General introduction 17 1 1.2.3.3 Leaf morphology In Arabidopsis, research has been done on lamina blade traits in rosette stage of development. Far-red light signals lead to a blade area reduction coupled with curling (Reed et al., 1993; Devlin et al., 1999; Cagnola et al., 2012). Chitwood and colleagues performed a metaanalysis on the morphological consequences of short-term or long-term exposure to shade in tomato (Chitwood et al., 2012; Chitwood et al., 2015). They found that leaf area, stomatal density, and chlorophyll abundance were changed in shade, and that alteration of leaf shape under shade is dictated by the expression of KNOX and other indeterminacy genes in tomato (Chitwood et al., 2012; Chitwood et al., 2015). A SAS study in maize (Zea mays) indicated that decline in biomass and leaf area growth during competition against weeds are likely occurring via low R:FR signaling from weeds to maize (Page et al., 2009). Woody species such as Populus, Acer, and Betula had faster growth rates in response to supplemental FR if they were late succession species in comparison to early succession species (weedy, fugitive) (Gilbert et al., 2001). In densely populated communities, the competition for living space among plants is influenced by leaf morphology and size within the canopy, impacting the generation of proximity signals and the ability to tolerate shade (Gilbert et al., 2001). 1.2.3.4 Root system architecture In shade avoidance response, plants undergo significant alterations in root morphology and architecture to acclimate to low light conditions and optimize their resource acquisition. Several studies have provided insights into the root modifications observed during shade avoidance. Shade has well-documented effects on above-ground plant tissues, yet understanding its impact on root development remains limited (Gundel et al., 2014). Applying FR-enriched light to whole seedlings is known to reduce main root length and lateral roots (LRs) (Salisbury et al., 2007; van Gelderen et al., 2018). However, the influence of shoot R:FR signaling on root development and the underlying mechanisms are unclear. Root growth is traditionally regulated by auxin transport and signalling (Bhalerao et al., 2002), with auxin playing a role in responses to various stimuli (Baster et al., 2013; Galvan-Ampudia et al., 2013; Zhang et al., 2013). It is plausible that auxin dynamics associated with shoot shade avoidance responses impact root auxin levels, thereby influencing root development. Light signalling further influences auxin transport by regulating auxin-transporter proteins (PINs), demonstrating its intricate impact on root development (Van Gelderen et al., 2018). The dynamic changes in root structure and function during shade avoidance demonstrate the plant’s ability to acclimated to shaded conditions and ensure optimal resource acquisition for sustained growth and development.
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