LIGHT ,PLASTIC FILM AND PLANTS

1. LIGHT ,PLASTIC FILM AND PLANTS WHAT ARE THEIR PROPERTIES AND WHAT IS THEIR RELATIONSHIP?

2. CONTENT OF PRESENTATION • TERMINOLOGY • LIGHT IT`S QUALITIES AND PROPERTIES • PHOTOSYNTHESIS AND THE LIGHT SPECTRUM • LIGHT ENERGY NEEDED BY THE PLANT • TECHNOLGY OF THE PRIVA CLIMATE CONTROL • PLASTIC FILMS AND THEIR DEGRADATION

3. TERMINOLGY • MOLE is Avogadros number of 6.02 x10 23 particles in a substance eg Gold has a molar mass of approx 197 g therefore in 197g there will be exactly 6.02 x 1023 atoms of gold.This relevant to all things that have mass even light as it is energy and Energy =mass x speed of light (c) • LUMENS = The amount of light irradiated by a source as perceived by the human eye(IR) • LUX = The amount of lumens per m 2 or specific area • CANDELAS = LUMENS(IR) • WATTS / M2 = The amount of energy arriving at a point over a period of time (1 Watt = 1 Joule/S) (E) • JOULES/ M2= The energy of light reaching a point each second(E) • LANGLEY =THERMOCHEMICAL UNIT /YR (E) • LANGLEY = 41840.00 J/m² (E) • SOME MEASURE IRRADIANCE(IR) AND OTHERS ENERGY (E)

4. WHICH UNITS ARE RELEVANT TO GREENHOUSE GROWERS? • 1 Amount of Light • 2 Direction of Light • 3 Spectral Quality of Light

5. WHAT UNITS ARE RELEVANT TO GREENHOUSE GROWERS? • Direction of light effects its energy per square meter • When light strikes an object from 90 Deg which is flat then energy of irradiance is highest • When light strikes an object at an angle which is flat then energy is lower • When light strikes a plastic film it is diffused then the plant receives light from all angles • Then the surface is not flat and omnidirectional reading must be taken ( Inside the greenhouse) • This measurement is called the fluence rate and can be expressed in watts per m2(E) or mol/m2/s(Mol)

6. WHAT UNITS ARE RELEVANT TO GREENHOUSE GROWERS? • Flat sensor measurement of photosynthesis can be measured in w/m2 or mol/m2/s • Photo active radiation (400-700 nM)is expressed in quanta and is an irradiance measurement • Before specialised light meters used for plants lux or candelas were used • These are based on the perception of light by the human eye which is in the region of 555 nM • Sensitivity either side of this falls off rapidly and is zero in the blue and red spectrum. • These measurements are of little use to plant physiology

7. WHAT UNITS ARE RELEVANT TO GREENHOUSE GROWERS? The relationship between PAR irradiance and PAR fluence rate is as follows In direct sunlight PAR Irradiance and Fluence rate are the same at 2000uMol/m2/s .This will be higher in Gauteng. This is equal to 400 W/m2 in energy NOTE THIS IS VERY IMPORTANT WHEN LIGHT IS COMPLETLEY DIFFUSE THEN IRRADIANCE IS ONLY 1 / 4 FLUENCE RATE. IT PAYS TO HAVE DIFFUSED LIGHT INSIDE YOUR GREENHOUSE

8. LIGHT FACTORS and ENERGY • Light has a wavelength and a frequency • A stream of energy in the form of light is a Quanta • Quantas are composed of Photons • Each Photon is equal to Planck`s constant multiplied by the frequency of the radiation (E=h.v) • ENERGY IS DIRECTLY PROPORTIONAL TO FREQUENCY • Not all colours of light have equal energy • Energy of a photon of a particular wavelength is thus Energy=h .speed of light /wavelength • ENERGY IS INVERSELY PROPORTIONAL TO WAVELENGTH

9. LIGHT FACTORS and ENERGY • 1 mole of photons (einstein) of 490 nM blue light will have an energy of 240 kJ • Whereas 1 mole of photons of 700nM of red light will have only 170kJ of energy • Plants require +2840 kJ /mole energy needed to convert 6 molecules of CO2 into 1 molecule of hexose sugar!!!! • Plants use 400 -700 nM wavelengths of light and bacteria use far above the infrared spectrum for energy(heat) They need heat not sunlight for energy • Fungi use UV rays for sporulation and survival as they do not have green pigment it does not effect them • Insects need UV light to see

10. SPECTRUMS OF LIGHT • Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence also invisible to the human eye). Owing to absorption by the atmosphere very little reaches the Earth's surface (Lithosphere). This spectrum of radiation has germicidal properties, and is used in germicidal lamps. • Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for the photochemical reaction leading to the production of the Ozone layer. • Ultraviolet A or (UVA) spans 315 to 400 nm. It has been traditionally held as less damaging to the DNA, and hence used in tanning and PUVA therapy for psoriasis. • Visible range or light spans 400 to 700 nm. As the name suggests, it is this range that is visible to the naked eye. • Infrared range that spans 700 nm to 106 nm [1 (mm)]. It is responsible for an important part of the electromagnetic radiation that reaches the Earth. It is also divided into three types on the basis of wavelength: • Infrared-A: 700 nm to 1,400 nm • Infrared-B: 1,400 nm to 3,000 nm • Infrared-C: 3,000 nm to 1 mm.

11. ELECTROMAGNETIC RADIATION FROM THE SUN

12. PHOTO ACTIVE RADIATION

13. LIGHT AND ENERGY NEEDED BY THE PLANT The fact that plastic film in a greenhouse creates diffuse light the issue of shadowing and the quantity of light is not limiting so plants will receive ample light UNLESS The film has started to deteriorate and is allowing in less total PAR light than outside the greenhouse. PAR irradiance and fluence rate or(diffuse light) is the same in direct sunlight We will touch on plastic deterioration in a later slide. The various growth stages of the plant are important but all are governed by the fact that light totality is as important as quality of light

14. LIGHT AND ENERGY NEEDED BY THE PLANT LIGHT AND PLANT QUALITYSome parts of the spectrum can influence the shape and height of the plant, branching and other aspects of plant quality. The main parts are:Blue light: plants respond to the intensity of blue light, and reducing the blue light will encourage plant elongation and leggy growth. This response is not relative to the strength of radiation in any other part of the spectrum - it is the absolute intensity of the blue light which influences plant height and quality.Red light and Far Red light: there is a more important response which depends on the relative intensities of red (660nm) and far red (around 730nm) light. Increasing the amount of far red light relative to the red makes plants grow tall and spindly. Increasing the red relative to the far red does the reverse. If the red/far red ratio is increased significantly, significant height reductions and changes in plant habit can be achieved.

15. LIGHT DISTRIBUTION ON A PLANT IN DIRECT SUNLIGHT

16. PRIVA CLIMATE CONTROL Priva developed the MAXIMISER unit to operate the greenhouse control This consists of a external and internal unit The external unit measures the following Tempreature Wind Speed and Direction Humidity Light intensity The internal unit measures only Tempreature Humidity

17. PRIVA CLIMATE CONTROL The Maximiser measures the light intensity in W/m2 which Is an irradiance reading. The light in the greenhouse needs to be measured ,as it is diffused, with an Energy type reading such as einsteins (mol/m2/s) The difference between the outer and inner tempreature and humidity readings Is used to monitor and control temp and humidity inside

18. PLASTIC DEGRADATION There are several types of degradation that occur when plastic is exposed 1 Photo Degradation is caused by harmful Ultra Violet Rays of Light namely UV A and UVB 2 Thermal Degradation as is caused by infra red and far infra red light 3 Chemo Degradation as is caused by exposure to chemicals such as sulfur 4 Oxidative degradation where oxygen reacts with the free radicals in the film and starts to oxidise the film Plastic film or polymer`s ability to resisit degradation is referred to as it`s STABILITY Degradation is inevitabile and it is a matter of when it will begin and how quickly it will happen until the basic qualities of the plastic are lost

19. Qualities of Plastic Lost due to degradation Degradation leads to changes in molecular weight and molecular weight distribution of the plastic . The following are some of the consequences Reduced Ductility and Brittling of the film Chalking or the film becoming opaque and losing light transparency Colour Changes from green to blue or clear to milky white Cracking or snapping of film General loss of characteristics of the film as it was designed for Polyethylenes are also susceptible to thermal and photo degradation and the resulting chain branching and cross-linking reduces the melt flow and produces embrittlement and color changes

20. Degradation Process INITIATION OF DEGRADATION Loss of a hydrogen ion from the polymer chain due to light or heat Hydrogen ion is now a free radical and can cause degradation PROPOGATION The free radical reacts with oxygen to form a peroxy radical This peroxy radical then free`s up another radical hydrogen ion This then forms hydroperoxide and it can free up 2 more hydrogen ion`s The process becomes exponential 2x2=4 and 4x4=16 ........ TERMINATION This is when all free radicals have been used and the film has no flexibility or qualities left

21. Plastic Qualities Stabilisers are added to the polyethylene to delay and prevent the process of decay Stabilisers absorb the free radicals and prevent initiation Degradation tests are done in an accelerated test and are extrapolated This is then averaged over a period of time to determine a film`s stability Summary Thermal degradation of plastics at elevated temperatures is an inevitable event For many polymers it can be a significant limitation to the application service life of a product. Even at moderate service temperatures long-term thermal degradation can represent a limitation to the service life.