Abstract
The responses of stomatal aperture to light intensity and CO2 concentration were studied in both Vicia faba (C3) and Kalanchoë fedtschenkoi (Crassulacean acid metabolism; CAM), in material sampled from both light and dark periods. Direct comparison was made between intact leaf segments, epidermises grafted onto exposed mesophyll, and isolated epidermal peels, including transplantations between species and between diel periods. We reported the stomatal opening in response to darkness in isolated CAM peels from the light period, but not from the dark. Furthermore, we showed that C3 mesophyll has stimulated CAM stomata in transplanted peels to behave as C3 in response to light and CO2. By using peels and mesophyll from plants sampled in the dark and the light period, we provided clear evidence that CAM stomata behaved differently from C3. This might be linked to stored metabolites/ions and signalling pathway components within the guard cells, and/or a mesophyll-derived signal. Overall, our results provided evidence for both the involvement of guard cell metabolism and mesophyll signals in stomatal responses in both C3 and CAM species.
Highlights
The waxy leaf surface is almost impermeable to carbon dioxide and water and most gaseous exchange between the leaf interior and the external environment passes through the stomatal pores on the leaf surface (Lawson, 2009)
We showed that stomata from isolated peels behaved in a similar manner to both intact leaf segments and epidermal transfers, with exception of those from the Crassulacean acid metabolism (CAM) “dark” chamber (Figure 5B)
The stomatal aperture in the CAM leaf sections/peels sampled in the light, as well as intact leaf segments sampled in the dark, increased in response to darkness (Figures 2, 5A), it was noteworthy that dark sampled K. fedtschenkoi epidermal peels did not respond to changes in light or [CO2] (Figure 5B)
Summary
The waxy leaf surface is almost impermeable to carbon dioxide and water and most gaseous exchange between the leaf interior and the external environment passes through the stomatal pores on the leaf surface (Lawson, 2009). In species with C3 and C4 photosynthetic metabolism, stomata open in response to low CO2 concentration, high light, and low VPD, whereas closure is driven by the reverse, high CO2 concentration, low light, and high VPD (Outlaw, 2003; Vavasseur and Raghavendra, 2005; Shimazaki et al, 2007; Lawson, 2009) It is well-established that stomatal conductance (gs) correlates with mesophyll rates of photosynthesis under a range of different conditions (Farquhar and Wong, 1984; Wong et al, 1979; Mansfield et al, 1990; Buckley et al, 2003). These findings had led to the proposal that there must be an alternative signal (see Lawson et al, 2018)
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