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Blossom End Rot phenomenon - description and prevention

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					                                  Strategy & Products Development
20.03.06



Blossom End Rot phenomenon - description and prevention
Introduction
Growing bell peppers and tomatoes in protected cultivation systems, e.g. greenhouses, plastic-houses, and screen-
houses in soil or on artificial substrates incurs heavy financial investment, which therefore requires high-quality
yield for an economically viable operation. However, the high incidence of blossom-end rot (BER) that adversely
affects pepper and tomato fruits exposed to various stress conditions impairs the required yield quality. BER is a
fruit physiological disorder, prevalent in peppers and tomatoes exposed to high salinity, high air temperatures and
low air humidity, water stress, high ammonium/nitrate ratio and high K/Ca ratio.
The economic losses caused by BER are very high; it is estimated, for instance, in Europe to be in the range of
20–40% of the total yield.
                                                   BER is often associated with local Ca deficiency in fruits (Adams,
                                                   2002; Ho et al., 1993, 1995; Marcelis and Ho, 1999; Marschner,
                                                   1995; Saure, 2001). Some researchers claim the causative
                                                   mechanism of BER is still obscure (Saure 2001), yet it is well
                                                   established that calcium is crucial for the proper development of
                                                   the membranes that bond the cells together into a cohesive unit.
                                                   BER is the result of insufficient rate of calcium flow to the
                                                   blossom (distal) end of the fruit to maintain cell integrity (Ehret
                                                   and Ho, 1986; Tachibana, 1991). Calcium is central to the proper
                                                   development of the membrane that bonds the cells together into a
                                                   cohesive unit. Since Ca is needed to strengthen cell walls and to
                                                   maintain membrane integrity Ca deficiencies lead to the collapse
of cells, resulting in tissue enzymatic browning, caused by polyphenol oxidase and peroxidase enzymes, as well as
tissue susceptibility to secondary infections, such as Phytophthora spp., Erwinia spp. and Botrytis spp. Leaky
membranes may also lead to chlorophyll losses or water-soaked areas. BER is the final expression of an
unbalanced interaction between the rate of fruit growth and the internal distribution of calcium towards the
susceptible tissue in the fruit (Adams and Ho, 1992).
Tomato blossom-end rot appears as round spots on the blossom end, opposite the stem
end of pepper and tomato fruits. These sunken spots may appear as if they are water
soaked, and will start out as brown spots, rapidly progressing to black as the area of the
blossom-end rot increases. The blossom-end rot spots will feel leathery to the touch and
may also develop moldy growths on the surface of the fruits where blossom-end rot is
spreading.

Calcium in the plant
In order to understand better the reasons causing BER it is important to understand the functions of calcium as
well as all the conditions that may affect the uptake and translocation of Ca within the plant.
Calcium is taken up from the soil as a divalent cation (Ca2+); it is a relatively large ion (ionic radius 0.412 nm).
Once inside the plant, some of the calcium gets bound in plant structures such as cell walls and some is
exchangeable at the cell walls and at the exterior surface of the plasma membrane. Most of the calcium functions
are related to the structural part, in particular to the cell walls where it supports intercellular linkages, contributing
to the overall tissue stability (Fig 1), however its importance was recognized in other plant mechanisms as
follows:
     - Calcium as a Secondary Messenger
          Calcium also plays a role in the plant similarly to hormones in the regulation of various cell functions.
          One such function is the regulation of the protein pump that controls the uptake and movement of
          nutrients into the root and throughout cells within the plant. This process is known as “facilitated
          diffusion” and is the means by which most nutrients are taken up by the plant. Calcium was found to
          stimulate the enzyme Calmodulin, which activates the protein pump involved in this process of nutrient
          uptake. Also auxin-regulated cell elongation seems to require Ca2+ as a secondary messenger.

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          (Marschner).
     -    Calcium and Photosynthesis
          At the first stage in the photosynthetic reaction in Photosystem I I (PSII) complex, water are being
          oxidized. Manganese (Mn) is an essential cofactor in the water oxidizing process. Four Mn ions are
          linked to amino acids, oxygen, chloride and calcium. Ca2+ containing cluster acts as a catalytic site of the
          photosynthetic water oxidation within the chlorophyll molecule. This catalytic center plays a crucial role
          in the photosynthesis process. (Rutherford 1989, Rutherford et al 1992, Debus 1992, Yachandra et al
          1993, 1996).
     -    Calcium and Aluminum relationship at the roots
          Adequate concentration of calcium at the root tips is required for facilitated diffusion – Apoplasmatic
          transport (responsible for nutrients uptake) to work properly. The mucilage secreted by the root cap
          needs to be high in calcium. Calcium functions as mediator of the signal substances and therefore is
          important for the secretory functions of the cap cells. Aluminum toxicity (caused when its concentration
          in the soil solution is greater than 400 ppm.) attacks the root cap reducing the secretion of mucilage and
          disrupting both the cytoplasmic calcium and the calcium in the mucilage. The root cap is a source of
          endogenous regulators of extension growth. Raising soil pH, which, in turn reduces the amount of Al3+
          cations released from soil minerals and made available to the root, can prevent aluminum toxicity.
          Liming the soil with calcium materials such as gypsum is a prevalent means to raise soil pH and to make
          calcium available to the plant roots, thereby reducing aluminum toxicity. (Moore et al., 1990)
     -    Calcium and temperature- related stress
          Heat stress generally increases stem length while reducing leaf size and area. As temperatures increase
          above 34o Celsius photosynthesis rate declines in most crops. Calcium can mitigate heat stress effects by
          improving stomatal function and other cell processes. Calcium is also believed to have an influence on
          the development of heat shock proteins that help the plant tolerate the stress of prolonged heat. Studies
          have showed that Ca2+ is involved in the regulation of plant responses to various environmental stresses,
          including heat (Bramm, 1992; Biyaseheva et al., 1993; Colorado et al., 1994). Increasing cytosolic Ca2+
          content under heat stress (Biyasheva et al., 1993; Gong et al., 1998) may alleviate heat injury and enable
          plant cells to better survive (Bamberg et al., 1998; Gong et al., 1998).
     -    Calcium was found to improve plant thermal protection even at lower temperatures (Zsoldos and
          Kaevaly, 1978), and in anaerobic conditions. Calcium also alleviates the damage caused to tissues by
          freezing thawing cycle stress.
     -    Calcium and Disease
          Calcium is often referred to as the plant’s first defense line against pathogens. Many microorganisms that
          infect plants do so by penetrating the cell tissue by pectinase enzymes like Polygalacturonase, which
          dissolve its pectin. The higher the calcium content in plant the higher the concentration of the plant
          pectin, which holds the cells together, and the greater the plant’s ability to withstand these enzymes. In
          some cases the pathogen’s pectinase is simply oxalic acid, which sequesters calcium from the leaf to
          form calcium oxalate. In these cases increased calcium levels in leaf tissue or the foliar application of
          calcium can greatly decrease the pathogens’ ability to invade the leaf. Adequate levels of calcium in
          plants also help in isolating the infection by suberifying the infected area. This process is called
          Demarcation or the ability to isolate the point of infection by suberification as a reaction to the wound
          (Bateman and Lumsden, 1965).




Fig 1. Schematic representation of two adjacent cells with a typical distribution of calcium ( ).
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    -    Calcium and Crop Quality
         It has long been understood that calcium plays a major role in improving and prolonging the shelf life of
         many crops and this is naturally due to further strengthening the bonds between the tissue cells. The
         effect is related to the function of Ca in strengthening cell walls and maintaining membrane integrity,
         preventing the collapse of the cells (Park S. et al., 2005).

Calcium transport
The Ca cation moves mainly through the xylem with a small amount of movement laterally into phloem cells
(Epstein 1961, Ferguson and Bollard, 1976). Some of the calcium seems to be transported as oxalate or by some
ion exchange mechanisms. Since Ca ions are passively taken up by the roots with the soil water and transported in
the xylem with the transpiration stream (Bradfield and Gutteridge, 1979), any factor affecting the uptake of water,
such as climatic conditions, root functioning, salinity, will also affect the Ca uptake.
Several soil- or plant- conditions may also enhance the development of Ca disorders such as inadequate root-zone
moisture, low availability of soil Ca, cation imbalances in the soil or fertigation solution, poor root growth and
saline or sodic soils; inadequate Ca movement to low transpiring, and rapidly developing plant organs due to poor
xylem development, high transpiration rates in leaf canopies and low night-time root pressures. Interplant factors
such as strong carbohydrate sinks, high growth rates and auxin and enzyme activities; and cultivar differences.
In the plant tissue, calcium may suffer from poor translocation within the plant, due to immobilization in the cell
vacuoles as oxalate, in the mitochondria as phosphate precipitate and as carbonate precipitates in the cell walls
(Raven 1977, Ferguson 1978).


Factors affecting the BER phenomenon
As stated before BER is a phenomenon controlled by several factors affecting the movement and distribution of
Ca2+ cations within the plant organs.

Transpiration rate
There is a close positive correlation between calcium concentration in a certain organ and its transpiration rate.
This is shown for example, by the low calcium content in the dry matter of low transpiring fleshy fruits (<0.3%
calcium) as compared with that of the leaves (3-5% calcium) in the same plant. In fact some claim that the leaves
are transpiring a higher ratio of water vapor, causing an uneven distribution of the calcium, forcing it to move
more to the leaves, and from there the movement to the fruits is restricted.
The transpiration flow is naturally, affected by several environmental conditions including radiation, humidity and
temperature.
     - Light intensity – Solar radiation influences the fruit growth rate by controlling assimilates
          supply from the leaves (source) to the fruits (sink), partial shading can be beneficial by reducing
          the growth rate. Calcium movement is very pronounced at high radiation, while in cloudy
          conditions it is considerably more limited.
     - Temperature – temperature affects the transpiration rates but also plays a predominant role in
          fruit growth rate. Temperature promotes metabolic activity, thus increasing the influx of photo-
          assimilates and water, resulting in an increase in fruit expansion (Pearce et al., 1993).
     - Humidity (Relative Humidity) – Low humidity levels during the day tend to favor calcium
          accumulation in the leaves and reduce the calcium transport to the fruit, thus exposing it to a
          greater chance of developing BER. At night high humidity tends to affect only the young leaves.
          High air temperatures will further exacerbate by accelerating fruit growth rate. It was found that
          the dry matter content of the leaves decreased with accordance to the increase in humidity.
          Under high humidity it was found that total amount of calcium per fruit was higher due to a
          higher dry weight accumulation (Banuelos et al., 1985). The conclusion is that constant high
          humidity appears to have greater benefit to proper fruit development than constant low
          humidity.
               o High humidity with low temperature – less BER.
               o Low humidity with high temperature – more BER.
An overall better expression of environmental conditions in relation to transpirational activity is the term VPD.

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VPD expressed in Kilo Pascals (kPa), is the difference between vapor pressure of the air when it is saturated to the
vapor pressure at the measurement time. In relation to the plant behavior, VPD is the “vapor pressure deficit”
between the inside and the outside of the leaf. VPD is combining both air humidity and temperature therefore
more accurate than just using RH (Relative Humidity). A change in VPD is always followed by a change in the
Evapotranspiration, unlike change in the RH. Measuring VPD is better when making decisions on heating,
ventilation, CO2 enrichment and humidity control. The VPD will affect the morphology of the leaves and their
ability to cope with changing weather conditions. At small VPD's the leaves will be small and light in color and
the growing point will be weak and thin. These are undesirable qualities in a producing crop.

Fruit growth rate
Higher growth rate will increase the BER phenomenon due to the dilution effect, while in low growth rate the
BER is expected at limited incidence. Fruits are at greatest risk of BER during the rapid phase of fruit
enlargement, some 14 to 21 days after anthesis. This is the time at which maximum dry matter but minimum
calcium accumulation occurs. Susceptible cultivars exhibit fewer and smaller xylem vessels in the distal end of
the fruit (Adams and Ho, 1992). The lowest concentration of calcium in the whole fruit was found in the distal
tissues. Efficiency of the internal calcium transport mechanism (absorption, transport and distribution) is a
function of the extensiveness of the developed network of vesicles (Minamide and Ho, 1993).


Plant nutrient supply
Calcium like other plant nutrients may be affected by other nutrients competing or decreasing its uptake by the
plant; therefore the right concentration should be applied for both Ca and its competitive nutrients.
    - Nitrogen source – (Ammonium/nitrate ratio), low ratio of NH4+ to NO3- will decrease the BER
          phenomena.
    - Nitrogen ratio – high nitrogen ratio above 100 ppm, will increase the risk of BER, but will be
          also dependent in the above source ratio.
    - Potassium concentration – high K ratio will increase the competition with Ca and will
          decrease its uptake.
    - Magnesium concentration – a common ration of 3:1 should be kept between Ca and Mg
          otherwise Mg will create a competition and will increase the BER problem.
    - Electrical conductivity – The electrical conductivity has several effects, when decreasing water
          volume, using fertilizers with high salt index or increasing fertilization the E.C will rise and will
          increase BER phenomenon. High E.C will decrease water absorption and xylem vessels
          lignification which reduce it’s optimal functioning. At high E.C levels, more calcium is retained
          in the root tissue. Electrical conductivity can be used to restrict fruit growth rate. In the case of
          salinity levels greater than 3 mS/cm but less than 8 mS/cm, growth rate is restricted without a
          concomitant reduction in dry matter portioning (Adams and Holder 1992).
    - Irrigation frequency and manganese concentrations – Increasing the irrigation frequency
          significantly increases the plant acquisition of nutrients, especially phosphorus and manganese
          (Silber et al., 2004). It was found that there is a negative correlation between the numbers of
          BER affected fruits and fruit-Mn concentrations. In recent works it was found that BER effect
          on the tissue is related to production of oxygen free radicals and diminution of anti oxidative
          compounds and enzymatic activities. Manganese plays a crucial role in enzyme activities and in
          detoxification of oxygen free radicals, this relationship between BER incidence and fruit Mn
          concentration may indicate that BER is related to Mn deficiency although further research is
          needed to validate this hypothesis.
    - Controlling pH - pH should be maintained at the range of 5.5-6.5 to avoid further acidification
          that would lead to reduction in the calcium cations availability.



Controlling the phenomenon
In order to assure proper growth and good control of the BER problem several steps in irrigation, fertilization and
control of the climate in the greenhouse should be taken.


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Fertilization
High care should be given to proper supply of Calcium and prevention of any possible competition.
    - In fertigation supply of 60-100 ppm of Ca should be enough, but when more cloudy weather is
         present the concentration should be increased to extent of 150-200 ppm of Ca.
    - A decrease in the share of ammonium (NH4+) should be made. The range should be between 10-
         20% maximum, but when affected even the rate should be even lower than 10%.
    - A decrease in the concentration of potassium (K+). K: Ca 1.6-2.0:1.0, since it is easer for plant
         to take the monovalent ions (X+/-) rather than the divalent ions (X2+/-).
    - Electrical conductivity (E.C) should be maintained in a reasonable level (2.5-3.0 mS/cm), to
         avoid over fertilization, which will increase the BER symptoms.
    - Foliar applications of Calcium are efficient although most of the calcium supplied in this way is
         not been translocated from the absorbing leaves; only the part absorbed by the fruit tissue plays
         a role.


Irrigation
Irrigation is important not for proper water supply but also as a vessel that should be used in efficient way to
deliver the nutrients.
     - Maintaining sufficient irrigation, avoiding water logging or water shortage that will lead to
          salinity problem.
     - Overcoming the transpiration calcium distribution is possible when adding some irrigation
          during the night. While during the day the water is been pulled by the negative pressure of the
          air forcing transpiration through the stomatas and cause an uneven distribution, where most of
          the calcium is dragged by the leaves and the least is reaching the fruits. During the night ‘root
          pressure’ is pushing the water and therefore the distribution of the calcium if supplied during
          this period is more even and uniform, and the fruits tissue can enjoy better supply. Night
          fertigation with calcium are most recommended for higher efficiency.
     - Irrigation intervals play a major role in nutrient supply. Continues replenishment of nutrients in
          the depletion zone near the root/medium interface and enhanced transport of dissolved nutrients
          by mass flow, because of the higher time-averaged water content in the medium during daytime
          (Silber et al., 2004) It was remained unclear why at high frequency there is less BER incidence,
          it was suggested that it is maybe related to the higher phosphorus, manganese availability, low
          E.C and high water availability.
          Irrigation frequency can reach up to 10-30 applications a day as long as the single irrigation volume is
          not too small to the extent it will cause salinity build-up.


Environmental control
Controlling the growth environment will directly affect the transpiration rate and therefore it is most important to
follow it with efficient nutrient supply. The main target is to maintain a proper transpiration rate that will not
damage the plant performance but will decrease the uneven distribution of the calcium.
      - In attempt to reduce the plant stress shading nets should be used to decrease the transpiration
          rate, temperature and radiation (better placed on top of the roof to avoid ventilation problems).
          It is also possible to color the roof white to reduce radiation (day transpiration). The desired
          radiation should be between 900 to 1000 Micro-Einsteins. Selecting the light intensity at which
          to start shading depends on the vigor of the plant (vegetative: generative balance) and how far
          into the season the crop has progressed. Crops with small leaves or small overall size with
          reduced vigor will need to be shaded at lower radiation levels while healthy vigorous growing
          crop with good canopy can withstand high levels of irradiance.
      - Temperature should be decreased using foggers that will increase humidity by creation of mist,
          another possibility is using ‘cooling mattress’.
      - Using foggers should also increase humidity. Under high humidity the total amount of calcium
          (milligram) per fruit is higher due to higher dry weight accumulation in these conditions
          (Banuelos et al. 1985).
It is possible to distinguish two major climatic conditions:
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Sudden changes occur from overcast to sunny conditions.
    - If CO2 enrichment exists it should be stopped.
    - The crop should be shaded
    - Temperature should be reduced
    - Calcium concentration in the feeding solution should be increased
    - Humidity should be increased
During prolonged sunny periods.
    - Irrigation should be done more frequently
    - Temperature difference between day/night should be reduced

Agrotechnique action
During the initial establishment phase of the crop a technique of leaf picking can be used. Leaves picking is
involved in removal of the leaf from behind the flowering truss. Leaf picking ensures a better distribution of Ca2+
between the leaves and the developing fruits. This action continues depending on the climate and the variety,
especially important for those varieties that are naturally vegetative. Older leaves at the bottom of the plant
canopy are big sink for Ca2+. These leaves will not be contributing to photosynthetic reaction. To minimize the
potential risk of Botrytis infection, leaf picking should be conducted in the morning to allow the wounds to dry.
On the other hand leaves are very important in relation to the fruit temperature. The fruit temperature affects the
rate of respiration and starch synthesis. Fruit growth and assimilates influx increases with the rise of the fruit
temperature. The fruit growth rate can be controlled by temperature reduction, when the fruit is well covered by
leaves. It was found that that tomato fruits at the top of the plant can have a temperature of 38.4oC while those at
third and forth truss would have only 29.5oC (Starver, 1991)




                                                 Written by:
                                                  Eyal Ronen
                                        Haifa Chemical Chief Agronomist




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                  Tel: 972-4-8469616, Fax: 972-4-8469953, E-mail: specialty@haifachem.com
                                             www.haifachem.com

				
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