Chapter 4 116 Initially, a whole-plant spray failed to elicit a significant internode 1 response at both 25 μM and 100 μM (Figure 4.5). However, when 100 μM IAA was applied through brushing on internode 1 and leaf 1, a mild elongation in both hypocotyl and internode was observed (Figure 4.5). This outcome serves as a promising indication of the efficacy of the brushing method. We also wanted to test if we could mimic the local FR treatment (Figure 4.4) by local IAA applications. We applied 100 μM IAA separately to both leaf 1 and internode 1, each with three applications of brushing over a week-long treatment period (Figures 4.5 and S4.3). We observed that the internode 1 elongated in response to IAA treatment in WL with both internode and leaf treatments, and the response was similar in WL for both tissues but did not reach the level of FR-responsive elongation (Figure 4.5g). However, we did not find any significant IAA-induced changes in supplemental FR conditions for either treatment location (Figures 4.5g,h and S4.3). We also quantified leaf growth, to assess negative effects that the IAA treatment might be causing. Application of IAA to leaf 1 or to Internode 1 led to a significant decrease in both leaf 1 and 2 areas (Figure 4.5i,j). We here considered some assumptions: first, auxins are known to promote apical dominance. When IAA is applied to the stem, it may enhance apical dominance, directing more resources towards the main stem and inhibiting lateral growth, including leaf expansion. Second, the application of IAA to the stem may disrupt the hormonal balance, leading to altered growth patterns in a complex network that regulates plant growth and development. Imbalances in these hormone levels can impact leaf development. Further investigation of this effect is needed, but it is not the focus in this chapter.
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