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Abstract
The Mediterranean region is recognized as a global biodiversity hotspot. However, over the last decades, the cessation of traditional farming in the north part of the Mediterranean basin has given way to strong afforestation leading to occurrence of abandoned agricultural lands colonized by pioneer expansionist species like Pinus halepensis. This pine species is known to synthesize a wide range of secondary metabolites, and previous studies have demonstrated strong allelopathic potentialities of its needle and root leachates. Pinus halepensis is also recognized to release significant amounts of volatile organic compounds (VOC) with potential allelopathic effects that have never been investigated. In this context, the objectives of the present study were to improve our knowledge about the VOC released from P. halepensis needles and roots, determine if these VOC affect the seed germination and root growth of two herbaceous target species (Lactuca sativa and Linum strictum), and evaluate if soil microorganisms modulate the potential allelopathic effects of these VOC. Thirty terpenes were detected from both, needle and root emissions with β‐caryophyllene as the major volatile. Numerous terpenes, such as β‐caryophyllene, δ‐terpinene, or α‐pinene, showed higher headspace concentrations according to the gradient green needles < senescent needles < needle litter. Seed germination and root growth of the two target species were mainly reduced in presence of P. halepensis VOC. In strong contrast with the trend reported with needle leachates in literature, we observed an increasing inhibitory effect of P. halepensis VOC with the progress of needle physiological stages (i.e., green needle < senescent needle < needle litter). Surprisingly, several inhibitory effects observed on filter paper were also found or even amplified when natural soil was used as a substrate, highlighting that soil microorganisms do not necessarily limit the negative effects of VOC released by P. halepensis on herbaceous target species.
Highlights
Plant community organization and dynamics are under the con‐ trol of biotic processes, plant–plant interactions such as resource competition, facilitation, and allelopathy (Callaway & Walker, 1997)
In addi‐ tion to be highly variable among species, the diversity and quantity of allelochemicals produced by a given species and their influence on a target species are strongly dependent on its phenological stage (Fernandez et al, 2009; Hashoum et al, 2017; Santonja, Le Rouzic, et al, 2018)
Needle physiological stage was a key factor of the observed allelopathic effects in the present study as they explained a large part of relative allelopathic effect (RAE) variance on both seed germination and root growth of (a) 0 β-caryophyllene concentraƟon
Summary
Plant community organization and dynamics are under the con‐ trol of biotic processes, plant–plant interactions such as resource competition, facilitation, and allelopathy (Callaway & Walker, 1997). Seed germination and seedling perfor‐ mance are the main life stages usually affected by allelochemicals, and frequent negative allelopathic effects are inhibition of seed germination (Fernandez et al, 2013; Herranz, Ferrandis, Copete, Duro, & Zalacain, 2006), delay of seed germination (Fernandez et al, 2013; Hashoum et al, 2017), and inhibition of seedling growth (Gavinet et al, 2019; Santonja, Le Rouzic, & Thiebaut, 2018) by al‐ tering physiological processes (e.g., photosynthesis, nutrient uptake, cell division, or elongation; Inderjit & Duke, 2003). Most of the published allelopathy studies were performed by using only green leaves (or needles) and neglected the allelopathic potentialities of chemicals contained in senescent leaves or leaf litter. Most of the published allelopathy studies were performed by using only green leaves (or needles) and neglected the allelopathic potentialities of chemicals contained in senescent leaves or leaf litter. Hashoum et al (2017) reported that the germination velocity of two target herbaceous species (Festuca ovina L. and Linum perenne L.) was inhibited by aqueous extracts of senescent leaves of woody species (Acer monspessulanum L., Cotinus coggygria Scop., and Quercus pubescens Willd.) while their seedling growth was affected by aqueous extracts of green leaves
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