2023-08-08-BIOL254-W4-Notes .docx

LECTURE 7: PRINCIPLES OF PLANT PHYSIOLOGY Tree Rings If you observe a pine tree for example, you will find rings and rings of cells usually called tree rings. These are because of secondary xylem and phloem that are formed. The secondary xylem every year is added by the vascular cambium. One year o...

LECTURE 7: PRINCIPLES OF PLANT PHYSIOLOGY Tree Rings If you observe a pine tree for example, you will find rings and rings of cells usually called tree rings. These are because of secondary xylem and phloem that are formed. The secondary xylem every year is added by the vascular cambium. One year of secondary xylem or phloem formed by the vascular bundle is called a tree ring. When the temperature warms up after winter, the vascular cambium becomes active again and produces secondary xylem and phloem until the temperature drops and it will stop as well. Secondary Growth and Lateral Meristem When the temperature is permissible, each of these vascular cambium cells will start cell division becoming two. One of the new cells will remain cambium cells, the other one will differentiate to become xylem cells. The same cambium cell will then undergo another cell division, one will remain the cambium cells and the other will then become phloem cell. And it keeps going like that increasing in size and thickness in cell wall. After one year of growth when the temperature begins to cool, it then stops. The secondary phloem does have a thin cell wall. Underneath the dermal tissue, some of the ground tissue will form woody tissue or fibre tissue. The secondary phloem is often crushed by the secondary xylem when the secondary xylem pushes and applies pressure towards the dermal tissue part of the plant. Anatomy of a Monocot Root Pericycle A single layer of parenchyma cells just located beyond the ground tissue. At some point during the maturity of the root, some of the cells in the pericycle differentiate and become a root forming meristem forming a lateral root. Auxin Signalling and Lateral Root Formation NAA – involved in changing the pericycle cells to become meristematic cells. Nitric oxide (NO) – hormone like molecule that causes the pericycle to form lateral roots. Sodium nitroprusside (SNP) – this can release nitric oxide. CPITO (destroys NO) – if added to plant tissue, it can destroy the Nitric oxide. Auxins Auxins are a class of plant hormones. Plant cells produce these hormones involved in cell division and plant cells growth. Example is IAA, IBA. They can be produced by plant cells under a natural condition but not in large amounts and are produced as at when needed. These plant hormones have similar chemical structures. NAA With this organic chemist can synthesis auxin in the lab. NAA is an example of synthesized compound that can work like a plant hormone in the cells of a plant. By increasing auxin concentration in the solution. The auxin concentration goes to the pericycle causing an increase in lateral root formation (See figure 3). Lateral Root Initiation Lateral roots emerge from meristem formed from the pericycle. Underneath the pericycle you see the vascular tissue. When the root is submerged in NAA, these pericycle cells begin to nitric oxide production triggered by the NAA in its cells. The pericycle cells in response to the NAA become meristematic and starts to divide forming a big bump (See figure). Looking at the graph bellow The roots were incubated in water. The roots were submerged in solution of SNP. The roots were submerged in solution of CPTIO – here the NO did not do its job because CPTIO destroyed the NO even though SNP was added. The roots were submerged in only CPTIO – here the CPTIO goes into the roots cells and destroys any NO it finds there resulting in no formation of lateral roots. See Figure C for visual results of the graph. The Leaf Features of a eudicot leaf Mid veins Lateral veins Petiole Lamina Whenever you see a vein on the leaf, you expect to see vascular tissue inside. In parts of the leaf, you expect to see the dermal tissue and ground tissue. In monocot, you see parallel veins and inside you see the vascular tissues as well. On the surface of leaf, you fond the first layer called the cuticle. The plant tissue inside synthesis a waxy substance and push it onto the outside called the cuticle, which is a waxy layer, this is to retain water in the cells of the leaves. You can also see the dermal tissue, these cells are arranged touching each other without spaces, or gaps. These dermal cells also function by minimizing the escape of water from the cells of the leaves. The leaves have an up and lower epidermis. The cells in the ground tissues of the leaves are parenchyma cells also known as chloroplast. Whose function is to carryout photosynthesis. The cells in the ground tissue of leaves tend to be underneath the plant surface called the palisade. Real inside the leaves, are other parenchyma cells and less organised and are called spongy mesophyll. The palisade is tightly packed and together and more organised, but the spongy mesophyll is less organised. The veins on the surface of the leaf of the xylem and the one at the lower part of the leaf is the phloem. The lower surface of the leaf has a different looking cell from the regular epidermal cells, they look like a pair of beans called guard cells. They are more defined epidermal cells. They are specialized epidermal cells which can expand or shrink. Once it expands, they create an opening which can make water to be lost or escape. It is necessary for photosynthesis because it needs to allow the inflow of carbon dioxide.