Permanently open stomata of aquatic angiosperms display modified cellulose crystallinity patterns

Most floating aquatic plants have stomata on their upper leaf surfaces, and usually their stomata are permanently open. We previously identified 3 distinct crystallinity patterns in stomatal cell walls, with angiosperm kidney-shaped stomata having the highest crystallinity in the polar end walls as well as the adjacent polar regions of the guard cells. A numerical bio-mechanical model suggested that the high crystallinity areas are localized to regions where the highest stress is imposed. Here, stomatal cell wall crystallinity was examined in 4 floating plants from 2 different taxa: basal angiosperms from the ANITA grade and monocots. It appears that the non-functional stomata of floating plants display reduced crystallinity in the polar regions as compared with high crystallinity of the ventral (inner) walls. Thus their guard cells are both less flexible and less stress resistant. Our findings suggest that the pattern of cellulose crystallinity in stomata of floating plants from different families was altered as a consequence of similar evolutionary pressures.