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Citrus under Knittex

Written by  Advertorial
| in Landbou
| June 25, 2014

Citrus, native to subtropical and tropical regions in Eastern Asia, has become widely cultivated around the world which has lead to many new challenges for growers because many of these areas are not ideally suited to citrus fruit production.

 Citrus trees thrive in a consistently sunny, humid environment with fertile soil and adequate rainfall or irrigation. However, most areas in South Africa where commercial production of citrus takes place, are relatively extreme due to excessive solar radiation and heat, coupled with low humidity. Typically, in the Upington, Citrusdal and Hoedspruit areas, the climate is characterised by hot to very hot summers, with temperatures reaching 42°C, coupled with high solar radiation and low humidity. Citrus has the ability to adapt and acclimate to and/or compensate for environmental stress for survival which generally results in less than acceptable quality and yields.  However, this does pay the bills and we don’t want our citrus trees to waste valuable energy trying to survive. Instead, they are there to produce good quality fruit and acceptable yields. Good photosynthetic response is essential and is the net result of many interacting processes which contribute to good growth, development and yield.
        The principal environmental parameters that effect the rate of photosynthesis of citrus are:

a) Light intensity, quality and duration; b) Carbon dioxide uptake; c) Water supply (quality, quantity and frequency); d) Mineral elements and composition of the soil; e) Temperature (ambient, leaf, fruit and soil)
        Light impacts on growth and development of plants in two ways:
Photosynthesis (primarily the conversion of light energy into chemical energy for plant fuel) and Photomorphogenesis (the control exerted by light over growth, development and differentiation of plants, namely size, shape, etc. that is independent of photosynthesis). The subject of photomorphogenesis will not be addressed in this publication.  Photosynthesis, is depicted by the following chemical equation:

LIGHT    +    6H2O      +      6CO2         →      C6H12O2    +   6O2        +        H2O
(light)          (water)   (carbon dioxide)       (sugars)     (oxygen)       (water)

        In other words: “Give the citrus tree the correct quality and quantity of light, water, minerals and carbon dioxide and it will be equipped to produce adequate sugars and energy to grow and develop into a healthy tree producing good quality fruit and acceptable yields”.
        For Citrus to photosynthesise optimally, we must manage light intensity and light quality reaching the tree effectively.
a) Light intensity - Firstly, all plants have their own “Photosynthesis Light Response Curve” (figure 1), wherein there is a “Light Compensation Point” (LCP) and a “Light Saturation Point” (LSP). As light increases from zero (darkness) to the LCP, the plant moves from a state of respiration to photosynthesis. Thereafter, the rate of photosynthesis increases as the light intensity increases until it finally reaches a plateau, namely the light saturation point. Plant leaves have no option but to absorb the excess energy received from the sun and are then required to dissipate this energy in some form or another. If not managed by the grower, plants will not be able to dissipate this excess energy successfully and will then be subjected to unnecessary stress.

knittex2Secondly, all plants differentially absorb Photosynthetic Photon Flux Density (PPFD) they are exposed to, which describes the quanta of photons (amount of energy particles) that is absorbed by a specific leaf area at a point in time. This is defined and expressed in micromole photons per sq meter per second (µmol m-2 s-1). This is very similar to light intensity but the term PPFD is reserved for those wavelengths that can potentially cause photosynthesis (between 400 to 700nm) and not the entire visible light spectrum. For example, the PPFD value for tomatoes is around 1500 µmol m-2 s-1, whereas for orchids it is between 200 and 800 µmol m-2 s-1, dependent on plant species.  In the case of Valencia sweet oranges, a PPFD of around 1000 µmol m-2 s-1 is found to be the saturation level for activation of citrus Rubisco, the enzyme which fixes atmospheric CO2. The quantum of light energy transmitted by the sun in summer in most parts of South Africa ranges between 2000 to 2450 µmol m-2 s-1. For most plants, this energy is far in excess of what is required to photosynthesise optimally without being over stressed. Thus, our first priority should be to reduce the quantity of light energy reaching the plant. This can be done using shade netting of different design densities, namely SpectraNet Code 80, 70, 60, 50, 40, 30 or 20. This will also reduce the high, energetic ultraviolet light energy by  similar amounts.

b) Light Quality - Photosynthesis is most efficiently driven by blue and red light. Blue light with a wavelength range of 425 to 500 nm is particularly important for the initial stage of plant growth as well as during the blooming and fruit colouring process due to its higher energy level. Red light on the other hand, with a wavelength range of 650 to 700 nm, provides for excellent plant growth and development due to its moderate energy level. In the production of fruit, vegetables or flowers, one must appreciate that in order to obtain best quality in fruit and vegetable size and colour or flower size and stem length, the quantity of blue and red light, as well as the ratio of blue to red light, must be manipulated by the grower. This process is done by using SPECTRANET shade netting in different colours or a combination thereof to absorb, reflect, transmit and diffuse selected light wavelengths as required for specific plants. The figure 3 below illustrates how Knittex SPECTRANET Code 80 reduces the UV and visible light intensity, whilst at the same time manipulating the ratios and quantities of blue, green, red and far-red wavelengths reaching the plant.

knittex1Knittex and Prof Leon van Rensburg of North West University have for the past twenty years been conducting light and temperature management trials on fruit, vegetable and flower production under SpectraNet shade netting, to study and analyse the influence of temperature and visible light energy on plant stress, growth, development, produce yield and quality. Creating the ideal light environment is all good and well, but as soon as citrus is subjected to high temperatures, new management practices have to be considered and carefully implemented. The optimum temperature for citrus photosynthesis ranges from 25°C to 30°C whilst temperatures above 35°C reduce photosynthetic activity. High temperatures, combined with low humidity, will cause heat stress to the citrus tree, inducing stomatal closure.  This reduces leaf transpiration rates and the absorption of CO2, negatively impacting on the photosynthesis efficiency of the tree. Surface temperature measurements taken on exposed citrus leaves and fruit in an open orchard compared to that of an orchard covered with SPECTRANET Code 20 Active Blue shade netting differed by between 4°C and 9°C at ambient temperatures of 23°C and 32°C respectively.  The higher the ambient temperature, the bigger the difference.  When ambient temperatures reach the 40°C region, exposed leaf and fruit temperatures exceed 50°C, causing bleaching  of leaves and the destruction of fruit skin cells.

Sunburn of Satuma Citrus                 Leaf bleaching from heat stress
                 Satsuma Citrus under SpectraNet Code 20 Black/White
        High solar irradiation and ambient temperatures also increase soil temperatures and water evaporation levels. Citrus trees place high reliance on their water resouces,  and therefore can ill afford losses through unnecessary evaporation. Inadequate water uptake or irregularities of soil moisture will rob the tree of its basic needs,
limiting the transportation of food and water to all parts of the tree and increase the possibility of fruit splitting and sunburn.
        High temperatures will also promote the abscission of both flowers and fruit as the tree protects itself against heat stress, resulting in much reduced yields because of reduced energy resources. SPECTRANET shade netting will reduce leaf, fruit, bark and soil temperatures as well as wind and sunburn damage, whilst increasing humidity and transpiration rates.  It has been proven to increase citrus yields and quality.
        Subject to the growing region, its climatic conditions and soil quality, different grades and colour combinations of SPECTRANET shade netting will be required for best performance.  “One size does not fit all.”