The stomatal crypts in some Banksia speacies does not reflect their assumed functions

Stomatal crypts, the depressions of the epidermis containing stomata, are among the most frequently cited as examples of an adaptation that reduces water loss. The effect of stomatal crypts, despite the considerable assumption regarding its possible impact on gas diffusion, has never actually been quantified. In accordance with Fick’s law of diffusion, assuming the structure of the crypt as a tube, we hypothesized that as the depth of the crypt increases, the diffusion path length increases, and as a consequence, the resistance to the diffusion of gas increases as well. Hence, at a constant cross-sectional area, leaves with a deeper crypt should have lower water loss. Despite assumptions about the function of stomatal crypts, there are surprisingly few published studies on the physiological effects of crypts. This study evaluated the micromorphology of stomatal modifications in a range of Banksia species as well as the impact of stomatal crypts on leaf gas exchange. We hypothesized that as crypt depth increased, transpiration and photosynthesis would decrease for a given VPD. If this were the case, this would support the idea that crypts are an adaptation to reduce water loss in arid environments. Leaf cross-sections and micrographs of over 110 species of the Proteaceae family were examined. Fourteen species of Banksia, as well as Dryandra praemorsa, were selected for this study. Two-year-old seedlings of the 15 species were obtained from Protea World, Adelaide, Australia, and grown for one year in 2 L pots containing premium potting mix (Premium Potting Mix, Australian Standard, AS3743) in a glasshouse at the University of Adelaide, Australia. The current study demonstrated that crypts occurring in the epidermis of the Banksia species examined at different depths and widths did not impact on gas diffusion through stomata. Also, the present results showed that deeper stomatal crypts did not have a significant impact on transpiration compared with leaves that had shallower crypts. The positive relationship between leaf thickness and depth of crypts and the negative relationship between leaf thickness and stomatal density in Banksia species found in this study might suggest that stomatal crypts possibly act as a means of overcoming mesophyll resistance to CO2 diffusion. Further studies are required to investigate this possibility.