STOMATAL DYNAMICS IN POALES

<p dir="ltr">Stomata are mobile epidermal valves that regulate CO₂ uptake at the cost of water loss in response to changes in light, CO₂, vapor pressure deficit (VPD), and other signals. Variation in stomatal function across species can have a significant impact on plant function; however, this variation is often examined within major land plant lineages or within a single species. Here, variation in stomatal function in response to light and water was investigated within the order Poales, one of the most ecologically and economically important plant orders. Light-driven stomatal responses operate through two independent pathways: red light (RL) signaling, mediated by mesophyll photosynthesis, and blue light (BL) signaling, mediated directly by guard cells (GCs). To investigate whether mesophyll anatomy influences stomatal responses to light, I examined stomatal responses to light in two subfamilies of Restionaceae with highly divergent substomatal mesophyll architecture: Restionoideae, which possess protective cells that line a large substomatal chamber, and Leptocarpoideae, which lack such cells. Restionoideae species have lost RL-induced stomatal opening and stomatal sensitivity to CO₂ concentrations. These results demonstrate that mesophyll anatomy plays a critical role in determining RL-driven stomatal responses. Within Poales, the Poaceae possess unique dumbbell-shaped GCs, while most non-grasses in the order have kidney-shaped GCs. It has been hypothesized that the dumbbell-shaped stomata of grasses confer greater mechanical advantage (MA) through interactions between GCs and subsidiary cells (SCs), but this has not been experimentally confirmed. By investigating stomatal responses to leaf excision, I found that grasses have exceptionally high MA. In contrast, non-grass Poales exhibited minimal MA, which is very different from most other angiosperms. High MA in grasses is driven by smaller GCs relative to larger SCs and increases stomatal responsiveness to light under highly evaporating conditions. These results may also explain the remarkable ecological success of grasses, compared to other families of Poales, and highlight that the considerable diversity in stomatal function within Poales is driven by epidermal and substomatal anatomy.</p>