agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 7.2 1999
GROWTH OF THE GRASS PLANT
P E Bartholomew
KwaZulu-Natal Department of Agriculture
Pattern of
Growth of the Grass Plant
Undisturbed Growth
Growth Following Defoliation
Carbohydrates and the Grass
Plant
Carbohydrates and Management
Factors Influencing
Tillering in Grasses
Resting: Carbohydrates
and Tillering
INTRODUCTION
Veld and pasture grasses maintain a cover of vegetation and continue producing under conditions of repeated defoliation. This continued production is a result of the morphological and physiological characteristics of the grass plant. These characteristics can, however, be modified by climate, soil and by management, all of which can influence production per se.
Although grass growth and yield can be described in physical and/or chemical terms (e.g. fresh matter, dry matter, organic matter, protein content), plants which make up a sward are living organisms. The grass plant grows by producing cells which expand and are modified, in the process of developing, into various organs such as tillers, leaves, stems, flowers and roots.
There are two phases of growth that are readily distinguishable in the grass plant. These phases are known as the vegetative phase and the reproductive (or flowering) phase, both of which are important in determining productivity and survival. It is the objective of this Production Guideline to give a brief outline of grass growth and the factors affecting production and survival of the grass plant. Such an outline is intended to provide guidelines for good management, and to provide some insight into why certain management practices are advocated by advisors and consultants.
PATTERN OF GROWTH OF THE GRASS PLANT
The grass plant consists of a collection of units called tillers (Figure 1). Each tiller is regarded as an independent unit which is made up of a short stem (from which arise the leaves which consist of a leaf blade and a leaf sheath), and roots.
At germination of the grass seed, the root and shoot systems develop from the embryo. The pattern of growth of the shoot is set by the stem apex which is present in the embryo at the outset. Successive primordia appear on alternate sides below the apex and develop into leaf initials which grow up around the apex, ensheathing it (Jewiss, 1981). Each segment, between successive leaf initials, forms an internode. Internodes are separated by nodes which are tightly packed one on the other to form the stem, which in the vegetative phase is only a few millimetres in length. Tiller buds (buds which can develop into tillers and which are replicas of the apical bud of the original stem) develop in the axil of each leaf (Figure 2). Normally the tiller bud in the axil of the first leaf emerges any time after the second leaf is fully expanded, although this can be delayed by unfavourable conditions, and can vary from species to species. During the vegetative phase of growth, little or no stem elongation occurs and the site of origin (and therefore the bases) of the leaf and tiller buds, remains close to the soil surface. Daughter tillers (which develop from the lateral buds described above) emerge adjacent to the parent tiller and at first appear as emerging leaves in the axil of older leaves on the parent tiller. Tillers root from the nodes and, although adjacent tillers remain in vascular connection with one another, tillers behave as if they were separate plants (Jewiss, 1981).Origin of tillers
Figure 1. The grass plant.
Figure 2. Diagrammatic sections of a vegetative grass showing the position of the stem apex and production of leaves and tillers from leaf primordia and buds (Jewis, 1981).
Tillering
As has been seen, tillers with their own roots arise from the axil of leaves. These tillers are of the utmost importance because the tiller, and not the whole grass plant, is the basic unit which should be considered in pasture and veld production. Continuous leaf pro-duction is what is required from a grass plant. Leaf production is a function of tiller production. For a given species of grass the number of leaves produced per tiller varies within narrow limits. Thus it is obvious that ‘grass management’ should be aimed at maximising tiller production. Furthermore, the continued pro-duction of new tillers is the basis of survival of perennial grasses, because once a tiller flowers, or the tiller apex is removed (by grazing or cutting), or destroyed (by trampling or burning), the tiller dies and needs to be replaced if production is to be sustained.Flowering
The developmental pattern discussed above changes with the onset of flowering. Instead of the apical bud producing tiller and leaf initials it elongates into a flower-bearing stem. The change from the vegetative phase to the reproductive phase is, in most grasses, determined by day-length. In some grasses a cold period (vernalization) is required to precede a specific photoperiod. Yet other grasses flower on reaching a certain physiological age, irrespective of day-length. Irrespective of the stimulus for flowering, a tiller must have reached a certain stage of physiological maturity before it can flower.
As soon as temperature, moisture and light are suitable for growth, tillers and leaves are produced. The general pattern of increase in the mass of the plant is shown in Figure 3.
Figure 3. Undisturbed growth curve of a grass plant.
The sigmoid curve shown in Figure 3 illustrates an initial slow increase in plant mass, followed by a rapid increase, and then a gradual decline. A closer look at this growth pattern reveals several interesting and important points.
- The initial growth in spring is dependent on the growing out of existing lateral buds, the production of new bud primordia and the production of leaves. This phase is represented by a slow increase in plant mass, even when climatic conditions are amenable to rapid growth.
- This initial growth, particularly of the leaves, is dependant on stored reserves (largely carbohydrates) and the higher the level of these reserves, the more rapid is this initial growth.
- Once sufficient leaf has been produced, energy for growth is supplied from current photosynthesis and the plant no longer needs to draw on stored reserves. Reserves (especially carbohydrates) will begin to accumulate once the leaves produce more carbohydrates than are needed for growth (tillering and leaf production); this represents a phase of rapid increase in plant mass.
Figure 4. The effect of defoliation height and frequency on regrowth of the grass plant (adapted from Booysen, 1966).
- As more tillers and leaves are produced, the plant mass increases further. The increased leaf area results in increased light interception, increased growth rate, and increased storage of photosynthetic products (mainly carbohydrates). Growth rates remain high during this phase.
- As the leaf area increases further, the amount of light reaching the base of the plant declines. Tillering rate is reduced, the lower leaves are shaded, and their photosynthetic activity is reduced. They begin to die off (turn yellow and brown), and growth rate declines.
- Since existing leaves are not removed and since the amount of light reaching the base of the plant is reduced, the development of new bud primordia is restricted and growth rate becomes more or less static, or may even decline.
Under practical grazing systems the grass plant is defoliated during the growing season. The effect of frequency and severity of defoliation on regrowth of the defoliated plant is illustrated in Figure 4.
Initial regrowth following lenient (and frequent) defoliation is more rapid than is the initial regrowth following severe defoliation (Figure 4). This is, in part at least, a result of regrowth from the leniently-defoliated plant, having a large amount of residual leaf, drawing substrate requirements for growth from current photosynthesis. The severely defoliated plant, on the other hand, would have a low residual amount of leaf to provide the substrate required for regrowth from current photosynthesis. Initial regrowth from the severely defoliated plant would thus be largely dependent on stored reserves. Since regrowth from current photosynthesis is faster than regrowth dependent on stored reserves, regrowth of the severely defoliated plant will be slower than regrowth from the leniently defoliated plant.
The effect of frequency and severity of defoliation on dry matter production is that frequent, lenient defoliation will provide for higher dry matter production, over the season, than will infrequent defoliation. It must, however, be borne in mind that frequency and severity of defoliation are relative concepts and that different grass species are adapted to different defoliation regimes.
CARBOHYDRATES AND THE GRASS PLANT
Carbohydrates, produced in the green parts of the plant during the process of photosynthesis, are used as a source of energy (for respiration), and as building material from which all other plant constituents are elaborated. When synthesis of carbohydrates is in excess of current requirements for respiration and growth, the excess carbohydrates are translocated to storage organs to be utilised at a later time. The principal storage organs are the stem bases and roots of the grass plant. The stored carbohydrates are utilised by the plant when the synthesis of carbohydrates (in green tissues) cannot meet the plant's requirements for growth and respiration. These times are in early spring, after defoliation, at flowering, and during periods of dormancy.
In early spring, when there is no green material for photosynthesis, environmental conditions conducive to growth (increased temperatures and available soil moisture) result in stored reserves being made available for growth. This results in a reduction of the stored carbohydrates. The higher the level of the reserves, the more rapid will be the initial growth and the sooner the plant can be grazed.Spring growth
Defoliation effects
Following defoliation, the photosynthetic area (amount of leaf) is reduced, thus reducing the photosynthetic capacity of the plant. If insufficient leaf area is left for current photosynthesis to meet the plants requirements for growth, the stored reserves will be drawn upon to fulfil these requirements. Once again the larger are the reserves, the more rapid will be the regrowth. As indicated earlier, regrowth is faster where the plant's requirements are supplied by current photosynthesis, than when stored reserves are mobilised to supply the energy for regrowth.Flowering effects
At flowering, there is a rapid elongation of the flower-bearing stem. This rapid growth requires a large amount of energy which is supplied largely by carbohydrate reserves. If there is a large leaf area at the time of flowering, the carbohydrate reserves need not necessarily be drawn on, and the energy needs for flowering may be supplied by current photosynthesis. Very little or no carbohydrates are translocated to storage organs at this time.Dormancy
When plants become dormant, either through lack of moisture or low temperatures, survival of the plant depends on continued respiration. When dormant, there is seldom green leaf tissue to photosynthesize and produce energy for respiration. Thus reserves are required for the plant to respire and so survive.
Although the carbohydrate reserves of the grass plant play a vitally important role in the growth and survival of the plant, this does not imply that the reserves must always be high. The grass plant must be managed intelligently, so as not to cause the plant to degenerate. This can be done in either of two ways.
- The pasture/veld can be managed in such a way that, following the accumulation of a reasonable leaf area, it is grazed short (little photosynthetic area left) in a short period of time (short period of occupation) and then allowed a long period of recovery. In this way the initial regrowth of the defoliated plant is largely dependent on stored reserves (because of the greatly reduced leaf area) and the reserves are drawn on to a large extent. Consequently a long regrowth period (rest period) must be allowed for the reserve carbohydrates to accumulate before regrazing. This system would correspond to a non-selective grazing (NSG) system.
- The pasture/veld can be grazed leniently (i.e. a relatively large amount of leaf area is left on the plant), but relatively frequently. In so doing there is still a relatively large photosynthetic area left following grazing, and the reserves are not drawn on to the same extent as they would be had the plant been grazed short. Consequently, the reserves are not depleted to any extent and are replenished, to pre-defoliation levels, within a relatively short period of time (i.e. short rest period). This system would correspond to a high production grazing (HPG) system.
From the above discussion, it is obvious that the shorter a grass plant is cut or grazed, the longer the rest period will need to be to restore the reserves used for regrowth. Conversely, the more leniently a grass plant is grazed, the shorter will the regrowth period need to be to restore reserves. No precise figures can be given for ‘short’ or ‘leniently’ grazed plants. Nor can accurate figures be given for ‘long’ or ‘relatively short’ regrowth periods. These vary with the grass species, the prevailing environmental conditions (largely moisture and temperature) and with soil type. Furthermore, the poorer the growth conditions, and the slower is the plant growth, the longer the rest period needs to be.
It is clear that intensity of defoliation is an important management factor affecting the vigour, regrowth and reserve status of the plant. Thus, for example, if a grass plant were to be grazed to a height of, for example, 30 mm every time it had made only 30 mm of regrowth, there would be insufficient leaf area to supply the requirements for regrowth, reserves would be drawn on and the reserve status of the plant could be exhausted and the plant could die. This then points to restricting the period of occupation, within a grazing cycle, to prevent animals from grazing regrowth within a period of occupation in a camp. Under conditions of continuous grazing and at low stocking rates, the plants are not defoliated short, provided the stocking rate is matched to growth rate, and current photosynthesis supplies the plants requirements for growth following lenient defoliation.
FACTORS INFLUENCING TILLERING IN GRASSES
As indicated earlier, continuous leaf production is what is required from grasslands. Continuous leaf production is a function of tiller production. Since each leaf has a subtending tiller bud in its axil, the potential number of tillers is, in theory, equal to the number of leaves on a plant. In practice not all these buds develop into tillers. Furthermore, tillering occurs at different rates at different times of the year.
The apical bud inhibits the development of lateral buds through the production of a growth-inhibiting hormone. Centres where this hormone is produced have preference for available energy and nutrient supplies. Since the actively-growing apical bud produces the hormone, energy and nutrients are channelled to the apical bud to fulfil its requirements for growth. Lateral, or daughter buds, are thus ‘starved’ and do not develop while the apical bud is active. However, under conditions of high growth rates (adequate leaf area, adequate moisture and optimum temperatures), excess substrate may be channelled to promote growth of lateral buds.Inhibitory factors
Once the apical bud becomes reproductive its demand for growth substrate increases substantially and its dominance over the growth of lateral buds strengthens. However, once active growth of the flower decreases, the production of the inhibitory hormone declines, dominance over lateral buds is reduced, and tillering may increase rapidly, provided energy supplies are adequate.
It follows, therefore, that if the site of inhibitory hormone production (i.e the apex) is removed, tillering will increase (i.e. lateral or basal buds will grow out). In other words, if the plant is defoliated, and in so doing the apical bud is removed, whether it be vegetative or reproductive, tillering will be induced. Once a tiller has flowered, or the apical bud is removed, or the tiller has produced its compliment of leaves (genetically controlled), that tiller dies once the leaves die.
Genetic factors
Some grass species have the potential to tiller more
than others. Thus tillering is, to a certain extent at least, dependent on the
genetic make up of the plant.
Temperature
Tillering is usually slow when day temperatures are
low (associated with low intensity radiation) and night temperatures are high
(i.e. conditions where energy supplies are formed slowly and dissipated rapidly
through high respiration rates at night). Conversely, tillering is generally
rapid during high day and low night temperatures. Tillering, in response to
temperature, will be species dependent since different species have different
optimum temperatures for growth.
Light
Adequate light, resulting in the rapid formation of
energy substrates, favours tillering. Conversely, excessive shading at the base
of the plant retards the development of both tillers and roots.
Water supply
Tillering is more rapid under conditions of favourable
moisture supply.
Nutrient supply
An adequate supply of all nutrients required for
growth is a prerequisite for rapid tillering. Nitrogen, in particular,
stimulates tillering.
Herbage removal
Two effects of defoliation are distinguishable with
respect to tillering.
Where the plant has a large leaf area, resulting in shading at the base of the plant, defoliation will stimulate tillering. However, repeated severe defoliation will adversely affect tillering, due to excessive demands placed on the reserves of the plant and the low residual amount of leaf available to photosynthesize.
The second effect of defoliation on tillering involves the stimulatory effect of the removal of the stem apex discussed earlier.
Flowering
The effect of flowering on tiller development has previously been discussed. Once the tiller flowers, apical dominance is removed, more nutrients are available for the growth of lateral tiller buds and tillering increases.
RESTING: CARBOHYDRATES AND TILLERING
Many veld management practices in South Africa are based on resting the veld during autumn and spring. Both these times are critical periods in the cycle of carbohydrate storage and carbohydrate use. The importance of high levels of substrate for tillering and for high growth rates in spring have been discussed. These high levels of substrate can be attained by resting the grass in autumn. In other words, an autumn rest is necessary for good spring production.
The need for a spring rest can be related to the development of tillers. In spring, it will be found that the younger tiller initials will develop first, followed by progressively older buds, as the amount of energy substrate increases with increasing leaf area and photosynthesis. If this leaf area is removed, by grazing or cutting, the stored carbohydrates will be drawn upon to maintain growth of existing tillers. Meanwhile, dormant buds will be steadily ageing and becoming less likely to sprout when management eventually allows for the production of excess energy supplies. The situation may then arise where, although those tillers which continue growing produce new buds, the tuft may break up and the basal cover may be reduced.
REFERENCES
BOOYSEN, P. DE V. 1966. A physiological approach to research in pasture utilization. Proceedings of the Grassland Society of southern Africa 1 : 77 - 85.
JEWISS, O.R. 1981. Shoot development and number. In : Hodgson, J.; Baker, R.D.; Davies, A.; Laidlaw A.S. & Leaver, J.D. (eds). Sward Measurement Handbook. British Grassland Society, 93-114.