The plant meristem is a type of tissue found at several locations on plants. This tissue is composed of cells which are totipotent. This means that these cells are able to divide and make all the types of cells of that particular plant at any given time. Meristem tissue allows continuous growth and the formation of new organs. Apical meristems are found at the tips of roots and shoots. The apical meristem is responsible for the elongation of roots and stems. It allows the stem to grow taller and the roots to increase in length. Also, the shoot apical meristem allows the formation of new leaves and flowers. The growth in height of the stem is important for photosynthesis while the lengthening of the roots is important for the plant to anchor deep into the soil and it is also vital for the uptake of water and nutrients found in deeper soil layers. The growth taking place at apical meristems is called primary growth. In addition, plants also grow by increasing the diameter of their stems and roots. This is called secondary growth and is a result of cell devision in the lateral meristems. It allows extra xylem and phloem tissue production and it also provides stability for the plant to grow taller.
For at least 400 million years before humans appeared on earth, plants were producing food consisting of leaves, stems, seeds, nuts, berries, fruits, tubers, etc., that made life possible for humans and animals when they evolved. Early plant evolution was essential not only for food but also for producing an oxygen environment necessary for animal and human survival. Plants introduced a very effective way of using the sun's radiation to transform carbon dioxide into food materials, such as sugars, starches, and cellulose, through the green pigment chlorophyll and the organelle that serves as the site for photosynthesis, the chloroplast.
Recently, it was shown that two species of the family Chenopodiaceae, Borszczowia aralocaspica Bunge and Bienertia cycloptera Bunge, each perform C4 photosynthesis in leaves through unique single‐cell systems, which has broken a paradigm that Kranz anatomy is required for function of C4 photosynthesis in terrestrial plants (b; ). In the present study, it is shown that mature cotyledons of Borszczowia have the same structural and biochemical features as previously reported in mature leaves, with chlorenchyma having dimorphic chloroplasts located at opposite ends of elongated cells. The structure and compartmentation, and expression of photosynthetic enzymes in cotyledons in seeds, and in cotyledons following germination, were evaluated to determine how the single‐cell C4 system develops. The results show a light‐ and developmentally dependent transition from C3‐like chlorenchyma cells to C4‐like chlorenchyma cells having specialized cytoplasmic compartments with dimorphic chloroplasts. This unique development of C4 photosynthesis in cotyledons of Borszczowia is discussed relative to that in species having Kranz anatomy, including another species in the family Chenopodiaceae which has been characterized recently, Salsola richteri (Moq.) Kar. Ex Litw. (b).
Seeds of Borszczowia aralocaspica Bunge were stored at 3–5 °C prior to use. Like many annual species of the family Chenopodiaceae, Borszczowia is characterized by heterocarpy and has two types of seeds, one with light‐brown, thin coats and the other with thick, black coats (Fig. A; see also ). Both types of seeds in this species are rather close in size. This type of heterocarpy in chenopods was classified based on differences in seeds and secondary seed coats (). Both types of the seeds were sectioned for microscopy to determine the structure of cotyledons. Seeds were germinated on moist paper in Petri dishes at room temperature. When the radical emerged, the seed coats were removed and fixation of material for various studies was made after 2–3 h of exposure to light. This point in time was designated day 0 (Fig. A). In thick‐coated seeds the process of imbibition can take a long time (up to several weeks), but we needed to have uniform material comparable with that from thin‐coated seeds. Thus, the covers of thick‐coated seeds were gently scarified after 2 d of imbibition and the coats were removed the next day. Seeds were germinated in either the dark or light to determine the effect of light on cotyledon anatomy relative to the development of the photosynthetic system.
While comparison of the C4 syndrome in related and unrelated species has provided valuable information about the range of features used in C4 photosynthesis, comparison of leaves and cotyledons of the same species can also shed light on development of the C4 syndrome. Leaves develop chlorenchyma from initially undifferentiated meristematic cells, while cotyledons that become photosynthetic generally must transform storage cells into chlorenchyma, which entails a complete shift in biochemical functions. There are dicotyledon C4 species, which have the same type of Kranz anatomy in leaves and cotyledons, including species of Chenopodiaceae (; , ; ), Amaranthaceae () and Asteraceae (). However, there are also chenopod species which have C4 photosynthesis in leaves, but have a completely different type of anatomy or photosynthesis in cotyledons. For example, there are species in the tribe Salsoleae having leaves with C4 salsoloid‐type Kranz anatomy, but cotyledons with either C3‐type photosynthesis and anatomy, or C4‐type atriplicoid anatomy (; , ; ). Identifying factors controlling this differential expression of photosynthetic anatomies between leaf and cotyledon will be useful to our understanding of the C4 syndrome and its potential integration into C3 species.
The highly efficient C4 system of photosynthesis is generally associated with differentiation of two cell types, bundle sheath and mesophyll, that are coupled in an anatomical arrangement referred to as Kranz anatomy. Because C4 photosynthesis has advantages over C3 photosynthesis under certain conditions where CO2 can be limiting (higher temperatures, drought, salinity), there is much interest in the possibility of introducing this feature into crop species, the majority of which exhibit the less efficient C3 photosynthesis. However, this will require a better understanding of the anatomical, developmental and biochemical features of the various types of C4 anatomies. The Chenopodiaceae may have the greatest structural and biochemical diversity in evolution of types of C4 photosynthesis of any plant family. It has five types of Kranz anatomy (atriplicoid, salsoloid, kochioid, conospermoid and suaedoid) if arrangement of mesophyll and bundle sheath cells versus other tissues (vascular and water storage, if present) is considered (; ), and two ultrastructural types of chlorenchyma cells related to two biochemical C4 cycle subtypes, NAD–malic enzyme (NAD–ME) and NADP–malic enzyme (NADP–ME) (; ; ; ). Thus, this family provides an excellent system for the study of the integration of anatomical and biochemical features of the C4 syndrome, as well as questions on evolution of various features.
• Background and Aims Previous work has shown that Borszczowia aralocaspica (Chenopodiaceae) accomplishes C4 photosynthesis in a unique, polarized single‐cell system in leaves. Mature cotyledons have the same structure as leaves, with chlorenchyma cells having biochemical polarization of dimorphic chloroplasts and C4 functions at opposite ends of the cell.
Plants with two embryonic leavesare termed ("dicots") and placed in theclass .In the case of dicot seedlings whose cotyledons arephotosynthetic, the cotyledons are functionally similar to leaves.
Tropisms are directional movement responses which occur due to external environmental stimuli. The direction of the stimulus affects the direction of movement. Tropisms can either be negative or positive. Positive tropisms are the directional movement towards the stimulus while negative tropisms are the directional movement away from the stimulus. Examples of stimuli causing tropisms in plants are gravity and light. Roots will grow towards gravity while the plant shoot will grow upwards in the opposite direction. The directional movement of plants in response to light is called phototropism. As seen with gravity, the plant's roots will grow away from the light, into the soil (negative phototropism) while the plant shoot will grow towards the light (positive phototropism). Positive phototropism seen at the tips of plant shoots is made possible due to plant hormones called auxins. Auxins are produced at the tips of plant shoots and then translocate to the darker side of the shoot tip and stem which is receiving less light. This translocation is made possible via auxin efflux carriers which are unevenly distributed in the plant tissue. Once auxins reach the shaded side of the plant, they cause the elongation of cells so that the shaded side grows faster than the brighter side, thereby promoting the bending of the plant shoot towards the light. Auxins do so by binding to auxin receptors on cells. The binding of auxin causes the transcription of certain genes within those cells and therefore the production of specific proteins which affect growth. Auxins allow the expelling of protons (hydrogen ions) into the cell walls of the cells on the shaded side, decreasing the pH inside the cells and in doing so activate specific enzymes which break down cellulose microfibrils within the cell wall. This loosens the cell wall and allows cell elongation. So to conclude, auxins are very important in the control of plant growth towards the light and thereby allow the plant to increase its rate of photosynthesis.
Vascular tissue - Consists of xylem and phloem which are found in the veins of the leaf. The veins in the leaf are positioned in the middle so that all the cells are in close contact with the vascular tissue. The xylem consists of xylem vessels (dead structure) which are long and tubular and transports water into the leaf to replace the water that has been lost through transpiration. The phloem is made up of living cells with pores in between them. It transports the products of photosynthesis out of the leaf.
In chlorenchyma cells in mature cotyledons, there is clear compartmentation of two types of chloroplasts, which are nearly agranal in the distal part of the cell and granal in the proximal part, similar to mature chlorenchyma in leaves (b, 2003a). Also, large mitochondria are located in the proximal part of mature chlorenchyma cells in cotyledons similar to those in chlorenchyma cells in leaf. The cotyledons thus develop a photosynthetic structure essentially identical to that of mature leaves.