Hormone interplay in the regulation far-red-responsive stem elongation in tomato 4 119 increased with an increase in IAA concentration to a similar level as FR (Figure 4.7). Our inability to synchronize the cell division rate in our analysis could potentially explain why IAA cannot induce internode elongation at a level similar to that observed under FR conditions. Moreover, we assessed periclinal pith cell divisions by counting cell layers and measuring pith radial thickness (Figure 4.7). It was evident that IAA promoted both cell elongation and cell division in internode 1. However, the overall phenotypic response to IAA was less pronounced compared to far-red light (FR) treatment, potentially due to missing hormone interactions and balance. Despite showing similar cellular responses to FR, the effects on overall plant phenotype were not as strong. In summary, IAA does influence cellular processes, including elongation and division in internode 1. However, the complex interplay of hormones and their balances might limit the full phenotypic expression of these cellular changes, resulting in a less prominent overall response compared to FR treatment (Figure 4.7). 4.2.5 NPA induced elongation and diameter decrease in tomato In our quest to understand how plants respond to light, we next tested N-1naphthylphthalamic acid (NPA), a polar auxin transport blocker, to disrupt signals from the leaf. We hypothesized that inhibiting auxin transport would disrupt the response to light. This approach aims to uncover the role of leaf-derived auxin in the complex interaction between light signals and plant growth. In our experiments, we tested various concentrations of NPA brushed on internode 1, namely 250 μM, 125 μM, and 50 μM (Figures 4.8-4.9). We applied the treatments one day prior to FR treatment and then phenotyped the plants after one week of exposure to FR light. We employed NPA in our initial test at a concentration of 250 μM, resulting in a significant impact on overall plant fitness. The stems exhibited substantial elongation, becoming thin to the point of fragility. Following this, we reduced the concentration. As we decreased the concentration to 125 μM, the plants required additional support to stand upright, while at 50 μM, the plants displayed a more robust growth (Figure 4.8). Therefore, we selected 125 μM and 50 μM as the concentrations for our subsequent experiments.
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