Stomata are structural elements of plant epidermis. They are composed of two guard cells divided by a stomatal pore.
Stomata control transpiration and gas exchange. Their guard cells are capable of reversible deformation. As a result of
this deformation, the stomatal pore opens or closes. The structural features of the guard cells responsible for their
deformation are still under debate. It is believed that stomatal movements depend mainly on turgor pressure in the guard
cells and on the structure of the guard cell walls, including their uneven thickness.
All guard cells have outer ledges formed by cuticle and located on their upper sides not far from the stomatal pore. The
guard cells can also bear folds that surround stomatal ledges forming marginal stomatal rings. Typically, the rings result
from the folding of the cuticle itself. The subcuticular space of such folds can be filled with pectin or with fibrous wall-
like materials. To elucidate the role of the rings, we have applied dynamic modeling using the finite element method.
The data on the shape of the guard cells, on uneven thickness of their walls, on localization and relative sizes of
stomatal ledges and rings were accurately reproduced during model stoma construction. Turgor pressure was simulated
by creating the load distributed over the inner surfaces of the guard cells. Stomata with marginal rings are located on the
subsidiary cells. The dynamic modeling has shown that the marginal stomatal rings are able to influence the movements
of the guard cells. The turgid guard cells without outer ledges and marginal stomatal rings bulge above the leaf surface.
The wide opening pore between such guard cells moves in the same direction and rises above the leaf surface as well.
The outer ledges prevent wide opening of the stomatal pore and cause its sinking below the leaf surface. Stomatal rings
can enhance this effect. The influence of marginal rings on stomatal movements depends on the mechanical
characteristics of the rings, namely on their rigidity and squeezing of stoma by them. The formation of rigid rings or
rings squeezing the stoma in addition to rigid ledges leads to the deepest sinking of the open pore below the leaf
surface.
The methods of light, scanning and transmission electron microscopy have shown that a stoma can bear not only one,
but many folds which either form several stomatal rings, or intertwine with each other, forming massive compound
stomatal rings. In some plant species, stomata are surrounded by peristomatal rims tightly pressed against the marginal
stomatal rings. Unlike the stomatal rings, the peristomatal rims are located not on the stomata, but on the cells around
them. It has also been established that stomatal rings can be induced by undulate morphology of the cellulose cell walls.
Taking into account all these data allows to adjust simulation results and reveals a stronger influence of stomatal rings
on the movements of the guard cells. Marginal stomatal rings have been found in plants that occupy different positions
in the APG IV taxonomic system, in both archaic and evolutionarily advanced plant groups. This indirectly indicates the
essential role of these structures.
The research was carried out with the support of the Russian Science Foundation grant No. 22-24-00572,
https://rscf.ru/en/project/22-24-00572 /.