Plant Processes – How Plants Grow
Basic plant processes
Any attempt to understand plant physiology must be prefaced by some discussion of the basic plant processes common to all green plants. While these will be grouped for convenience under separate headings, it should be appreciated that all processes are closely inter-related, each having a bearing on the other.
Breathing is a function essential for all living organisms. In the case of green plants air is taken in through the pores on the leaves (and to a lesser extent through the stems and roots), its oxygen content extracted as energy for various chemical processes, and carbon dioxide gas expelled as a waste product—in a similar way to a fuel being burned by a fire or an internal combustion engine. The speed of respiration varies according to the age of the plant, the temperature of the atmosphere, and the rate of growth, this last activity not always being completely in keeping with temperature, as a combination of other factors may have a bearing on the growth rate irrespective of age or temperature.
Respiration is a continuous destructive process as compared with other functions such as photosynthesis which are carried out only during daylight hours. The destructive nature of respiration therefore draws on the reserves of the plant night and day, which means that food manufactured by the process of photosynthesis is utilized at night, a fact that has considerable significance in relation to day and night temperatures. If, for example, a warm bright day when a great deal of carbohydrate is manufactured in the plant leaves is followed by a cold night, there will usually be a surplus of carbohydrate, which in the case of the tomato plant will result in starch filled curled leaves. The converse is also true, and cool dull days with limited photosynthesis, followed by warm nights, will in fact dissipate the reserves of carbohydrate resulting in drawn spindly plants.
Process of respiration
Air taken in through
pores in leaves, stem and roots —->
energy extracted for
chemical processes —->
expelled through pores
This is the process whereby the air taken in through the pores in the leaves has the carbon dioxide gas contained in it extracted (at approximately 300 volumes/parts per million) and the oxygen content of the air expelled, more or less the converse of respiration. Carbon dioxide is the vital source of carbon needed for the build-up of carbohydrates, which are synthesized with the aid of the catalyst chlorophyll, the green pigment present in all green plants. The supply of water and nutrients taken up by the plant roots (which will be discussed shortly) is also closely involved with photosynthesis.
Research has shown that there is a direct relationship between the rate of photosynthesis and the prevailing light intensity, provided that temperature is sufficiently high and there is no restriction in water and nutrient supply. Adding to the natural complement of carbon dioxide, up to threefold enrichment, by various means under conditions of high light and temperature levels can, in certain cases, increase the rate of photosynthesis, which has various benefits largely centring round the increased supply of carbohydrates for the plant’s vital processes.
No research has yet shown precisely what physiological processes are brought into play by carbon dioxide enrichment but it is quite clear that photosynthesis is an extremely complex function. The practical implication for greenhouse gardening is to ensure that there is no light restriction for light-demanding crops except when there is excess sunlight, and that all other environmental factors must simultaneously be given strictest attention.
Process of photosynthesis
Air taken in by plant —->
carbon dioxide content extracted —->
Water absorbed by the plant roots and other parts of the plant passes up through the xylem tissue in the stem, and into the leaves where quantities in excess of requirements for various chemical processes are expelled as water vapour through the pores in leaves and stems, and to a limited degree through the epidermis or plant ‘skin’. Apart from helping to keep the plant ‘cool’ by loss of heat through evaporation, a transpiration stream is created, this being essential for the movement of dissolved foodstuffs throughout the growing plant. Transpiration rate is controlled in an interesting way, as the pores or stomata are opened and closed by the activity of guard cells which close the stomata when flaccid and open it when turgid. Thus the rate of water loss is directly related to the temperature and humidity of the air.
Once again it would be wrong to consider transpiration in strict isolation, as there are in addition many other interrelated issues: (1) the healthy state of the roots and their ability to absorb sufficient water, (2) freedom from disease or restriction on the part of the xylem and other conducting tissue, (3) the age and size of the plant with particular emphasis on the leaf area, (4) the respective osmotic pressures of bothand plant which will greatly affect the ability of the plant to take up moisture and nutrients in the first case, (5) the species of plant involved and whether it is botanically adapted to moisture conservation, as indeed is the case with most cacti when the leaves are modified to ‘needles’.
Process of transpiration
Water taken in through the process of osmosis passes up through the xylem and a proportion is given off as water vapour at a rate dependent on temperature, humidity, the osmotic pressures of plant and soil and other related issues.
This is undoubtedly not only one of the most important physiological processes, but an activity over which the gardener can exert considerable control. Any botany book will state that osmosis is the process whereby a higher concentration of salts in solution on one side of a semi-permeable membrane will absorb the water of a less concentrated solution by pulling it through the semi-permeable membrane. Relating this to plants, the stronger solution contained in the cell of the root (or other tissue) absorbs the water from the less concentrated solution of salts in the soil or growing medium, the net result of which is to set up suction pressures which are always endeavouring to equalize.
Botanists make much of the fact that only water is absorbed by osmosis and that soluble plant nutrients move in by diffusion and absorption, as few semi-permeable membranes are perfect; but for all practical purposes the two processes are so closely related that they can be considered simultaneously. Once the salt solution is contained in the cell of root or other tissue a chain reaction process goes on, as the cell receiving water has its concentration lowered, altering the osmotic pressure, which allows the more concentrated solution in the next cell to pull in the now less concentrated solution to even up the pressure, and so on.
The rate at which osmosis proceeds depends on many factors such as transpiration rate and the speed of other processes, but undoubtedly the salt concentration of the soil water is of paramount importance. When concentrated and soluble chemicals (some weedkillers and many fertilizers) are applied to the soil or growing medium, they alter the salt concentration. Where near equilibrium exists there can only be limited osmotic activity, if indeed there is any at all, whereas should the salt solution in the growing medium become more concentrated than that of the plant cell, osmosis in reverse can and will take place. The importance of salt concentration in the growing medium and successful osmosis on the part of the growing plant cannot be divorced from the availability of water supplies, as when water supplies diminish, salt solutions will become more concentrated and vice versa. This is a factor of considerable importance in greenhouse culture where the majority of plants will be growing in limited quantities of media. It must be appreciated that some measure of growth control can be achieved by adjustment of the salt concentration of the growing medium, by applying either solid or liquid fertilizers, something which intuitive gardeners have been unconsciously carrying out for years.
Process of osmosis
The stronger solution of salts in the plant cell pulls in the water from the weaker solution of the soil or growing medium and simultaneously there is absorption of liquid nutrients. A measure of growth control can be achieved by paying strict attention to the salt concentration of the growing medium.
This function relates to the transport of elaborated foodstuffs throughout the plant tissue, the main conducting tissue being the phloem. Many gardeners fail to realize that the food synthesized in the leaves is moved into the flower or swelling fruit as required, there often being the allusion that leaves andor fruit have a separate unrelated existence. Further credence is given to this philosophy by the various feeding formulae prepared for the swelling of fruits or the size and colour of flowers. Neither flower, fruit nor leaf of the plant exists in limbo, and it is the successful culmination of all processes in which translocation is merely one vital part which results in good flowers, fruits, or for that matter foliage.
Disruptions of any kind such as irregular watering, unsuitable or variable temperatures, or the wrong dilution of liquid feeding, can frequently affect the usual movement of elaborated foodstuffs around the plant, resulting in physiological trouble such as blotchy ripening or black bottoms of, flower drop in many pot plants, lack of hearting in greenhouse lettuce, to mention only a few malfunctions which are closely linked to faulty circulation of elaborated foodstuffs or the direction of simple elements into their respective roles. Here again is a case, however, for emphasizing that there is a very close relationship between all physiological factors and that compartmentalized thinking can be confusing.
Pollination and fertilization
Sexual reproduction is a feature of most flowering plants and is a process which depends on the minute pollen grain or male sperm successfully and in good condition reaching the ovary of the same or other flower of similar species, and thereafter germinating, producing a pollen tube through which fusion between male gamete (sperm) and female gamete (egg or ovum) can take place, resulting in fertilization and the production of new cells which form the fruit. Flowers which contain both male and female organs producing pollen capable of reaching, and acceptable to the ovary, are said to be self-fertile. When the pollen from another flower of the same species is necessary, a flower is said to be self-sterile; where male and female organs are produced on separate flowers, the flower is unisexual. Both of these factors can give rise to problems when the fruit of the plant being grown is important. In some cases, however, fertilization is not desirable and before it happens the flowers of male blooms are generally removed.
There are many cases where fertilization is unsuccessful, either because the pollen is not fertile or it dried out before germination, a frequent state of affairs indue either to poor environmental conditions or the production of infertile pollen under conditions of poor light. Fertilization of can be aided by damping down frequently. In the case of peaches and nectarines pollination is assisted by collecting the pollen on a rabbit’s tail or cotton wool and transferring this to open flowers, whereas with the male flower is removed and inserted into the female flower.
By contrast, it is inadvisable to allow fertilization offlowers, as this results in the production of bitter fruit. Modern breeding programmes have now produced ‘all female’ .
Germination of seeds
A seed contains a new plant in an embryonic state, and given suitable conditions the young plant will develop into an adult plant. The initial awakening of the embryonic plant is called germination, which will occur when moisture has penetrated through the protective seed coat or testa to trigger it off. Air, moisture and suitable temperature are the prerequisites for successful germination, but these must be balanced to the particular inherent requirements of each species. Primed seed is now available.
Most seeds have a dormancy period during which they cannot be readily induced to germinate and this period varies according to species. Various ways and means are practised to break this dormancy period, for example by subjecting the seed to periods of low or high temperature. The time taken for a seed actually to germinate will vary with the species, and also the age of the seed. Seeds vary considerably in their viability or ability to germinate, and reference to both these facts will be made in the respective cultural notes where applicable, it being worth noting that the particular germinating procedure which has evolved for a particular type of seed has probably come partly by usage and partly by research over many years. Some seeds germinate below the compost surface (hypogeal), others above (epigeal).