Although stomata have been extensively studied in many plant families, there are still some plant families for which there is limited knowledge on stomata. Some examples of these families include:
Podostemaceae: This is a family of aquatic plants that are found in fast-flowing streams and waterfalls. There is limited information on stomatal morphology and function in these plants.
Hydnoraceae: This is a family of parasitic plants that grow underground and have no leaves. As a result, there is very little known about the stomata in these plants.
Balanophoraceae: This is another family of parasitic plants, which are mostly subterranean and lack chlorophyll. The stomata of these plants have not been well studied.
Corsiaceae: This family contains about 15 species of mycoheterotrophic plants that live in symbiosis with fungi. There is limited information on the stomata of these plants.
It is worth noting that even in plant families for which there is some knowledge on stomata, there may still be gaps in our understanding. Ongoing research is constantly expanding our understanding of stomatal morphology and function across the plant kingdom.
The compilation of new data on stomatal ontogeny from the literature and the finding of a rather unknown type inMarcgravia have shown the need of a new classification of the ontogenetic types of stomata. Pant (1965) recognized 10 main types; this number is now enlarged to 26 and a modified terminology is chosen. From the name of each type not only the ontogenetical pattern but also the morphological nature of the adult stoma can be deduced. Thus the gap between morphological and ontogenetical classifications has been bridged. Two important differences from Pant’s classification and definitions are introduced. In this classification any other new type can be included; all the possibilities for the introduction of supplementary data are left open.
CURRENT DISCUSSIONS ON STOMATA / INTERESTING THEMES FOR FUTURE RESEARCH
Stomata and climate change:
Scientists currently discuss the role of stomata in plant responses to climate change. Some suggest that stomata may help plants adapt to changing environmental conditions, while others argue that stomatal responses may be too slow or limited to provide adequate protection against the effects of climate change. More research is needed.
Regulation of stomatal opening and closing:
Scientists still discuss the importance of different factors, such as light, CO2, humidity, and hormones, in the regulation of stomatal behavior. It is an interesting domain in the plant physiology. More research will deliver interesting views.
Stomatal density and plant functions:
It has been suggested that there may be a relationship between stomatal density and some plant functions, such as photosynthesis and water use efficiency. However, a number of scientists discuss the extent to which stomatal density is really a reliable indicator of plant performance. More research is needed.
Evolution of stomata: Some researchers argue that stomata have evolved quite early in the history of land plants. Our knowledge about the mechanisms and selective pressures that drove their evolution is rather limited. The debate is going on.
Leaf (or assimilation branch) epidermal micromorphology of desert plant in arid and semi- arid areas of China
Liu Y.-B., Li X.-R., Li M.-M., Liu D., Zhang W.-L. (2016)
LIU Yu-Bing1, LI Xin-Rong1, LI Meng-Meng1, LIU Dan1, ZHANG Wen-Li1,
1.Shapotou Desert Research & Experiment Station, and Key Laboratory of Stress Physiology and Ecology in Cold and Arid Regions of Gansu Province, Northwest Institute of Eco-environment and Resources, Chinese Academy of Sciences, Lanzhou, China 730000)
Leaf epidermal micromorphology is an important adaptation of desert plants to arid environment. A micromorphological analysis of leaf epidermal tissue of desert plants was carried out in order to obtain qualitative and quantitative data on epidermal characteristics and to evaluate the long-term adaptive strategy of desert plants to aridity in desert conditions.
The leaf (or assimilation branches) materials were sampled for more than 200 natural populations of 117 desert plant species from 74 genera and 28 families, in arid and semi-arid areas of China. The characteristics of leaf epidermal micromorphology of desert plants were then measured by scanning electron microscopy (SEM). The characteristics of epidermal cell, trichome, stomatal, cuticular wax on adaxial and abaxial surface were presented.
Leaf epidermal micromorphology of desert plants showed abundant diversity in different classification levels. The desert plants adapted to environmental stress could be divided into 11 basic morphological types according to the structure of the epidermis, and their characteristics of leaf epidermal morphology were classfied into 6 main types according to the relationships between stress resistance and structural characteristics of epidermal micromorphology and their appendages.
The main epidermal appendages of desert plants (such as trichome, cuticular wax) and epidermal structures (concave-convex and papillary structure, stomata) could cooperate with each other to improve the resistance of desert plants to drought and other adverse environmental stress by resisting the strong light and reducing leaf transpiration.
Based on SEM examinations of about 5000 species of seed plants, this is a survey of their epidermal surface characters with an aim to application in taxonomy. Surface characters may be grouped into four categories: (1) Cellular arrangement or cellular pattern. (2) Shape of cells (the “primary sculpture” of a surface). (3) Relief of outer cell walls (the “secondary sculpture” superimposed on the primary sculpture), caused mainly by cuticular striations and superficially visible wall inclusions and wall thickenings. (4) Epicuticular secretions (the “tertiary sculpture” superimposed on the secondary sculpture), i.e. mainly waxes and related substances.
The systematic applicability is discussed for each of these structural groups. Epidermal characters are only slightly influenced by environmental conditions. Their high structural diversity provides most valuable criteria for the classification between species and family level. There is also some evidence for their systematic applicability above the family level.
The possible evolutionary–ecological significance of surface sculpturing is discussed briefly. There is evidence that these features may be seen primarily under the aspects of reduced ability of plants to contaminate and as temperature control mechanisms of the surfaces.
The acquisition of stomata is one of the key innovations that led to the colonisation of the terrestrial environment by the earliest land plants. However, our understanding of the origin, evolution and the ancestral function of stomata is incomplete. Phylogenomic analyses indicate that, firstly, stomata are ancient structures, present in the common ancestor of land plants, prior to the divergence of bryophytes and tracheophytes and, secondly, there has been reductive stomatal evolution, especially in the bryophytes (with complete loss in the liverworts). From a review of the evidence, we conclude that the capacity of stomata to open and close in response to signals such as ABA, CO2 and light (hydroactive movement) is an ancestral state, is present in all lineages and likely predates the divergence of the bryophytes and tracheophytes. We reject the hypothesis that hydroactive movement was acquired with the emergence of the gymnosperms. We also conclude that the role of stomata in the earliest land plants was to optimise carbon gain per unit water loss. There remain many other unanswered questions concerning the evolution and especially the origin of stomata. To address these questions, it will be necessary to: find more fossils representing the earliest land plants, revisit the existing early land plant fossil record in the light of novel phylogenomic hypotheses and carry out more functional studies that include both tracheophytes and bryophytes.
The average number of stomata per unit area of a leaf was found to be inversely proportional to the area of the lamina. The constant obtained by the multiplication of these values is termed as the Absolute Stomatal Number. This value is constant in those leaves where differentiation of the stomata has been completed. This has been shown with shown with the developing leaves of five Solanaceous plants.
Controlling the distribution of stomata is crucial for the adaptation of plants to new, or changing environments. While many plant species produce stomata predominantly on the abaxial leaf surface (hypostomy), some produce stomata on both surfaces (amphistomy), and the remaining few produce them only on the adaxial surface (hyperstomy).
Various selective pressures have driven the evolution of these three modes of stomatal distribution. Despite recent advances in our understanding of stomatal development and dorsiventral leaf polarity, the genetic basis for the evolution of different stomatal distributions is still unclear.
Here, we propose the genus Callitriche as a new model system to investigate patterns in the evolution of stomatal distribution. Callitriche comprises species with diverse lifestyles, including terrestrial, amphibious, and obligately aquatic plants. We found that species in this genus cover all three modes of dorsiventral stomatal distribution, making it a desirable model for comparative and evolutionary analyses on distribution modes. We further characterized the genetic basis of the different distribution modes, focusing on the stomatal key transcription factor SPEECHLESS. Future research using the promising model system Callitriche would open a new direction for evolutionary developmental biology studies on stomata.
To summarize, we characterized the stomata of Callitriche species that exhibit different stomatal distribution modes, namely amphistomy (C. terrestris), hyperstomy (C. palustris, C. stagnalis), and hypostomy (C. japonica) (Figure 1). The occurrence of all three distribution modes in the same genus will facilitate evolutionary analyses of stomatal distribution. For example, future investigations could focus on how the different lifestyles of Callitriche species have affected the evolution of stomatal distribution. Various factors including light, growth rate, leaf thickness, and herbaceous or woody nature influenced the evolution of stomatal distributions.6–8,12 However, previous studies examined few or no hyperstomatous species, many of which have amphibious or floating lifestyles. Therefore, the mechanisms by which the transition to aquatic lifestyles affected the evolution of stomatal distribution are currently unclear. The genus Callitriche could be an excellent model system to bridge this knowledge gap. Moreover, genetic pathways involved in stomata development could be investigated further. Here, we found that the spatial expression pattern of SPCH corresponds with the different stomatal distribution modes (Figure 2). Recently, the regulation and expression profiles of SPCH were investigated not only in Arabidopsis26 but also in tomato,27 another potential model for the evolution of stomatal distribution.24 A comparison of Callitriche SPCH sequences and functions with those of such other emerging model systems could provide insight into evo-devo studies of stomatal distribution. Collectively, this study illuminates the potential of the genus Callitriche as a new model system for studying the evolution of plant stomata.
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