Last week we looked at the three stages of dormancy: induction, endodormancy, and ecodormancy.
Some species enter dormancy when the number of daylight hours reduces, while others seem to respond to decreased temperatures. In contrast, most species appear to respond to increasing temperatures predominantly, rather than increasing daylight hours, when breaking dormancy. This may be due to a lack of leaves, which are the only organs a plant has that responds to light.
The Four Physiological Processes Which Regulate Dormancy
There are four physiological processes which regulate the three stages of dormancy: transport, phytohormones (plant hormones), genetic and epigenetic regulation, and carbohydrates. I’ll cover below what each of these do in general for now, and go into more specific detail over the next few weeks.
Transport in our bodies is via our vascular, or circulatory, system, which comprises the blood and lymph vessels. A plant’s vascular system comprises xylem and phloem tissues. (Definitely topics for future posts!)
(More simple plants, such as mosses and algae, lack these tissues and are called non-vascular plants. They are limited to a small size as their own specialised transportation tissues are less effective than those of vascular plants.)
Xylem transports water and dissolved nutrients from the roots up to the stems and leaves, and phloem transports food produced from photosynthesis (photosynthates) from the leaves down to the non-photosynthesising stems and roots of a plant. (My mnemonic is that phloem carries food, thus xylem carries water! Bonus aside: that linked page has a brilliant mnemonic for working out which months have which number of days that is well worth checking out. Hubby taught me that one years ago, and I have never met anyone else apart from his family who also knows it, so I love sharing it every chance I get — much easier than that song!)
Xylem moves water (and nutrients) passively and against gravity via a negative pressure generated by water evaporation from the leaves — water molecules are ‘pulled’ upwards as water higher up is removed. (Water’s unique properties make this possible at all.) This transpiration will slow down as temperatures drop, as there is less water vapour in the leaves being ‘pulled out’ into the surrounding cooler autumn air.
Phloem transportation on the other hand is not passive, though flow does rely on the help of gravity to move downwards through a plant. Rather, transport is via active diffusion across cell membranes, and that diffusion in turn is controlled by microscopic channels called plasmodesmata (plural form of plasmodesma).
Phloem maintains connectivity between all the plant’s organs and facilitates the transportation of hormones, minerals and sugars to them.
‘Phytohormone’ simply means ‘plant hormone’. Phytohormones are the signallers within a plant, and act in very low concentrations over short and long distances. They regulate plant growth and development, as well as its responses to stresses. You may have heard of abscisic acid, gibberellic acid, auxins, and cytokinins — these are all phytohormones.
Genetic and Epigenetic Regulation
Genes are strands of DNA which code for proteins. Some hormones and all enzymes are proteins, coded by genes.
Genetic regulation is the switching on and off of particular genes so as to regulate the production of the product they code for. This helps conserve an organism’s resources as well as minimise adverse effects — it makes sense for the genes that guide flower or bud development to not be actively behind hormone production during the coldest winter months.
Epigenetic regulation is a more difficult concept to grasp, so let’s define epigenetics first. ‘Epigenetics’ means ‘above genetics’ or ‘on top of genetics’. A gene’s DNA sequence is not changed, but the DNA structure is modified in a way that changes a gene from ‘readable’ to ‘unreadable’ in a cell, or from ‘unreadable’ to ‘readable’. It is a deeper level of ‘on’ or ‘off’ than the genetic regulation described briefly above. A very common modification is for a ‘cap’ (often a methyl group, –CH3) to be added to part of the DNA molecule, which prevents some genes from being read and expressed.
Every cell in a plant or animal contains the exact same DNA, yet we have skin cells, brain cells, kidney cells, and so on, while plants have leaf cells, stem cells, root cells, and so on. It is epigenetics that causes that differentiation — a leaf cell, not a root cell, develops in a leaf, because the genes that make a cell a leaf cell have been switched on while the genes that make a cell a root cell have been switched off.
Epigenetic regulation ensures a plant seed remains dormant, often for months or even years, until conditions are right for germination. Epigenetic regulation also controls photoperiodism and the timing of flowering and other growth and development responses.
Plants produce carbohydrates for both structural and non-structural purposes. Structural carbohydrates make up plant cell walls, and you’d know these as ‘fibre’ and ‘cellulose’. Non-structural carbohydrates are the primary photosynthates and include glucose, fructose, sucrose (or table sugar: glucose molecules joined to fructose molecules in a 1:1 ratio) and starch (a polymer of glucose).
Non-structural carbohydrates are a store of energy when not used for growth and development, and have additional roles in transport and supply. They are also key in osmotic regulation (maintaining the correct balance of water and salts in cells) and signalling.
Non-structural carbohydrates vary in quantity from season to season, and this helps a plant adapt to changes in light, temperature, water and nutrient availability.
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