Chapter 5 168 observed in the WL+FR, although the response was mild (Figure 5.2a). Next, later we subjected Arabidopsis to LD with 150 PAR with same R:FR=0.2, and the result was stronger (Figure 5.2a). We then continued with test LD and SD with R:FR=0.11 to see if lower R:FR ratio would increase the response (Figure 5.2b). In LD, the response was significant already at 5 days with 150 PAR, accompanied by a significant increase in inflorescence stem diameter (Figure 5.2b, c). Then we tested the effect of light intensity and if it could induce a stronger or a more robust response (Figures 5.2d, e). With higher light intensity of 200 PAR and R:FR 0.20 in LD, we observed FR-induced elongation also at 5 days, with the diameter first narrowing before the radial growth was observed (Figure 5.2d, e). All in all, we concluded that the robustness of the FR-induced elongation in the Arabidopsis inflorescence stem was highly dependent on light level and the day length, and observable only in a brief window of time. 5.2.2 Infloresence stem elongation in SAS is not associated to pith elongation in Col-0 We investigated the cellular morphology, including the pith cell elongation response in Arabidopsis inflorescence stems in response to low R:FR. Previous literature (Franklin, 2008) had reported consistent elongation of inlorescence stem structures in such conditions. Inspired by the phenomenon that we observed in Chapter 2 in tomato cultivars Moneymaker and M82, where pith cells exhibited elongation, we applied a similar sectioning method to assess pith cell response in Arabidopsis Col-0. We used 200 PAR in LD with R:FR=0.11 for further work. The pith cell sizes appear heterogeneous already by eye (Figure 5.3a), so we quantified them from different locations within the pith cylinder. We measured the length of pith cells in the outermost two layers and the middle layers, comparing different light treatments in Col-0 (Figure 5.3). Surprisingly, pith cell length in either location did not exhibit significant elongation in the WL+FR treatment. However, we observed a significant increase in pith thickness in WL+FR, and radial diameter, suggesting some level of cell expansion in the radial dimension. So, the pith cell response in Col-0 was not consistent with our expectations but the radial growth aligned with tomato data. Given that the radial growth phenotype was observed, we wanted to test what would happen to the FR-responsive influorescence stem elongation in radial growth mutants
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