*3.3. OfXTHs Response to Ambient Temperature in Regulation of Flower Opening*

Flowering in many species is influenced by ambient temperature. In *Arabidopsis*, the transition to flowering is delayed in low ambient temperatures [32]. Phased constant temperature treatment and a lower temperature could shorten the flower differentiation process and promote the flowering of *Crocus sativus* [33]. Similarly, the change in ambient temperature affected flower-opening processes significantly in *O. fragrans*. The flower opening is significantly promoted by low ambient temperature (19 ◦C) in *O. fragrans*, and the flower buds could not open until ambient temperature was below 23 ◦C [15]. However, there are fewer reports on the role of *XTHs* in responding to low ambient temperature; most studies focused on the high temperature regulating cell growth. For example, the expression of *AtXTH9* and *AtXTH11* was strongly increased by high temperature in stem and root, respectively, to accelerate cell elongation [8,34]. Through transcriptome analysis, we found that five and sixteen *OfXTH* genes were differentially expressed at the second and fifth day under 19 ◦C treatment compared with the control 23 ◦C, respectively (Table 3). Specifically, *OfXTH24*, *27*, *32*, *35*, and *36* were significantly upregulated at both the second and fifth day after 19 ◦C treatment, which indicated that *OfXTH24*, *27*, *32*, *35*, and *36* might respond to ambient temperature changes and contribute to the process of low ambient temperature accelerated flower opening of *O. fragrans.*
