Resisting Aridification: Adaptation of Sap Conduction Performance in Moroccan Wild Olive Subspecies Distributed Over an Aridity Gradient

Abstract

In the current context of global change, the increasing frequency and the length of drought periods are testing the resistance capacities of plants of dry habitats. However, although the adaptation of plants to drought has been widely studied, the anatomical features of wood influencing the functional responses of plants to drought are still lacking at the intraspecific level, especially for species with a wide geographical distribution. As a result, we have studied the variation of wood anatomical traits related to sap conduction (i.e., vessel surface area, vessel density, and number of vessels joined by radial file) in two wild olive subspecies distributed in Morocco (i.e., Olea europaea subsp. europaea. var. sylvestris and Olea europaea subsp. maroccana), in relation to various drought conditions. This functional study, based on wood trait measurements of 351 samples from 130 trees and 13 populations, explores potential sap conduction in relation to environmental parameters and as a result, strategies to resist water stress. We found that (1) branch diameter (BD) captured 78% of total wood trait variation, (2) vessel size (SVS) expressed 32% of intraspecific variation according to cambium age, and (3) the positive relationship between SVS and BD could be explained by climate type, vegetation cover changes, and therefore available water resources. Taking into consideration the diameter of the branch as the main factor of anatomical variation, established reaction norms (linear models) at the intrapopulation scale of vessel lumen area according to aridity show for the first time how the functioning of the cambium modulates and controls sap conduction, according to aridity and thus available water resources. They pinpoint the risks incurred by the wild olive tree in the perspective of a dramatic increase in aridity, in particular, the inability of the cambium to produce large enough vessels to efficiently transport sap and irrigate the leaves. Finally, this study opens new and interesting avenues for studying at a Mediterranean scale, the resistance and the vulnerability of wild forms and cultivated varieties of olive to heterogeneous and changing environmental conditions.

Image analysis of Moroccan carob seeds (Ceratonia siliqua L.) revealed substantial intraspecific variations depending on climate and geographic origin

“…On a large geographic scale, Soper Gorden et al (2016) showed that climate is the dominant factor explaining between-species variability in seeds size. Likewise, the aridity gradient found in our study could play an important role in trait variation patterns (Kassout et al 2021). Indeed, we found that increasing aridity (low MAP and high MTWM) has a significant effect on seeds' shape (e.g., circularity, roundness).…”

“…Therefore, the variation expressed in each of the studied ecological scales will have been affected by genotype, phenotypic plasticity (Geber and Griffen 2003;Albert et al 2010a), and potential differences in phenology, land use, and history of the study area. However, several studies pointed to the predominance of environmental factors in the expression of intraspecific variability (Jung et al 2014;Siefert et al 2015;Kassout et al 2019Kassout et al , 2021Kuppler et al 2020). Partitioning of the variance in carob seeds shows that area, length, width, and perimeter expressed an important amount of variation between geographic zones.…”

“…For instance, in the widespread wild olive trees in Morocco, Kassout et al (2019) showed that ecophysiological leaf-related traits express important variability depending on the large-scale gradient of aridity. In contrast, hydraulic conductance traits (e.g., vessel lumen size) were largely controlled by variations in local conditions, such as water availability and vegetation type (Kassout et al 2021). Fenollosa et al ( 2021) revealed high inter-population variability in seed traits of Carpobrotus edulis according to both geographic and local conditions.…”

Effects of climate change on the distribution of the native Carob tree (Ceratonia siliqua L.) in Morocco

“…The correlations between precipitations-related variables and plant physiological responses were also reported for the thermophilous and Mediterranean wild native olive tree (Olea europaea subsp europaea var. sylvestris) in Morocco (Kassout et al, 2019(Kassout et al, , 2021. This observed pattern might be regarded as the results of the biogeographic history of Mediterranean ora (Blondel et al 2010).…”

Linkages between stomatal density and minor leaf vein density across different altitudes and growth forms

“…The evolution of stomata undergone a transformation from large and rare to small and dense ( Liu et al., 2019 ), which is closely associated with decrease in atmospheric carbon dioxide concentration ( Feild et al., 2011 ). Dense stomata contribute to high transpiration ( Kassout et al., 2021 ), whereas water loss is unavoidable as a result of the exchange of water vapor through stomata ( Brodribb and Jordan, 2011 ; Zhao et al., 2020 ), thus an urgent water supply is needed to compensate for these moisture losses ( Scoffoni et al., 2017 ). Leaf veins transport water for transpiration and photosynthesis and ensure water supply within leaves ( Brodribb and Field, 2010 ; Zhu et al., 2020 ).…”