Chapter 2 34 In addition to these measurements, we also assessed stem diameter. Our observations revealed a notable increase in the diameter of internode 1 of M82 in response to FR+WL (Figure 2.6). This augmented diameter response could serve as a critical mechanism to bolster the tomato stems against potential breakage and prevent tomatoes from toppling over during rapid stem elongation. It indicates that there is not only longitudinal elongation in supplemental FR, but also radial expansion in M82, but not so much in MM. Figure 2.6. Stem diameter of internode 1 of three-week-old M82 and Moneymaker after seven-day WL and FR+WL treatments. Asterisks indicate significance between WL and WL+FR as follows: * p≤ 0.05; ** p≤0.01, *** p≤0.001. Biological replicates n=18, the experiment was repeated three times In the literature, the effect of FR on leaf area is not always consistent. In Arabidopsis leaf area reduces in SAS (Keiller and Smith, 1989; Robson et al., 1993) while leaf area increases in Trifolium repens and tomato (Weijschedé et al., 2006; Chitwood et al., 2015). In our experiments (Figure 2.7), no obvious changes to the leaf area were observed. Building upon prior research that highlighted the pivotal role of phytochromes in redirecting carbon metabolism towards growth in Arabidopsis (Yang et al.), and given the limited knowledge about the specifics of leaf cell structural changes in tomato SAS (Courbier et al., 2021), the collection of fresh and dry leaf weights in shade avoidance experiments enables a deeper understanding of how plants adjust their resource allocation and biomass accumulation strategies in response to fluctuations in light conditions. We measured a series of leaves, starting from the bottom of the stem (oldest) to the shoot apex (youngest) through image analysis. Petiole and rachis were removed as much as possible from the image. We observed no significant responses of leaf area to WL+FR as compared to WL controls (Figure 2.7). Since the leaves looked more yellow in WL+FR than in WL, we assumed the cellular
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