variation of wood anatomical traits involved in sap conduction. It aims to bring new insight onto the functioning of tree, to understand how it optimizes the conduction of sap and resists to an increasing aridification of environmental conditions and therefore a decrease in water resources.