Grass growth and development - Terms and definitions
By Phil Sharples
Photosynthesising is the plant's way of producing its own food for consumption and therefore growth. The grass plant uses the energy from sunlight to produce sugar (glucose). Cellular respiration then converts this sugar into ATP, the 'fuel' used by all living things. This conversion of 'unusable' sunlight energy into 'usable' chemical energy is closely associated with the actions of a green pigment found in the plant called chlorophyll. This photosynthetic process uses water and releases oxygen that obviously is vital to us as human beings.
The photosynthetic reaction is expressed as:
6H20 + 6CO2 Light C6H12O6 + 6O2
In simplistic terms the above equation can be described as; SIX molecules of water plus SIX molecules of carbon dioxide produce ONE molecule of glucose plus SIX molecules of oxygen.
The process of respiration goes on continuously in the plant. The plant uses glucose as a source of stored energy, one by-product of photosynthesis. From the leaf it is dispatched via the vascular tissue (phloem) to the living cells in every part of the plant. Here, it is converted back into carbon dioxide and water again with the help of oxygen from the air. During respiration, oxygen is absorbed, carbohydrates oxidised, carbon dioxide and water formed, and the chemical energy is then converted to usable energy. Living cells capture a portion of this usable energy and utilise it in support of many vital life processes.
The uptake of water by the roots is simply caused because of loss of water from the leaves. This is known as transpiration. Transpiration causes a hydraulic gradient in the xylem tissue, this stimulates the upward movement of water through adhesion (contact and fixing of water molecules to xylem) and cohesion (contact and fixing of water molecules to each other).
Osmosis is the transfer of water from cell to cell. This movement is due to a water potential gradient found in individual cells. This gradient can be explained in the following way. The cells near the leaf posses a lower water potential than those found in the roots due to water loss from transpiration. Absorption of water by root hairs increases the root cortex cells' water potential (or capacity). This absorption is due this time to the lower water potential in the cells of the root than that of the soil water solution held in the soil. Once this occurs cells will become high in water potential, resulting in the neighbouring cells (around and above) having a lower water potential. Water will then move from high to low water potential (or pressure), and so on, so forth.
Root growth and morphology
The turfgrass root functions to allow (a) anchorage of the plant (b) absorption of nutrients, water and gasses such as oxygen, (c) stores some carbohydrates (food), and (d) synthesis of plant hormones. Once established, the grass root system is fibrous with no single root more prominent than the others. However, during establishment a dominant root called the 'seminal root' is produced to aid initial growth and development. These roots are able to absorb more nutrients than fibrous (or adventitious) roots, but they are short-lived with a life expectancy of around 6 - 8 weeks.
Root morphology is made up of three important regions.
1. Region of division
2. Region of elongation
3. Region of maturation
In the region of division the cells are produced from apical meristem, located just behind the root cap. From here the cells divide into their various type or structure (this is determined through chemical signalling) and elongate to their correct size and form. Once the cells have elongated they can be classed as mature. Once matured, the roots outer cells (epidermal) extend outward or laterally to produce what we know as root hairs. It is at the root hairs that water and nutrients are taken up from the soil and gasses are exchanged. Root hairs penetrate the spaces between the soil particles (soil pores) and dramatically increase the surface area of the root.
It is during the stage of elongation that the root grows as the extension of the cells pushes the root cap further into the soil. Interestingly the root cap is coated with a substance known as 'mucigel'. The mucigel covers the root cap and protects it (or more importantly the apical meristem or region of initial cell production) from wearing out as it is pushed though the soil.
Turfgrass roots do not instinctively grow downward; they can grow horizontally, even upward! It is certain however that roots will grow when stimulated. If the soil has adequate and correct supplies of available water and nutrients, growth will occur.
Grass plant and water relationships
There is a continuous transfer of water within and among the soil, the turfgrass plant, and the atmosphere. The transfer of this water may occur in the liquid or vapour form. Water is absorbed in far greater quantities than any other substance required for growth. However, it is thought that only 1 - 3 % of absorbed water is utilised in plant metabolic processes. Most of the absorbed water is lost through transpiration. The internal water balance of a plant is influenced by; water absorption, internal movement and transpiration.
Turfgrass plants absorb water primarily from the soil through the root system. Most water absorption occurs in the 'root hair' zone located a short distance behind the root apex. Root hairs increase the absorption surface of roots. The root hair zone is continually replaced as the root elongates. Note: The number of root hairs decreases as the soil moisture level increases, therefore, constant wetting or watering of the soil can be detrimental to root growth and water uptake.
The quantity of water absorbed is dependent on,
1. The depth of the root system
2. Root number
3. Amount of available water in the rootzone
4. Root extension rate
5. Transpiration rate
6. Soil temperature
Turfgrasses typically have a large number of roots but their depth is limited by mowing. The rooting depth generally will increase in proportion to the cutting height. This is simply a result of the associated increase in carbohydrate synthesis made possible by a fuller leaf blade.
Root activity and water absorption rates will be restricted by,
1. Excessive nitrogen fertilisation
2. Over watering
3. Acidic compacted soils
4. Low cutting heights
Water and nutrients can also be absorbed by the leaves (known as foliar absorption) and stems of turfgrasses. It may enter leaves in either a liquid or vapour state. The path of water movement is usually through the epidermal cells rather than the stomata. This type of absorption occurs with liquid applications of nutrient.
Water movement in the plant
The xylem is the principal water conducting tissue in plants and is continuous from the root hair zone of the roots, through the stem, and to the cells of the leaves. This transports water upward through the plant. A hydrostatic gradient develops between the evaporation zone of the leaves and the water absorption region of the roots with water flowing along gradients of decreasing water potential (see osmosis).
Most transpirational water loss occurs through the leaves although some may occur through any plant part exposed to the atmosphere. The amount of water contained in a plant at any one time is only a small quantity absorbed and transpired. Nearly all the water lost from the turfgrass plant will be lost through the stomata on the leaves.
The stomata are important structures facilitating the gaseous exchange of CO2 and O2 so vital to photosynthesis. These same stomatal features that enhance efficient gaseous exchange also result in extensive water loss by transpiration. Although composing of only 2 - 3 % of the total turfgrass leaf area the stomata are responsible for as much as 90% of the total water lost to the atmosphere by transpiration.
The loss of moisture from the soil is termed evaporation. As we have already discussed loss of moisture from the plant is termed transpiration. Loss of moisture from both the soil and plant is termed Evapotranspiration. As much as 80 - 85% of the soil moisture depletion can be attributed to evapotranspiration. The evapotranspiration is influenced by; (a) light duration, (b) temperature, (c) atmospheric water pressure, (d) wind, (e) water absorption rate, and (f) soil moisture tension.