Physiological Disorders Related to Irrigation and Fertilization
Blossom-end rot (BER)
Description
At the anatomical level, the earliest symptoms are areas of white or brown
locular tissue. Symptoms next appear in the fruit placenta in the case
of internal blossom-end rot or in the blossom-end pericarp in the case
of external blossom-end rot (Adams and Ho,
1992). Externally, the disorder begins as a small, water-soaked spot
at or near the blossom scar of green tomatoes. As the spot enlarges, the
affected tissue dries out and turns light to dark brown, gradually developing
into a well-defined, sunken, leathery spot (Fig.
3).
Causes
It has long been known that BER develops when calcium and/or water levels
in the root zone are low. Only recently, however, have Ho and co-workers
explained the complex interplay of anatomical, genetic and environmental
factors determining whether or not a given fruit develops the disorder.
According to Adams and Ho (1993):
"The basic cause of BER is a lack of co-ordination between
the transport of assimilates by the phloem and of calcium by the xylem
during rapid cell enlargement in the distal placenta tissue, i.e. an interaction
between the rates of fruit growth and of calcium acquisition at the distal
end of the fruit. Whilst changes in the environment have a marked influence
on the incidence of BER, genetic susceptibility is also a major cause
of the disorder."
At the anatomical level, lack of calcium is the immediate cause of the tissue breakdown that leads to the development of the disorder. This lack of calcium can occur even when calcium is relatively abundant in the root zone because it represents a localized deficiency in the distal (blossom end) locular tissue of the fruit. There are several reasons for these low calcium concentrations. Deposition into the calcium pectate and calcium phosphate fractions in the distal pulp tissue is low under all conditions (Minamide and Ho, 1993). When calcium import by the tomato fruit is further reduced by external factors, the requirements for cell walls and cell membranes may not be met. Leakage of cell contents, resulting from a loss of semipermeability of the cell membrane or weakened cell walls may be the direct cause of BER symptoms.
A scarcity of vascular tissue, especially as the fruit enlarges, predisposes the distal regions of the fruit to calcium deficiency. The number of vascular bundles decreases from the proximal (stem) end to the distal end of the fruit (Belda and Ho, 1993) and during the two weeks after anthesis, rapid expansion of the fruit means that the density of bundles falls dramatically. This parallels a sharp decline in the calcium concentration in the fruit tissue (Ehret and Ho, 1986).
Water stress, low daytime relative humidity, high light, and high temperature are often cited as causes of BER (e.g. Tan and Dhanvantari, 1985; Pill and Lambeth, 1980). It seems likely that all these stresses reduce calcium transport to the fruit. Calcium is transported only in the water-conducting tissues (xylem). When water uptake is reduced, calcium uptake is also reduced. Humidity affects calcium transport to the fruit because the fruit and the leaves normally compete for water. Daytime low humidity and high temperature increase transpiration, forcing more calcium to the leaves and less to the fruit. Conversely, high humidity decreases transpiration, thereby decreasing accumulation of calcium in the leaves and increasing the calcium content of the fruit (Adams and Ho, 1993). High humidity at night also increases fruit calcium uptake, but more calcium is absorbed during the day than at night on as absolute basis. The increase in calcium in the fruit due to high humidity at night is relatively small compared to humidity effects on calcium uptake during the day (Adams and Ho, 1993). The increase in calcium uptake at high nighttime relative humidities has been attributed to high root pressure (Gutteridge and Bradfield, 1983; Banuelos et al., 1985). Constant high (95%) relative humidity in growth chambers, however, reduced calcium concentration and increased BER relative to constant 55% relative humidity (Banuelos et al., 1985). The authors felt that maintaining constantly high relative humidity prevented the buildup of nighttime root pressure, and the associated high levels of calcium uptake.
Ho et al. (1993) reported a positive relationship between BER incidence and the product of the average daily solar radiation integral and temperature during the period of rapid fruit growth. They separated high light and high temperature effects from each other in a series of greenhouse experiments in which the effects of raising temperatures by 2°C were compared to those of shading to reduce light. Added heating was found to increase BER incidence to a much greater extent than added sunlight, presumably because high temperatures increase the rate of fruit expansion more than does extra light. A rapid rate of fruit enlargement (Ho et al., 1993) would increase the demand for calcium in plasmalemma synthesis because of the higher rate of cellular enlargement. This may explain the observation of DeKock et al. (1982a) that thinning tomatoes to 1 or 2 fruit per truss initially increases the size of the fruit initially but subsequent trusses were severely affected by blossom-end rot.
The incidence of BER also increases in saline conditions. Salinity decreased both total calcium uptake and the calcium content of the fruit. (Adams and Ho, 1993). Salinity reduced calcium uptake mainly by restricting water uptake. Raising the salinity by adding major nutrients such as Mg and K rather than NaCl, increased BER even more (Adams and Ho, 1993), presumably because of competition of these elements with calcium. Similarly, providing nitrogen in the ammonium, as opposed to the nitrate form has been associated with higher BER levels (Pill and Lambeth, 1980; DeKock et al., 1982b). Xylem development inside the fruit was also restricted by salinity (Belda and Ho, 1993), decreasing the fruit's ability to transport calcium to the distal end. Since high salinity increases such fruit quality parameters as fruit dry matter content, sugar content, acidity and shelf life, it is sometimes raised in soilless culture, despite lower fruit production and the risk of BER. Nederhoff (1999) found that increases in EC (8mS cm-1) at night but normal levels (2 mS cm-1) during the day showed some potential for improving tomato fruit quality with minimum loss of production. However, BER was not reduced in this treatment.
A typical scenario in which BER develops is as follows: When a bright period follows a period of cloudy, dull days, the increase in air temperatures reduces the relative humidity, thus increasing the rate of transpiration, drawing more water and calcium to the leaves and away from the fruit. In the greenhouse, the vents would be opened under these conditions to reduce the temperature, which would further reduce relative humidity and impede movement of calcium to the fruit. The increase in light would stimulate an increase in photosynthesis, which will move into the fruit. This increase and the increase in air temperature, which increases the rate of fruit expansion would both stimulate fruit growth rate, increasing the demand for calcium. If salinity or NH4-N levels are also high, the chances of BER development are even higher.
Control
BER is now relatively well understood, but control is still not always
achievable in practice. For example, large fluctuations in soil water
deficits increase BER (Pill and Lambeth,
1980; Shaykewich et al., 1971) so
mulching should decrease BER by reducing soil moisture losses. Elmer and
Ferrandino (1991) however, reported
that black plastic mulch increased early season BER, but did not significantly
affect the seasonal total. They felt this was because the plastic mulch
reduced penetration of early season rainwater to the roots.
The following general guidelines should be helpful however. First in importance is assuring that the rootzone calcium supply is adequate and that concentrations of competing cations are not excessive. High levels of K and Mg in the nutrient solution replace Ca. These nutrients should be maintained at about 400 and 80 mg l -1, respectively, but up to 500 mg l-1 Na has little effect on Ca uptake or BER incidence (Adams, 1999). Second, the water supply must be conducive to uptake, i.e., not too saline, flooded or otherwise restricted. Salinity should be reduced if it rises above 4-5 mS cm-1 in hot weather (Adams, 1999). Third, water must go to the fruit, as opposed to the leaves, which means avoiding daytime high temperatures and low humidity. Misting or fogging inside the greenhouse should reduce BER incidence. Finally, as with all physiological disorders, cultivars differ in susceptibility.
