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The physiological disorder blossom end rot (BER) can result in significant losses for growers.

It is mostly visible as a water-soaked dark spot at the end of the fruit occurring when the young fruit is in rapid expansion but can also develop only internally as black seeds and/or pulp. Although it has been widely studied and described prediction and prevention remains a challenge. Below is a summary of the major factors influencing BER in tomatoes.


Physiological Factors

The incidence of BER is observed when the water-soluble Ca concentration in the distal part of immature fruits decreases below a critical minimum (a critical minimum of 0.2% has been reported). Ca plays an import  physiological role in plants as central part of cell walls, as signalling molecule and in optimising the plant stress response.  Although BER is defined as a Ca related disorder, it is rarely solely a result of low concentrations of soluble Ca in the soil but also the plant’s ability to take the Ca from the rootzone to the fruit. Ca uptake is passive and coupled to water uptake along an electrochemical gradient and its movement depends on transpirational water flow and root pressure.

The development of BER is often related to a reduction in Ca uptake associated with reduced transpiration and/or a change in partitioning within the plant. Reducing for instance the number of leaves that can contribute to transpiration or increasing relative humidity and thereby decreasing the transpiration rate will reduce xylemic Ca transport to the leaves and increase its movement into the fruits. In addition to being coupled to the uptake and transport of water through the xylem, Ca can in turn also regulate water flow in plants through changes in the cell wall structure and
stomatal opening and Ca itself is also linked with xylem development.

The critical period for BER occurs at about the second week after anthesis when the relative growth rate of fruit is at a peak and cell enlargement is starting. During this phase the fruit act as a major sink. A stronger sink to source ratio is also related to a higher incidence of BER. Upper trusses
are more prone to BER, especially under stress inducing conditions – salinity, drought, high temperatures – and within a truss fruit from the basal part is often most severely affected.

As Ca moves predominantly in the xylem, Ca ions absorbed by the leaves are not translocated to the fruits. The efficacy of foliar applied Ca sprays to prevent BER therefore remains low. Spraying Ca directly onto young fruits can however prevent the onset of BER to some extent, but only when done during early fruit development.

Environmental Stresses

The tomato plant is well adapted to most climatic regions; however, environmental stresses are the primary constraints of tomato potential yield and quality. The mode of action of environmental stresses increasing the incidence of BER is related to either affecting water and Ca uptake and translocation or the release of toxic compounds in young fruits apoplast.
The presence of these toxic compounds increases the leakiness of membranes, leading to loss of apoplastic Ca ions, which is linked to blossom end rot.

Root zone temperature can influences water and Ca uptake. At cooler temperatures more oxygen will be dissolved in the soil solution, improving root development. This is important as most of the Ca is taken up at the young root tips. High air temperatures and low relative humidity increase the translocation of water and Ca to leaves for transpirational cooling, decreasing the movement of Ca to the fruit. This is also due to the limited potential of fruit to transpire – since there are fewer stomata. High air temperature (together with high light intensity) increases photosynthetic
rate and rapid fruit cell enlargement and transpiration that will bring about a
preferential transport of Ca to leaves rather than fruits. 

Soil properties, chemical and biological, affect nutrient availability and thus BER occurrence. The tomato plant is moderately sensitive to soil salinity and sodicity.  Soil salinity will restrict water uptake by roots and can change nutrient uptake and even damages cells resulting in impaired functioning of membranes, salt accumulation in cells and a decrease in the Ca concentration. 

Genetic Factors

Tomato cultivars vary in their susceptibility to BER. The primary factor determining the likelihood of blossom end rot developing is the growth rate of the fruit and the potential fruit size. Small fruits lacking the rapid fruit expansion phase is rarely affected while large fruits, where a rapid expansion of cells in the distal portion of the fruit, associated with reduced Ca translocation and lower water soluble Ca concentrations in the fruit,
is more often prone to blossom end rot. High yielding varieties tend to be more susceptible, likely due to more assimilates accumulating in fruit via the phloem, against limited quantities of water and Ca moving to the fruit via the xylem.

Agronomic Factors

Best practice to reduce the incidence of BER includes starts with a proper assessment of the soil characteristics, choosing the best varieties – including the use of grafted plants. In addition, optimal irrigation scheduling, taking water quality into consideration, and a well balanced fertiliser program can help avoid plant stress that can result in BER.

Ca uptake and distribution is decreased by high concentration of monovalent cations in soils such as K, Na, and NH4. This reduces the ratio of Ca to other elements and can increases the prevalence of BER.  Reducing the K has been shown to improve Ca uptake and reduce the occurrence of BER. At pH>5.0, increasing soil acidity is associated with increased  vailability of Ca in the soil solution. As the pH decrease, H replaces Ca2  ions on exchange sites making them available in the soil solution for uptake through roots. BER incidence is also positively correlated to the concentration of NH4 versus NO3 in the nutrient solution, especially under high temperatures.

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