Internal tipburn is one of the most common physiological disorders affecting a wide range of vegetables and fruit. A necrotic breakdown of the marginal tissues of leaves is seen in cabbage heads; both Chinese and European cabbage are afflicted. Tipburn is usually attributed to localized calcium deficiency and related in incidence to genotype and prevailing weather conditions, and the availability of nitrogen fertilizer. Large applications of easily available nitrogen at transplanting increase the shoot to root ratio, unbalancing the plant metabolically and leading to physiological disorders such as tipburn. Magnusson (2002) suggested that rapidly increasing growth and high total nitrogen and nitrate concentrations at harvest increased the incidence of tipburn. He made comparisons of growth of Chinese cabbage with 'green mulch' grass leys used as intercrops (Chapter 6) and in combination with mineral fertilizers in which intercropping decreased the prevalence of tipburn.
An extensive review of tipburn in Brassica was published by Everaarts (2001), who subscribes to the view that a number of interacting factors lead to the expression of tipburn symptoms. This physiological problem is increasing with all forms of European and Oriental Brassica; possibly this is analogous to the upsurge in pathogenic and pest problems and reflects the increasingly concentrated genetic base from which commercial Brassica cultivars are drawn. Normally, there are no external symptoms, while internally they vary between genotypes and between storage and fresh market types.
In the fresh market types, the symptoms are desiccated papery thin leaf margins extending in zones from several millimetres to eventually covering the entire leaf. In genotypes intended for storage, the symptoms are dry, papery, thin, dark brown circular to oval spots with deep brown to black margins of several millimetres to several centimetres in size.
Localized calcium deficiency in the leaves is found in rapidly growing tissues with low transpiration rates and is exacerbated by constant high relative humidity, which stimulates calcium-related disorders. Since calcium is transported mainly in the xylem, the amount reaching the growing and meristematic tissues is closely related to the rate of transpiration; where this is low, then calcium deficiencies will develop.
Calcium accumulates in the outer leaves during the day by mass flow in the transpiration stream, and in heads at night when growth takes place and root pressure forces water and calcium into the head. Some genotypes exhibit levels of resistance or tolerance to tipburn. Since calcium is only taken up by the very young unsuberized roots, the number of young roots and the position in the soil of root apices relate to calcium uptake; consequently, root architecture is an important factor in the expression of this disorder. The large vigorous and deeply rooting genotypes are less likely to be prone to tipburn. Cultivars producing high yields and having rapid growth are prone to tipburn, but it is discouraged by the use of wider planting distances. Tipburn is most frequent where plants grow rapidly but fail to develop a sufficient root system, and in consequence there is a high leaf to root ratio.
The rate of biomass production in Brussels sprout is known to be proportional to the intercepted radiation. High radiation rates will increase growth rate and, thus, the amount of tipburn in this crop. The incidence of tipburn is associated with the use of high levels of nitrogen fertilizer. This is due to accelerated growth resulting from the availability of nitrogen, not the effect of the element per se. Ammonium as a nitrogen source reduces the uptake of calcium due to competition between the two cations. Husbandry factors such as planting date also affect the incidence of tipburn. Cabbages intended for long-term storage are normally planted between the end of April and mid-May, with head formation starting 60-75 days later. Early planting is associated with increased tipburn resulting from high growth rates during periods of limited duration of darkness. Delaying harvests increases the likelihood of tipburn developing; probable interacting factors are the age of tissue, and storage which increases tipburn with cool (low temperature) storage.
White cabbage roots can penetrate to 100 cm or more and the root shape is obconical reaching 150 cm deep, but the greatest intensity of rooting is in the top 20 cm of soil. Such size and vigour tend to minimize the incidence of tipburn. Soil characteristics can affect the incidence of tipburn, thus for example Dutch growers stop liming when the soil calcium content exceeds 2%. Normally they use well-cultivated and drained, moisture-retaining fertile soils for cabbage.
Soil where waterlogging is present leads to dysfunctional root growth, resulting in anaerobiosis. Even when anaerobiosis only lasts for short periods, it is sufficient to cause root death and, in consequence, reduces calcium uptake and initiates subsequent tipburn. Calcium applications made directly to the crop, however, are unlikely to be of benefit except possibly sprays of calcium chloride or nitrate. Tipburn is associated with inadequate calcium uptake by young, rapidly growing leaves. Several soil chemical and environmental factors that increase plant growth and decrease calcium mobility and transpiration have been implicated.
Soil cations (Ca2+, K+, Mg2+ and NH4+) play critical roles in the development and prevention of physiological disorders usually involving competition for plant uptake resulting in an excess or deficiency of a particular element or elements in the tissues (Cubeta et al., 2000). Rapidity of growth in the presence of excessive nitrogen and minimal calcium appears to be the dominating factor in the development of this syndrome and affects all Brassica crops that form heads or inflorescences. The association with calcium deficiency is shown in Table 8.4
The association between tipburn and root size was demonstrated by Johnson (1991). Root systems of a tipburn-susceptible cultivar were smaller than those of a tipburn-resistant one, hence the plants were more susceptible to moisture stress, but per se this does not necessarily correlate with the development of tipburn. The calcium efficiency ratio (CaER = mg of dry matter produced per mg of calcium in tissue) in young leaves was greater, however, for tipburn-tolerant cultivars than for susceptible ones, and provides a valuable gauge to the likelihood of symptom expression (Table 8.5).
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