Investigating the growth of Lemna (Duckweed)

Authors Avatar

Investigating the growth of Lemna (Duckweed)

Aim: investigate one factor which affects the growth of Lemna.

Introduction:

Lemna is a floating aquatic plant that is generally known as “duckweed”. There are several common species of duckweed including the large-leaved Spirodela (Lemna) polyrhiza, the three-lobed duckweed, L. triscula, and the common duckweed, L. minor. These duckweeds thrive on lakes and multiply until a layer of it covers the entire surface of the water, but are not usually a pest as the layer is thin and light and oxygen can still pass into the water beneath their leaves.

There has been a case in Britain where a particular species of duckweed has been so successful that it blanketed the entire lake in the nature reserve of Canterbury with a mat that was five to twelve centimeters thick, thus isolating the lake from light and oxygen and killed the rich growth of underwater vegetation which then decomposed. The result of the decomposition was high concentrations of hydrogen sulphide which proved toxic to most organisms living in the water. Once rid of its competitors, the duckweed proceeded to use the decomposition products, particularly ammonia, as their source of nitrogen.

The success of L. minuta in the Canterbury lake can be credited to a number of human-generated factors. Firstly, the water in the lake is enriched by fertilizers and fully treated sewage thus providing a rich source of nutrients for the duckweed to thrive upon. Secondly, humans accidentally move organisms around the planet. Examples of the latter include the introduction of sycamores to Britain, rabbits to Australia and probably L. minuta to England. Thirdly, there may have been increased amounts of sunlight (insolation) warming the lake over the past three years.

        Lastly, one crucial factor appears to be that the Canterbury lake is sheltered from the wind by trees, and water disturbance is known to reduce the success of L. minuta.

        From this example, I can deduct from it, the factors which will affect the growth of Lemna.

Variables to Investigate:

  • Temperature – Lemna grow particularly well in warm water and is deterred by colder climates.
  • Area – when the leaves of the Lemna touch, they stop reproducing, as there is no space left.
  • Water Disturbance – wind and waves are known to slow the growth of Lemna
  • Light Intensity – as Lemna is a plant, it photosynthesizes which requires light thus the importance of light intensity as a factor.
  • Nutrient concentration – as Lemna is a life form, it needs nutrients to survive thus the importance of nutrient concentration as a factor.
  • Nutrient deficiency – see “nutrient concentration”.
  • Light wavelength – As chlorophyll is green, it has different levels of receptivity to various wavelengths and reflect some wavelengths thus the importance of light wavelength as a factor.

Scales of Growth to consider:

  • Counting
  • Area Covered
  • Biomass (dry weight)
  • Number of Leaves
  • Health

My choice of Variable to investigate is: “Nutrients and the effects of their deficiency”.

The Nutrients involved:

Phosphorus / Phosphates:

Phosphates promote root establishment and formation as well as flowering. Phosphorus is also used in production of DNA and RNA during cell mitosis. Therefore if a plant is phosphorus deficient, it may have a purplish cast on the underside of the leaves (corn) or purplish/red leaves and stems (tomatoes). Flowering and fruiting will be limited. Plants will be slow to form new roots and establish. Plants are slow growing. Premature fruit drop may occur. (Include diagram of cell mitosis and “before a cell can divide.doc”). Mention RNA and its roles and illustrate the importance of phosphorus in creating DNA and RNA. Phosphorus is also involved in the energy dynamics of plants. Without it, plants could not convert solar energy into the chemical energy needed for the synthesis of sugars, starches, and proteins.

Potassium:

Potassium is involved in photosynthesis, sugar transport, water and nutrient movement, protein synthesis, and starch formation. Potassium helps to improve disease resistance, tolerance to water stress, winter hardiness, tolerance to plant pests, and uptake efficiency of other nutrients. Strong, stiff stalks are also an effect of potassium. Additionally, potassium promotes productions of sugar, starches and oils, increases size of grains and fruits and improves the quality of the crop's yield. This is because potassium plays a vital role in activating enzymes throughout the plant. Potassium deficiency includes reduced vigour, increased disease problems, thin fruit or vegetable skins as well as small fruit.

Join now!

Calcium:

Calcium is an element which contributes towards the strengthening and building of cell walls. Calcium is a constituent of the cell wall and keeps the cell membranes stable. Visual evidence of calcium deficiencies generally occurs in growing points of the plant at the fruit, stem, leaf, and root tips. Calcium deficiency will show up as weak stems or trunks as well as a reduction of any new growth from the plant's growing points.

Nitrogen:

Nitrogen is combined with sugar molecules or smaller carbohydrate molecules to create amino acids that are then combined to form the proteins that make up ...

This is a preview of the whole essay