Chapter 2 30 epidermal cell numbers and lengths increase in response to shade, contributing to stem and petiole elongation. While these cellular responses have been observed in Trifolium repens and soybean (Weijschedé et al., 2008; Lyu et al., 2021), similar investigations in tomato are scarce (Courbier et al., 2021). In this chapter, we measured various architectural traits in response to low R:FR in two commercial tomato cultivars: M82 and Moneymaker. M82 is a model for field-grown processing tomatoes. M82 has medium-sized oval fruit, originates from the USA, and has been used for many gene function studies (Brooks et al., 2014; Xu et al., 2015; Gupta and Van Eck, 2016; Kajala et al., 2021). Moneymaker is a greenhouse crop from the UK, cultivated for more than 80 years worldwide, and is a typical model for greenhouse tomatoes in research (https://www.ufseeds.com/product/moneymaker-tomato-seeds/ TOMO.html; Courbier et al., 2021).As these two cultivars have differences in usage and growth environment, we chose to compare them in this study to provide a more comprehensive understanding of the SAS among tomato cultivars. After the architectural traits, we used microscopy to characterize the cellular anatomy SAS of the first internode and identified the most significantly responding cell types, 2.2 RESULTS 2.2.1 FR promotes hypocotyl length, but has no effect on root growth in tomato seedlings In SAS studies of Arabidopsis, many responses at seedling stages are of interest, including hypocotyl elongation, hyponasty (increased leaf angle), and changes in root system architecture (Kasperbauer, 1971; Pierik et al., 2004; Bou-Torrent et al., 2014; Pantazopoulou et al., 2017; van Gelderen et al., 2018). Arabidopsis seedlings root system responds to FR enrichment with decreased lateral root density (van Gelderen et al., 2018; Kang, 2018). We wanted to see how Arabidopsis knowledge translates to a crop species, so we set out to characterize phenotypes of tomato seedlings in white light supplemented with far-red (WL+FR) compared to white light (WL) control. First, we conducted seedling characterization. We grew S. lycopersicum cv. Moneymaker seedlings on plates for 5 dag (days after germination) and then treated them with supplementary FR or continued with control WL for 7 days. At this point, the cotyledons had emerged but the seedlings did not yet have true leaves (Figure 2.1). We measured hypocotyl length (Figure 2.2a), primary root length (Figure 2.2b), lateral root length (Figure 2.2c), total lateral root length (Figure 2.2d), lateral root number (Figure 2.2e), and calculated the lateral root density as lateral root number divided by primary root length (Figure 2.2f).
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