In this article we will discuss about:- 1. Factors Affecting Growth 2. Regions of Growth 3. Pattern.
Factors Affecting Growth:
The growth of a plant occurs in the range of about 0°C to 35°C. Within most of this range, raising of the temperature by 10°C increases growth rate by 2-3 times. There are three temperatures known as the cardinal points for growth: the minimum or the lowest temperature at which growth can be detected; the optimum or temperature of maximum rate of growth; and the maximum, or highest temperature at which growth can be detected.
These are not very sharp temperatures and they vary from species to species. It must be mentioned that the optimum temperature for growth may be different for a particular plant and also at a particular stage of development of the same plant. Growth depends on other processes such as photosynthesis and respiration and these processes also have their cardinal temperatures.
Since the whole process of plant growth is due to chemical reactions which are enzymatically controlled, this implies that cardinal temperatures for growth, therefore, must be controlled by the denaturation temperature of a plant enzymes.
The amount of water present inside the cell and the conditions of protoplasm are closely related with its resistance to extremes of temperature. The values for three cardinal temperatures vary from plants of arctic, temperate and tropical zones.
Though growth of higher plants eventually depends upon photosynthesis, light as such is not essential for the process of plant growth, as long as sufficient amounts of organic materials are available. Some plants can complete their life cycle in the dark, e.g., tuberous or bulbous plants.
The higher plants grown in dark show poor growth known as etiolation. Usually the leaves remain free of chlorophyll and the colour is, therefore, pale-yellow, although some ferns, gymnosperms, seedlings, algae and embryos can synthesize chlorophyll in the dark. Light affects variously and the effects depend upon its intensity, quality and periodicity.
Weak light promotes shortening of internodes and expansion of leaf. Very weak light reduces the rate of overall growth and also photosynthesis. Development of chlorophyll is dependent on light and in its absence etiolin compound is formed which gives yellow colour to the plant. Similarly high light intensity affecting indirectly increases the rate of water loss and reduces the rate of growth.
Different wavelengths of light affect growth of plant. Blue-violet light enhances the internodal growth while green light reduces the expansion of leaves as compared with complete spectrum of visible light. Red light favours growth. Infra-red and ultraviolet lights are detrimental to growth.
Vegetative as well as reproductive structures are remarkably affected by the duration of light. The induction and suppression of flowers are dependent on duration.
Since growth depends on a hydrostatic turgor pressure, water deficiency will, of course, retard or completely stop it. On the other hand, excess of water may result in an abnormal type of growth. So, in a saturated atmosphere the development of leaves is poor and the differentiation of tissues is retarded.
This is the result of excessive stretching of the cell walls because of the abnormally high turgor pressure. Plants adapted to aquatic conditions have low osmotic pressures and, therefore, cannot develop such excessive turgor pressure even when their tissues are saturated.
(iv) Chemical stimulants and inhibitors:
Even nutrient salts required by the plants for normal growth may inhibit growth or actually kill the plant if applied in an unbalanced state. On the other hand, they stimulate growth when applied in suitable quantities and in balanced solutions. Growth is frequently inhibited by non-essential mineral elements. Salts of heavy metals (copper, lead, silver, mercury, etc.) are toxic.
Even the metabolic products of a plant like oxalic acid may be poisonous to the protoplasm instead of being stored in the vacuole. Several of the poisons when applied in very weak doses stimulate growth. Thus phenol is poisonous in 1: 1000 concentration but stimulates when used in 4 to 8: 100,000; ethyl alcohol checks growth in 25 to 75: 100,000 and stimulates it in 25 to 75: 100,000. Mercury compounds used to disinfect seeds sometimes stimulate growth.
Air pollution, in particular, has been recognized for well over 100 years as influencing plant growth. Ethylene, sulphur dioxide and fluorides are shown to damage a wide variety of plants. Recently, several new classes of air pollutants have been recognized: hydrocarbons from vehicle exhausts and photochemical products resulting from the interaction of hydrocarbons with nitrogen oxides in the presence of sunlight.
Ozone and peroxyacetyl nitrate are two of the very active compounds that are formed in this way. Automobile exhaust fumes also contain large amounts of lead and boron, which are added to gasoline as antiknock compounds and these may have a detrimental effect on plant growth. Finally, the widespread use of organic pesticides, herbicides, fungicides, insecticides, etc. have harmful effects on plant growth.
It is likely that air pollution will remain a problem so far as plant growth is concerned since the sources of air pollutants are increasing as new industries and highways are built and the urban population grows. It has already become almost impossible to grow certain plants in heavily polluted areas.
For example, citrus fruits and gladioli are extremely sensitive to fluoride and are no longer grown in regions where the atmospheric fluoride concentrations are high. Tobacco grows poorly where ozone and photochemical products reach high atmospheric levels.
Cotton is very sensitive to ethylene and white pine is killed by high levels of ozone and other atmospheric oxidants. It has been reported that air pollutants reduce the crop yield by 5 to 15%.
As stated above, gladioli are extremely sensitive to atmospheric fluorides. But some of the varieties are more susceptible than others. Similarly, occasional white pines appear to tolerate high concentrations of atmospheric ozone and other oxidants.
The physiological basis for resistance to air pollution damage is not known, but it might be possible to select and develop plants resistant to air pollution as is currently done in developing insect and fungi resistant plants.
Plants themselves emit volatile substances of a complex organic nature. The blue haze that hangs over the smoky mountains and other densely vegetated areas is due in part of these plant volatiles. Chemical studies show that terpenes and photochemical decomposition products of terpenes as well as methane, are present in plant volatiles.
These substances influence the growth of other plants and are part of the natural environment in which we live. Recent studies on the dynamics of ecosystems have shown that plant volatiles, i.e., allele-chemicals play an important role in controlling plant growth and development.
(vi) Oxygen Supply:
Oxygen causes increase in growth because it helps in respiration which ultimately provides energy of vital activities of the plant during growth and development.
(vii) Ionizing Radiations:
During growth, plants are also exposed to very short wavelength, high energy radiations known as ionizing radiations and some of these are part of the natural energy environment to which all living things are exposed, while other radiations are man-made. Cosmic rays, e.g., originate in nuclear reaction in outer space.
In addition to cosmic rays, the natural environment contains several sources of ionizing radiations to which all living organisms are continuously exposed. These include radioactive carbon and potassium and several isotopes of radium and uranium. The pollution of the environment by radioactive material from nuclear reactions, atom bomb tests, wastes from nuclear reactors is of great concern to plant physiologists.
(viii) Nutrition Supply:
The supply of nutritive materials is directly proportional to the rate or growth and with deficient food supply to growing regions, the rate of growth decreases and ultimately stops. Photosynthetic processes supply the growing plant with carbon skeletons which are incorporated into amino acids, proteins, phospholipids, nucleic acids, carbohydrates and other cytoplasmic and structural constituents. These metabolic processes require adequate supplies of inorganic elements.
(ix) Genetic control of development:
The concept of totipotency indicates that information needed for the development of a complete plant is contained within the genetic complement of each cell including highly differentiated cells. In a way cell continues to have genes though several of the genes may not be expressed or may be turned off with progressive differentiation and development.
The activation of genes is sequential, orderly and programmed and activated in a precise manner to yield specific gene products i.e., proteins at the pertinent time. The cells must have capability to respond to those products. The increasing evidences in molecular biology have shown that change in gene expression is a pivotal factor in regulating development at the intracellular level.
Genes comprise specific sequences of nucleotides in the DNA molecule. A sequence of three nucleotides-a codon, codes for each amino acid. These sequence in the gene thus determine the primary structure or sequence of amino acids of proteins, especially enzymes which regulate the speed and course of metabolism.
Gene expression, thus, implies synthesis of specific proteins encoded by specific genes. Not all the genes are active all the time; they may be turned on or off depending upon the state of development, environmental conditions, etc. The complement of enzymes in the cell, and thus the rate and state of metabolism in a cell is linked with differential gene (s) expression.
In higher plants and organisms gene expression is divisible in five main stages: gene activation; transcription; RNA processing; translation and protein processing. Further, regulation may occur through any one of the five stages.
The transcription of messenger RNA (mRNA) from gene. It was stated that eukaryotic gene contained regions that coded for proteins (exons) alternating with regions that do not code for proteins (introns) (Fig. 19-3).
Transcription was carried out by RNA polymerase II, which moves along the DNA molecule as the mRNA chain elongates. It may be stated that mRNA must be processed before it is extruded from the nucleus.
This process involves capping the 5′-end with methyl-GTP and the addition of polyadenylate tail containing 100 to 200 adenylate molecules to the 3′ end. Subsequently the introns are removed and the remaining exons spliced together. Finally, the transcript is ready to be exported through a nuclear pore into the cytosol.
Once in the cytosol the mRNA attaches to a ribosome, where the message is translated into the sequence of amino acids that constitute the protein (Fig. 19-4). Many proteins are not useful immediately after their release from ribosomes, but are required to experience some post- transcriptional processing before they become active.
Proteins intended for membranous compartments like chloroplasts contain hydrophobic amino acid sequences-the leader sequences which facilitate transport of the proteins through the membrane. Once the protein attains a specific position, the leader sequence is removed and degraded. Other proteins remain in an inactive state until the peptide chain is shortened or modified by the action of protelytic enzymes.
Some need addition of carbohydrate groups. In some cases protein enzymes require phosphorylation for activation. In summary, for successful expression of a gene several steps are required and each step represents a pivotal point at which the expression of the gene may be regulated during development.
Now evidences are available which suggest differential transcription as well as control of translation and post-translational processing throughout plant development.
(x) Hormonal regulation:
Nutrient balance and pertinet hormones/PGRs when added to the basal medium, parenchyma cells are stimulated and give rise to callus which differentiate into shoots or roots and then a fully grown plantlet.
The role of hormones in the differentiation and organ formation has been well demonstrated and are supposed to be chemical messengers that carry information from one cell to another. Several classes of hormones are known, which either promote or inhibit different developmental responses, either alone or in combination.
(xi) Environmental regulation:
The plant development may also be regulated by several external stimuli and most of these stimuli are physical factors e.g. light, temperature, gravity, wind, sound. Other environmental factors like water, nutrition also have a great impact on plant development.
Recently role of pollutants in modifying plant development patterns has also been demonstrated. The outside signals are perceived by the plant and are transduced to the region of evocation. Several studies have demonstrated the metabolic changes which occur in the chain between perception and response. Several of these environmental stimuli act partially through modification of gene expression or hormonal activities.
Several environmental factors and crop responses to the complexity of cropping environments is reflected in different ways. The concept of light interception by crop canopies and its relationship to crop productivity is expressed in different ways and these include leaf area, interception of solar radiation pertinent to crop growth; crop growth rate, net assimilation rate (CAG = NAR x LAI).
Further critical and optimum leaf area indexes, radiation attenuation through crop canopies (e.g., leaf inclination and efficiency of photosynthesis; crop growth rate; role of leaf inclination variation within canopies) are also crucial in dry matter production.
Plants adopt several strategies for maximizing solar energy utilization and these are: leaf area duration (LAD), solar energy-temperature interaction. Plant density and its distribution over the land surface is also vital for efficient interception of radiant energy.
It may be mentioned that several environmental factors affect optimum plant density and is linked with several factors e.g. plant size, tillering and branching, lodging, reduction in fruit set, irradiance, moisture, soil fertility, periodicity and extent of weed around the crop species. Plant distribution and row spacing are all crucial factors for dry matter accumulation.
(xii) Organismal Control:
Development of an organism or an organ is controlled by hormones. These hormones are of diverse types and at low concentrations regulate growth, development and even metabolism. Hormones may cause their effects in several ways, e.g., by inducing or repressing genes, regulation of metabolites or nutrients, etc. Chemical structure and mode of action of these hormones in detail. In addition several growth substances (e.g., florigen, rhizocauline) have also been suggested to occur.
Regions of Growth:
Growth regions in plants are the apical regions (apices) of shoot and root where growth is restricted to meristematic regions. Such growing regions are known as apical meristems, primary meristems, or regions of primary growth.
These apical meristems, are responsible for the increase in length, differentiation and formation of tissues. In some plants, for example Mentha, however, the increase in length also occurs by intercalay meristems (Fig. 19-5).
These intercalary meristems are considered as the parts of apical meristem separated by permanent tissues and are temporary regions of growth. Lateral meristem constitutes the third kind of growth region which is responsible for secondary growth.
Patterns of Growth:
After germination, there is development of primary root followed by growth of aerial parts of the plant. The germination is either hypogeal or epigeal. Root meristem is protected at the tip by the root cap whereas the apical meristem of monocots is covered by coleoptile. In dicots, the stem is recurred near the tip and forms the plumule hook.
Quality and quantity of light as perceived by the plumule regulates the seedling response, plumule tip is highly sensitive to light. Phytochrome, which absorbs light, regulates several aspects of plant growth and development. Several of the responses regulated by light can be substituted by the exogenous application of specific hormones.
Light triggers several developmental steps in the seedling. Its quality, quantity and periodicity in collaboration with temperature constitute most important factors in controlling patterns of growth. However, light, temperature, nutrients, etc. set a chain of biochemical and genetic responses which initiate and influence development.
Compared with stem, root growth is simple since it produces only one axis, and lacks nodes. Thus, number of initials vary. An existence of a quiescent centre comprising several hundred cells with a group of actively dividing cells, has been reported (Fig. 19-6).
This gives rise to several cells which form the tissue of the root. Root cap is produced from a meristem close to the apical surface of the root. The growth of root seems to be under the control of auxin concentration. Usually, IAA is needed in very small quantities. The general thinking is that IAA needed for root elongation is transported from the shoots.
Cytokinins are required for cell division in roots and their quantity and balance regulates the type and pace of growth. The general assumption is that auxin stimulates early root tip growth and checks its lateral expansion. Cytokinins change auxin concentration and alterations of such concentration may be causing the double function of auxin as far as root tip is concerned.
The cells which are produced from the root tip meristem are left behind and begin to enlarge, elongate and differentiate into several tissues of the mature root. The pattern of stem is variable. Several questions interest the root developmental biologists and these are whether pattern of organization is controlled by meristem or if the stimulus for specific differentiation is transmitted directly from cell to cell.
Recent evidences show the origin of hormones or some growth factors from the root tip which control the specific differentiation at the sites beyond their production. Tissue culture studies have indicated that the pattern of root growth was established through the root tip area and was not because of the consequence of the influence of mature tissue.
Further, IAA was involved in establishing the pattern of vascular tissue and meristem size influenced the pattern of vascular tissue formation in some way. Further, the pattern set by primary vascular tissue regulates the pattern of secondary vascular tissue.
Studies have also indicated that some factors from the leaves brought about secondary cambium and vascular tissue development in roots when IAA at high concentration along with sucrose solution was used in some roots.
Secondary cambium developed and there was formation of secondary vascular tissue. In some roots cytokinins and myoinositol were required for successful secondary growth of root. Evidently, a specific balance of nutrition and hormones was needed to achieve a precise pattern of root growth. Moreover, leaves/shoots provide carbohydrates, vitamins, amino acids and growth factors, etc.
In the region of vascular differentiation, some of the pericycle cells become meristematic. These cells lie opposite to the xylem star. New meristematic cells divide, differentiation and grow outwardly through the cortical tissue of the root. Gradually, the vascular elements of lateral roots connect with those of the main axis.
In all probability auxin or its precise balance with cytokinins control lateral root formation. Cytokinins are produced at the root tip and their high concentration may be an important factor contributing towards preclusion of root hair formation in this area.
However, in the region of cell differentiation, there is low cytokinin and comparatively high auxin content. In summary gradients of translocated substances may be responsible for the initiation of lateral roots.
Stem has apical meristem which develops into stem-shoot, leaves and branches. All these structures arise at the surface of the apical meristem as outgrowths. It is highly complex though small, usually a fraction of a millimeter in diameter. Leaf primordia are produced at regular intervals and stem consists of nodes and internodes.
Cell division in the meristem is organised on the basis of tunica corpus theory when tunica (outer layers) divides anticlinally and corpus (cells laying inside the tunica) divide variously. Incidentally, most part of the internal tissue of the new stem and organs arise from these cells. However, shoot and leaf primordia consist of both the tissues.
Shoot elongation is a complex phenomena which is controlled by several factors including hormones and environments, etc. Variability in the level of growth factors produces differences in different parts of the plant.
Auxins affect shoot elongation whereas gibberellins affect stem growth in different ways e.g., the latter stimulates the cell division as well as cell elongation and the former causes cell elongation. Briefly endogenous auxin is essential for the gibberellins to exert its full effect. Moreover, the importance of a specific balance between different growth substances is highly vital.
On the periphery of the meristem small protuberances arise in a regular fashion which develop into leaves. The angle between successive leaves is determined by the angular distance and the primordium next to it.
Specific arrangement of leaves on the stem provides efficiency of light trapping. In most of the arrangements overlapping of leaves is avoided in order that maximal efficiency and advantage for light procurement exists.
The formation of leaf primordia at definite places and originating at defined times appears to be linked with the origin of specific signals. Recently concept of leaf initiating stimulus has been advanced, but it does not seem to be supported by any experimental evidence. However, the physical size of available space in which primordia are expected to appear, seems highly important.
In recent years, several physiologists have reported a close relationship between auxin production in leaves and xylem regeneration in a mound in the stem. The rate of IAA production is also correlated with the rate of xylem element formation in young leaves. Apparently xylem formation was affected by the auxin and nutrients balance.
Further studies by R.H. Wetmore and J.P. Rier have demonstrated that presence, and concentration of sugar affected the type of tissue differentiation. Thus, xylem formation is promoted by low sugar concentration whereas phloem formation occurred with high concentration. Intermediate concentrations resulted in the formation of both xylem and phloem.
The leaf shape is regulated by several factors including duration, quality and intensity of light. Further concentrations of auxin and CO2 control the leaf shape especially in heterophyllous plants by regulating the length of the petiole of floating leaves. The influence of a gradient of growth regulators arising in primordia also control the leaf shape and size.
Mechanism of floral initiation which is a change over from vegetative to floral conditions. The type of signal and the possibility of other factors in inducing flower formation has also been brought out. The foliar nature of floral parts has also been demonstrated.
In summary, it may be stated that very little is known regarding development responses, growth initiation, developmental processes and triggering of new responses.